Dynamic Effects of Current Fluctuations on Moored Vessel
Authors: de Kat J.O. , Wichers J.E.W. 
Published: October 2008

The behavior of a bow hawser moored tanker exposed to wind, waves and current has been studied by means of computer simulations. An overview is given of the theory used in the simulations. Some comparisons are shown with experiments. Tanker motions and bow hawser forces were investigated both in a stationary and in a dynamic, time-varying environment. In the stationary environment steady wind and current are considered and in the dynamic environment the effects of waves and unsteady wind were studied. In particular, the occurrence of unstable behavior is investigated. Lastly, it is shown that time-dependent current can influence bow hawser forces significantly.

1. INTRODUCTION

While there exists an extensive amount of knowledge concerning the behavior of moored tankers in a steady environment, certain dynamic characteristics of wind, waves and current still deserve attention.

It is becoming increasingly common to moortankers to single points for marginal and medium sized oil fields. The moored tanker may have processing equipment on board and be served by shuttle tankers, or it may act as a storage unit and loading terminal. Single point mooring (SPM) systems have been installed in moderate to severe weather areas. The behavior of the moored system in wind, waves and current can affect design and operation aspects.

In a typical ocean environment a moored vessel will exhibit both high and low frequency motions. Wind and waves may subject the vessel to steady and slowly varying drift forces, which together with the current loading govern the excitation of the system. The high frequency wave-induced motions are not considered in the theory described here.

In this paper a tanker will be considered moored by means of a bow hawser to a fixed point and exposed to wind, waves and current. A sketch of the considered terminal is shown in Fig. 1. The system consists of a 200,000 DVT tanker, for which the particulars are given in Table 1 and the body plan in Fig. 2. Both the fully loaded condition and 40 per cent of the full load draft condition are considered. Hawser lengths between 45 and 90 m have been investigated. One stiffness curve is used for the bow hawser line, as shown in Fig. 3. The water depth is taken to be 30.24 m. The following basic environmental conditions are used:

(MATHEMATICAL EQUATION AVAILABLE IN FULL PAPER)

where Vc is the current speed, Vw is the wind speed, Hs is the significant wave height, and Tp is the peak period of the irregular wave spectrum (assumed to be of the JONSWAP type). The rather extreme wind speed of 60 knots is used for computational purposes only.

The objective of this investigation is to study the influence of the various environmental components on the tanker motions and forces in the bow hawser. A description is given of the derivation of the complete equations of (low frequency) motion in the time domain. The motions are assumed to occur in the horizontal plane only, so that all attention is focused on surge, sway and yaw motions. In the simulations a distinction is made between a stationary and dynamic environment.