Double Hull Tankers: The Need for Proper Maintenance and Operation, Notas de estudo de Engenharia de Produção

Baixe Double Hull Tankers: The Need for Proper Maintenance and Operation e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! OCIMF Double Hull Tankers — Are they the answer?  2 Summary Double Hulled vessels are regarded by some as the answer to all the problems of transportation of oil at sea without pollution. Whilst it is acknowledged that double hulled vessels have some advantage over single hulled vessels, indeed they will provide added security in low impact collisions and groundings, both designs will be inadequate if poorly maintained and operated. Double Hulled tankers because of their complex design and structure are potentially more susceptible to problems of poor maintenance and operation. Double Hulled tankers may only be the answer if combined with; high quality operation, maintenance, classification surveys, and proper policing by flag state and port state. © OCIMF 2003 OCIMF would like to thank Shell Shipping Technology for the technical assistance given by them for this information paper. 5 pollution by cargo leaking into the pipes and through into the clean ballast. It does mean, however, that replacement of the ballast pipes will be more difficult since they will be threaded through access holes in the floors of the double bottom tanks, as they are in bulk carriers and other dry cargo ships with double bottoms. Removal of old pipes will require the cutting of large openings in the bottom shell plating or inner bottom. There is no doubt that significant advantages in cargo operations will accrue from the change to double hulls. This will especially benefit product carriers, which carry high value cargo, and where even better outturns can be expected. The smoother internal tank surfaces and pump suctions recessed into wells in the double bottom will make cargo discharge and tank washing much easier and lead to reduced cargo residue in the tanks. 3. Construction Issues Modern shipyards adopt 'factory' techniques to improve productivity and thus reduce ship construction times. Whereas a VLCC might have taken two years to build in the early 1970s, double hull VLCCs are now being built, from first steel cutting to delivery, in about eight or nine months. This puts a great deal of emphasis on doing the job correctly first time - corrective work or request by owners to follow “Industry Best Practices and Guidelines” causes delay and disrupts the shipyard's production programme. This in turn puts pressure on quality and an owner's supervision team, needs to be alert to several critical aspects of the construction of double hull tankers. Probably the most significant of these is the protection of the ballast tanks. The ballast tanks, which carry sea water as ballast when the tanker is on its return (empty) voyage are the areas most prone to corrosion because of the extremely corrosive nature of salt water. This aspect attains far greater significance in a double hull tanker because of the increased surface area of the structure inside the ballast tanks. Because these tanks are much longer and narrower than those in single hull tankers, their surface area is two to three times that of the ballast tanks in a single hull ship. Although protective coatings are an obligatory requirement of the major classification societies, it is up to the owner to choose the type and number of coats, ensure that they are properly applied and decide whether to fit anodes as well. Shipyards will argue that their standard preparation procedure is adequate even though it fails to meet the standards recommended by experts and paint manufacturers. The standard coating specification from the builder will usually be inadequate for the expected lifetime of the ship, again to keep the build cost down and minimise the impact on production. The confined spaces of the double hull ballast tanks, whether sides or bottom, are far more unpleasant to work in than the comparatively spacious ballast tanks of the single hull tanker and good ventilation through the design of openings is very important. Maintainability also needs to be designed into the vessel and here is an area where currently there are no enforceable standards, anything included at present is generally at the request and expense of, diligent managers and operators, as it adds complication to production for the shipyard. Some features of the double hull design make life easier for the builder. The double sides and double bottom form natural three-dimensional rigid building blocks, less susceptible to deformation than the predominantly two-dimensional components of the single hull ship. However, the number of cruciform joints where primary structural members terminate on double skin structure is significantly increased. Many of these are located in critical areas (defined as areas where high stress levels combined with potential stress concentration features may lead to premature failure of primary structure). It is vital that in these locations, the importance of enhanced standards of fit up, alignment and inspection, which vary greatly from yard to yard, are recognised and implemented by the Designers, Builders and Class. Another area where there is guidance available which is generally at odds with shipyard production practice. 4. Salvage of a double hulled tanker If a double hull ship should run aground and rupture the outside shell, the available damage statistics suggest that the inner hull will, in most cases, not be breached and no oil will be spilt. However, on smaller tankers the damaged space might easily be a 'U' shaped tank, allowing free flooding right across the double bottom and up to the outside water level each side of the cargo tank. Thus a considerable weight of flood water would be admitted, making the ship sit more firmly on the bottom and more difficult to re-float. A single hull tanker, by contrast, would spill a small amount of cargo which would lighten the ship and make it easier to re-float (Figure 3). Damage to an 'L' shaped double bottom tank on the other hand, would cause flooding on one side resulting in considerable list should the ship not come to rest on the rock but remain free-floating. This would need to be corrected by the filling of an opposite tank. If the ship remained 6 aground with damage to an 'L' shaped tank, then the consequent heel when the ship floated free would need to be considered in the salvage plan. In the Prestige incident one side was flooded and the ballast tanks on the opposite side were filled to bring the ship upright, this caused the stresses to exceed the design limits by some 68%. The relative merits of single and double hull designs depend on weather conditions at the time and on the availability of salvors. It might in some circumstances be advantageous to have the ship sitting more firmly aground until salvage equipment arrives. Salvors have indicated that they would prefer to salvage a ship with a double bottom as it gives them the option of using air pressure to expel the flood water. This is certainly true of dry cargo ships, which do not have to meet the raking damage criterion and have greater subdivision in their double bottom tanks. Nor do they have interconnected side and bottom tanks. The use of air pressure then becomes a feasible salvage technique. Almost certainly it will take longer to re-float a damaged double hull tanker than a damaged single hull tanker, during which time the weather conditions could be critical. 5. Design Issues Structural Design The structural integrity of oil tanker hulls relies not only on good quality of initial design and construction but also on an effective programme of inspection, maintenance and repair being conducted by the owner or his manager. However well designed and built a tanker may be, it will not provide trouble-free service unless it is well operated. The tanker designs produced by today’s shipbuilders although approved by all the major classification societies, are based on the assumption that the owner will undertake all necessary repairs to the fabric during its lifetime. There is no such thing as a maintenance-free tanker. The design and construction process therefore, although important, is not the sole factor in the long term integrity of the structure. The history of ship structural design is one of evolution rather than revolution. Designers learn from past experience and each new ship tends to be a development of a previous successful design. Whenever this course has been abandoned - as in the rapid growth in size of tankers in the 1960s and the large open hold container ship designs of the 1970s - structural problems have materialised sooner or later due to the lack of relevant service experience. A shift from single hull design to double hull design represents a similar departure from established successful designs. Despite the advent of ever more powerful computers and increasingly sophisticated structural analysis programs, structural design remains a largely empirical process and it is still impossible to produce a successful design exclusively from first principles. This is because of the extremely complex interaction of the many variables which affect the stresses in the structure: • Structural design – plate thicknesses, local stress concentrations, stiffness and proper transmission of loads; • construction quality - for instance alignment, local imperfections, the quality of steel and welding; • distribution of the cargo weight in the ship; • static and dynamic forces of the sea and waves resulting from heaving, pitching, rolling and possibly slamming; • vibration from machinery; • random corrosion; and • the complex internal distribution of stresses between primary, secondary and tertiary structures. Clearly, the 'design' or calculated stress levels in any element of the structure should have a safety factor based on previous successful experience. It is impossible to calculate accurately the true stress levels in service throughout the tanker's structure entirely from first principles. However, safety factors for large double hull structures are not yet available for the simple reason that there is little service Fig 3. Effect of Bottom Damage 7 experience. Although there is already some successful service experience from gas ships and smaller double hulled product carriers, it will be several years before the structural design of double hulled VLCCs can be proved. In the meantime, generous safety factors need to be incorporated into the design. The utilisation of higher tensile (HT) steel in double hull designs, driven by builders to minimise steel weight and retain competitive edge in the world markets, is again an important factor. When used extensively for primary and secondary structure, particularly within the ballast tank spaces, the resulting increases in deflections and stress levels will impact negatively on their fatigue lives and also upon the effective lifetime of coating systems. Thus the incorporation of extensive amounts of HT steel will potentially have more impact on the operational performance of a double hull design compared with single skin. Therefore, it falls upon the Owner/Operator to try and limit the extent of HT steel incorporated in new builds, usually at extra cost to themselves. The difficulty of accurate stress prediction is compounded by the higher hull girder bending moments of double hull tankers (Table 1). These arise because of the uniform distribution of cargo and ballast over the length of the ship, whereas in a single hull tanker the ballast tanks can be positioned to minimise longitudinal bending and shear stresses, resulting in values well below the classification society maximum (Figure 4). Double hull tankers will operate with global stress levels some 30% higher than those with single hulls - close to the maxima acceptable to classification societies - unless an owner spends a substantial amount on extra steel thicknesses and suffers the attendant increase in design draught, or builds in extra ballast tanks to reduce bending moments. Commercial pressures mean that few, if any, owners are willing to suffer these financial penalties. These higher stresses will increase the risk of buckling failure - especially after several years in service and the consequent reduction in plate thickness from corrosion. They will also increase the likelihood of the development of small fatigue cracks. Many shipyards worldwide designed double hull tankers for the first time based on their own direct calculations and guided by the experience of the classification society, which, in the case of some of the smaller societies, may also be limited. This could put the design of double hull VLCCs, for example, closer to 'revolution' than 'evolution' in the absence of service and operational experience and safety factors. The consequence is most likely to be small fatigue fractures in the early years of service, especially in larger double hull tankers, unless great care is exercised in the design detail and workmanship during construction. Some of the major classification societies have put much effort into studying these problems and are confident that they can achieve a successful structural design first time. Some of the first generation of double hull tankers suffer from defects in poor design details, such as poor alignment of the cruciform joints, poor support of the lower knuckle between cargo tank and ballast tank and lack of understanding of the need for good weld profiling in areas of higher stress. None of these issues was relevant on single hull tankers. There is still a learning process as these problems and others are detected and solved. In the meantime all operators of double hull tankers have to be on their guard to detect fatigue cracks as quickly as possible to prevent crude oil leaking into ballast tanks or the contamination of valuable product cargoes with ballast water. Perhaps the greatest concern is that of an accumulation of hydrocarbon gas inside an empty ballast tank. Table 1