Up to the fifties, classification assessment of ship’s strength was mainly based on past experience, static and quasi-static wave profile loads, as the natural forces and behaviour of the sea were deemed at the time to be largely unpredictable. This rule and minimum standards framework ensured safety for existing ship types but was more difficult to apply to new types of ships. Furthermore, the requirements referring to the ship’s structure scantlings had tabular form and were not expressed in non-dimensional format, normally derived from the principles of structural mechanics. At the time the ship structure was appraised in terms of separate structure members. It was conservatively assumed that if each structure member satisfied the minimum requirements then the whole hull structure would be safe. On larger ships, the verification of deck cross section was additionally required. Nevertheless, trends to optimise the fleet led to new ship types reflecting the diversity of carried cargo and means of loading and unloading. The safety standards applied at the time appeared to be inadequate to the new types of ships. Classification societies started to develop new safety standards in response to this new situation.
Safety standards in the present rules correspond to the division of the hull structure strength into hull girder, zone (hold) and local problems, i.e. 3 problems in total. Theoretically 4 criteria , namely yielding of the structure material, buckling of the structure, fatigue of structure details and ultimate strength, have to be applied to each problem, effectively resulting in 12 problems. In practice, the ultimate strength criterion, in current rules, is only applied to certain structures (e.g. bulkheads) and the fatigue strength criterion is applied only to the design requirements of some structural connections (e.g. for connections of longitudinals); thus, reducing the number of problems. In the yield check the allowable stress is divided into components, i.e. the criterion for hull girder, zone and local strength components. However, the decomposition of the allowable stress into components is not simple. This is mainly due to the fact that class rules require application of different wave loads, in the form of formulae, which are likely to occur once in a ship’s lifetime. These loads do not appear “simultaneously”; there is a phase shift between them. Therefore, summing the stresses in a particular structural member (e.g. bottom longitudinal) resulting from the application of the rule load components (for example, wave bending moments, wave pressure and ship’s accelerations) results in a value greater than that caused by superposition of the loads taking into account their phase shift. The proper combination of the stress components is important for deriving the total stress value, giving rise to the question : “How can one combine the dynamic load components, determined either by the rules formulae or predicted separately through the use of suitable software?”.
Decades of applying such rules and ship structure casualties, notably affecting the structure of bulk carriers and tankers and mainly due to uncertainties in wave loads determination, gave rise to the development and implementation, sometimes retroactively, of new requirements in direct reaction to such casualties. This unsatisfactory state of regulations for ships’ structure triggered both:
the development of Common Structural Rules for Oil Tankers (JTP) and Bulk carriers (JBP) by IACS (2004a, b), which have recently been presented to the industry for comments, and
the development of the Goal Based New Ship Construction Standards (GBS) by IMO (2004 a, b, c), namely MSC 78/26, MSC 79/26 and MSC 80/6.
The proposed Common Rules are intended to embrace more aspects of safety, such as ultimate strength, fatigue strength and strength in damaged conditions, than the presently binding rules. Design loads - the most uncertain aspect affecting safety - are in the form of a combination of static and dynamic, local and global loads and they “consider the most unfavourable combination of load effects”, as given by JTP (IACS 2004a).
The requirements referring to the dynamic load components (wave bending moments and shear forces, external sea pressures, internal dynamic pressures, ship motion and accelerations) are presented in the form of formulae. The loads for scantling requirements and strength assessment are at the probability level of 10-8 (10-4 being the reference level for fatigue strength), as given by JTP and JBP. The load combination factors are given as tabulated values and are calculated by application of the equivalent design wave approach , as also given by JTP and JBP (IACS 2004a, b).
Shigemi and Zhu (2003), and Zhu Shigemi (2003) developed methods for practical estimation of the design loads which, based on the following definitions:
design sea state (irregular wave) is the sea state that generates response value equivalent to the long-term prediction of stress,
design regular wave is the regular wave that generates response values equivalent to the response values generated by the design irregular wave, and
design loads are loads generated by the design regular wave and used to design the hull structure.
The values of stresses estimated with the use of these proposed design loads are claimed to be equivalent to the long-term predictions of stresses for typical load cases (see section 5). In the design regular wave approach the dominant load is determined for wave heading angle, wave period and height values which produce a maximum response. The dominant load is computed for each load case. Then the load combination factors, representing the relationship between responses to dominant and secondary loads, are determined (Shin et al 2004b). This regular wave approach has been widely used
in local scantling and FE analysis of ship structure. An irregular wave approach is more appropriate in combining load/stress components for fatigue assessment. Shin et al (2004b) presented a consistent and complete method for the combination factors in multiple sea states. The formulation is exact when applied to a single random sea. To determine the load combination factors in long term the probability of the sea state occurring was taken into account.
Direct methods of determining the loads acting on a ship and its structural response are based on hydrodynamic and structural mechanic theories. The actual shape of the ship, mass distribution, randomness of the sea and loading conditions are taken into account in these theories. Theoretically, an infinite number of loads acting on the ship should be considered; in practical calculations, however, a sufficient but finite number of representative cases is implemented. Most important of all, the actual phase shifts between loads are determined in the evaluation of the stresses on the structure. Therefore, it is questionable whether:
the simple formulae approximating the amplitudes of wave load components - with the assumed probability of exceeding thereof,
the phase shifts between loads in the form of factors, and
the small number of load cases assumed in the simplified methods
can approximate the randomness of the sea and ship operations that is represented in its mathematical models by a family of functions (stochastic approach).
The Maritime Safety Committee of IMO has commenced the development of Goal Based Standards, which was initiated in 2002. So far this Committee has formulated (IMO 2004 a, b, c):
the goals, which assume that “ships are to be designed and constructed for a specified designed life to be safe and environmentally friendly when properly operated and maintained under the envisaged operating and environmental conditions, in intact and foreseeable damage conditions, throughout their life”, and
the functional requirements which, amongst others, refer to the design life, environmental conditions, fatigue life, structural strength and residual strength.
The problem of quantification of the functional requirements is under discussion.