Issue 8 - February 2005
"The carrier shall properly and carefully load, handle, stow, carry, keep, care for and discharge the goods carried."
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The carriage of liquefied gases
Introduction
The renewed interest in gas, which started in the 1990s due to its excellent environmental credentials, has seen an increase in the order book for LNG carriers - LNG carriers being the leviathans of the gas carrier fleet. Yet, while attracting great interest, the gas trade still employs relatively few ships in comparison to oil tankers, and hence its inner workings are little known except to a specialist group of companies and mariners.
Considering the fleet of gas carriers of over 1,000 m3 capacity, the total of nearly 1,000 ships can be divided into five major types according to the following table:
By contrast, the world oil tanker fleet for a similar size range is over 16,000 ships! Given the relative paucity of knowledge on gas tankers in comparison to oil tankers, the purpose of this article is to describe the gas carrier genre, its particularities within each type and its comparison with other tankers. The aim is to provide basic knowledge about gas carriers and an overview of their strengths and weaknesses, both from design and operational viewpoints. A second article describes the liquefied natural gas (LNG) carrier in more detail and a third article, to be published later, will describe the liquefied petroleum gas (LPG) carrier.
The introduction of a tanker designed to carry compressed natural gas (CNG) is anticipated in the near future. A number of designs have been produced but, due to the relatively low deadweight and high cost of these ships, the first commercial application of this technology cannot be predicted.
The gas carrier is often portrayed in the media as a potential floating bomb, but accident statistics do not bear this out. Indeed, the sealed nature of liquefied gas cargoes, in tanks completely segregated from oxygen or air, virtually excludes any possibility of a tank explosion. However, the image of the unsafe ship lingers, with some administrations and port state control organisations tending to target such ships for special inspection whenever they enter harbour. The truth is that serious accidents related to gas carrier cargoes have been few, and the gas carrier's safety record is acknowledged as an industry leader. As an illustration of the robustness of gas carriers, when the Gaz Fountain was hit by rockets in the first Gulf War, despite penetration of the containment system with huge jet fires, the fires were successfully extinguished and the ship, together with most cargo, salved.
The relative safety of the gas carrier is due to a number of features. One such, almost unique to the class, is that cargo tanks are always kept under positive pressure (sometimes just a small overpressure) and this prevents air entering the cargo system. (Of course special procedures apply when stemmed for drydock). This means that only liquid cargo or vapour can be present and, accordingly, a flammable atmosphere cannot exist in the cargo system.
Moreover all large gas carriers utilise a closed loading system with no venting to atmosphere, and a vapour return pipeline to the shore is often fitted and used where required. The oxygen-free nature of the cargo system and the very serious limitation of cargo escape to atmosphere combine to make for a very safe design concept.
The liquefied gases
First let us consider some definitions in the gas trade. According to the IMO, a liquefied gas is a gaseous substance at ambient temperature and pressure, but liquefied by pressurisation or refrigeration - sometimes a combination of both. Virtually all liquefied gases are hydrocarbons and flammable in nature.
Liquefaction itself packages the gas into volumes well suited to international carriage - freight rates for a gas in its non-liquefied form would be normally far too costly. The principal gas cargoes are LNG, LPG and a variety of petrochemical gases. All have their specific hazards.
LNG is liquefied natural gas and is methane naturally occurring within the earth, or in association with oil fields. It is carried in its liquefied form at its boiling point of -162ºC. Depending on the standard of production at the loading port, the quality of LNG can vary but it usually contains fractions of some heavier ends such as ethane (up to 5%) and traces of propane.
The second main cargo type is LPG (liquefied petroleum gas). This grade covers both butane and propane, or a mix of the two. The main use for these products varies from country to country but sizeable volumes go as power station or refinery fuels. However LPG is also sought after as a bottled cooking gas and it can form a feedstock at chemical plants. It is also used as an aerosol propellant (with the demise of CFCs) and is added to gasoline as a vapour pressure enhancer. Whereas methane is always carried cold, both types of LPG may be carried in either the pressurised or refrigerated state. Occasionally they may be carried in a special type of carrier known as the semi-pressurised ship. When fully refrigerated, butane is carried at -5ºC, with propane at -42ºC, this latter temperature already introducing the need for special steels.
Ammonia is one of the most common chemical gases and is carried worldwide in large volumes, mainly for agricultural purposes. It does however have particularly toxic qualities and requires great care during handling and carriage. By regulation, all liquefied gases when carried in bulk must be carried on a gas carrier, as defined by the IMO. IMO's Gas Codes (see next section - Design of gas carriers) provide a list of safety precautions and design features required for each product.
A specialist sector within the trade is the ethylene market, moving about one million tonnes by sea annually, and very sophisticated ships are available for this carriage. Temperatures here are down to -104ºC and onboard systems require perhaps the highest degree of expertise within what is already a highly specialised and automated industry. Within this group a sub-set of highly specialised ships is able to carry multi-grades simultaneously.
Significant in the design and operation of gas carriers is that methane vapour is lighter than air while LPG vapours are heavier than air. For this reason the gas carrier regulations allow only methane to be used as a propulsion fuel - any minor gas seepage in engine spaces being naturally ventilated. The principal hydrocarbon gases such as butane, propane and methane are non-toxic in nature and a comparison of the relative hazards from oils and gases is provided in the table below:
Design of gas carriers
The regulations for the design and construction of gas carriers stem from practical ship designs codified by the International Maritime Organisation (IMO). This was a seminal piece of work and drew upon the knowledge of many experts in the field - people who had already been designing and building such ships. This work resulted in several rules and a number of recommendations.
However all new ships (from June 1986) are built to the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (the IGC Code). This code also defines cargo properties and documentation, provided to the ship (the Certificate of Fitness for the Carriage of Liquefied Gases in Bulk), shows the cargo grades the ship can carry.
In particular this takes into account temperature limitations imposed by the metallurgical properties of the materials making up the containment and piping systems. It also takes into account the reactions between various gases and the elements of construction not only on tanks but also related to pipeline and valve fittings.
When the IGC Code was produced an intermediate code was also developed by the IMO - the Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (the GC Code). This covers ships built between 1977 and 1986.
As alluded to above, gas carriers were in existence before IMO codification and ships built before 1977 are defined as 'existing ships' within the meaning of the rules. To cover these ships a voluntary code was devised, again by the IMO - the Code for Existing Ships Carrying Liquefied Gases in Bulk (the Existing Ship Code).
Despite its voluntary status, virtually all ships remaining in the fleet of this age - and because of longevity programmes there are still quite a number - have certification in accordance with the Existing Ship Code as otherwise international chartering opportunities would be severely restricted.
Cargo carriage in the pressurised fleet comprises double cargo containment - hull and tank. All other gas carriers are built with a double hull structure and the distance of the inner hull from the outer is defined in the gas codes. This spacing introduces a vital safety feature to mitigate the consequences of collision and grounding. Investigation of a number of actual collisions at the time the gas codes were developed drew conclusions on appropriate hull separations which were then incorporated in the codes. Collisions do occur within the class and, to date, the codes' recommendations have stood the test of time, with no penetrations of cargo containment having been reported from this cause. The double hull concept includes the bottom areas as a protection against grounding and, again, the designer's foresight has proven of great value in several serious grounding incidents, saving the crew and surrounding populations from the consequences of a ruptured containment system.
So a principal feature of gas carrier design is double containment and an internal hold. The cargo tanks, more generally referred to as the 'cargo containment system', are installed in the hold, often as a completely separate entity from the ship; i.e. not part of the ship's structure or its strength members.
Herein lies a distinctive difference between gas carriers and their sisters, the oil tankers and chemical carriers. Cargo tanks may be of the independent self-supporting type or of a membrane design. The self-supporting tanks are defined in the IGC Code as being of Type-A, Type-B or Type-C.
Type-A containment comprises box shaped or prismatic tanks (i.e. shaped to fit the hold).
Type-B comprises tanks where fatigue life and crack propagation analyses have shown improved characteristics. Such tanks are usually spherical but occasionally may be of prismatic types.
Type-C tanks are the pure pressure vessels, often spherical or cylindrical, but sometimes bi-lobe in shape to minimise broken stowage.
The fitting of one system in preference to another tends towards particular trades. For example, Type-C tanks are suited to small volume carriage. They are therefore found most often on coastal or regional craft. The large international LPG carrier will normally be fitted with Type-A Tanks. Type-B tanks and tanks following membrane principles are found mainly within the LNG fleet.
The pressurised fleet
Pressurised LPG carrier with cylindrical tanks
The first diagram, on the previous page, and the photograph above show a small fully pressurised carrier. Regional and coastal cargoes are often carried in such craft with the cargo fully pressurised at ambient temperature. Accordingly, the tanks are built as pure pressure vessels without the need for any extra metallurgical consideration appropriate to colder temperatures. Design pressures are usually for propane (about 20 bar) as this form of LPG gives the highest vapour pressure at ambient temperature. As described above, ship design comprises outer hull and an inner hold containing the pressure vessels. These rest in saddles built into the ship's structure. Double bottoms and other spaces act as water ballast tanks and if problems are to develop with age then the ballast tanks are prime candidates. These ships are the most numerous class, comprising approximately 40% of the fleet. They are nevertheless relatively simple in design yet strong of construction.
Cargo operations that accompany such ships include cargo transfer by flexible hose and in certain areas, such as China, ship-to-ship transfer operations from larger refrigerated ships operating internationally are commonplace.
Records show that several ships in this class have been lost at sea because of collision or grounding, but penetration of the cargo system has never been proven.
In one case, a ship sank off Italy and several years later refloated naturally, to the surprise of all, as the cargo had slowly vaporised adding back lost buoyancy.
The semi-pressurised fleet
Semi-pressurised LPG carrier
In these ships, sometimes referred to as 'semi-refrigerated', the cargo is carried in pressure vessels usually bi-lobe in cross section, designed for operating pressures of up to 7 bars. The tanks are constructed of special grade steel suitable for the cargo carriage temperature. The tanks are insulated to minimise heat input to the cargo. The cargo boils off causing generation of vapour, which is reliquefied by refrigeration and returned to the cargo tanks. The required cargo temperature and pressure is maintained by the reliquefaction plant.
These ships are usually larger than the fully pressurised types and have cargo capacities up to about 20,000 m3. As with the fully pressurised ship, the cargo tanks are of pressure vessel construction and similarly located well inboard of the ship's side and also protected by double bottom ballast tanks. This arrangement again results in a very robust and inherently buoyant ship.
The ethylene fleet
Ethylene, one of the chemical gases, is the premier building block of the petrochemicals industry. It is used in the production of polyethylene, ethylene dichloride, ethanol, styrene, glycols and many other products. Storage is usually as a fully refrigerated liquid at -104ºC.
Ships designed for ethylene carriage also fall into the semi-pressurised class. They are relatively few in number but are among the most sophisticated ships afloat. In the more advanced designs they have the ability to carry several grades. Typically this range can extend to ethane, LPG, ammonia, propylene butadiene and vinyl chloride monomer (VCM), all featuring on their certificate of fitness. To aid in this process several independent cargo systems co-exist onboard to avoid cross contamination of the cargoes, especially for the reliquefaction process.
The ships range in size from about 2,000 m3 to 15,000 m3 although several larger ships now trade in ethylene. Ship design usually includes independent cargo tanks (Type-C), and these may be cylindrical or bi-lobe in shape constructed from stainless steel. An inert gas generator is provided to produce dry inert gas or dry air. The generator is used for inerting and for the dehydration of the cargo system as well as the interbarrier spaces during voyage. For these condensation occurs on cold surfaces with unwanted build-ups of ice. Deck tanks are normally provided for changeover of cargoes.
The hazards associated with the cargoes involved are obvious from temperature, toxic and flammable concerns. Accordingly, the safety of all such craft is critical with good management and serious personnel training remaining paramount.
The fully refrigerated fleet
Fully refrigerated LPG carrier
These are generally large ships, up to about 100,000 m3 cargo capacity, those above 70,000 m3 being designated as VLGCs. Many in the intermediate range (say 30,000 m3 to 60,000 m3) are suitable for carrying the full range of hydrocarbon liquid gas from butane to propylene and may be equipped to carry chemical liquid gases such as ammonia. Cargoes are carried at near ambient pressure and at temperatures down to -48ºC.
Reliquefaction plants are fitted, with substantial reserve plant capacity provided. The cargo tanks do not have to withstand high pressures and are therefore generally of the free standing prismatic type. The tanks are robustly stiffened internally and constructed of special low temperature resistant steel.
All ships have substantial double bottom spaces and some have side ballast tanks. In all cases the tanks are protectively located inboard. The ship's structure surrounding or adjacent to the cargo tanks is also of special grade steel, in order to form a secondary barrier to safely contain any cold cargo should it leak from the cargo tanks.
All cargo tanks, whether they be of the pressure vessel type or rectangular, are provided with safety relief valves amply sized to relieve boil-off in the absence of reliquefaction and even in conditions of surrounding fire.
The LNG fleet
Although there are a few exceptions, the principal ships in the LNG fleet range from 75,000m3 to 150,000m3 capacity, with ships of up to 265,000 m3 expected by the end of the decade. The cargo tanks are thermally insulated and the cargo carried at atmospheric pressure. Cargo tanks may be free standing spherical, of the membrane type, or alternatively, prismatic in design. In the case of membrane tanks, the cargo is contained within thin walled tanks of invar or stainless steel. The tanks are anchored in appropriate locations to the inner hull and the cargo load is transmitted to the inner hull through the intervening thermal insulation.
LNG carrier with Type-B tanks (Kvaerner Moss system)
LNG carrier with membrane tanks
All LNG carriers have a watertight inner hull and most tank designs are required to have a secondary containment capable of safely holding any leakage for a period of 15 days. Because of the simplicity and reliability of stress analysis of the spherical containment designs, a full secondary barrier is not required but splash barriers and insulated drip trays protect the inner hull from any leakage that might occur in operation. Existing LNG carriers do not reliquefy boil-off gases, they are steam ships and the gas is used as fuel for the ship's boilers. The first ships to burn this gas in medium speed diesel engines will be delivered in 2005/6, and ships with reliquefaction plant and conventional slow speed diesel engines will enter service late in 2007. It is likely that gas turbine propelled ships may appear soon after this.
Crew training and numbers
As they did for oil tankers and chemical carriers, the IMO has laid down a series of training standards for gas carrier crews which come in addition to normal certification. These dangerous cargo endorsements are spelt out in the STCW Convention. Courses are divided into the basic course for junior officers and the advanced course for senior officers. IMO rules require a certain amount of onboard gas experience, especially at senior ranks, before taking on the responsible role or before progressing to the next rank. This can introduce checks and balances (say) in the case of a master from the bulk ore trades wanting to convert to the gas trade. The only way, without previous gas experience, to achieve this switch is to have the candidate complete the requisite course and sail as a supernumerary, understudying the rank for a specified period on a gas carrier. This can be costly for seafarer and company alike.
Accordingly, as the switch can be difficult to manage, especially at senior ranks, current requirements tend to maintain a close-knit cadre of 'gas men or women' well experienced in the trade.
In addition to the official certification for hazardous cargo endorsements, a number of colleges operate special courses for gas cargo handling. In the UK a leader in the field is the Warsash Maritime Centre.
While this situation provides for a well-trained and highly knowledgeable environment the continued growth in the fleet currently strains manpower resources and training schedules and it is possible that short cuts may be taken. While the small gas carriers normally operate at minimum crew levels, on the larger carriers it is normal to find increased crewing levels over and above the minimum required by the ship's manning certificate. For example, it is almost universal to carry a cargo engineer onboard a large gas carrier. An electrician is a usual addition and the deck officer complement may well be increased.
Gas carriers and port operations
As gas carriers have grown in size, so too has a concern over in-port safety. Indeed, the same concerns applied with the growth in tanker sizes when the VLCC came to the drawing board. The solutions are similar; however, in the case of the gas carrier, a higher degree of automation and instrumentation is often apparent controlling the interface between ship and shore.
Terminals are also protected by careful risk analysis at the time of construction so helping to ensure that the location and size of maximum credible spill scenarios are identified, and that suitable precautions including appropriate safety distances are established between operational areas and local populations. Regarding shipping operations, risk analysis often identifies the cargo manifold as the area likely to produce the maximum credible spill. This should be controlled by a number of measures. Primarily, as for all large oil tankers, gas carriers should be held firmly in position whilst handling cargo, and mooring management should be of a high calibre.
Mooring ropes should be well managed throughout loading and discharging. Safe mooring is often the subject of computerised mooring analysis, especially for new ships arriving at new ports, thus helping to ensure a sensible mooring array suited to the harshest conditions. An accident in the UK highlighted the consequences of a lack of such procedures when, in 1993, a 60,000 m3 LPG carrier broke out from her berth in storm conditions. This was the subject of an official MCA/HSE inquiry concluding that prior mooring analysis was vital to safe operations. The safe mooring principles attached to gas carriers are similar to those recommended for oil tankers (they are itemised in Mooring Equipment Guidelines, see References).
Hard arms at cargo manifold, including vapour return line (below, centre arm)
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Hard arm connection to manifold, showing double ball valve safety release
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Hard arm quick connect/disconnect coupler (QCDC)
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The need for such ships to be held firmly in position during cargo handling is due in part to the use of loading arms (hard arms - see photos above) for cargo transfer. Such equipment is of limited reach in comparison to hoses, yet it provides the ultimate in robustness. It also provides simplicity in the connection at the cargo manifold.
The use of loading arms for the large gas carrier is now quite common and, if not a national requirement, is certainly an industry recommendation. The alternative use of hoses is fraught with concerns over hose care and maintenance, and their proper layout and support during operations to prevent kinking and abrasion. Further, accident statistics show that hoses have inferior qualities in comparison to the hard arms. Perhaps the worst case of hose failure occurred in 1985 when a large LPG carrier was loading at Pajaritos, Mexico. Here, the hose burst and, in a short time, the resulting gas cloud ignited. The consequent fire and explosion impinged directly on three other ships in harbour and resulted in four deaths. It was one of those accidents which have led directly to a much increased use of loading arms internationally. The jetty was out of action for approximately six months. Fortunately the berth was in an industrial area and collateral damage to areas outside the refinery was limited.
As ships have grown in size the installation of vapour return lines interconnecting ship and shore vapour systems has become more common. Indeed, in the LNG industry it is required, with the vapour return being an integral part of the loading or discharging system. In the LPG trades, vapour returns are also common, but are only opened in critical situations such as where onboard reliquefaction equipment is unable to cope with the loading rate and boil-off. A feature common to both ship and shore is that both have emergency shutdown systems. It is now common to interconnect such systems so that, for example, an emergency on the ship will stop shore-based loading pumps. One such problem may be the automatic detection of the ship moving beyond the safe working envelope for the loading arms. A further refinement at some larger terminals is to have the loading arms fitted with emergency release devices, so saving the loading arms from fracture (see centre photo opposite). Given good moorings and well designed loading arms, the most likely sources of leakage are identified and controlled.
Hazards to shore workers and crewmembers at refit
While the gas carrier accident record is very good for normal operations, and exemplary with respect to cargo operations and containment, the same cannot be said for the risks it faces in drydock. Statistics show that the gas carrier in drydock presents a serious risk to personnel, particularly with respect to adequate ventilation through proper inerting and gas-freeing before repairs begin. Most often the risk relates to minor leakage from a cargo tank into the insulation surrounding refrigerated LPG tanks. A massive explosion occurred on the Nyhammer at a Korean shipyard in 1993 for this very reason, where considerable loss of life occurred. Although the ship was repaired, it was a massive job.
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Checklist
The following checklist, made available from SIGTTO, may be used as guidance in a casualty situation involving a disabled gas carrier.
- What cargo is onboard?
- Is specialist advice available in respect of the cargo and its properties?
- Are all parties involved aware of cargo properties?
- Is the cargo containment system intact?
- Is the ship venting gas?
- Is the ship likely to vent gas?
- What will be the vented gas and what are its dispersal characteristics?
- Is a gas dispersion modelling tool available?
- Is the ship damaged?
- Does damage compromise the ship's manoeuvring ability?
- What activities and services are planned to restore a seaworthy condition?
- Is ship-to-ship transfer equipment available if required? ? When is it expected the ship will be seaworthy again?
- Is prevailing shelter (and other dangers) suitable for the intended repairs?
- What contingency plans are required?
- Who will control the operation?
- How will the ship operator and port or public authorities co-operate?
- Will customs and immigration procedures need facilitation for equipment and advisers?
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Liquefied natural gas
Background
It was as far back as 1959 that the Methane Pioneer carried the first experimental LNG cargo, and 40 years ago, in 1964, British Gas at Canvey Island received the inaugural cargo from Arzew on the Methane Princess. Together with the Methane Progress these two ships formed the core of the Algeria to UK project. And the project-based nature of LNG shipping was set to continue until the end of the 20th century. LNG carriers only existed where there were projects, with ships built specifically for employment within the projects. The projects were based on huge joint ventures between cargo buyers, cargo sellers and shippers, all in themselves large companies prepared to do long term business together.
The projects were self-contained and operated without much need for outside help. They supplied gas using a purpose-built fleet operating like clockwork on a CIF basis. Due to commercial constraints, the need for precisely scheduled deliveries and limited shore tank capacities, spot loadings were not feasible and it is only in recent years that some projects now accept LNG carriers as cross-traders, operating more like their tramping cousins - the oil tankers.
Doubtless the trend to spot trading will continue. However, the co-operative nature of LNG's beginnings has led to several operational features unique to the ships. In particular there is the acceptance that LNG carriers burn LNG cargo as a propulsive fuel. They also retain cargo onboard after discharge (the 'heel') as an aid to keeping the ship cooled down and ready to load on arrival at the load port. Thus matters that would be anathema to normal international trades are accepted as normal practice for LNG.
Again, looking back to the early days, there was also great interest in this new fuel in the USA and France. Receiving terminals sprouted. However, gas pricing difficulties in the USA saw an end to early American interest while Gaz de France consolidated rather than expanded. Indeed, the American pricing problems, and the failure of an early US-built shipboard Conch containment system on new buildings, blanketed any spectacular progress in the Atlantic basin until the regeneration of interest initiated by the Trinidad project in 1999.
At that time, the stifling of European interest was also due to the discovery of natural gas in the North Sea, so quantities to replace town gas were available in sufficient volume on the doorstep without the need for imports. This being so, the first LNG project from Algeria to UK eventually faltered, with the receiving terminal at Canvey Island switching to other interests. The stagnation of LNG in the 70s and 80s applied the world over, with the singular exception of imports to Japan and Korea. Here interest in LNG's potential as an environmentally-friendly fuel stayed vibrant; as it does today. LNG projects are massive multi-billion dollar investments. Major projects in the Far East included Brunei to Japan, Indonesia to Japan, Malaysia to Japan and Australia to Japan, comprising some 90% of the LNG trade of the day.
Consequently, the Japanese defined much of what is seen best today in way of safety standards and procedures. It is worthy of note, however, that some early safety standards and practices are being questioned today in the light of experience in a more mature industry.
LNG as a fuel
Because the ships, terminals and commercial entities were all bound together in the same chain, advantages could be seen in limiting 'unnecessary' shipboard equipment, such as reliquefaction plant, and allowing the boil-off to be burnt as fuel. One way or another the ship would need fuel, be it oil or gas and, if gas, it was only then a matter to quantify usage and to direct the appropriate cost to the appropriate project partner.
Interestingly, this concept was recognised in the IMO's Gas Codes from the very earliest days, and with the appropriate safety equipment in place the regulations allow methane to be burnt in ship's boilers. This is not the case for LPG, where reliquefaction equipment is a fitment, but specifically because the LPGs are heavier than air gases and use in engine rooms is thereby disallowed.
LNG quality
LNG is liquefied natural gas. It is sharply clear and colourless. It comprises mainly methane but has a percentage of constituents such as ethane, butane and propane together with nitrogen. It is produced from either gas wells or oil wells. In the case of the latter it is known as associated gas. At the point of production the gas is processed to remove impurities and the degree to which this is achieved depends on the facilities available. Typically this results in LNG with between 80% and 95% methane content. The resulting LNG can therefore vary in quality from loading terminal to loading terminal or from day-to-day.
Other physical qualities that can change significantly are the specific gravity and the calorific value of the LNG, which depend on the characteristics of the gas field. The specific gravity affects the deadweight of cargo that can be carried in a given volume, and the calorific value affects both the monetary value of the cargo and the energy obtained from the boil off gas fuel.
These factors have significance in commercial arrangements and gas quality is checked for each cargo, usually in a shore-based laboratory by means of gas chromatography. LNG vapour is flammable in air and, in case of leakage, codes require an exclusion zone to allow natural dispersion and to limit the risk of ignition of a vapour cloud. Fire hazards are further limited by always handling the product within oxygen-free systems. Unlike oil tankers under inert gas, or in some cases air, LNG carriers operate with the vapour space at 100% methane. LNG vapour is non-toxic, although in sufficient concentration it can act as an asphyxiant.
Gas quality is also significant from a shipboard perspective. LNGs high in nitrogen, with an atmospheric boiling point of -196ºC, naturally allow nitrogen to boil-off preferentially at voyage start thus lowering the calorific value of the gas as a fuel. Towards the end of a ballast passage, when remaining 'heel' has all but been consumed, the remaining liquids tend to be high on the heavier components such as the LPGs. This raises the boiling point of the remaining cargo and has a detrimental effect on tank cooling capabilities in readiness for the next cargo.
The good combustion qualities attributed to methane make it a great attraction today as a fuel at electric power stations. It is a 'clean' fuel. It burns producing little or no smoke and nitrous oxide and sulphur oxide emissions produce figures far better than can be achieved when burning normal liquids such as low sulphur fuel oil. Natural gas has thus become attractive to industry and governments striving to meet environmental targets set under various international protocols such as the Rio Convention and the Kyoto Protocol. The practice of firing marine boilers on methane provides the further environmental advantage of lesser soot blowing operations and much fewer carbon deposits.
Cargo handling
The process of liquefaction is one of refrigeration and, once liquefied, the gas is stored at atmospheric pressure at its boiling point of -162ºC. At loading terminals any boil-off from shore tanks can be reliquefied and returned to storage. However, on ships this is almost certainly not the case. According to design, it is onboard practice to burn boil-off gas (often together with fuel oil) in the ship's boilers to provide propulsion. In the general terms of seaborne trade this is an odd way to handle cargo and is reminiscent of old tales of derring-do from the 19th century when a cargo might have been burnt for emergency purposes. It is nevertheless the way in which the LNG trade operates. Boil-off is burnt in the ship's boilers to the extent that it evaporates from its mother liquid. Clearly cargo volumes at the discharge port do not match those loaded.
Accounting however is not overlooked and LNG carriers are outfitted with sophisticated means of cargo measurement. This equipment is commonly referred to as the 'custody transfer system' and is used in preference to shore tank measurements. These systems normally have precise radar measurement of tank ullage while the tanks themselves are specially calibrated by a classification society to a fine degree of accuracy. The system automatically applies corrections for trim and list using equipment self-levelled in drydock. The resulting cargo volumes, corrected for the expansion and contraction of the tanks, are normally computed automatically by the system.
Cargo tank design requires carriage at atmospheric pressure and there is little to spare in tank design for over or under pressures. Indeed, the extent to which pressure build-up can be contained in a ship's tanks is very limited in the case of membrane cargo tanks, although less so for Type-B tanks. Normally this is not a problem, as at sea the ship is burning boil-off as fuel or in port has its vapour header connected to the terminal vapour return system. Clearly, however, there are short periods between these operations when pressure containment is necessary. This can be managed. So taken together, shipboard operations efficiently carried out succeed in averting all possible discharges to atmosphere, apart that is from minor escapes at pipe flanges, etc. Certainly this is part of the design criteria for the class as it is recognised that methane is a greenhouse gas.
Boil-off gas (BOG) is limited by tank insulation and new building contracts specify the efficiency required. Usually this is stated in terms of a volume boil-off per day under set ambient conditions for sea and air temperature. The guaranteed maximum figure for boil-off would normally be about 0.15% of cargo volume per day.
While at sea, vapours bound for the boilers must be boosted to the engine room by a low-duty compressor via a vapour heater. The heater raises the temperature of the boil-off to a level suited for combustion and to a point where cryogenic materials are no longer required in construction. The boil-off then enters the engine room suitably warmed but first passes an automatically controlled master gas valve before reaching an array of control and shutoff valves for direction to each burner. As a safety feature, the gas pipeline through the engine room is of annular construction, with the outer pipe purged and constantly checked for methane ingress. In this area, operational safety is paramount and sensors cause shutdown of the master gas valve in alarm conditions. A vital procedure in the case of a boiler flameout is to purge all gas from the boilers before attempting re-ignition. Without such care boiler explosions are possible and occasional accidents of this type have occurred.
Cargo care
The majority of LNG shippers and receivers have a legitimate concern over foreign bodies getting into tanks and pipelines. The main concern is the risk of valve blockage if (say) an old welding rod becomes lodged in a valve seat. Such occurrences are not unknown with a ship discharging first cargoes after new building or recently having come from drydock. Accordingly, and despite discharge time diseconomies, it is common practice to fit filters at the ship's liquid manifold connections to stop any such material from entering the shore system. The ship normally supplies filters fitting neatly into the manifold piping.
In a similar vein, even small particulate matter can cause concerns. The carryover of silica gel dust from inert gas driers is one such example. Another possible cause of contamination is poor combustion at inert gas plants and ships tanks becoming coated with soot and carbon deposits during gas freeing and gassing up operations. Subsequently, the contaminants may be washed into gas mains and, accordingly, cargoes may be rejected if unfit. Tank cleanliness is vital and, especially after drydock, tanks must be thoroughly vacuumed and dusted. A cargo was once rejected in Japan when, resulting from a misoperation, steam was accidentally applied to the main turbine with the ship secured alongside the berth. The ship broke out from the berth, but fortunately the loading arms had not been connected. This action was sufficient however for cargo receivers to reject the ship, and the cargo could only be delivered after a specialised ship-to-ship transfer operation had been accomplished. The ship-to-ship transfer of LNG has only ever been carried out on a few occasions and is an operation requiring perfect weather, great care and specialist equipment. Another case of cargo rejection, this time resulting in a distressed sale, involved a shipment to Cove Point in the USA, where the strict requirements which prevail on in-tank pressures on arrival at the berth were not adhered to. The ship had previously been ordered to reduce pressure for arrival. This is a difficult job to perform satisfactorily and, if it is to be successful, the pressure reduction operation must progress with diligence throughout the loaded voyage by forcing additional cargo evaporation to the boilers. This cools the cargo and hence reduces vapour space pressure. The process of drawing vapour from the vapour space at the last moment is ineffective, because the cargo itself is not in balance with that pressure and once gas burning stops the vapour space will return to its high equilibrium pressure. This process is known in the trade as 'cargo conditioning'.
Ship care
LNG carrier with Type-B tanks (Kvaerner Moss system)
A temperature of -162ºC is astonishingly cold. Most standard materials brought into contact with LNG become highly brittle and fracture. For this reason pipelines and containment systems are built from specially chosen materials that do not have these drawbacks. The preferred materials of construction are aluminium and stainless steel. However these materials do not commonly feature over the ship's weatherdecks, tank weather covers or hull. These areas are constructed from traditional carbon steel. Accordingly, every care is taken to ensure that LNG is not spilt. A spill of LNG will cause irrevocable damage to the decks or hull normally necessitating emergency drydocking. Accidents of this nature have occurred, fortunately none reporting serious personal injury, but resulting, nevertheless, in extended periods off-hire.
Moss design (courtesy of Moss Maritime)
LNG carriers are double-hulled ships specially designed and insulated to prevent leakage and rupture in the event of accident such as grounding or collision. That aside, though sophisticated in control and expensive in materials, they are simple in concept. Mostly they carry LNG in just four, five or six centreline tanks. Only a few have certification and equipment for cross trading in LPG. The cargo boils on passage and is not re-liquefied onboard - it is carried at atmospheric pressure. Although there are four current methods to construct seaborne LNG tanks, only two are in majority usage. There are the spherical tanks of Moss design and the membrane tanks from Gaz Transport or Technigaz (two French companies, now amalgamated as GTT). Each is contained within the double hull where the water ballast tanks reside. The world fleet divides approximately 50/50 between the two systems.
Regarding spherical tanks, a very limited number were constructed from 9% nickel steel, the majority are constructed from aluminium. A disadvantage of the spherical system is that the tanks do not fit the contours of a ship's hull and the consequent 'broken stowage' is a serious diseconomy. In general terms, for two LNG ships of the same carrying capacity, a ship of Moss design will be about 10% longer. It will also have its navigating bridge set at a higher level to allow good viewing for safe navigation. On the other hand the spherical tanks are simple in design and simple to install in comparison to the membrane system, with its complication of twin barriers and laminated-type construction.
Tank designs are often a controlling factor in building an LNG carrier. Shipyards usually specialise in one type or the other. Where a yard specialises in the Moss system, giant cranes are required to lift the tanks into the ships and limits on crane outreach and construction tooling facilities currently restrict such tanks to a diameter of about 40 metres.
LNG carrier with membrane tanks
Membrane design (GTT)
Early LNG carriers had carrying capacities of about 25,000 m3. This swiftly rose to about 75,000 m3 for the Brunei project and later ships settled on 125,000 m3. For some years this remained the norm, giving a loaded draught of about 11.5 metres, thus stretching the port facilities of most discharge terminals to their limits. Since then, however, there have been some incremental increases in size, usually maintaining draft but increasing beam, resulting in ship sizes now of about 145,000 m3. That said, one of the newest in class is the Pioneer Knudsen, trading at only 1,100 m3 capacity from a facility near Bergen to customers on the Norwegian west coast. At the end of 2004 the first orders were placed for LNG carriers of more than 200,000m3 and ships to carry over 250,000m3 are expected to be delivered by the end of 2008.
Large modern LNG carriers have dimensions approximately as follows:
LNG having a typical density of only 420 kg/m3 allows the ships, even when fully laden, to ride with a high freeboard. They never appear very low in the water as a fully laden oil tanker may do. Ballast drafts are maintained close to laden drafts and, for a ship having a laden draft of 12 metres, a ballast draft of 11 metres is likely. This means that for manoeuvring in port in windy conditions the ships are always susceptible to being blown to one side of the channel, and restrictions on port manoeuvring usually apply with extra tug power commonly specified.
Another salient feature of the LNG class is the propensity to fit steam turbine propulsion. This is an anachronism brought about by reluctance to change over the years, together with a fear that a system as yet untried on LNG carriers may not find favour with the principal charterers - the Japanese. Most other ship types of this size have diesel engines and the engineers to run diesel equipment are plentiful and suitably trained. On the other hand, engineers knowledgeable in steam matters are few and their training base is the ship itself. This situation is changing though, with both diesel electric dual fuel systems and slow speed diesels now finding favour. With slow speed diesel propulsion, reliquefaction plants will be required onboard to handle boil-off gas, and all diesel systems will require back-up gas disposal facilities - also known as 'gas combustion units' (Gus) - for when either the reliquefaction plants or the duel fuel diesel engines are not available to process boil-off gas.
LNG ships are expensive to build. They comprise very valuable assets: generally far too good to let rust away. Shipowners and ship managers alike recognise this and, together with inspection regimes, the overall quality of LNG tonnage is kept to a high standard. Age for age, they are probably the best maintained ships in the world. Of course some of these ships are now old and only a few have ever been scrapped; some are over 30 years old. This is very old for a large tanker trading all its life in salt water, when 25 years is already considered by many as a cut-off date. On termination of their original projects we are now seeing many of the older ships as surplus to requirements. Sometimes the project wishes to continue but only with new ships. So the older ships are laid off. In the past this would have been their death knell but today this is not necessarily the case. The slow development of a spot market has allowed the shipowner to consider life extension programmes of considerable cost; all this set against the value of a very expensive new building. Today life extension programmes are common with old ships making handsome profits in the spot market.
SIGTTO
Valuable assistance in the preparation of these articles has come from the Society of International Gas Tanker and Terminal Operators (SIGTTO).
SIGTTO is the leading trade body in this field and has over 120 members covering nearly 95% of the world's LNG fleet and 60% of the LPG fleet. SIGTTO members also control most of the terminals that handle these products. The Society's stated aim is to encourage the safe and responsible operation of liquefied gas tankers and marine terminals handling liquefied gas; to develop advice and guidance for best industry practice among its members and to promote criteria for best practice to all who have responsibilities for, or an interest in, the continuing safety of gas tankers and terminals.
The Society operates from its London office at 17 St. Helens Place EC3.
Further details on activities and membership is available at http://www.sigtto.org
References
Liquefied Gas Handling Principles on Ships and in Terminals - SIGTTO
Safe Havens for Disabled Gas Carriers - 2003, SIGTTO
Mooring Equipment Guidelines - 2001, OCIMF
Ship-to-Ship Transfer Guide (Liquefied Gases) - 1995, SIGTTO
The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, (IGC Code) - IMO
A Contingency Planning and Crew Response Guide for Gas Carrier Damage at Sea and in Port Approaches - 1999, SIGTTO
The aforementioned publications are available from Witherby & Company Ltd, London.
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Bulk liquid cargoes - sampling
Introduction
Sampling is a vitally important factor in the custody transfer of bulk liquid cargoes. Acquisition and subsequent care and retention of representative samples can provide an important means of rebutting unfounded allegations of cargo contamination. This applies equally to chemical, petrochemical, petroleum product and crude oil shipments.
Cargo surveyors attending the loading or discharge of any given cargo are often working on behalf of shippers or consignees (or both, on a joint basis) and are not obliged to provide samples to the ship, albeit that it is common practice to place samples in the custody of the master at the load port for delivery to the disport receivers. However, these samples are not the property of the ship and only on rare occasions are official-sealed custody transfer samples provided. Whether samples are provided by the cargo interests to the ship or not, it is recommended that the vessel's crew draw samples for the ship's protection.
Retention and sealing
Due to the inability of the ship's officers to undertake analysis of samples, only the most obvious contamination problems will be apparent at the outset, such as:
- Change in colour.
- The presence of water (if water is not soluble in the cargo).
- Foreign particulate matter.
- Odour taint.*
Samples taken at the initial stages of cargo operations showing such obvious cargo quality deviations should give cause to halt cargo operations in order to carry out further investigations** and to note protest.
* Safety: Odour is not an issue on all cargoes. Toxic and highly odiferous cargoes should not be tested for odour.
** A P&I surveyor should be summoned.
All samples drawn should be sealed, labelled, retained and recorded. Wherever possible, samples drawn by the ship's crew should be clearly labelled with the following:
- Operational status
i.e. before loading, after loading, before discharge.
- Sample source
i.e. tank number, manifold number.
- Sample type
i.e. top, middle, bottom, dead bottom, running, composite.
- Identity of sampler
i.e. surveyor, crewmember.
- Location
i.e. port, berth, anchorage.
Seals are customarily applied to samples by an independent surveyor in order to preserve sample provenance in the event of dispute. Nowadays, seals are widely available and relatively inexpensive and it is increasingly common for ships to be equipped with their own seals. Alternatively, some owners use self-sealing tamper-evident bottle closures which may not be individually numbered but, nonetheless, preserve sample provenance.
Marked samples should be retained in a dedicated locker, ideally for at least 12 months. Space considerations may make this impractical in which case the samples should be retained for as long as possible. However, where the cargo is known or expected to be the subject of dispute, samples should be retained for at least 12 months in any event. Samples should not be exposed to extremes of temperature and should be kept in darkness. When no longer required, disposal should be by appropriate means; many owners use the services of local cargo surveyors who invariably have disposal methods already in place.
Sample bottles
Sample bottles vary in size and in the materials from which they are made. Glass and plastic bottles can be dark or clear. Most samples can generally be stored in clear glass bottles. Light sensitive samples, however, should be stored in brown bottles*. Certain samples, such as caustic soda or potash require plastic containers. Petroleum products/crude oil samples are often retained in lacquer-lined tinplate containers. These types of containers are, in general, unsuitable for retention of chemical cargo samples. Where possible, a range of containers should be available. Sample bottle closures vary in the chemical resistance of the sealing insert. Waxed cardboard disc type should only be used for petroleum products/crude oils. Aluminium foil-faced cardboard discs are unsuitable for acid or alkaline samples. Preferred inserts are polypropylene or PTFE.
Sample bottle size may be determined, to some extent, by storage capacity, balanced against the need to retain sufficient sample volume to allow analysis in the event of a dispute arising. Generally, 500ml is a realistic compromise.
Where to take samples
During the custody transfer of a bulk liquid cargo, the principal sampling points where cargo quality can be adequately monitored are:
- Loadport shore tank(s).
- Shoreline sample following any 'packing' or flushing operation.
- Vessel's manifold at commencement of loading and spot checks during loading.
- Vessel's cargo tanks first foots.
- Vessel's cargo tanks post-loading.
- Vessel's cargo tanks pre-discharge.
- Vessel's manifold at commencement of discharge.
- Disport shore tank(s) pre- and post discharge.
Ideally, all of these samples should be taken on each cargo carrying voyage, but in any event, onboard ship samples 3 to 7 should always be taken by the crew for protection of the owner's interests. Further samples might be considered, such as 3, following changeover of shore tanks at a mid-loading stage.
Method of drawing samples
Samples should be drawn in compliance with industry practice as set out in publications such as those issued by ASTM, API and BS (see References). In general, a 'running' sample taken by use of a bottle and sample cage is the preferred method of obtaining a representative sample in a homogeneous bulk cargo. Where the cargo may not be homogenous, careful zone sampling is required to produce a representative composite sample. The properties of some chemical cargoes require that special sampling procedures are adopted such as excluding air, using specialist sample valves or indeed 'closed' sampling methods due to the toxicity or flammability of the cargo. Here, the sampling procedure is prescribed by the specialist equipment in use. Appropriate safety procedures must be observed and the sampler protected from exposure to the cargo during sampling.
Conclusion
It is unquestionably the case that a vessel's adherence to the above sampling procedure can provide the necessary evidence to rebut cargo quality claims in circumstances where unfounded allegations are made against shipowners. A rigorous sampling system should form an essential part of a vessel's ISM operational procedures.
References
ASTM D 4057
Standard Practice for Manual Sampling of Petroleum and Petroleum Products.
ASTM E 300
Standard Practice for Sampling Industrial Chemicals.
BS 3195
Methods for Sampling Petroleum Products.
BS 5309
Methods for Sampling Chemical Products.
IP
Petroleum Measurements Manual Part IV Sampling - Section I Manual Methods.
API
Manual of Petroleum Measurement Standards Ch 8, Standard Methods of Sampling Petroleum and Petroleum Products.
* Brown bottles impede inspection of the sample for colour/water/particulates. It is suggested that clear glass bottles are used initially and, after inspection, the sample transferred to a dark brown bottle for storage.
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Carriage of potatoes
Introduction
The potato tuber, Solanum tuberosum L., is an annual of the Solanaceae family and originally native to South America. The edible tuber forms at the end of the underground stems or stolons of the plants and within which the starch-rich nutrients are stored. Colour together with other criteria form important characteristics for identifying the numerous varieties of potatoes:
- Skin colours - brown, russet, white, yellow, pink or red.
- Skin textures - rough or smooth.
- Flesh colours - white, cream, yellow, blue/purple/red or striated.
- Tuber shape - round, oblate, oval, or kidney shaped.
- Usage - table, processing or seed.
- Harvest time - early/new or immature, or late/mature.
Three basic type of potato, left to right: early/new; late/mature;
seed (notice fragile "eyes" which produce new growth)
Potatoes are grown throughout the world, except in humid tropical lowland areas. They are one of the world’s most important food crops, and thus are an important commodity of trade. For the purposes of this article we shall refer to three basic types of potato, which are:
- Early/new or immature.
- Late/mature.
- Seed.
All of which require special considerations for stowage and carriage. Early or new potatoes have thin, relatively loose, skins that are easily removed and are thus readily liable to damage. Over more recent years, demand for this type of potato has increased and large quantities are shipped from Cyprus, Greece, Israel, Turkey and the Canary Islands during the northern winter and spring seasons. Late/mature potatoes have firm skins and are therefore more resistant to damage and much easier to carry than immature potatoes.
Seed potatoes for shipment comprise small whole tubers each with at least one eye to produce the new growth. Seed potatoes are grown under a regulated certification programme to ensure that they are as disease-free as possible.
Pre-shipment considerations
Once potatoes have been harvested they must be stored under optimal conditions until released for shipment. However no storage is able to improve the product placed therein, but much can be achieved to minimise losses.
High temperatures cause the tuber respiration rate to increase, whereby oxygen and food reserves are used, potentially resulting in excessive shrinkage. Freezing or chilling temperatures can damage and kill tuber cells. If the air surrounding the tubers has a low humidity then water will move from the tubers to the air, resulting in weight loss. Should the oxygen content of the air fall to a low level, cells within the tubers die and 'blackheart' forms.
Sprouting is a natural function of the tuber, however, during shipment it is not desirable as, in the event, quality and condition will suffer. Sprout suppressant chemicals or other methods may be used prior to shipment to preclude sprouting but control in stowage can only be maintained by application of the correct temperature(s).
Potato tuber diseases may be the result of micro-organisms or adverse preshipment storage conditions. They may also be the result of improper stowage and conditions of carriage. Potatoes are grown under the soil and, as such, when harvested will always contain on their surfaces spores of invading micro-organisms, which will attack the tubers if the natural defence mechanism is ruptured. This can result from mechanical damage, either during harvesting or subsequent handling or, alternatively, can result from other forms of deterioration such as sun-scald. It may also result if the tuber is subjected to wetting such that a film of water is present over its surface.
Some of the principal diseases found at the time of harvesting may include Phytophthora infestans (potato blight); a dry mealy rot due to species of Fusarium (dry rot); a bacterial soft rot caused by Erwinia ssp. (black leg); or brown rot caused by the bacterium Ralstonia solanacearum and ring rot caused by the bacterium Clavibacter michiganensis subsp. sepedonicus, both of which are notifiable diseases in the UK and other countries.
Post-harvest deterioration i.e. storage/ stowage deterioration will normally result from the development of bacterial soft rot, usually the result of infection by Erwinia ssp. which causes collapse of the cells of the infected potatoes exuding heavily infected fluid and gives rise, by contact, to soft rot developing in adjacent tubers. Hence over a period of time the contents of whole bags may collapse to a malodorous slime.
Another cause of deterioration is infestation by insects, which has been a problem since potatoes have been grown. The two most serious infestants of potato crops are the North American black and yellow striped beetle (Colorado Beetle) and the Potato Tuber Moth (Phthorimaea operculella).
It is necessary for shippers or charterers to provide phyto-sanitary certificates, attached to the bill(s) of lading or other trade documents. These certificates are produced by the Authority of the country of origin indicating that the specified consignment(s) have been inspected or treated according to the importing country's requirements. Recent legislation The Potatoes Originating in Egypt (England) Regulations 2004 came into force on 15 May 2004.
Potato tubers infested with Colorado Beetle
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Signs of infestation by the Potato Tuber Moth
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Whereas the master should be able to rely upon a valid phyto-sanitary certificate he does have a continuing duty in relation to cargo in his charge. For example, if infestation is noticed during the voyage, the master/owners must take reasonable steps to deal with the situation.
Fumigation prior to berthing at an arrived port, or alternatively rejection of a cargo of potatoes as a result of infestation or infection by serious bacterial diseases, not only may cause massive delays to a vessel but also considerable additional problems for the shipowners.
Greening may occur in any part of a tuber exposed to light. Exposure to bright light during post harvest handling, or longer periods (7 to 14 days) of low light, can result in the development of chlorophyll (greening) and bitter, toxic glycoalkaloids, such as solanine. Experts advise that whereas in cultivated varieties green discolour of the flesh does not cause substantive harm to health, it undoubtedly will, depending upon extent, result in a loss of value of consignments. Green flesh of potatoes tastes bitter and must be cut away before cooking.
When presented for shipment, consignments should be inspected for external condition of the packaging. Evidence of wet patch staining of the bags, or any associated malodours, should alert crewmembers to likely problems and the vessel's P&I association should be requested to appoint an expert surveyor to investigate and ensure only healthy and undamaged potatoes are shipped. Since potatoes have been shipped in woven polypropylene bags of varying dark colours it has become extremely difficult to recognise wet patches from superficial examinations; close inspections are thus recommended. Mechanical damage is one of the most important factors affecting potato condition, since it is largely preventable. Special care is therefore essential during handling to and from the vessel, especially when immature/new potatoes are being shipped. Bags of potatoes should not be walked over or handled roughly, with special care taken if palletised units of bags are over-stowed by a second tier of pallets. In light rain, snow, or damp weather cargo must be protected from moisture to preclude the onset of premature spoilage by bacterial soft rot. Do not load or discharge potatoes during heavy rain.
Summary
Subsequent to harvesting and prior to packing for shipment:
Early or new potato tubers should be graded and sorted:
- without mechanical damage;
- sound, without disease;
- dry;
- without greening;
- free from adherent soil and stones;
- and stored at optimum temperatures.
Late or mature potato tubers should, in addition to the above:
- be fully mature and firm skinned;
- have been stored for a specific post harvest period of 10 to 14 days (wound healing and curing).
Seed potato tubers may, in addition to those points noted under 'early potatoes':
- consist of unwashed tubers and may contain loose soil and foreign material but should generally be free of caked soil.
Packaging
Potatoes may be packed in hessian bags, woven polypropylene bags, sacks lined with an internal perforated polyethylene bag and sometimes cartons or crates. Various sizes of bags are utilised, however the bags will usually contain about 25 kg of tubers.
A more recent innovation is to pack potatoes in large open-top lift bags weighing some two to three tonnes. New potatoes are frequently packed in moist or dry peat moss. The main purpose for including moist peat moss within the bags is to protect the 'new' tubers and to preclude skin-set and thus maintaining their value. However, excess free water or release of water from the peat moss during carriage can cause problems leading to bacterial soft rot of the tubers.
Stowage
As for any product which may enter the human food chain, preparation of stowages will include ensuring that the cargo spaces are clean and dry. Potatoes are highly sensitive to odours and readily absorb foreign smells from chemicals, mineral oils, and some fruits, etc. All compartments destined for stowage of potatoes must be free from malodours and volatile substances.
Potato tubers are living organisms that consume oxygen and evolve carbon dioxide, water and heat. The principal problem as far as stowage and carriage is concerned is the heat produced, and therefore good climate control is required to maintain the condition of tubers. Condensation in the form of ship or cargo sweat should not be allowed to develop during a voyage. Long voyages therefore demand more critical control than short-term voyages.
An example of the heat produced by cargoes of potatoes is noted in the table below.
From these figures it is evident that new / immature potatoes produce considerably more heat per 1000 kg than late / mature potatoes and are commensurately more difficult to carry.
When potatoes are presented for loading in bags, stow heights of up to eight tiers are preferable. To ensure adequate ventilation of cargo blocks, maximum stow heights of twelve to thirteen bags should never be exceeded. The stowage must be so arranged to ensure a free flow of air throughout the compartments.
Bags shipped on pallets are usually stacked to a height of eight/nine bags and are often secured to the pallet baseboards by means of nylon netting. Care must be taken, (especially when the bags are constructed of woven polyethylene) to ensure that the contents of pallets are fully and properly secured.
The frictionless nature of this type of outer bag frequently results in the pallet loads becoming deformed and, in some cases, detached from the base-boards. This slippage can result in additional stevedoring costs for re-making the pallets. Slippage of woven polyethylene bags from pallets, and also when loose stowed, into ventilation channels will cause restrictions of air flow and must be prevented by the use of timber dunnage or dunnage nets.
Stowages in refrigerated cargo vessels
As previously noted, not only do growing and harvesting conditions influence the post harvest/pre-shipment behaviour of potatoes but, additionally, post-harvest storage conditions are also critical to the optimum temperature requirements for their carriage. Therefore written instructions for the carriage temperature regime should always be obtained from the shippers and should be complied with throughout the voyage. Transport temperatures must be such that respiration and weight losses due to evaporation are maintained to a minimum.
The approximate lowest safe temperature for the carriage of potatoes is plus 4° Celsius (39° Fahrenheit) and carriage is usually recommended at plus 4° to 5° Celsius (39° to 41° Fahrenheit) at a relative humidity of between 90 and 95%. However potatoes destined for processing will require to be carried at temperatures depending upon their cultivar. In these cases, it is thus essential for shippers to provide detailed instructions and for those instructions to be rigorously followed.
The exact stowage patterns adopted for potatoes will depend upon the permanent air circulation systems incorporated in a vessel. Strict supervision of cargo stowage must ensure that airflow will be evenly distributed throughout the compartments for maintenance of optimal temperature control. Detailed records of cargo compartment / flesh temperatures should be maintained throughout the transit period.
At the time of discharge from refrigerated stowages, the cargo should ideally be landed to stores at similar temperatures to that of carriage. If cold cargoes are discharged into ambient warm humid conditions then a risk of condensation forming on the tubers may exist and bacterial soft rot will ensue. Some shippers/consignees will request the vessel to undertake a dual temperature regime during transit and require the vessel to slowly raise the temperature of the cargo, to above the anticipated ambient dew point at the discharge port, commencing some two to three days before discharge is due to commence.
Stowages in mechanically ventilated general cargo spaces
The usual system adopted is to use block stowage with air channels around each cargo block. This system relies on convection cooling. The cargo is stowed clear of the deck either by placing it on double dunnage or alternatively on pallet boards. Cargo blocks should normally not exceed 3 metres by 3 metres square. Smaller blocks may be preferred under certain circumstances; however stability of each block is critical and when loose stowed, bags must be key-stacked to construct a locking stow precluding slippage or collapse of bags into the air channels potentially causing a breakdown in the air circulation.
High stows may not only cause compression damage/bruising to the potatoes (especially new/immature tubers) but may also result in excessive heating due to metabolic processes. Bags should be stowed ideally to eight tiers in height, but never more than twelve to thirteen. The width of the air channels around the cargo blocks should be in the order of 20 to 30 cms. constructed using dunnage and/or the locking stow noted above. Cargo should be stowed clear of transverse bulkheads and ship's sides to promote air circulation with exposed steel work protected by paper mats or other sheeting to preclude condensation damage.
Potato cargoes should be kept well clear of engine room bulkheads and any other local heat source situated on the vessel.
The stowage on any vessel should be designed to suit the type of permanent ventilation system fitted. Potato cargoes make heavy demands on ships' ventilation systems and a capacity of at least fifteen air changes per hour in each empty hold is required. At these rates the ventilation system should be run continuously except when weather and climatic conditions prevent e.g. risk of shipping water through the weatherdeck ventilators or condensation forming on the cargo or internal ship's structures. At higher rates of air changes per hour consideration should be given, especially on longer voyages, to either run the fans on lesser power (reduction of speed) or for lesser times (ventilate intermittently) in order to maintain humidity and preclude water loss from the tubers (desiccation).
Details of ambient air wet and dry bulb temperatures, hold wet and dry bulb air temperatures / flesh temperatures and the ventilation regime undertaken according to the acquired data regularly obtained must be recorded in a dedicated ventilation logbook or alternatively the deck log book.
Ro-Ro vessels
Cargoes of new/immature potatoes have for some time been shipped from Eastern Mediterranean ports in the holds of Ro-Ro vessels. Packed in woven polypropylene bags, shipped on pallet boards with bags secured by nylon nets, losses and/or additional costs have been experienced due to the displacement of bags from the pallet boards.
Bearing in mind the practice of keeping the Ro-Ro deck lights illuminated throughout the voyage the problem of tuber greening has been experienced. Attempts to prevent this have included covering stowages with polythene sheets, which unfortunately reduce the effectiveness of the hold ventilation system. Hold lights should never remain continuously illuminated throughout a voyage, even of short duration.
Transport of potatoes in ISO containers
Cargoes of potatoes may be carried in fan assisted ventilated containers, open sided containers, insulated refrigerated containers and 'port-hole' insulated containers. For voyages of a short Carriage of potatoes continued duration, closed cargo containers may be used but doors should remain open when ever possible to promote ventilation. Stowage on deck must include provisions to protect the cargo from rain, sea-spray and sunlight.
Flat racks are also used for below deck stowages in well-ventilated compar, provisions should be made to afford exposed bags protection against rain and sunlight prior to loading and subsequent to discharge.
Seed potatoes
Seed potatoes are usually shipped around the world in smaller consignments than those of new or mature potatoes. The value of seed potatoes is much greater than potatoes destined for consumption and special care should be taken as any loss in quality or condition will potentially result in substantial claims. They may be carried in a mechanically ventilated stowage but for longer voyages involving any prolonged period in warm climatic conditions, say in excess of 20° Celsius, they should be carried under refrigeration at a temperature of 2° to 4° Celsius.
Safety
Inadequate, or failure of, ventilation in spaces containing cargoes of potatoes can cause life threatening concentrations of carbon dioxide (CO2) or oxygen (O2) depletion to arise. Thus under these or suspected conditions the compartment(s) must be fully ventilated and a gas measurement conducted. The threshold limit value (TLV) for CO2 concentrations is 0.49 % by volume.
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Fumigation of ships and their cargoes
Introduction
Fumigation is a procedure that is used throughout the world to eradicate pests that infest all types of goods, commodities, warehouses, processing factories and transport vehicles including ships and their cargoes.
1 What are fumigants and how do they work?
Fumigants are gases, which are toxic to the target infestation. They can be applied as gas, liquid or in solid formulations, but after vaporisation from liquids or reaction products from solids, always act in the gaseous phase. They act either as respiratory poisons, or as suffocants in the case of controlled or modified atmospheres. On release, they mix with air at a molecular level. They are capable of rapidly diffusing from one area to another and through commodities and buildings.
Fumigants should not be confused with smokes, which are solid particles in air, or with mists, aerosols or fogs, which are liquid droplets, of various sizes, in air. Smokes, mists, aerosols or fogs are not fumigants as they are unable to diffuse (i.e. they do not mix with air at a molecular level) and do not reach deep-seated infestations in commodities or structures.
The fumigant gases used to carry out the fumigation process are numerous, but the most commonly used currently for the treatment of ships cargoes are phosphine and methyl bromide. Others used are carbon dioxide and more recently sulfuryl fluoride, which is starting to replace the use of methyl bromide.
1.1 How does a fumigant gas work effectively?
The critical parameters, which need to be considered for fumigants to be effective are:
- Nature of infestation (type of pest, e.g. rodent, insect or beetle, and stage of its life cycle).
- Type of fumigant applied.
- Concentration and distribution of gas.
- Temperature.
- Length of time fumigant must be applied.
- Method by which fumigant is administered.
- Containment of fumigant.Nature of commodity.
- Nature of commodity packaging.
- Monitoring system.
- Ventilation system.
1.2 Aim of fumigation
Fumigation aims to create an environment, which will contain an effective concentration of fumigant gas at a given temperature, for a sufficient period of time to kill any live infestations. Both the time measured (hours or days) of exposure and concentration of gas is critical to fumigation efficiency. Dosages applied are usually expressed as grams per cubic metre, concentrations measured during the fumigation are usually expressed in parts per million (PPM) or grams per cubic metre, and total concentrations actually achieved, as concentration-time-products (CTPs). The fumigation process is not completed until ventilation has been effectively carried out, and removal of any residues is completed.
2 When can ships' cargoes be fumigated?
The ship's cargo can be fumigated and ventilated:
- In warehouse or storage silos before loading.
- In freight containers before loading.
- In the hold of the ship with fumigation and ventilation completed before sailing.
or
- In the hold prior to sailing with fumigation continued during the voyage (intransit).
- In freight containers before loading with fumigation continuing during the voyage (intransit).
In these situations the fumigation continues during the voyage and is not finished until the ventilation and removal of residues is completed, which is normally at the first discharge port.
3 Rules, regulations and guidelines that affect the fumigation process
3.1 The United Nations International Maritime Organisation (IMO) Safety of Life at Sea (SOLAS) Convention places an obligation on all governments to ensure all shipping activities are carried out safely.
3.2 The Recommendations on the Safe Use of Pesticides in Ships (IMO Recommendations) published by the IMO (revised 2002) are intended as a guide to all those involved in the use of pesticides and fumigants on ships and are recommended to governments in respect of their legal obligations under the SOLAS Convention.
These recommendations are referred to throughout this document as within the IMO Recommendations.
3.3 Individual countries (e.g. US and Canadian Coastguard) have their own requirements, but some governments have chosen to make the IMO Recommendations mandatory on all vessels in their territorial waters (e.g. UK).
3.4 The IMO International Maritime Dangerous Goods (IMDG) Code, which is mandatory in many parts of the world under SOLAS, specifically relates to the fumigation of packaged goods only and will be referred to under section 8 on freight container fumigation.
The fumigation of packaged goods and freight container recommendations, are referred to throughout this document as within the IMDG Code.
3.5 The International Maritime Fumigation Organisation (IMFO) Code of Practice (COP) provides clear guidance to fumigators and ships' masters in respect of bagged and bulk cargoes, in addition to packaged goods.
IMFO is an organisation of independent maritime fumigation servicing companies with members in many countries. See Annex 2.
4 Fumigants that can be used for intransit fumigation of bulk and bagged cargoes in ships' holds
4.1 The most widely used fumigant for intransit fumigation is phosphine (PH3). The gas is normally generated from aluminium phosphide or sometimes magnesium phosphide, but can also be applied direct from cylinders.
4.2 Methyl bromide should never be used for fumigation intransit (IMO Recommendations, Annex 1D).
4.3 Insecticides such as dichlorvos, pirimiphos-methyl, malathion, permethrin and others may be sprayed on to the grain during loading. These are not fumigants and should be allowed provided data is provided to the master as set out in IMO Recommendations 6.2 and 6.4 and Annex 1A.
5 Intransit fumigation of bulk and bagged cargoes with phosphine gas
5.1 Phosphine is only fully effective if a lethal concentration is maintained for a period of time that can be as little as 3 days or as much as 3 weeks.
The actual time needed will vary according to the cargo temperatures, insect species that may be present, and the system of fumigation (refer to Annex 1 of this article for brief details of the types of system).
This is the reason why fumigation with phosphine is almost always carried out during the voyage (intransit) so that the voyage time can be used to ensure a fully effective treatment.
Probing aluminium phosphide in retrievable sleeves into a bulk cargo
5.2 When the owners/charterers/master agree to fumigation being carried out intransit with phosphine, the master should ensure he is familiar with the requirements of IMO Recommendations 3.4.3.1. - 3.4.3.20. This will enable the master to be clear what the obligations of both fumigator and master are.
A checklist of these obligations is as follows:
5.2.1 Fumigator
To provide written documentation in respect of the following:
- Pre-fumigation inspection certificate.
- Standard safety recommendations for vessels with fumigated grain cargoes.
- Gas tightness statement.
- Statement of vessel suitability for fumigation and fumigant application compliance.
- Manufacturers information or safety data sheet.
- First aid and medical treatment instructions.
- Fumigation certificate.
- Fumigation plan.
- Instructions for the use of the phosphine gas detecting equipment.
- Precautions and procedures during voyage.
- Instructions for aeration and ventilation.
- Precautions and procedures during discharge.
- Also to provide sufficient additional respiratory protective equipment (RPE) where necessary to the vessel, to ensure the requirements of IMO in respect of RPE are available for the duration of the voyage. (Note; the RPE may consist of SCBA or canister respirators or a combination of both but the minimum requirement is for 4 sets of RPE).
Refer also to IMO Recommendations Annex 4.
5.2.2 Master
- Appoint a competent crewmember to accompany the fumigator during the inspections/testing of empty holds prior to loading to determine whether they are gas tight, or can be made gas tight and, if necessary, what work is to be carried out to ensure they are gas tight.
- Ensure the crew is briefed on the fumigation process before fumigation takes place.
- Ensure the crew search the vessel thoroughly to ensure there are no stowaways or other unauthorised personnel onboard before fumigation takes place.
- Appoint at least two members of the crew to be trained by the fumigator to act as representatives of the master during the voyage to ensure safe conditions, in respect of the fumigations, are maintained onboard the ship during the voyage.
- After the fumigant has been applied and appropriate tests have been completed, the master should provide his representative to accompany the fumigator, to make a check that all working spaces are free of harmful concentration of gas (IMO Recommendations 3.4.3.11).
- When the fumigator has discharged his responsibilities, the fumigator should formally hand over in writing responsibility to the master for maintaining safe conditions in all occupied areas, which the master should accept (IMO Recommendations 3.4.3.12).
- It must be clearly understood by the master that, even if no leakage of fumigant is detectable at the time of sailing, this does not mean that leakage will not occur at some time during the voyage due to the movement of the ship or other factors. This is why it is essential the master ensures regular checks are carried out during the voyage.
- During the voyage, the master should ensure that regular checks for gas leakage should be made throughout all occupied areas and the findings recorded in the ships log (IMO Recommendations 3.4.3.13). If any leakage is detected appropriate precautions to avoid any crew being exposed to harmful concentrations must be taken. If requested to do so by the fumigator, the master may, prior to arrival at the first discharge port, start the ventilation of the cargo spaces.
- Prior to arrival at the first discharge port the master should inform the authorities at the port that the cargo has been fumigated intransit. (IMO Recommendations 3.4.3.16).
- On arrival at the discharge port the master should not allow discharge of the cargo to commence until he is satisfied that the cargo has been correctly ventilated and aluminium phosphide residues that can be removed have been removed, and that any other requirements of the discharge port have been met (IMO Recommendations 3.4.3.17). Refer also to IMO Recommendations, Annex 4.
6 Fumigation of bulk and bagged cargo with ventilation in port
This procedure can be used either after loading and prior to sailing (6.1) or on arrival at the discharge port prior to discharging (6.2).
6.1 After loading and prior to sailing
Phosphine fumigation is the only fumigant that should be accepted for this procedure, as methyl bromide (though frequently used) is not recommended (IMO Recommendations, Annex 1D). Phosphine fumigation and ventilation in port, prior to sailing, will normally take from 1-2 weeks to complete and therefore is only occasionally specified. All procedures as for intransit fumigation should be followed to ensure a safe and effective fumigation.
6.2 At discharge port prior to discharge
Methyl bromide is the most common fumigant used for this purpose as it is normally possible to achieve an effective fumigation of the cargo in 24-48 hours. The crew should be landed and remain ashore until the ship is certified 'gas free' in writing by the fumigator in charge. The fumigator is responsible for the safety and efficiency of the fumigation, though crewmembers may remain in attendance to ensure the safety of the ship provided they adhere to safety instructions issued by the fumigator in charge.
The ventilation of methyl bromide from cargoes can be a very slow process if sufficient powered ventilation is not available and the master (or his representative) should ensure that the fumigator has ensured that residues of gas are below the TLV (IMO Recommendations, Annex 2) throughout all parts of the cargo and holds. Phosphine fumigation and ventilation in port, prior to discharge, will normally take from 1-2 weeks to complete and therefore is only occasionally specified. All procedures as for intransit fumigation should be followed to ensure a safe and effective fumigation.
7 Fumigation of empty cargo holds and/or accommodation to eradicate rodent or insect infestation
7.1 Methyl bromide is the most common fumigant used for this purpose (although hydrogen cyanide (HCN) or sulfuryl fluoride may be used in some countries) as it is normally possible to achieve an effective fumigation of the empty spaces in 12-24 hours.
7.2 The crew should be landed and remain ashore until the ship is certified 'gas free' in writing by the fumigator in charge as for 6.2 above.
8 The intransit fumigation of freight containers
8.1 The reason for the fumigation of containers is normally to try to ensure that when the goods arrive at the discharge port they are free of live pests/ insects.
8.2 Containers are normally fumigated and subsequently ventilated prior to being loaded onboard the ship.
Containers that have been fumigated and subsequently ventilated and where a 'certificate of freedom from harmful concentration of gas' has been issued, can be loaded onboard ships as if they had not been fumigated (IMO Recommendations 3.5.2.1).
8.3 Frequently containers are fumigated but not ventilated prior to loading and these containers are therefore fumigated intransit, as the ventilation process will not take place until after they have been discharged from the ship. The carriage of containers intransit under fumigation is covered by the IMDG Code whereby these containers are classified in Section 3.2 Dangerous Goods List as 'Fumigated unit Class 9 UN 3359'. Also refer to the IMDG Code Supplement Section 3.5.1 and 3.5.2 of chapter called 'Safe use of pesticides in ships'.
WARNING - Containers are still sometimes shipped under fumigation with no warning notices attached and no accompanying documentation stating they have been fumigated. This process is in direct contravention of the IMDG Code. There may be dangerous levels of fumigant gas inside the container when it arrives at its destination which is both illegal and dangerous.
8.3.1 Obligations on the fumigator
- The fumigator must ensure that, as far as is practicable, the container is made gas tight before the fumigant is applied.
- The fumigator must ensure that the containers are clearly marked with appropriate warning signs stating the type of fumigant used and the date applied and all other details as required by the IMDG Code and IMO Recommendations Annex 3.
- The fumigator must ensure the agreed formulation of fumigant is used at the correct dosage to comply with the contractual requirements.
8.3.2 Obligations on the exporter
- The exporter must ensure that the containers are clearly marked by the fumigator with appropriate warning signs stating the type of fumigant used and the date applied and all other details as required by the IMDG Code and IMO Recommendations Annex 3.
- The exporter must ensure that the master is informed prior to the loading of the containers.
- The exporter must ensure that shipping documents show the date of fumigation and the type of fumigant and the amount used all as required in the IMDG Code, volume 1, page 35 and specifically section 9.9.
8.3.3 Obligations on the master
- The master must ensure that he knows where containers under fumigation are stowed.
- The master must ensure he has suitable gas detection equipment onboard for the types of fumigant present, and that he has received instructions for the use of the equipment.
- Prior to arrival of the vessel at the discharge port the master should inform the authorities at the discharge point that he is carrying containers under fumigation.
- If the master (or his representative) suspects that unmarked containers may have been fumigated and loaded onboard they should take suitable precautions and report their suspicions to the authorities prior to arrival at the discharge port.
8.3.4 Obligations on the receivers
- The receiver (or his agent) must ensure that any fumigant residues are removed, and the container checked and certificated as being free from harmful concentrations of fumigant by a suitably qualified person before the cargo in the container is removed.
For further information: International Maritime Organisation, 4 Albert Embankment, London, SE1 7SR
Tel: 0207 735 7611. Fax: 0207 587 3210 Website: http://www.imo.org
International Maritime Fumigation Organisation, Friars Courtyard, 30 Princes Street, Ipswich, Suffolk, IP1 1RJ or any member worldwide. See - http://www.imfo.com.
Annex 1
A summary of the various methods of phosphine application methodology that can be considered for intransit fumigation of bulk or bagged cargoes in ships' holds.
1 Application of tablets or pellets to cargo surface (or into the top half metre).
High concentrations of gas build up in the head space, potentially resulting in a lot of leakage through the hatchcovers unless they are very well sealed. Very little penetration down into the cargo. Powdery residues cannot be removed. Good kill of insects in top part of cargo but negligible effect on eggs or juvenile or even adults in lower part of cargo.
2 Application of tablets or pellets by probing into the cargo a few metres.
Less loss of gas through hatchcovers than in 1. Better penetration of gas than when applied on surface only but unlikely to be fully effective unless holds are relatively shallow and voyage time relatively long. Powdery residues cannot be removed.
3 Application of tablets or pellets by deep probing into the full depth of the cargo.
This is difficult to achieve and currently practically impossible if the cargo is more than 10 metres deep. Ensures effective fumigation provided voyage time is relatively long to allow gas to distribute. Powdery residues cannot be removed.
4 Application of aluminium phosphide in blankets, sachets or sleeves, placed on the surface of the cargo (or into the top half metre).
All points the same as 1, except that with this method powdery residues can be removed prior to discharge.
5 Application of tablets or pellets by probing into the cargo a few metres in retrievable sleeves.
All points as for 2, except that with this method powdery residues can be removed prior to discharge.
6 Fitting of an enclosed powered re-circulation system to the hold and application of aluminium phosphide tablets or pellets to the surface.
This will ensure the gas is distributed throughout the cargo evenly and rapidly making maximum use of the fumigant in the shortest possible time. Powdery residues cannot be removed.
7 Fitting of an enclosed powered re-circulation system to the hold and application of aluminium phosphide in blankets, sachets or sleeves on the surface or probed into the top one or two metres.
As for 6, except that with this method, powdery residues can be removed. Also gaseous residues can be removed more easily than with other methods, as once the powdery residues have been removed the re-circulation system can be used to assist this to happen rapidly.
8 Deep probing into the full depth of the cargo (however deep) with tablets or pellets (in retrievable sleeves when required).
This is being developed in Canada but is not yet available. Also deep probing using pre-inserted pipes.
Will enable good distribution of gas to be achieved without the requirement for a powered re-circulation system, provided the voyage is long enough.
9 Use of powered re-circulation system with phosphine from cylinders.
This is not yet available but could be in the future and will enable phosphine fumigation to be carried out without using aluminium phosphide. This will mean no powdery residues to deal with and therefore residue and safety problems at the discharge port will be minimised. A powered re-circulation system will be needed to enable this system to work with maximum efficacy.
Annex 2
References
International Maritime Organisation Recommendations on the Safe Use of Pesticides in Ships revised 2002. Published by IMO, 4 Albert Embankment, London, SE1 76R
International Maritime Organisation The International Maritime Dangerous Goods Code (IMDG Code) Volumes 1, 2 and Supplement (which includes the Recommendations on the Safe Use of Pesticides in Ships referred to above). Published by IMO London as above. Refer to Dangerous Goods List under entry UN 3359.
The International Maritime Fumigation Organisation (IMFO) Code of Practice (COP) Obtainable from the IMFO website http://www.imfo.com
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Scrap metal
(borings, shavings, turnings, cuttings, dross)
Ferrous materials in the form of iron swarf, steel swarf, borings, shavings or cuttings are classified in the IMO Code of Safe Practice for Solid Bulk Materials as materials liable to self heating and to ignite spontaneously.
Turnings are produced by the machining of steel, turning, milling, drilling, etc. When produced the turnings may be long and will form a tangled mass but they may be passed through a crusher or chip breaker to form shorter lengths. Both forms of turnings are shipped and shipments are frequently a mixture of short and long chips. The density of the short chips is of the order of 60 pounds per cubic foot, twice the density of the longer chips as they tend to compact more readily.
Borings are produced during the making of iron castings. Because of the nature of the parent metal, borings break up more readily than turnings. They tend to be finer and the bulk density is greater than turnings.
Turnings and borings may be contaminated with oils - cutting oils for instance - used in the manufacturing processes. Oily rags and other combustible matter may also be found among the loads.
Iron will oxidise, (rust) and iron in a finely divided form will oxidise rapidly. This oxidation is an exothermic reaction, heat is evolved. In a shallow level mass of turnings this heat will be lost to the surrounding atmosphere. However in large compact quantities as in a cargo hold this heat will be largely retained and as a result the temperature of the mass will increase. This oxidation process is accelerated if the material is wetted or damp, contaminated with certain cutting oils, oily rags or combustible matter.
The turnings may heat to high temperatures but will not necessarily exhibit flames. In one incident temperatures in excess of 500°C were observed six feet below the surface of the cargo. Temperatures of this order may cause structural damage to the steelwork of the carrying vessel. Flames are frequently seen in cargoes of metal turnings but these flames are usually the result of ignition of the cutting oils, rags, timber and other combustible materials mixed with the turnings.
Spontaneous heating of metal turnings has caused several major casualties. In the incident mentioned above spontaneous heating was detected, the vessel was moved from port to port in attempts to agree discharge. After weeks of delay all the holds were eventually flooded to reduce the heating for safe discharge of cargo. Following discharge of the turnings the vessel loaded a cargo of conventional scrap. During the subsequent voyage rough weather was encountered, cracks developed in the shell plating, the holds flooded and the vessel was lost with 29 lives.
In another incident heated turnings formed a solid mass in the hold which had to be mechanically broken into pieces before discharge by grab. In a further incident, following a normal passage it was not possible to discharge the cargo by grabs. The surface of the stow had crusted to a hard mass. Bulldozers were used to loosen the surface of the cargo and several hours later fire was observed in all of the holds.
The IMO Code of Safe Practice for Solid Bulk Cargoes has special requirements for the loading of turnings and borings which include:
- Prior to loading, the temperature of the material should not exceed 55°C. Wooden battens, dunnage and debris should be removed from the cargo space before the material is loaded.
- The surface temperature of the material should be taken prior to, during and after loading and daily during the voyage. Temperature readings during the voyage should be taken in such a way that entry into the cargo space is not required, or alternatively, if entry is required for this purpose, sufficient breathing apparatus, additional to that required by the safety equipment regulations, should be provided.
If the surface temperature exceeds 90°C during loading, further loading should cease and should not recommence until the temperature has fallen below 85°C. The ship should not depart unless the temperature is below 65°C and has shown a steady or downward trend in temperature for at least eight hours. During loading and transport the bilge of each cargo space in which the material is stowed should be as dry as practicable.
- During loading, the material should be compacted in the cargo space as frequently as practicable with a bulldozer or other means. After loading, the material should be trimmed to eliminate peaks and should be compacted.
Whilst at sea any rise in surface temperature of the material indicates a self-heating reaction problem. If the temperature should rise to 80°C, a potential fire is developing and the ship should make for the nearest port.
Water should not be used at sea. Early application of an inert gas to a smouldering fire may be effective. In port, copious quantities of water may be used but due consideration should be given to stability.
- Entry into cargo spaces containing this material should be made only with the main hatches open and after adequate ventilation and when using breathing apparatus.
It will be noted that compacting the cargo as loaded with a bulldozer is recommended. This will tend to form a dense mass, pushing the short turnings into the bundles of long turnings, tending to exclude air from the stow. However some authorities argue that compacting the stow tends to break up the long turnings, creating greater surface areas for the oxidation process. However shorter turnings should compact more readily than the longer forms and thus reduce the area exposed to oxidation. The reference to trimming level ensures that there is less cargo surface exposed to the air than cargo in a peaked condition. Furthermore, theoretically air will pass across the top of a level trim, but can pass through the stow if loaded in a peaked condition creating a 'chimney' effect, thus accelerating the heating process.
The requirements for entry into cargo spaces are very important, many lives have been lost by officers and crewmembers entering a hold to inspect a heating problem without taking adequate precautions. Oxygen is essential for the oxidation process and in a sealed space the oxygen is reduced by the heating reaction of the turnings or borings. The concentration of oxygen in air is 20.8%. Exposure to an atmosphere of 16% oxygen concentration causes an impairment of mental and physical state.
Concentrations of 10% will cause immediate unconsciousness and death will follow if not removed to fresh air and resuscitated. The symptoms which indicate an atmosphere is deficient in oxygen may give inadequate notice to most people who will then be too weak to escape when they eventually recognise the danger. Ventilation of the hold and testing the atmosphere or use of breathing apparatus is essential for safe entry to a hold which is loaded with these cargoes.
Metal dross and residues
Aluminium dross
Aluminium dross is formed during the recovery of aluminium from scrap and in the production of ingots. Dross may constitute about 5% of the metal where clean mill scrap is involved but will constitute greater quantities where painted or litter scrap is recovered. The main components of dross are aluminium oxide and entrained aluminium. Small amounts of magnesium oxide, aluminium carbide and nitride are also present.
The dross is recovered and re-melted under controlled conditions to provide aluminium metal which is then treated to remove hydrogen and other impurities including trace elements. Storage or transport of aluminium dross should be conducted under carefully controlled conditions. Contact with water may cause heating and the evolution of flammable and toxic gases, such as hydrogen, ammonia and acetylene. Hydrogen and acetylene have wide ranges of flammability and are readily ignited.
Aluminium dross, aluminium salt slags, aluminium skimmings, spent cathodes and spent potliner as aluminium smelting by-products are included in the IMO Code of Safe Practice for Solid Bulk Cargoes. The Code recommends that hot or wet material should not be loaded and a relevant certificate should be provided by the shipper stating that the material was stored under cover or exposed to the weather in the particle size in which it is to be shipped for not less than three days. The material should only be loaded under dry conditions and should be kept dry during the voyage. The material should only be stowed in a mechanically ventilated space. In our opinion the ventilation equipment should be intrinsically safe.
Zinc dross
Zinc dross, zinc skimmings, zinc ash and zinc residues are all materials obtained from the recovery of zinc. The zinc types may be recovered from galvanised sheets, batteries, car components, galvanising processes, etc. Zinc ashes are formed on the surface of molten zinc baths, and whilst primarily zinc oxide, particles of finely divided zinc will also adhere to the oxide. The various types of zinc are treated by processes to produce pure zinc metal.
The ashes, dross, skimmings and residues are all reactive in the presence of moisture liberating the flammable gas hydrogen and various toxic gases. The materials are also listed in the IMO Code for Solid Bulk Cargoes which states that any shipment of the material requires approval of the competent authorities of the countries of shipment and the flag state of the ship.
The Code recommends that any material which is wet or is known to have been wetted should not be accepted for carriage. Furthermore the materials should only be handled and transported under dry conditions. Ventilation of the holds should be sufficient to prevent build up of hydrogen in the cargo spaces. All sources of ignition should be eliminated which would include naked light work such as cutting and welding, smoking, electrical fittings etc.
We have knowledge of one incident where the cause of an explosion in a hold containing zinc ashes was said to be a lamp used to warm the sealing tape used to seal the hatchcovers. The flame of the lamp was stated to have ignited hydrogen gas leaking from the hold. The flame flashed back into the hold to ignite an explosive concentration of hydrogen/air. The explosion lifted the hatchcovers and collapsed a deck crane.
Unfortunately there was also loss of life. The hydrogen had been generated by reaction of the zinc ashes with water, zinc ashes which had been loaded in a damp condition.
The zinc ashes were discharged and later spread on the quayside in a thin layer to dry. Seven days later hydrogen was still being evolved to the atmosphere, as proved by tests with a hydrogen gas detector.
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Hold cleaning: bulk cargoes - preparing a ship for grain
Surveyors inspection/requirements
Prior to loading grain, all ships are usually subject to a survey by an approved independent surveyor. The surveyor will require the vessels particulars and details of at least the last three cargoes carried. He will then inspect the holds for cleanliness and infestation, or the presence of any material which could lead to infestation.
When the surveyor is satisfied with the condition of the hold, he will issue the ship with a certificate stating which holds are fit to load grain.
Purpose:
To ensure cargo holds are prepared to receive the next cargo.
Large claims have arisen when cargo holds have not been cleaned sufficiently to prevent cargo contamination. The requirements for cleaning the holds are dependent upon the previous cargo carried, the next cargo to be carried, charterers' requirements, the requirements of shippers and/or the authorities at the port of loading and the receivers.
It is becoming common practice for receivers to have an inspector at the load port.
General
Regardless of the previous cargo, all holds should be thoroughly cleaned by sweeping, scraping and high-pressure sea water washing to remove all previous cargo residues and any loose scale or paint, paying particular attention to any that may be trapped behind beams, ledges, pipe guards, or other fittings in the holds.
If the ship has been carrying DRI (direct reduced iron), the dust created by this particular cargo during loading or discharging, will be carried to all areas of the ships structure and the reaction between iron, oxygen and salt will create an aggressive effect wherever the dust may settle. This is particularly noticeable on painted superstructures. (The IMO Bulk Cargo Code contains guidelines). Whenever salt water washing is used to clean hatches, the relevant holds should always be rinsed with fresh water to minimise the effects of corrosion and to prevent salt contamination of future cargoes. In this respect, arrangements should be made in good time to ensure sufficient fresh water is available for this operation.
Before undertaking a fresh water rinse, the supply line (normally the deck fire main or similar) will need to be flushed through to remove any residual salt water. Accordingly, it is suggested that fresh water rinsing of the holds is left until the end of hold cleaning operations to minimise the amount of fresh water required.
Grain preparation and safe carriage
One of the most difficult hold cleaning tasks is to prepare a ship for a grain cargo after discharging a dirty or dusty cargo such as coal or iron ore, particularly if the last cargo has left 'oily' stains on the paintwork or other deposits stubbornly adhering to the steel surfaces. Greasy deposits which remain Cargo hold, coal sticking and discharging salt. on the bulkheads will require a 'degreasing chemical wash' and a fresh water rinse in order to pass a grain inspection. The degreasing chemical used should be environmentally acceptable for marine use, and safe to apply by ships staff, who have had no special training and do not require any specialised protective equipment. Product safety data sheets of the chemical should be read, understood and followed by all persons involved with the environmentally friendly degreasing chemical.
To avoid taint problems, fresh paint should not to be used in the holds or under the hatch lids at anytime during the hold preparation, unless there is sufficient time for the paint to cure and be free of odour as per the manufacturer's instructions. Most marine coatings require at least seven days for the paint to be fully cured and odour free. All paint used in the holds and underside of the hatchcovers should be certified grain compatible and a certificate confirming this should be available onboard. Freshly painted hatches or hatchcovers will normally result in instant failure during the grain inspection, unless the paint has had time to cure.
Processed grains or grain cargoes that are highly susceptible to discolouration and taint should only be stowed in holds that have the paint covering intact. It is important that there is no bare steel, rust, scale, or any rust staining in the hold.
Dependent upon the quality of the grain to be carried, the charter may require the holds to be fumigated. This may be accomplished on passage with fumigant tablets introduced into the cargo on completion of loading. Fumigation can also be undertaken at the port of loading (or occasionally discharge). The ship will normally be advised how the fumigation is to be carried out and of any special precautions that will have to be taken. In all cases, the preparations (i.e. inspecting the holds and hatchcovers for gas-tight integrity) and fumigation must be carried out in accordance with the IMO document Recommendation on the Safe Use of Pesticides on Ships. Gas detectors and proper personal protective equipment should be available and relevant ship's officers should receive appropriate training in their use. After introduction of the fumigant, an appropriate period should be allowed (normally 12 hours) for the gas to build up sufficient pressure so that any leaks can be detected: the vessel must not depart from port before this period has expired. The entire process should be certified by a qualified fumigator. The holds must not be ventilated until the minimum fumigation period has expired, and care must be taken to ensure that subsequent ventilation does not endanger the crew.
Alongside the discharge port
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| Hatchcover underside and clean hatch rubber
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On non-working hatches, remove all cargo remnants, loose scale and flaking paint from the underside of the hatch lids and from all steelwork within the hold, provided safe access can be obtained. Then commence washing the underside of the hatchcovers |
