The Institute for Marine Dynamics has recently completed a test program on a moored offshore structure in the clear-water tow tank. The program is the first commercial project of this type completed at IMD and required development of techniques to model mooring systems and simulate wind and current.
A 1:45 scale model of a moored, six-column, semisubmersible floating production system was tested in the simulated environment of waves, wind and current to deter-mine motions and mooring forces. The model was moored in the tow tank of dimensions 200 m x 12 m with a still water depth of 7 m. The full scale spread mooring consisting of twelve catenary lines was modelled using four thin cables attached to vertical springs supported at the tank walls. The model mooring system was designed to model horizontal stiffness only and hydrodynamic effects on the mooring lines were assumed to be negligible.
Both regular and irregular waves were generated for the test program. The irregular spectra were analyzed to identify grouping patterns used to compute the slow drift forces on the vessel. Wind was simulated using a bank of twelve analog controlled fans. Two gusting spectra were simulated by fluctuating the fan speeds. Currents were produced in the tank using two rows of nozzles positioned one above the other below the still water surface. Two submersible pumps supported from the main carriage approximately 23 m from the nozzles were used to set up a surface current. The vessel was subjected to the three environments separately. The two irregular wave spectra were then run with wind and current included.
Test results were compared to those predicted by a computer program based on the traditional 3-dimensional source distribution technique using an idealized wetted hull surface of 576 panels. A comparison between the measured and computed results was made with respect to first-order oscillatory motions, second-order drift displacements and mooring forces.
