To investigate the differences between a truncated mooring-line model and the corresponding fulllength mooring-line model, model tests regarding the llne tensions and motions of truncated and fulllength mooring llne systems were conducted in the Offshore Technology Research Center (OTRC) wave basin recently. The tests involve three models: a 1:80 scale full-length mooring line model, a 1:40 scale model truncated vertically and a 1:40 scale model truncated both vertically and horizontally. Both truncated moorlng-llne models were designed to match their static offset curves with that of the full-length moorlng-llne model in prototype scale. When the upper end (fairlead) of the mooring lines oscillated regularly or irregularly in the horizontal plane, the tensions and trajectory of a mooring llne were recorded. Numerical simulations of the motions and tensions of mooring lines are made using a nonlinear FEM (Finite Element Method) code called CABLE3D. The numerical results are compared with the corresponding measurements. The differences in dynamics between the truncated and full-length mooring lines are significant. The understanding and results of this study may be helpful to develop a method for designing a truncated mooring-line model that can better dynamically match a full-length moorlng-llne model.
Recent exploration and production of gas and oil have been pushing their activities to deeper and deeper ocean. Currently they are in the range of water depth from 1000 to 3000 m. The design of a mooring system in the unprecedented water depth presents a challenge to the model tests in wave basins. Due to the limitation of the existing wave basin, either the model is made in extra small scale, or the mooring system has to be distorted. The mooring system is truncated vertically and sometimes even horizontally as well due to the limited horizontal dimensions of a basin [Bychner, etc. 1999, Ormerg, etc. 1999].
