Dynamic Positioning in Single-Point Moorings

ABSTRACT This paper deals with the concept of and a system for complete control of the slowly varying bow motion of a bow-moored ship using dynamic positioning. System performance is shown by simulated case studies with an offshore 1oading tanker. Significant improvements of motion behaviour and mooring force performance are demonstrated. A complete positioning system is presently runn1ng successfully onboard a 125000 dwt tanker in the North Sea. Some preliminary results are shown. INTRODUCTION Mooring of large ships at offshore terminals are done by single-point mooring. The ship is connected to the terminal through a single mooring line, or hawser, which allows H to position itself according to the preventing weather and sea conditions, like a weather-vane, and thereby to remain moored with a minimum mooring force when the direction of forces changes. A major problem faced by single-p01nt mooring In hostel 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 motionstems from unstable ship mooring configuration and/or dynamic force excitations from wind and wavesl. From practical experience a certain amount of astern thrust is found to have a favorable effect on the slow motion, but it is questionable as a remedy for reducing peak hawser tension. Dynamic positioning (dp) has been applied with great success to freely moving ships and moored vessels for more than ten years. While keeping the ship In a fixed position is typical to most dp system applications, 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 capability. In this paper a review of the essential slow motion behaviour of a bow moored tanker 1s presented with emphasis on stability and mode dynam1cs. The des1gn and the theoretical performance of a dp-system to meet the single-point mooring positioning objective Is presented and, finally, results from full scale tests with a 125000 dwt tanker during offshore loading are shown. ANALYSIS OF SLOW MOTIONS IN SINGLE-POINT MOORING Only static (elastic) properties of the mooring terminal affects the slow motion behaviour, while the high frequency (wave following) propert1es are reflected in the mooring force. Hence, the terminal is left out in the following analysis. Governing equations The governing equations describing the slow motion of a bow moored ship In wind, waves and current are reasonably well establ1shed. A detailed description is presented In1. Applying some symmetry assumptions, the equations of motion In moored ship coordinates.