The DP equipment class 3 pipelaying vessel (s-lay) was operating in open bus in a two way split configuration, with all thrusters and generators online. There were four DGNSS selected to DP, however no further position reference systems were selected. Two tensioners set on tension mode provided set-point and load cell feedback to the DP system in order to update the mathematical model. 

Whilst pipelaying in shallow water, the vessel experienced a tension sensor failure on one tensioner. An alarm for sensor failure was generated on DP when the tensioner sensor failed and the DP system stopped receiving measured tension. 

At this point the tension load was transferred to the remaining on-line tensioner. 

Six minutes later the second tensioner experienced the same fault on the load cell and went offline. Now with no tensioner inputs to the DP system, the tension mode was lost and the tensioners applied the brake. 

The sudden force applied by the pipe tension caused the vessel to become unstable and the vessel lost position, exceeding the allowable pipe integrity and causing damage to the pipe. 

Troubleshooting of the event concluded that: 

Training, training, training. Mechanical failures happen and that is a fact, how we respond to them is based on knowledge and preparedness. 

It was not immediately apparent to the personnel present on the Bridge that the tensioners were no longer in tension mode and the DPO was unaware of the critical nature of the situation. 

The action that was required in this situation was to minimise the vessels surge motions by applying a fixed input as a manual force along the surge axis to try and maintain the adequate tension on the pipe. 

Since the incident the company implemented multiple upgrades in hardware and training: 

The following IMCA Guidance would be relevant to these case studies: