In general, a three degree-of-freedom (DOF) horizontal motion control has been regarded as adequate in the design of a dynamic positioning system (DPS). However, for certain marine constructions, especially those with a small waterplane area and low metacentric height, which results in relatively low hydrostatic restoring compared to the inertia forces, an unintentional coupling phenomena between the vertical plane and the horizontal plane through the thruster action can be invoked. A novel roll–pitch motion controller with horizontal motion control law is proposed in this paper. The roll and pitch proportional controller is first introduced into the DP control law. The performance of the newly proposed controller was demonstrated by time-domain simulations for a dynamically positioned semi-submersible.

Traditionally, dynamic positioning systems for ships depend mainly on a feed-back control using the position error as input, supplemented by a feed-forward control signal based on the instantaneous wind and current speed and direction. The feed-forward control is further extended to wave feed-forward, thanks to the research development on the real-time estimate of the wave drift force. Wave feed-forward allows the dynamic positioning system (DPS) to directly compensate the slowly varying wave drift force in order to avoid sudden drift off caused by large unexpected wave force. A novel real-time estimate method of wave drift force is proposed in this paper. The estimated wave drift force is used in the dynamically positioned control system for wave feed-forward dedicated to enhancing the performance of the DPS. A time domain simulation for a dynamically positioned semi-submersible is conducted. It is demonstrated from the results that the novel proposed wave drift force estimate method is effective and efficient to be applied in wave feed-forward. And the positioning performance of the vessel is improved under the employment of wave feed-forward.

This paper numerically and experimentally investigates the hydrodynamic interaction between two semi-submersible type VLFS modules in the frequency domain. Model tests were conducted to investigate the relationship between interactions and wave headings. Numerical studies were performed by solving the radiation-diffraction problem and were validated against the experimental results. Motion Response Amplitude Operators (RAOs) were obtained from numerical and experimental studies. The dependency of the hydrodynamic interaction effect on wave headings is clarified. The influence of different wave periods on the motion responses of two-module VLFS and wave elevations in the gap is studied. The results indicate that the hydrodynamic interactions of the two modules are directly related to the wave headings and the periods of the incident wave. The shielding effect plays an important role in short wave, and the influence decreases with the increase of the incident wavelength. The numerical results based on the diffraction-radiation code can give a relatively good estimation to the responses in short wave while for long wave, it would over-predict the response.