ADVANCES IN MOORING LINE DAMPING

This paper discusses recent fundamental and applied work investigating the damping forces induced on floating vessels and in particular catenary mooring lines. A description of new tests being performed at University College London on large scale sections of mooring line to reveal damping contributions is also presented. The contributions to damping from the morring lines is highly topical in that a number of floating production and storage units are currently being planned for hydrocarbon exploitation. Indeed a number of schemes are already in operation at North Sea sites. Design of the mooring system required to hold the vessel within a specified radius above the wellhead depends on an understanding of the imposed static and dynamic environmental loads. The low frequency excitation caused by the random waves, and, to a certain extent, wind loading results in resonant motion responses in the horizontal plane leading to high mooring line forces. Many of the loading mechanisms are well understood. However, until recently the fluid induced forces acting on the moorings were assumed to have little influence on the vessel dynamic response. Recent work has shown that the mooring system may under certain circumstances provide up to 80% of the total damping available thus significantly reducing the peak line tensions.