The hawser tension in a mooring system is classically decomposed into three components: mean, lowfrequency and wave-frequency. The low-frequency component is often the most significant contribution. This component is produced by relatively small second-order wave forces whose frequency coincides with the natural surge period of the tanker/mooring system but which, because of the small damping, induces large dynamic amplifications.
The Paper will demonstrate that it is possible to considerably decrease such dynamic amplification by means of small DP controlled thrusters installed on the tanker. In this case, the power of the thrusters is not used to maintain the ship at a given location but to increase the damping by a control on the low-frequency velocity of the tanker.
A detailed description of the positioning system is given with regard to the type of mooring, i.e., offloading or permanent mooring. Particular attention is paid to the system's redundancy and reliability.
In 1981, a purpose built dynamic positioning system for an offshore loading tanker in a single point mooring situation was developed by KONGSBERG ALBATROSS. This type of systems has been in successful operation on the Statfjord field since 1981.
The experience gained during this period is, in our opinion, useful for development of new offshore loading concepts and floating production systems.
This paper will give an introduction to the dynamic positioning system tailormade for single point mooring.
Schemes showing how this experience can be utilized to optimize the offshore loading concept will be given.
The same principles for positioning as described for offshore loading will be applicable also for floating production. In the floating production situation very high attention has to be paid to the safety and regularity question, which will be discussed in this article.
The practical aspects of Dynamic Positioning assisted mooring will be illustrated by three examples. It will be shown to which extent a moored tanker can benefit from DP assistance, with regard to the safety of operations and reduction of weather downtime.
The major problem faced by single point mooring in hostile areas is due to the motion of the ship and the terminal. The ship tends to oscillate slowly relative to its mooring point with a following oscillatory behaviour of the mooring stresses, and with the effects of wave following motions superimposed. When the mooring force exceeds a prescribed value, loading must be stopped and mooring abandoned. The low frequency ship motion stems from unstable ship mooring configuration and/or dynamic force excitations from wind and waves.
The desirable objective in single point mooring is to keep the distance between the mooring terminal and the ship's bow as nearly constant as possible, while maintaining the overall weather-vane capabi1ity.
To be able to develop this tailormade DP-system a detailed analysis of vessel movements in single point mooring was necessary. This was done by full scale recordings and theoretical studies, resulting in a complete simulation program for dynamic analysis of the total system including buoy and vessel.