Survivability Analysis of a MODU Platform With Fibre Ropes and Automatic Thruster-Assisted Mooring: Intact and Damaged Conditions

Abstract This study analyses the interaction between mooring systems equipped with fibre components and an Automatic Thruster Assist (ATA) for 8-point and 12-point configurations, focusing on the polyester fibre ropes’ behaviour, such as elongation and dynamic stiffness. The stiffness of fibre ropes is characterised by a nonlinear load-elongation curve that varies as a function of the mean load. Recent drilling units have relied on thrusters to increase their station-keeping capability, which increases the interaction with the mooring system. This work investigates how the ATA system interacts with fibre ropes under survivability mode for intact and damaged conditions, namely, line and thruster failures (single thruster, group, and blackout). The analyses are conducted for distinct mooring configurations: an 8-point and a 12-point system. In both systems, the interaction between the ATA and the fibre rope stiffness damaged conditions was shown to be important, especially for the blackout conditions. This occurred due to increased mean tension acting on the fibres during a failure case, leading to increased fibre stiffness and worse dynamic results. This paper also discusses the implications of the fibre and ATA interaction and provides suggestions.