The Performance, Safety and Production Benefits of SPS Structures ...

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construction related failures and insufficient maintenancecontinue to be addressed.In the early days of double hull tankers there was littleknowledge on which to draw, other than the experienceof single hulled tankers. Whilst this was extensive, theindustry was unsure how it would extrapolate to doublehulled vessels.and 3.4. Such failures had been experienced on singlehull tankers and much work in the marine industry hasbeen undertaken to understand and avoid such problems.3.2(a)Early Concerns of Double Hulled TankersFollowing the introduction of OPA90, it became clearthat double hulled tankers would supersede single hulldesigns. Concerns were raised about the universaladoption of a standard requiring tankers to be of doublehull construction, many of which centred around theconsequences of an accident involving a double hullvessel, compared to that of a single hull.Figure 3.2 Critical Areas for Fatigue and StructuralFailures on Double Hulled TankersAMSA has recently published a brief summarycomparison of single and double hull tankers (Ref 1),which gives a very clear description of the merits anddisadvantages of double hulled vessels. These includeoil outflow and considerations of hydrostatic balancefollowing collisions and grounding. The difficulties ofsalvaging a double hulled vessel due to flooding and lostbuoyancy following a grounding incident are alsoaddressed. The intact and damaged stability of doublehulled tankers was also questioned, due to concerns overthe raised centre of gravity associated with doublebottom tanks and the greater free surfaces of wide tanks.Other concerns centred around structural issues and werelegitimately raised in light of the experience borne outover the previous 20 to 30 years. Double hulled tankershave a high level of structural complexity, the number ofstructural intersections is almost twice that of anequivalent size single hull ship. Leakages of cargo intothe ballast spaces from cracks in the inner hull structurewere seen as a major risk. Fatigue and corrosion werehighlighted as potential causes of such failures.Fatigue:The well documented service histories of single hulltankers were used to highlight potential areas for fatigueproblems. Figure 3.2 shows the areas that are critical forfatigue on double hulled tankers. These includeconnections of inner hull plating at the upper and lowerintersections of the sloping hopper, connections at thetoes of brackets and stringers and the intersections ofsecondary longitudinal stiffeners with transversemembers.Secondary stiffeners and their associated intersectiondetails are well known sources of structural problems.Depending on their location, geometric configuration andconstruction, a combination of service loads, residualstresses and corrosion can result in coating breakdownand fatigue cracks. This can lead to ongoing propagationand eventual structural failure as shown in Figures 3.3Figure 3.3 Typical Fatigue Crack Found on Side ShellLongitudinal Stiffeners of Single Hull TankersUse of higher tensile steel has also contributed to fatiguefailures. Apart from fatigue lives being reduced by ageneral increase in working stresses associated withreduced scantlings, problems have also been encounteredwith increased residual stresses introduced duringconstruction. These arise in areas such as stiffenerintersections, where a welding sequence has createdresidual stresses that are difficult to “relieve” in HT steel.When using this material, care is required with detaildesign and with construction processes.Figure 3.4 Early Experiences of Fatigue Damage Led toCrack Propagation and Larger Structural Failure4
Corrosion:Undetected corrosion has been a major cause of some ofthe most noteworthy marine disasters. A paper producedby OCIMF (Ref 2) provides a very concise summary ofthe potential problems facing the operators of doublehulled tankers. In particular it highlights the difficultiesof coating, inspecting and maintaining the ballast spaces,which in conventional steel designs are cellular, with acomplex arrangement of internal stiffeners, brackets andassociated structural details. In today’s double hulledtankers it is common practice to provide full coating ofthe ballast tanks, but to coat only the bottoms of cargotanks. A second paper by OCIMF (Ref 3) provides avery clear view of the various sources of corrosionaffecting tankers and their influence on corrosion rates.These include coatings, washing processes, sulphurcontent of cargo, inert gas quality, construction materialsand others.It is clear that control of corrosion plays a major part inthe safe operation and maintenance of a double hulledtanker. Conventional steel designs, with their inherentcomplexity are costly to protect and maintain. This isdue to the large surface areas, difficulty of access andlarge numbers of points for potential initiation of coatingbreakdown.Experience of double hulled tankers to date:To date there is little documented operational experienceon the structural performance of double hulled tankers.Anecdotal evidence suggests that the experience has sofar been satisfactory. Whilst there are cases of minorstructural failures, such as fatigue cracks in stiffenerintersections and inner hull connections with the slopinghopper, these have been found and addressed with nomajor incidents. Regular inspection and maintenanceremains a clear necessity.A paper by Bergesen, produced in 2000 (Ref 4) providesa comprehensive summary of experience in operatingthree double hulled VLCCs. The paper concludes with anumber of measures for protecting both ballast and cargotanks from corrosion. The paper also notes that waveloads on the bow have resulted in local buckling ofinternal structure and hull plating deformation, resultingin a “starved horse” appearance.Today’s double hulled tankers are not known to besuffering from structural failures. However, the effortrequired to design and construct intersections with goodservice performance adds considerably to the productioncost and the problems have not been completelyeliminated.3.3 USE OF SPS IN DOUBLE HULLED TANKERSIntelligent Engineering believes that SPS technology willprovide improved safety through greater resistance toimpact, collisions, fire and explosion. Furthermore, thestructural simplification that SPS provides will givecommercial benefits in terms of reduced costs forproduction, inspection and lifetime maintenance. Byremoving secondary stiffeners from the structure, thehigh risk areas of coating breakdown, acceleratedcorrosion and fatigue cracking are eliminated.3.3(a)Design principles in using SPSUsing SPS in ship structures requires a fresh view ondesign, especially in areas of structural intersections.Basic longitudinal strength is assessed in a similar way tocurrent established methods, using classic navalarchitecture theory and standard hull section propertycalculations. With SPS structures, the continuous innerand outer face plates of the composite sandwich areincluded in the section properties, but the elastomer coreis not included in the calculation. Depending on localstrength requirements, a portion of the steel area, aboutthe neutral axis, may be redistributed to the deck andouter hull to maintain the section modulus whilereducing overall weight.SPS design for local strength involves a new approach.Conventional ship structures are commonly designedusing established ‘empirical’ classification society rules.These do not yet exist for SPS structures, which behaveas true composite materials with isotropic properties.Design assessment rules for sandwich plate shipstructures are being developed by Lloyd’s Register andare expected in draft form in 2005.The scantlings of SPS plating are generally in the rangeof 3 mm to 8 mm for the steel face plates and 20 mm to50 mm for the core thickness. The required scantlingsare a function of the loads and spacing of the supportingframing, and are currently determined by direct designcalculations, using normal stress criteria for yield andshear. Corrosion margins are added to the face plates, asrequired.SPS panels are very stable, since the stiff elastomer coreprevents local buckling effects. Semi-empiricalequations have been developed to determine the in-planeload carrying capacity for any combination of transverseloads. These have been verified using full scale tests andnon-linear finite element analyses. Current designpractice uses finite element analysis to check the in-planeload carrying capacity of SPS panels. This is achievedwith models using solid or layered shell elements toaccount for both material and geometric non-linearity.The maximum transverse framing should be in the rangebetween 3.2 to 4.0 m. SPS plating should have an aspectratio (the ratio of length of the long to short plandimension) between 1.4 and 1.7, to produce an economicplate with the maximum buckling strength. SPS panelsshould be arranged so that the largest dimension isaligned along the length of the ship.5
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