ABSTRACT

This paper is concerned with the evaluation of the extreme environmental loading on fixed steel offshore platforms. The evolution of the design practice for environmental loading is reviewed, noting the physical phenomena not explicitly accounted for In design. Conclusions with respect to the accuracy of the design models are drawn based on evidence from the historical performance of platforms, notably during very severe storms. It is demonstrated that in order to bring about a substantial improvement in the accuracy of the design environmental loading model and to rationally eliminate some conservative elements, it is necessary to replace the present ‘design package1 by a new one rather than to adjust’ the existing package. The main ingredients of the new refined environmental loading model are:

  • explicit account of joint probability of occurrence of waves, winds and currents in the definition of the design storm;

  • use of a reduced current to account for the blockage effect;

  • use of a new, improved wave kinematics model;

  • realistic values of hydrodynamic coefficients.

Although the new proposed model is more refined than the present simplistic practice, it will, nevertheless, be as convenient to apply as the present one, once a number of outstanding tasks are completed.

INTRODUCTION

The design of offshore platforms for operation in the harsh waters of the North Sea is largely governed by the extreme storm environmental loading. The ability to predict accurately the extreme storm loading remains an important factor in the continued safe and economic exploitation of the hydrocarbon reserves in this area. This paper is confined to a sub-class of offshore platforms, namely fixed steel platforms whose response to environmental loading is essentially quasi-static and which comprise a space frame of 'slender members which do not influence the gross characteristics of the incident waves (no wave diffraction). The majority of offshore platforms fall in this category.

The phenomena which cause environmental loading are complex and generally defy rigorous description which is why a huge amount of research effort has been spent towards their resolution. The studies have comprised extensive environmental data gathering and hindcasting projects for winds, waves and currents; measurement of water particle motion in real seas; instrumentation of offshore platforms to measure imposed loads; laboratory experiments in wave tanks, towing tanks, wind tunnels, etc. and supplementary theoretical studies to explain the observed phenomena and to establish appropriate models for predictive purposes

At the heart of the complexity of the problem lies the natural sea environment during a storm which comprises a complex system of wind-driven waves having different wavelengths, amplitudes and periods, travelling at different speeds and in different directions, plus storm-driven and tidal currents. This complexity is reflected in the resulting water particle motions which cause the loading. Last but not least, is the problem of estimating the magnitude of loading for given, known water particle motions. This problem, too, has defied theoretical solution and reliance has had to be placed on extensive measurement programmes.

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