Beware the solitons

An equipment class 2 pipelay vessel was engaged in pipelay operations in an area known for Solitons – an oceanographic phenomenon where large solitary waves of large amplitude are generated.

A support vessel that was operating with the pipelay vessel observed surface anomalies approximately 5 nautical miles (nm) upstream. The support vessel relayed radar imagery to the pipelay showing irregular surface banding – consistent with soliton-induced roughness.

Following standing orders and ASOG procedures the pipelay operations were immediately suspended and the pipelay vessel was purposely driven off station pre-emptively, prior to soliton arrival.

The DP system reported an excursion of approximately 9 metres from the setpoint, just within alert thresholds, as the Soliton passed the vessels location. Thruster activity spiked and the generator load increased by over 35%. The taut wire system showed inconsistent readings, and acoustic PRS began to degrade as the subsurface currents intensified.

The pre-emptive response likely prevented a DP incident. However, operations were delayed by 6 hours, and the event highlighted the non-existent predictive capability of standard DP models against non-linear internal waves.

The lessons learnt were:

• Early warning from support vessels can be critical.
  
  
• Radar alone may not be sufficient; a combined radar + Acoustic Doppler Current Profiler strategy is more effective.
  
  
• Standing orders and ASOG procedures must reflect soliton-specific risk scenarios.

As a result of the event presented above, IMCA has released Information Note 1710 which provides more details of what solitons are, and how to plan, predict and optimise your DP system to help ‘ride the wave’.