A test program has been carried out on a 1:50 scale model of a 150,000 tonne tension leg platform in a water depth of 350 m. The same model was tested under the same wave and wind loading conditions in two separate facilities. The first was a 200 m by 12 m tow tank having a water depth of 7 m (350 m full scale). The second facility was a 75 m by 32 m basin having water depth of 2 m (100 m full scale) with the additional 5 m (250 m full scale) accommodated using a pit. The paper compares results found in the tow tank and those found in the basin. Presentation is based on comparison of the tether tension and rigid body motion response of the TLP to a number of wave and wind spectra simultaneously applied to the model. These results are also compared to numerical predictions.
The tension leg platform (TLP) is one of the most popular concepts in compliant offshore structures employed in water depths ranging form approximately 100 to 1500 meters. The TLP, similar in appearance to a semisubmersible, is characterized by its vertical tethers (or legs) kept taut by excess buoyancy in the floating structure. The objective is to eliminate heave, roll and pitch motions of the platform by taking the associated loads axially in these vertical legs. The platform does however experience motions of surge, sway and yaw.
Among other considerations, hydrodynamic analysis of a TLP is concerned with response of the platform in terms of its motions, and response of the tethers. Normally, in the shallower range of water depths the tether dynamics are assumed to be very small and have negligible effect on the platforms motion, whereas the platform motion is the primary source of excitation to the tethers. In deeper water, tether dynamics can be significant. In this case there are two main methods of analysis; one analyzes the tethers independent of the platform, while another considers the coupled response of the structure-tether system. Similar consideration is given to the riser system attached to the platform.
Scaled prototype modelling is normally used to investigate the behaviour of such structures. The primary objective is to subject a model to an accurate representation of the environment which the full scale version will encounter. Usually, the largest physical model possible is sought and choice of scale is governed by available water depth at a particular facility. Consequently, a problem associated with test facilities for model test programs is the water depth. Most facilities intended for deep water model testing are equipped with a pit extending below the main floor of the tank. This assumes that the floor is sufficiently deep to avoid interference with kinematics of water particles in the water column. These effects are significant when considering the model tether (or riser) responses.