Extensive research carried out by experts in hydrodynamics and ship structures at ABS has identified critical areas within the structure of ultra large containerships (ULCs) may not be adequately addressed by traditional, prescriptive class rules. These vessels are inherently more flexible and behave very differently from their smaller counterparts. To better understand and predict the motions and structural behavior of ULCs, a non-linear hydrodynamic sea load approach, integrated with a full ship finite element structural analysis must be used to augment standard classification review, according to ABS.
According to Chris Wiernicki, Chief Technology Officer at ABS, they have found that the traditional prescriptive approach has not adequately addressed some load effects. The unique hull form of these vessels with large, flat, overhanging sterns and pronounced bow flare, coupled with high service speeds, introduces non-linear ship motions and load effects that will impact the magnitude of the wave induced bending moment, the torsional loads, as well as whipping, bow and stern slamming and springing. Using the sophisticated ABS SafeHull Dynamic Load Approach (ABS SH-DLA), coupled with the Large Amplitude Motion Program (LAMP), the society has been able to review a number of existing and proposed shipyard designs and has clearly identified the high stress areas that may be susceptible to cracking within a relatively short in-service period. That is why loads that have been considered implicitly in traditional class rules must now be treated explicitly, using engineering first principles, if adequate strength is to be introduced at the design stage.
ABS researchers have found that these non-linear sea loads can result in significantly higher wave induced sagging moments and shear forces than have been considered in traditional prescriptive rule-making. Whipping induced bending moments and shear forces, combined with higher sagging bending moments, result in higher forebody structure loads. Of specific concern is buckling in the forebody deck structure, fatigue of the hatch corners and distortions of the hatch openings which can impact container lashing. Careful analysis of these sections of the structure is required to determine appropriate scantlings, according to Mr. Wiernicki.
Greater attention must be paid to the allocation of steel within the structure, to the design details and the overall structural arrangement. The overall steel weight of the vessel should not be affected but the distribution of the steel within the structure should properly address the specific stress concentrations that can be determined through a non-linear hydrodynamic sea loads analysis. Appropriate criteria are being incorporated into the ABS Rules.