ABSTRACT

This paper is concerned with the application of the near field based added resistance gradient method of predicting the wave drift damping coefficients of a moored structure and their use in time domain simulations. The simplified 3D fluid structure interaction analysis formulation used is a generalisation of the earlier reported enhanced strip theory approach. The predictive capability of the near field added resistance gradient procedure has been demonstrated in the analysis of ship forms through comparison with experimental measurements. This particular 3D prediction procedure has been demonstrated to be a better predictor of wave drift damping coefficients than an alternative 3D far field approach in the low frequency regime. In this paper the method is applied to the prediction of the wave drift damping coefficients of a semisubmersible with comparisons of the predicted values and experimental measurements. The effect of the wave drift damping coefficients upon the responses of a moored semisubmersible is then examined by comparison of mooring line tension predictions obtained from time domain simulations including and excluding the predicted wave drift damping coefficients. Finally mooring line tension statistics related to the time domain simulations are compared with those determined from experimental measurements.

INTRODUCTION

The use of semi-submersible vessels for exploration, drilling and construction has been well established within the offshore oil and gas industry. Such vessels are moored during operations by composite cable and chain lines forming a catenary array. Whenever the weather becomes unfavourable the operation is ceased, the moorings dropped and the vessel rides out the storm using its own propulsion system. However whereas these vessels have retained their mobility, a new generation of semi-submersibles are being designed as floating production platforms for application in marginal and deepwater fields. The vessels incorporate a purpose built semi-submersible hull carrying an oil or gas production facility as a payload. The requirements for station keeping l due to riser excursion limits, and the inability to leave station and the need to survive the extreme environmental conditons has therefore led to more stringent design requirements for permanent catenary moorings. Previously moorings were not considered as permanent with the mariner preferring to run with the storm. Furthermore traditional mooring designs have normally been carried out using the quasi-static approach. However Ramzan and Robinson1 have argued that the quasi-static approach is inadequate and time domain simulations are necessary.

The major shortcoming in the use of the time domain approach is in the evaluation of the second order damping. Large amplitude motions of moored structures at sea are generated by the second order wave drifting forces at low frequency due to the resonant response of the mooring system. Methods exist2 that can predict the second order wave drifting force with satisfactory accuracy. However, the damping mechanism deserves more attention since it is well known that the response amplitude of a resonant system is critically dependent upon the associated damping. Uncertainty in the evaluation of the second order damping force could lead to a large over-prediction of line loads leading to very costly mooring systems and at worst, an uneconomical development scenario.

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