The main aim of this thesis is to develop a probabilistic methodology, based on structural reliability theory, which allows information gained during manufacture, fabrication, construction and service life of a particular structural system to be systematically combined with data available at the design stage in order to improve reliability predictions about the structure's performance during its remaining service life. Broadly speaking, the objectives of the thesis can be summarised as follows: • To develop generic strategies and methods for reliability-based structural integrity assessments focussing on specific structures or plant • To further develop reliability-updating procedures so as to sharpen predictions of remaining life • To determine the value of obtaining additional in-service data in terms of improved SI calculations and reduction of model uncertainty. To achieve these objectives, consideration is restricted to two factors affecting the SI of offshore steel jacket structures, namely fatigue crack growth and system reliability performance under the influence of environmental loading.

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Topside structures of offshore installations have to support heavy process plant dealing with large volumes of oil and gas under high pressure. Many of these platforms have to be operated in very remote areas in a harsh environment with little supporting infrastructure. It is therefore necessary to design these high risk installations to various types of extreme loadings. One of these extreme scenarios is blast loading from a possible hydrocarbon explosion. Although this is a comparatively low frequency accidental event, it has the capability to cause major fatalities to personnel and serious structural damage which could lead to the complete loss of the platform. At present, most topside structures are designed based on working stress design (WSD), or load and resistance factor design (LFRD), which is quite safe but not economical due to the uncertain extent of the levels of protection. For these reasons, a Performance Based Design Methodology is proposed, which emphasizes the structure’s predictable behaviour and the protection of personnel and assets. The end result will be an optimum design which satisfies the function of a system without compromising safety. A performance based design guideline for the assessment of topside structures subjected to blast loading is proposed. The guideline, which reasonably incorporated some statistical findings, simplifies the evaluation of performance levels for the topside structure without quantitative risk assessment (QRA) data. The assessment of topside structures is not complete if the proper behaviour and response is not fully understood. It has been shown in the study that the roles of secondary structural members i.e. deck plates and stringer beams cannot be overlooked. Having substantial deformations on secondary members averts severe damage on primary members. A simplified deck plate analytical model is proposed and the optimum slenderness ratio for deck plate design is recommended. Although accuracy of the proposed analytical method is found slightly offset from the finite element result at extreme overpressure, the model is straightforward and provides a quick method to assess the deck plate capacity. The study has also highlighted the weakness of sniped bottom flanges for stringer beams, a necessary condition to facilitate practical fabrication. This shortcoming is overcome by strengthening with angles, a novel idea which is simple and practical with minimum interference to the existing structural configuration. Based upon a typical topside framing, the performance level of the topside is evaluated for reference which can be applied to other topsides. The study has investigated a number of mitigation techniques for improving beam to beam connections. The techniques comprise studies based on some conventional approaches, typical fabrication methods and a new proposal with tubular braces. Finally, the effect of equipment on the topside structure is investigated and recommendations are made to minimise unnecessary damage.

Stainless and other high quality steels are used extensively in the topside construction of oil rigs. Steel is heavy, expensive and even the special grades are prone to corrosion in the aggressive marine environment. New materials are needed which are lighter, more cost effective and free from corrosion related problems. Glass fibre reinforced plastics (GRP) have the required properties but their performance in fire conditions is not known. Fire is a very real and possibly catastrophic threat. Before specifying the use of GRP components it is essential to quantify their reaction to fire. Panels and pipes to be used in fire risk areas were the components of interest, and the objectives of the research, based on experimental testing, were as follows: 1) To evaluate GRP laminates for use as structural panel skins, noting their structural and fire performance. 2)To develop incombustible, low cost cores for sandwich panels. 3) To produce sandwich panel design proposals which satisfy specified fire exposure requirements. 4)To assess the fire performance of empty and dry, stagnant water filled and flowing water filled polymer composite pipes with or without fire protection. 5) To use finite difference modelling as part of the design process for fire exposed pipes and panels. Factors of water content for hygroscopic cores and the ablation mechanism of fire exposed GRP were taken account of. 6) To assess the validity of the standard furnace fire resistance test with respect to combustible materials, and with respect to the reproducibility of results between different furnace arrangements.

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Offshore structures are required to survive in extreme wave environments. Historically, the design of these offshore structures and vessels has relied on wave-tank experiments and linear theory. Today, with advances in computing power, it is becoming feasible to supplement these methods of analysis with fully nonlinear numerical simulation. This thesis is concerned with the development of an efficient method to perform this numerical modelling, in the context of potential flow theory. The interaction of a steep ocean wave with a floating body involves a moving free surface and a wide range of length scales. Attempts to reduce the size of the simulation domain cause problems with wave reflection from the domain edge and with the accurate creation of incident waves. A method of controlling the wave field around a structure is presented. The ability to effectively damp an outgoing wave in a very short distance is demonstrated. Steep incident waves are generated without the requirement for the wave to evolve over a large time or distance before interaction with the body. This enables a general wave-structure interaction problem to be modelled in a small tank, and behave as if it were surrounded by a large expanse of water. The suitability of the boundary element method for performing this modelling is analysed. Potential improvements are presented with respect to accuracy, robustness, and computational complexity. Evidence of third order diffraction is found for an FPSO model.

An understanding of extreme waves is important in the design and analysis of offshore structures, such as oil and gas platforms. With the increase of interest in the shipping of LNG, the design of import and export terminals in coastal water of slowly varying intermediate depth requires accurate analysis of steep wave shoaling. In this thesis, data from laboratory experiments involving random wave simulations on very gentle slopes have been analysed in terms of a model of large wave events, and the results interpreted by observation of the shape and magnitude of the large wave events. The auto-correlation function of the free surface elevation time histories, called NewWave, has been calculated from the wave spectrum and shown to fit very well up to the point where waves start to break (when compared to the ‘linear’ surface elevation time history). It has been shown that NewWave is an appropriate model for the shape of the ‘linear’ part of large waves provided kd > 0.5. A Stokes-like expansion for NewWave analysis has been demonstrated to match the average shape of the largest waves, accounting for the dominant vertical asymmetry. Furthermore, an appropriate local wave period derived from NewWave has been inserted into a Miche-based limiting criterion, using the linear dispersion equation, to obtain estimates for the limiting wave height. Overall, the analysis confirms the Miche-type criterion applies to limiting wave height for waves passing over very mild bed slopes. A derivation of general Green-Naghdi (GN) theory, which incorporates non-linear terms in its formulation, is also presented. This approach satisfies the boundary conditions exactly and approximates the field equations. The derived 2-dimensional vertical GN Level 1 model, capable of simulating steep waves on varying water depth, is validated against solitary waves and their interactions, and solitary waves on varying water depth and gives good qualitative agreement against the KdV equation. The developed and validated numerical model is used to simulate focussed wave groups on both constant depth and gentle slope. In general, the behaviour of waves simulated by the numerical model is very similar to that observed in the experimental data. There is evidence of vertical asymmetry as the water depth is reduced, owing to the non-linearity. Although the main physics is still controlled by linear dispersion, the higher order harmonics become increasingly important for shoaling waves. The numerical results also show a slope-induced wave set-up that keeps on increasing in amplitude as the wave group travels on the gentle slope.