The Effect of Wind Spectra on the Low-Frequency Motions of a Moored Tanker in Survival Condition
Authors: Feikema G.J. , Wichers J.E.W. 
Published: October 2008

In the design of moored tankers the knowledge of the wave drift forces is of importance. As is known, the low frequency part of the wave drift forces is responsible for the low frequency motions of the vessel. In general the low frequency motions may cause a substantial part of the mooring forces.

Another source of the low frequency excitation can be found from wind spectra. The wind spectra originate from the turbulence in the wind field. Besides the wave drift forces also the fluctuating wind force will induce low frequency oscillations of the tanker.

The purpose of this study is to investigate the effect of a wind spectrum based on an hourly mean wind speed in relation to the 1- minute steady wind gust, both in combination with a co-linearly directed storm wave spectrum on the low frequency motions of a 200 kDWT tanker.

The tanker is moored in 82.5 m deep water. The computations have been applied to a linear type mooring system. To confirm the reliability of the computation procedure model tests have been carried out.

INTRODUCTION

For the design of a mooring system the environment in terms of storm wave spectrum, current speed and wind velocity has to be established. The irregular waves generate the wave loads, which induce the wave frequency ship motions and the wave drift forces. The wave drift forces consist of a mean wave drift force, a mean wave drift damping and the low frequency wave drift forces. In the design, normally a steady current velocity is taken, while for the wind speed either the steady 3-second, the l-minute or the 10-minute wind gust speed is taken.

Except for the wave frequency motions, the motions can be distinguished in a mean offset part and a low frequency oscillating part. The mean displacement of the moored vessel is caused by the mean wave drift force, the steady current and wind force. From the point of view of steady offset the mean wave drift force and the steady wind force are often important components in the mooring force.

The oscillating part of the wave drift force is responsible for the low frequency oscillating motions of the vessel. In general the low frequency motions may cause a substantial part of the mooring forces.

Another source of the low frequency Excitation can be found from wind spectra. The wind spectra originate from the turbulence in the wind field. The description of the turbulence is very complex. However, it is possible to express the turbulence in terms of so-called wind spectra or gust spectra. Studies on wind spectra have been carried out by several researchers as, for instance, Harris [1], DnV [2], Forristal [3], Davenport [4], Ochi-Shin[5] and Wills [6].

Most of the wind spectra contain energy of the wind velocities at the natural periods of the tanker. Because of the relatively light damped system the fluctuating wind velocities generate fluctuating wind forces, which induce low frequency tanker motions.

The effect of the wind spectra on the low frequency motions will be studied. For that purpose a moored tanker has been exposed to the following weather conditions:wind spectrum based on an hourly mean wind speed, (VW = 30.9 m/s)storm wave spectrum,