ABSTRACT

Second order wave (drift forces on a barge type hydrodynamic ally compact structure are measured in a wave tank and compared with theoretical calculations. The theory used is the generally accepted method based on perturbation theory and on integration of the hydrodynamic pressure over the instantaneous wetted surface of the structure (near field solution). The theory assumes drift forces to be proportional to the square of the wave amplitude. Comparison of the theoretical and experimental results show that both the water depth as well as the wave height affect the drift forces in regular waves. In contrast to the theoretical assumptions, the measured wave drift forces clearly indicate that they are proportional to a power of wave amplitude greater than two.

The consequences of these results are extensively studied using the probabilistic analysis. Extreme drift excursions of a large displacement offshore power plant are calculated for five wave spectra, three mooring systems and three water depths. Results indicate that drift excursions show a large variation and depend on the choice of drift coefficients used. Therefore, caution is recommended when designing mooring configurations for stationing such structures in more severe sea states with higher waves and larger characteristic periods.

INTRODUCTION

Recent developments in ocean engineering structures show a continuous increase in size and weight of fixed and floating platforms. Consequently, their installation procedures become more sophisticated and the requirements put on mooring systems used during this installation procedure are more demanding. This is especially true for mating operations of platforms consisting of a foundation structure connected to a jack-up unit, which platforms are used for the accommodation of special purpose plants.

One of these platforms is the 360 MW offshore power plant EPOS (Fig. 1), designed by DEUTSCHE BABCOCK for marginal oil fields in coastal waters [1]. During installation, the upper deck structure is moored and is jacked up after mating with its foundation structure (Fig. 2).

In a natural seaway this mating operation is a very delicate procedure as exemplified by the requirement that during the installation the maximum motions of the leg ends are limited to 0.5 m. This requirement is particularly difficult to meet because of the following facts:

  • The displacement of the floating unit (V = 27 880 m2)as well as its water line area (approximately 5 000 m2) are unusually large for a jack-up type structure.

  • The platform's pitch and roll resonance frequencies are close to the frequency of maximum wave energy (Fig. 3). This may lead to relatively large vertical and horizontal motions of the leg ends [2].

  • In the relatively small water depth at the planned location (36 m), the floating unit experiences significant surge motions endangering the jack-up operation.

This paper describes some of the problems associated with the analytic prediction of the motion behavior of the moored barge. The analysis consists of theoretically determining second order wave (drift) forces and deducing theoretical drift force coefficients. In parallel, an extensive experimental program has been carried out to measure drift forces in regular waves.

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