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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This thesis describes an integrated approach to fatigue cracking, reliability and inspection of offshore structures. The basis of the approach is statistical in nature and draws on recent experimental data and field measurements. It is intended as a working tool for those engaged in design, structural appraisal and sub-sea inspection of steel jacket structures. A review of current practice has been made and the requirements of an integrated approach are established. An approach is proposed comprising a series of compatible models dealing with fatigue cracking, the reliability of cracked joints and the inspection of welds for fatigue cracks. The primary linking parameter is the distribution of fatigue crack size which is considered as a time dependent variable. An integral part of the approach is a new statistically-based fatigue crack growth model. This is developed and the parameters involved in the model estimated from an analysis of experiment and oceanographic data. For any fatigue calculation the model allows the corresponding fatigue crack growth distribution to be estimated for any time during, or beyond, the nominal fatigue life. A number of example calculations are included; and using one of these a Bayesian procedure for revising fatigue lives in the light of inspection results is demonstrated. The effect of fatigue cracking upon the various modes of tubular joint failure is considered using linear statistical models. Example calculations are performed for a typical joint. An inspection strategy is proposed based on the concept of minimising life costs, including risk costs arising from the consequences of possible structural failure. This allows alternative inspection plans to be evaluated and compared, and a typical example calculation is included. The approach is discussed in the context of possible alternative approaches and areas for further related research are identified.

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The dynamic motion of floating wind turbines is studied using numerical simulations. Floating wind turbines in the deep ocean avoid many of the concerns with land-based wind turbines while allowing access to strong stable winds. The full three-dimensional Navier-Stokes equations are solved on a regular structured grid, using a level set method for the free surface and an immersed boundary method for the turbine platform. The tethers, the tower, the nacelle and the rotor weight are included using reduced order dynamic models, resulting in an efficient numerical approach which can handle nearly all the nonlinear wave forces on the platform, while imposing no limitation on the platform motion. Wind is modeled as a constant thrust force and rotor gyroscopic effects are accounted for. Other aerodynamic loadings and aero-elastic effects are not considered. Several tests, including comparison with other numerical, experimental and grid study tests, have been done to validate and verify the numerical approach. Also for further validation, a 100:1 scale model Tension Leg Platform (TLP) floating wind turbine has been simulated and the results are compared with water flume experiments conducted by our research group. The model has been extended to full scale systems and the response of the tension leg and spar buoy floating wind turbines has been studied. The tension leg platform response to different amplitude waves is examined and for large waves a nonlinear trend is seen. The nonlinearity limits the motion and shows that the linear assumption will lead to over prediction of the TLP response. Studying the flow field behind the TLP for moderate amplitude waves shows vortices during the transient response of the platform but not at the steady state, probably due to the small Keulegan-Carpenter number. The effects of changing the platform shape are considered and finally the nonlinear response of the platform to a large amplitude wave leading to slacking of the tethers is simulated. For the spar buoy floating wind turbine, the response to regular periodic waves is studied first. Then, the model is extended to irregular waves to study the interaction of the buoy with more realistic sea state. The results are presented for a harsh condition, in which waves over 17 m are generated, and linear models might not be accurate enough. The results are studied in both time and frequency domain without relying on any experimental data or linear assumption. Finally a design study has been conducted on the spar buoy platform to study the effects of tethers position, tethers stiffness, and platform aspect ratio, on the response of the floating wind turbine. It is shown that higher aspect ratio platforms generally lead to lower mean pitch and surge responses, but it may also lead to nonlinear trend in standard deviation in pitch and heave, and that the tether attachment points design near the platform center of gravity generally leads to a more stable platform in comparison with attachment points near the tank top or bottom of the platform.