This case study examines a DP incident that occurred on a DP equipment class 2 vessel carrying out cable laying operations in an area known for the strong tidal currents. At the time of the incident the vessel was operating in 4 kts current (DP) from 193°, 2m wave height from 333° and wind speeds of 24kts with gust up to 29kts from 153°. 

All thrusters were selected to DP with the A-frame selected as the centre of rotation. Three of the five generators were running and the vessel was operating with four position reference systems selected to DP, three DGNSS and one HPR. 

The vessel was engaged in critical operations, with the DP system set up in automatic position mode and due to the nature of the operation required the heading to be maintained at 017°. The weather conditions had been noted as ‘on the limit’, but no formal discussion had taken place with regards to ceasing operations. 

At 04:09 the vessel experienced sudden wave slamming towards the stern and an ‘Insufficient Thrust from tunnel/bow’ alarm was generated on the DP console. One minute later this was followed by ‘position heading out of limits <3°’. 

This was the start of a 43-minute period where frequent high force and insufficient thrust alarms were generated. The stern thrusters started to oscillate which transferred to an oscillating motion of the vessel and high forces were being used to try to maintain position. 

The DPO took immediate action moving to ASOG yellow status. Operations immediately stopped and deck evacuated, with the teams on standby for emergency abandonment of cable. 

The Master was called to the Bridge. 

Seven minutes after the initial alarm the centre of rotation was changed to the centre of gravity and the SDPO made adjustments to the DP settings to increase station keeping capability, by using quick current model and reducing the gain. 

At 04:52 the DP system stabilised, the oscillating movement stopped, the thrusters loading reduced to normal operating limits and the vessel was holding position. The DP current was also reducing to <4.0 kts. 

At 05:00 the ASOG status was moved to Advisory Status Blue. 

Figure 2 – A Frame Centre of Rotation 

Following the event an investigation was conducted by the vessel and DP system OEM, the following was discovered: 

This event highlights the importance on preparation and training. All DP personnel must know what actions to take and what to expect when operating parameters are exceeded. As it was the DPO took quick and correct actions, which resulted in the operation not having to emergency disconnect; however, for nearly 45 mins the vessel oscillated in a dangerous manner. 

Knowledge and training on the importance of centre of rotation was provided to the DP Personnel to help understand the effects of choosing the centre of rotation on the aft A-frame, in those conditions. 

When the sudden high amplitude/short period waves came into the stern, this caused the initial excursion. The DP system then had to combat additional forces present, led by the sudden increase in wave action. When the rotation was on the aft A-frame there was a long arm for the forces for the heading (bow and tunnel thruster) but very short arm for the 3 aft thrusters controlling the surge towards the setpoint (centre of rotation a frame). As the gain settings were unknown to the DPOs, (assumption that the same as previous DP system) the additional forces to counter the excursion used were too high, leading to an oscillation sway and yaw (movement on the thrusters). It needs to be fully understood that current indicated was the “DP current” and not a measured current. A DP current value could be affected by other disturbing factors and needs to be relied on with care with regard to magnitude and direction. The vessels mission was a contributing factor as the tension from the connected cable is an unknown factor affecting DP current. 

The question that is raised, though, is should the operation have already been suspended? 

At the time of the incident the strength of the DP current was above the threshold of those specified in the vessels ASOG, yet the vessel was still operating and in Green status. It is essential the ASOG is continually reviewed to ensure it covers all possible hazards during a specific operation and that all crew are aware of the limits. The responsibility lies on all the DP Personnel, all should be aware of the ASOG limits and have no fear of repercussions for suspending the mission.