ABSTRACT

The offshore structures of the Forties Field were investigated to determine their static and dynamic response to wave action. The dynamic analysis took into account the nondeterministic nature of waves by representing the sea as a wave spectrum.

A comparison of the results of the dynamic response analysis and the static analysis demonstrate that for extreme weather conditions, the static design is adequate.

INTRODUCTION

Offshore platforms are traditionally designed on the basis of a static application of a design wave. Hostile sea environments, which exist in the North Sea, require that the dynamic behavior of structures in intermediate and deep water be studied. The procedure used to determine the dynamic response of the Forties Field structures to wave action takes into account the nondeterministic nature of ocean waves. This is done by describing the sea state as a wave spectrum which specifies the distribution of wave energy with respect to frequency. Because of this approach, the results of the analysis are described in a statistical way. Such a dynamic wave analysis was performed for platforms 'FB' and 'FC l to ensure that the overall dynamic response did not exceed the static design criteria. A mathematical description of the analytical procedure used to perform the spectral analysis is included in the appendix, while a more detailed description of the procedure is presented in references (1), (2) and (3).

DISCUSSION OF ANALYSIS PROCEDURE

The dynamic response of the structures to wave forces was calculated using the computer program "TOWER" (4) which determines the frequencies and shapes of the lowest modes of vibration and the overall response of the structure to random wave forces described by a wave spectrum.

The dynamic response of the structure was determined considering vibrations in the horizontal direction only with the masses of the structure lumped at the levels of horizontal bracing, which were assumed to act as rigid diaphragms. (A description of the structures and the dynamic models are presented in Figures 1 and 2.)

The total dynamic lumped masses include the mass of structure and equipment, enclosed water in flooded members and virtual (added) hydrodynamic mass. The damping of the dynamic system includes both structural and hydrodynamic damping. The flexibility matrix is determined by modeling the structure as a three dimensional space frame and applying a series of unit loads at the levels corresponding to the nodes of the dynamic model which represents a series of lumped masses. The flexibility coefficients are then the lateral displacements of the three-dimensional model.

The computer program uses the wave spectrum, linear wave theory and the Morison wave force equation to determine spectral densities of the wave forces on the structure, while an iterative procedure is used to linearize the drag force. The tributary projected areas and volumes of the members are apportioned at the two extreme faces of the jacket at discrete levels below the water line to obtain a more accurate assessment of the wave forces on the structure.

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