New equipment, unexpected consequences

A DP equipment class 2 vessel was conducting subsea operations with two remotely operated vehicles (ROVs) deployed. The vessel was operating with three DGPS position reference systems only selected to DP.

The vessel was maintaining a safe distance from nearby infrastructure, operating in conditions of 2m wave height, 14 knots of wind and a DP current of 2.3 knots.

6 of the 7 thrusters were selected and 4 of the 6 generators running and connected to an open bus.

During the operation, a loss of DGNSS correction signals was experienced across multiple vessel DGNSS units.

At 14:47 LT, a loss of DGNSS corrections was experienced on DGPS No. 1 and DGPS No. 2. DGPS No. 3 continued receiving corrections, but only intermittently.

The DPO notified the relevant departments, operations were ceased and the ROV was released and moved clear of the pumping unit. The ROV was landed on the seabed to ensure a stable and reliable acoustic position reference.

The combination of the degraded DGPS No. 3 signal and the HPR system provided sufficient redundancy to maintain vessel position during the event, allowing time to investigate the loss of DGNSS corrections.

The initial symptoms suggested the possibility of an external influence affecting DGNSS correction services. Discussions with correction providers indicated elevated solar activity in the region, while troubleshooting measures were undertaken onboard.

The onboard Survey Team was consulted to verify whether their DGNSS units (two independent DGNSS systems separate from the vessel’s DGNSS) were receiving corrections normally. The surveyor also reported a drastic degradation of their correction signal. However, other vessels operating in the same field were contacted, and none of their units were experiencing any loss of DGNSS corrections.

With support from the DGNSS correction provider, several troubleshooting actions were carried out on the vessel’s systems, including rebooting the demodulator, replacing the antenna, swapping antennas, and reconfiguring constellations and frequencies.

Further investigation identified that an independently operated survey DGNSS system was generating interference that affected the vessel's DGNSS equipment. Isolation testing confirmed that a recently installed survey antenna was causing interference to all three vessel DGNSS units. Once the survey equipment was disconnected, normal DGNSS functionality was immediately restored. Subsequent testing identified defective newly installed antennas as the source of the problem.

Key findings

• Initial assumptions focused on external influences rather than onboard sources.
  
  
• Multiple troubleshooting activities were completed before interference testing was considered.
  
  
• Recently installed equipment had not previously exhibited any obvious faults.
  
  
• Independent systems operating onboard can adversely affect critical DP-related equipment.
  
  
• Sufficient position reference redundancy remained available throughout the event, allowing operations to be safely managed.
  
  

The event demonstrates the importance of adopting a structured and systematic troubleshooting process. Although external causes appeared credible, particularly during a period when scintillation and jamming were commonplace, standard fault finding should also thoroughly consider the possibility of hardware failure. The root cause was ultimately traced to onboard equipment interference.

The incident highlights the value of considering all recently installed or modified systems during fault investigations and reinforces the need to verify compatibility between vessel and project equipment before commencing operations.

• IMCA M103 – Guidelines for the design and operation of dynamically positioned vessels
  
  
• IMCA M220 – Recommended practice on operational activity planning
  
  
• IMCA M252 – Guidance on position reference systems and sensors for DP operations
  
  

Additional reference

• IMCA HSS001 – Guidelines for management of change