Ship structures are joined with thousands of metres of weld lines. The presence of welds reduces fatigue life by introducing potential discontinuities and residual stresses in the parent material. These effects are often further amplified by the presence of defects that are inherent to welding process. The closer the weld is to a perfect weld, the longer its fatigue life will be. Rules, standards and guidelines require manufactures to carry out certain procedures to enhance a weld’s reliability. The techniques include weld toe grinding to improve weld profile geometry, heat treatment to improve welded joint’s toughness and non-destructive examination to find weld defects. Regarding welding defects, performing non-destructive examination on finished welds has been the best way to find possible defects, as relevant Rules require the manufacturer to do so.
Apart from visual inspection it is not feasible (particularly from a commercial viewpoint) to perform NDE for all of welded joints. Hence classification societies tend to devise partial inspection regimes by specifying a number of checkpoints for examination of welds instead of 100% inspection. The objective is for verifying the general quality of welding as well as ensuring that critical structural elements are defect free. This is achieved through recommending tables, formulas and clauses that define the minimum extent of NDE required to be carried out.
Prescribed tables and formulas have developed over the time primarily based on engineering judgment, historical experiences of cracks found during service, and somewhat on structural analysis of ships. Each classification society has its own set of rules, the development of which has a basis from its own experience. This has resulted in significant variance amongst Classification Societies of the final inspection plans, both in terms of the number of checkpoints and their locations. Current IACS procedures let individual Class Societies set their own guidelines - there is no common approach. This study aims to develop a robust methodology to optimise the extent of NDE inspection such that there is improved confidence in the structural integrity (and hence safety) of the ship.