Fatigue in the side shell of ship shaped structures

摘要

In ordinary ship structures such as oil tankers, the side shell has been severely exposed to fatigue damages. This problem has escalated recent years due to the introduction of high tensile steel in such structures in the late 80s. Fatigue cracking is caused by stress cycles due to the time varying forces acting on the ship, such as sea pressure. The fatigue capacity of the side shell is strongly dependent on the design of local details. Small variations in the local design may strongly improve the fatigue capacity. Current fatigue design of ships is based on rules and regulations from Classification Societies. In fact, only limited fatigue requirements are given for ordinary ships. In order to validate the safety of ships used for offshore applications, design from first principles should be applied. Det Norske Veritas (DNV) has a possibility within the Nauticus [14] and Sesam [16] program package to carry out such analyses. These tools may also be used to improve fatigue strength of ordinary vessels. Possibilities and limitations of such analyses are outlined and discussed. A typical VLCC is used as a case study. The fatigue life is dependent upon factors such as global and local design, workmanship during construction, maintenance, corrosion protection, trade and load history. How current rules deals with some of these factors are outlined and compared to how they can be treated in a refined assessment. Some typical ship details such as bracket toes are covered in a proper manner. Other areas such as slots and scallops are not given as much attention in the current rules and regulations. Applying direct calculations, all details may be assessed, and different designs may be compared directly. Advantages and limitations of the direct fatigue design approach are discussed based on results from the case study.