The Likely Cause of Loss of Position for Dynamically Positioned Semi-Submersibles Under Moderate Wave Conditions

A number of incidents of wave-induced loss of position for dynamically positioned semi-submersible under normal operating conditions have occurred in the Norwegian offshore sector in recent years. A study has been carried out to seek their cause. One hypothesis for the cause has been slamming load from an extremely tall and steep wave that is not effectively counteracted by the DP system due to delays in signal filters and thruster response. Another cause could the loads from a train of tall and steep waves. A linear single-degree-of-freedom numerical model is made for a vessel with DP. The model is essential in that it represents the basic characteristics of the DP system: State observer/filter and feedback control. The model is used to calculate the frequency response and the impulse response of the dynamically positioned semi-submersible. By calculating extreme impulsive load based on published theory it is established that slamming will not cause great vessel excursion. Still, due to delays, the DP system gives more than twice as large excursion as a mass-spring-damper system with identical restoring stiffness and damping. Using an advanced model for vessel with DP, three-hour simulations of stochastic vessel response are carried out for five steep wave states of moderate significant height, The likely cause of large vessel excursion is found to be wave-drift. Due to additional viscous loads on the semi’s columns the wave-drift loads will be significantly larger than predicted with potential theory.