ABSTRACT

A method is presented which is based on statistical evaluation of loads and corresponding stresses in irregular seas. Observed wave data in the potential areas of operation are used as a basis for simulating characteristic sea states on the computer, i.e. the time variation of the irregular wave pattern. On this basis the maximum stresses over the lifetime of the structure are predicted.

I INTRODUCTION

The most commonly used method for evaluating wave loads on an offshore structure has until now been to base the calculations on one or more design waves. This was also the case in the shipbuilding industry some years ago. When ship size increased, however, it became obvious that the design wave method led to unreasonable results in many cases. It was realized that the random nature of the ocean waves, and consequently of the responses of a structure to these waves, could only be described by statistical methods.

Recent research had revealed that the irregular wave pattern could be described by linearly superimposing regular waves of different frequencies. In this way it was made possible to take into account all the wave components present in a realistic sea. The development of methods for calculating the response of a floating structure to the regular waves was also initiated in this period. Det norske Veritas has played its part in this development and a number of computer programs relating to ship problems have been the result, see for instance /1, 2/. Calculations for ships based on the principles indicated have proved successful and these methods are now commonly used.

It was a natural step to try to extend the same principles and methods to offshore structures like drilling platforms.

The following will give a description of these methods as applied to semisubmersibles. Fig. 1.1 illustrates the procedure in principle. A particular feature is that the wave load program is directly connected to the stress analysis programs.

2 HYDRODYNAMICS
2.1 Equations of motion

The following forces acting on the platform are accounted for in the present analysis:

• Hydrodynamic forces due to the relative motion between platform and waves

• Inertia forces due to platform Motion

• Restoring forces set up by the Anchors

• Hydrostatic forces and other static forces applied at any point and acting in. arbitrary direction, e.g. wind and current forces.

The static forces are included in the list for completeness only, of course they do not influence the dynamics of-the problem.

The set of equations used to solve the motion problem in six degrees of freedom may be written in matrix notation as:

• (Mathematical equation available in full paper)

where M, Band C are six by six symmetric matrices. M is associated with the platform mass and added mass, B with the damping, and C with restoring. The displacement matrix may be written more explicitly as:

• (Mathematical equation available in full paper)

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