Myers, Peter Terence; (1998) Corrosion fatigue and fracture mechanics of high strength jack up steels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Jack up platforms are self elevating mobile units used for drilling and production of offshore oil and gas reserves. These platforms utilise high strength weldable steels for the fabrication of the truss leg structures. The yield strength of these steels is typically around 700MPa compared with 350MPa in standard fixed jacket structures. Concern regarding the performance of such steels was heightened by the discovery of cracking in and around the spud can of several jack up platforms operating in the North Sea. It is thought that the higher strength steels are more susceptible to material degradation due to Hydrogen Embrittlement. An experimental investigation has been performed to investigate and quantify the magnitude of such deleterious effects. Seven fatigue tests have been performed on large scale welded tubular joints fabricated from SE702, a 690MPa steel commonly used in jack up construction. Cathodic protection levels of -800mV and -1000mV (versus Ag/AgCl) were used. Evidence from these tests suggests that this particular steel performs at least as well as lower strength steels in air and under cathodic protection. The shape of the tubular joints in jack up structures can be significantly different from those found in conventional fixed structures. This can have a significant effect on the stress distribution at the intersection of the tubular members. Accurate knowledge of the stresses at the intersection is needed for design and assessment purposes. Although the stress distribution in unstiffened joints has been heavily researched, little work has been done for joints representative of jack up legs. For this reason an extensive thin shell finite element investigation has been performed to determine the stress distribution across a wide range of jack up leg geometries. The distribution of stresses at the surface and through the thickness of the chord members has been quantified. The results show that the 'hot spot' SCF can be reduced by as much as 50% compared to the corresponding unstiffened joint. The results of this stress analysis study have been integrated into a fatigue fracture mechanics analysis of jack up representative geometries. Additionally, contemporary fracture mechanics models have been assessed against fracture mechanics data extracted from the experimental fatigue tests.

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