THREE-DIMENSIONAL DIFFRACTION HYDRODYNAMICS

In this section, we will examine three-dimensional diffraction results for semisubmersible vessel motions, comparing MOSES to the Netherlands Ship Model Basin (NSMB) motions programs and to the model test results taken at the NSMB (NSMB, 1984). It should be noted that NSMB is the former name for MARIN, the Maritime Research Institute Netherlands.

The vessel used was the semisubmersible crane vessel Balder, with length, breadth, and depth dimensions of 118×86×42 meters, respectively, at a 22.5 meter draft. Before comparison, the differences in the mathematical and test basin models should be noted. The displacement computed by MOSES was 3.3% greater than either NSMB model, and the MOSES KG was 1.87 meters lower than NSMB’s. The KG shift was necessary in order to achieve equivalent transverse GM values. These indicate different values of waterplane inertia, which cannot be compared since they were not published in the NSMB report. Figure 27 illustrates the hydrostatic model used by MOSES.

Figure 27: Isometric View of Crane Vessel Balder

This mesh was automatically refined by the MOSES program to generate a detailed mesh suitable for hydrodynamic analysis, as seen in Figure 28. Imposing a constraint that the maximum panel side be less than four meters resulted in a diffraction mesh consisting of 1130 panels. For the MOSES analysis, viscous damping was introduced by using Morison tubular elements that attract only drag forces. Diameters for these tubular elements were chosen to approximate the cross section of the pontoons and columns.

Figure 28: Isometric View of Detailed Mesh Model

The response amplitude operators for a number of combinations of surge, sway, heave, roll, and pitch in head, quartering, and beam seas are shown in Figures 29 through 36. The results cover periods ranging from 6 seconds up to 18 seconds. These figures compare the results obtained from MOSES diffraction computations (denoted as MOSES) with the results obtained from the NSMB diffraction program (denoted as NSMB Diffraction), as well as the results from NSMB model tests (denoted as NSMB Tests). The agreement shown here by the MOSES computational results is excellent.

Figure 29: Surge Comparison of MOSES Diffraction of Semisubmersible Balder in Head Seas

Figure 36: Roll Comparison of MOSES Diffraction of Semisubmersible Balder in Beam Seas