Viscous Drift Forces on a Semisubmersible in Waves and Currents

Abstract

Starting from the early sixties and then in the early eighties, it was well established that second-order wave excitation forces exist for floating bodies in regular waves. This force is not only steady (mean) in nature; it also has a low-frequency (difference frequency) and a high-frequency (sum frequency) component force in irregular waves, e.g., in a wave envelope of irregular waves. While potential drift force calculations were entirely accurate for large body structures like tankers, FPSOs, and other bluff bodies, the potential theory-based drift force calculations fell far short (seriously under-predicted) for slender body offshore structures like a semisubmersible. The drift force in a regular wave is dominated by viscous effects resulting in the force itself becoming a function of the third order in the splash zone of the semisubmersible column. Experimental results have validated this. This happens at the low diffraction parameter (κD) and high viscous parameter (H/D). In a waves-only field, the potential drift force remains purely second-order in regular waves due to first-order and second-order pressure. The presence of currents with waves increases the potential drift forces with the additional contribution. The presence of currents along with waves increases the viscous drift force in the splash zone and increases viscous effects on the submerged zone of the semisubmersible like the pontoons and the part of the columns below the splash zone. As the semisubmersible consists of vertical columns (vertical cylinders) and submerged pontoons, extensive experimental works were conducted with a vertical cylinder (resembling the column of a semisubmersible) and with fully submerged pontoons. These tests provided a fundamental understanding of the physics and mechanism of viscous effects in drift forces on semisubmersibles. Finally, numerical works were done using some of the experimental findings. The prediction matched quite well with experimental results for a complete semisubmersible in waves and waves and currents. The prediction method is also helpful in calculating the higher-order environmental forces in station keeping, like static and dynamic mooring analysis, Dynamic Positioning (DP) calculations, especially in time domain DP predictions.