Extensive comparative studies have been undertaken on various theoretical approaches used for analysing spiral strands, in order to identify their appropriate ranges of applicability. Based on the findings of this exercise, a theoretical model is proposed for analysing the coupled axial/torsional response of a stranded wire rope with either an independentwire rope or fibre core. A theoretical model is also proposed for the analysis ofaxial/torsionai coupling of locked coil cables, regarding which, there was previously a . paucity of reliable analytical or experimental information, despite their wide range of applications. Axial hysteresis of locked coil cables is estimated, and it is shown that it is significantly higher than the axial hysteresis associated with spiral strands. Simplified routines which are amenable to hand calculations are developed for obtaining the no-slip stiffness coefficients for the coupled axial/torsional stiffness matrix of axially preloaded spiral strands, with the effect of hydrostatic pressure on sheathed strands catered for. Moreover, the concept of orthotropic sheets for calculating the pattern of interwire/interlayer contact forces in spiral strands is used to determine the recovery length of a broken wire in any layer of a multi-layered spiral strand. Wherever possible, the final theoretical results are compared with experimental data from the available literature and very encouraging correlations have been found in all cases.

Fixed jacket offshore structures. which react environmental wave loading. are generally constructed using tubular steel members. When subject to load. severe surface and through-thickness stress gradients occur due to local bending of the tubular wall. The cyclic nature of the environmental wave loading results in high stress concentration at the joints. which can lead to fatigue cracking. British Gas currently operates twelve fixed offshore structures. two of which. the Rough A-Complex structures. are now ageing and showing signs of fatigue cracking. The objective of the work described in this thesis is the development of a fracture mechanics-based methodology for the assessment of fatigue cracking in these structures. The fracture mechanics approach uses the stress intensity factor (SIl) to characterise crack-tip conditions. and provides a means of analysing the behaviour of cracks. The SIF is defined in terms of the crack site stress distribution and the change in structural compliance with crack size. Difficulties in the application of fracture mechanics lie in the derivation of accurate solutions for the SIF. The tubular joints of offshore jacket structures present particular difficulties due to their complex loading and geometry. The current work starts with a review and assessment of tubular joint fracture mechanics models. followed by a numerical study of cracked tubular joints using shell finite element (FE) models incorporating line spring crack representation. Based on the results of this study. a general fracture mechanics model for the prediction of SIF solutions for tubular joints. is derived and assessed. The general fracture mechanics model is incorporated into a crack growth model. which is best implemented using sophisticated commercial software. Crack growth and fatigue life predictions obtained are validated against full scale tubular joint fatigue data. Finally. a complete methodology for the assessment of fatigue cracks in any tubular joint is proposed, and applied to the assessment of fatigue cracking in the Rough A-Complex structures.

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