Thrust incident due to DP system mode change on vessel

This case study examines an incident involving a Construction vessel that experienced an insufficient thrust alarm while holding Dynamic Positioning (DP) position during offshore installation crew transfer operations.

The incident was caused by an unintended change in the Power Management System (PMS) mode, leading to a temporary loss of positioning.

The vessel was engaged in offshore construction and crew transfer alongside an installed jacket.

Initially, the vessel was operating in “Closed Bus” mode, utilising two generators. Wind was 190 degrees/15 knots; DP current was 205 degrees/2 knots, heading was 38 deg.

Mode change and insufficient thrust alarm: The vessel’s PMS mode was inadvertently changed from “Closed” to “CAM Island,” during the operation which leads to increasing the number of generators online from two to four. The incident report notes that this transition period (for unknown reasons) resulted in insufficient thrust being available, causing the vessel to drift and experience a position excursion of approximately 5 metres.

DPO response: The Dynamic Positioning Operator (DPO) responded to the effect the mode change was having and promptly reverted the system back to “Closed”. After assessing the situation, the DPO confirmed the vessel’s stability and resumed operations.

Human Interaction: Log file analysis revealed that the mode change was due to human interaction, not a system fault. The change from “Closed” to “CAM Island” was manually initiated.

System Response: The transition to “CAM Island” mode required time to bring additional generators online, during which the vessel lacked sufficient thrust to maintain its position against environmental forces.

Considering this incident, the vessel should have been in ‘CAM Island’ mode rather than ‘Closed’ prior to this operation. The DPO ought to have transitioned to ‘CAM Island’ sooner. They should have halted the operation, relocated to a secure location, and then switched to ‘CAM Island’ before resuming activities once they realised that they were in the incorrect mode. It is not clear from the incident report that suitable ASOG were in place and that the DP system set-up procedures (and field entry checks) prior to starting the mission were followed.

This incident underscores the critical importance of precise control and monitoring of DP system modes during sensitive offshore operations. The swift action by the DPO mitigated the potential risks associated with the unintended mode change. The subsequent software modification and procedural reviews enhance the safety and reliability of DP operations, emphasising the need for continuous vigilance and robust system controls in dynamic maritime environments. It also highlights the importance of robust procedures around field entry and activity specific guidelines.