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

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