XXVIII Symposium "Zmęczenie i Mechanika Pękania" - Selected Problems and Current Practice of Analysis of Fatigue Life of Ship Welded Joints in the Requirements of Classification Societies
Polski Rejestr Statków S.A., Gdańsk
Polski Rejestr Statków S.A., Gdańsk
The development process of Rules for construction of ship and offshore structures is still challenging. In one hand, guidelines should result in safely-designed construction to ensure that during exploitation period, no accidents related to major structural damages will occur. To provide that, the calculation and analysis procedures that are based on the current state-of-the-art knowledge should be adopted. In other hand, the proposed proce-dures should not be too complicated and sophisticated to ensure that there can be utilized during design process. Recently, International Association of Classification Societies (IACS) issued Common Structural Rules for Bulk Carriers and Oil Tankers (CSR) and one of the chapters includes consolidated fatigue assessment procedures. In many cases, the adoption of them in designing practice is still challenging.
In presented work, the fatigue assessment guidelines required by Common Struc-tural Rules are briefly introduced. The both, simplified approach together with advanced Finite Element Method based approach are explained. To quantify the stress range levels, the so-called Equivalent Design Wave approach is adopted, and the basic assumptions of this methodology are outlined. To show the difficulties that can be faced during analysis, two numerical examples of the fatigue assessment of the tanker ship hull welded joints are presented. First one is related to the connection between inner bottom longitudinal and transverse double bottom girder and the joint is analysed with the used of simplified approach. In second example, the three-cargo hold FE model of the tanker ship is pre-sented and the regions that are prone to crack are shown. In these regions, the very fine mesh was adopted, and the hot spot stress ranges were identified. The results of fatigue assessment for one of these joints are presented. The reference tanker ship was built with the use of pre-CSR Rules and the resulting fatigue life was lower than the target one. This led to the conclusion, that current rules are more conservative from the fatigue point of view.
Basically, the CSR are applicable only to bulk carriers and oil tankers and only for specific main dimensions of the ship. In case of tankers and bulk carriers that are outside the scope of CSR and other ship types, the requirements for fatigue assessment of welded joints are the prerogative of each individual Classification Society (CS), such as Polish Register of Shipping. To show the differences between specific guidelines, the bench-mark study was conducted by International Ship and Offshore Structures Congress. In this benchmark, the simplified fatigue assessment of the typical connection between deck longitudinal stiffener and deck transverse girder of bulk carrier was conducted with the use of different CSs Rules. In presented work, the results of this study are briefly outlined and discussed.
Although the CSs are preparing guidelines that should prevent from the cracking of structural members, such damages are still occurring during ship exploitation. This could be caused by the wrong design that not prevent from stress concentration and still our limited knowledge about fatigue strength. In presented paper, the fatigue damage case study is presented and discussed.
The Industry influence into the CSR preparation is briefly explained and several conclusions are derived.