When designing a rig for dynamic positioning, it’s important to know the maximum expected load on the thrusters to be able to size the thrusters according to defined maximum weather criteria. The force on the thrusters is depending on the hull geometry for wave and current loads and the topside structures for the wind load.
For each semi-submersible and drillship design, force coefficients for the wave, current and wind loads are defined based on previous design experience (including model tests and CFD (computational fluid dynamics) analysis). These coefficients are then used with six representative data points from weather data for different locations to calculate the thruster force for each data point. The forces are then summarized in accordance with the frequency of occurrence for each weather to give a representative average thruster force.
The thruster force is then converted to thrust using the ABS (American Bureau of Shipping) bollard pull formula including efficiency and thruster interference factors based on design experience.