Residual static strength of high strength steel cracked tubular joints

Jack-up platforms are to an increasing extent being considered for use as long-term production support structures. The implications of this new application for structural integrity assessment and maintenance are significant. Traditional regular dry dock inspection and repair is no longer possible when a platform is stationed for medium to long-term production support periods. For this reason the extent to which jack-up platforms can tolerate fatigue damage must now be a design consideration. Jack-up platforms by their nature do not have the degree of structural redundancy as found in traditional fixed jacket production structures. This means that the push-over or static strength capacity of jack-ups is far more sensitive to reduction in strength due to fatigue cracking in individual members than equivalent cracking in jacket structures. In addition, high strength weldable steels are regularly used in the construction of jack-up rig legs due to their excellent strength-to-weight ratio. However, there is often little information available concerning the mechanical behaviour in fatigue and fracture particularly under protected corrosion conditions. This has, to some extent, limited the designer's ability to maximise the potential benefits offered by high strength weldable steels. This paper considers the problem of residual static strength of jack-up platform components by examining the results of two sets of static strength destructive tests performed on full-scale pre-cracked tubular welded T- and Y-joints manufactured from a commercially used high strength weldable steel. The paper also highlights the need to conduct further work so that a thorough understanding of jack-up platform defect tolerance can be attained.