Higher Order Wave Excitation Forces and Station-Keeping Capability of a Semisubmersible
Authors: Dev A. K. 
Published: February 2024

A floating body like a semisubmersible in regular or irregular waves experiences first-order and second-order wave excitation forces due to potential flow effects (radiation-diffraction theory). However, for a slender structure like columns of a semisubmersible, higher-order forces originate due to viscous effects, resulting in mean and slowly varying forces much higher than those calculated by potential flow effects. Such a significant difference may severely underestimate the wave excitation force calculation while dealing with mooring analysis or dynamic positioning (DP) capability of a semisubmersible regarding station-keeping capability.

Experimental works have shown that regular waves have a much higher-order wave excitation force (cubic). This force is much higher than second-order wave excitation forces (potential flow effects) while dealing with the mean/steady drift force. Two different diameter model cylinders have proved it well. The theoretical part has also been established while dealing with the viscous drag term up to its instantaneous wave elevation. Experiments were also done for the pontoons of a semisubmersible. These experimental works resulted in an engineering rationale formula introducing a mean drag coefficient dependent on the Keulegan-Carpenter (KC) number. Further demonstration with currents has proved that these drift forces enhance considerably with the presence of currents with waves. As the natural sea is irregular, experiments with a floating semisubmersible have proven that, like the higher-order drift force, the slowly varying (low-frequency) force also increases due to viscous effects.

The experimental results with cylinders and pontoons prove that second-order forces due to potential flow effects are insufficient. The viscous effects cause both mean and slowly varying forces to be considerably larger in waves and currents and waves. As a result of these variations in computation techniques, only dealing with potential flow effects may seriously underestimate the wave excitation forces while carrying out both mooring and dynamic positioning (DP) capability analysis of a semisubmersible. So, having a sufficient design safety margin, especially in storm conditions when wave frequencies are extremely low and wave heights are high, will be attractive for viscous effects to dominate both the mean and slowly varying forces.

Experimental works have well-established the viscous effects in wave excitation forces (higher-order). There has also been theoretical development for engineering solutions in waves and currents. Both mean, and slowly varying forces are essential in the mooring and DP capability of a slender-body structure like a semisubmersible. Thus, additional sources due to viscous effects resulting in higher-order forces should be addressed, especially safeguarding the floater for storm conditions’ safety. These novel findings are equally applicable to new floating solutions for wind turbine floaters, which are very slender structures. So, wave excitation forces’ analysis without taking viscous effects may also underestimate the mooring and dynamic positioing analyses.