Abstract

The mean forces induced by waves, wind and current on tankers, barges and other structures, are very important for the selection of an appropriate passive or active positioning system. In the present paper tools are given for the determination of these forces specially for tanker shaped bodies, taking into account the main influence of water depth. The way in which the estimation of the mean forces has to be used for a preliminary design of a mooring system will be dealt with.

Introduction

The problem of designing an adequate mooring or anchoring system for floating offshore structures is rather complicated not at least due to the non-linear nature of the problem. It is a problem of dynamics and the moored structure may be regarded as a mass-spring system. The total load, exerted by the environment, consists of the oscillating wave forces and of forces, which vary at a frequency much lower than the wave frequency: the wind, current and wave drift forces. When designing an anchor system, three aspects of these very slowly varying forces are of importance.

1. As will be illustrated at the end of this paper the almost steady wind, current and wave drift forces are most significant for the final determination of the diameter of the anchor lines. This is due to the fact that the main purpose of an anchoring system is to hold the structure on an average position. Basically the force required for this station keeping equals the steady or the slowly Description of steady forces and moments

Wind forces and current forces are frequently presented as being composed of a drag force, in the direction of the flow, and a lift force, perpendicular to the flow direction. However, when describing the dynamic behaviour of a floating object, the equations of motion are usually related to a structure bound system of coordinates, with its origin in the mid-ship section of the structure. Therefore it is convenient to describe the steady forces as a longitudinal and a transverse component applying in the midship section and a moment around the vertical axis. Figure 2 shows a sketch in which the sign of the forces and moment, and the direction of wind, waves or current are defined.

THE FORCE DUE TO WIND

Like all environmental phenomena, wind has a stochastic nature which greatly depends on time and location. It is usually characterized by fairly large fluctuations in velocity and direction. It is common meteorological practice to give the wind velocity in terms of the average over a certain interval of time, varyingfrom 1 to 60 minutes or more. The variation in mean velocity is very slow compare with the wave period. The fluctuation around the mean value will impose dynamic forces on the structure, but in these aerodynamic forces may be neglected in comparison with the hydrodynamic forces, when considering the dynamic behaviour of the floating body.

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