On Fatigue Design Rules For Welded Structures

Abstract

A comparison between the design rules in use in various countries has shown that there is a wide difference between them on such questions as design stresses, the treatment of high tensile steel vis-a-vis mild steel, the question of whether or not a cumulative damage approach should be used in design and on what factors of safety should be adopted. However, published test data exist for many types of detail and the paper quotes the results of a statistical analysis of these data which could form the basis for a universal set of ‘master rules’. It is suggested that all design standards should be related to the same basic data. In the context of offshore structures much additional data are required, particularly reliable information about the fatigue strength of tubular joints and about corrosion fatigue. INTRODUCTION Offshore structures, whether they be floating or fixed platforms, are subjected to loading of an extremely complex nature arising from a variety of sources, but particularly from wave action. The severity, both in terms of magnitude and frequency of application, of these loads varies considerably from one part of the world to another, but in some places at least it can be such that the fatigue strength of these structures has to be treated as a most important design criterion. Thus, in the North Sea several structures are known to have suffered fatigue cracking, sometimes of major proportions and after only a relatively short period in service. In contrast, there is reputed to be no fatigue problem in the Gulf of Mexico.(1) The difference stems from the much more, severe fatigue loading spectrum in the North Sea. In view of the current, and future, large investment in offshore structures and equipment, and the potential cost in terms of lost production, environmental pollution and new capital requirements, which could result from major structural failures, it is pertinent to consider what steps can be taken to reduce the risk of fatigue failure to acceptable levels. Clearly, there is a need for a realistic set of design rules which can readily be interpreted by designers. In the context of offshore structures, fatigue can be taken to mean high cycle fatigue. It is the purpose of this paper to review the current situation regarding high cycle fatigue design rules for welded structures, to suggest ways in which improvements could be made and to point out regions where further research seems to be necessary. EXISTING RULES AND THEIR BASIS Historically, the majority of the design standards relating to high cycle fatigue have been concerned primarily with bridges, and particularly railway bridges.

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