Abstract

Jack-up rigs have traditionally been used for short term drilling and maintenance operations. The deployment of jack-up platforms fm extended periods increases the risk of long term problems such as fatigue due to the reduced frequency of dry dock inspections and the enhanced susceptibility to fatigue crack initiation and propagation. Furthermore, there is less data available on high strength steels used in the fabrication of jackups when compared with conventional offshore structural steels such as BS 4360 and BS 7191 and there is no existing fatigue guidance: current fatigue guidance is limited to steels with yield strengths up to 500MPa. There is need to establish the fatigue behavior of high strength steels used in jack-up construction and this paper presents results from a major large scale.: fatigue testing programme on the fatigue performance of a typical high strength jack-up steel, SE 702, which investigated (a) the effects of cathodic protection on the corrosion fatigue behaviour of large-scale high strength welded tubular joints tested under constant amplitude loading, and (b) the corrosion fatigue behaviour of the same steel under simulated service loading and environmental conditions. The effects of cathodic protection and variable amplitude loading on the fatigue performance of SE 702 high strength steel are assessed and compared with results from conventional offshore structural steels.

INTRODUCTION

High strength steels have been used ill the fabrication of jack-ups for many years. Steels with nominal yield strengths ill the range 450 - 700MPa have commonly been used for the fabrication of the leg structure (see figure 1), rack and pinions and spud cans. Several jack-ups are now being used for production, thus limiting the opportunity for inspection and repair. As such then! is a higher risk of fatigue crack growth and subsequent structural failure.

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