The Fatigue Behaviour Of Welded Joints Under North Sea Environmental And Random Loading Conditions
Authors: Holmes Robert 
Published: October 2008

Offshore installations have been designed to withstand fatigue loading using existing codes derived mainly from constant amplitude fatigue information. Additional fatigue data that are much closer to service conditions experienced in the North Sea are urgently required. Corrosion fatigue tests in a simulated sea-water environment have been carried out on 38 mm thick cruciform welded joints to investigate the effect of different types of narrow-band random load spectra. These load spectra are representative of the loading conditions experienced on offshore structures due to wave action. The results show a gradual decrease in the fatigue strength of the joints as the loading changed from a stationary Rayleigh type random input to a non-stationary Laplace type input. Further tests were carried out under impressed current cathodic protection conditions to determine if the beneficial effect experienced under constant amplitude loading is maintained under random loading.

INTRODUCTION

Corrosion and fatigue have always been problems in structural engineering but their combination, namely corrosion fatigue, has become increasingly predominant in offshore installations due to the use of relatively low strength steels. The large increase in the use of such steel for structures on the European Continental Shelf has resulted in designers having to work without the normal service experience, since never before have offshore installations been built for such a hostile environment as that experienced in the northern North Sea. During its life, a typical structure will experience upwards of 100 million wave cycles, with half of the cycles resulting in loads greater than 35 per cent of the design load. In the light of these problems and, following an investigation1 of the fatigue and fracture problems associated with offshore structures, it was realized that further detailed information on the behaviour of welded joints in a marine environment was urgently required before platforms could be designed with the required reliability. This led to the formation of a major research and development programme, the United Kingdom Offshore Steels Research Project [UKOSRP] 2, of which the work reported in this paper is a small part.

The corrosion mechanisms of steel under static loading 3 are to some extent understood. When steel is subjected to random loading, the variables which affect the corrosion fatigue performance of welded joints become numerous; hence any estimate of the safe working life of an offshore platform is necessarily approximate unless validated by relevant data. As the National Engineering Laboratory, UK, had considerable experience of component fatigue testing under random loading conditions 4 and full-scale service simulation fatigue testing5, it was appropriate that NEL should carry out part of the basic studies section of UKOSRP. This section was primarily designed for investigating the effect of a marine environment and random stress histories on the fatigue strength of cruciform welded joints with and without cathodic protection, and for establishing if the design standards 6,7 in use today, which are derived mainly from small joint testing under constant amplitude loading, are still appropriate when applied to offshore structures.