Simultaneous DGPS Jump During FPSO Approach
A DP2 construction support vessel is holding about 60 m off an FPSO, preparing to connect a hose. The DP configuration includes two DGPS receivers and one laser position reference mounted on the FPSO. Both DGPS displays suddenly jump about 15 m to starboard at the same instant. The laser remains steady and agrees with the visual bearing and the FPSO radar. The DP controller is using a 'best position' solution that favours the two DGPS because the laser has had intermittent signal quality. The thrusters begin to ramp and the vessel starts moving toward the FPSO. The DPO sees the position jump and the closing rate on the laser display. Wind is 12 kt, current 0.4 kt, and the vessel is in the 500 m zone. The FPSO control room is not yet aware of the problem.
1. What immediate actions should the DPO take, and why is the 'two out of three' PRS logic not sufficient here?
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The DPO should treat this as a common-mode position reference failure, not as a normal single-sensor fault. Two DGPS receivers that jump together are likely sharing the same satellite constellation, correction service, multipath environment, or ionospheric disturbance; majority voting will therefore reinforce the false position instead of rejecting it. The DPO should immediately deselect both DGPS inputs, select the laser as the sole trusted reference if its quality is acceptable, and monitor the laser, visual bearing, and FPSO radar. If the DP system cannot hold with the laser alone, or if the laser quality is too poor, the DPO should take manual/joystick control and stop the approach.
The DPO must also inform the FPSO control room and the OIM immediately, warn the deck crew, and be ready to abort the hose connection. Do not wait for the DP controller to resolve the conflict; by then the vessel may have closed the 60 m gap. The fact that two sensors agree does not prove they are correct, especially when they are the same technology and the independent laser disagrees.
2. How should the team determine which position reference is false, and what evidence should be used?
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Use independent evidence, not the DP controller's majority vote. The laser is a different measurement principle and is mounted on the FPSO, so it should be compared with visual bearings to the FPSO, the FPSO radar, and the vessel's own radar. If the laser, visual bearing, and radar all show the vessel stationary relative to the FPSO while both DGPS show a 15 m jump, the DGPS data are false. Check the DGPS quality figures, correction-service status, satellite geometry, and any recent notices about ionospheric activity. Also check whether both DGPS share the same antenna, correction link, or software; a shared failure path explains simultaneous behaviour.
Do not use dead reckoning as proof. Record screenshots of the DGPS jump, laser display, and DP alarm history. If possible, compare with a third independent reference such as a taut wire or separate radar-based system, but do not delay the immediate control action to gather evidence. The decision to reject the DGPS should be made on the weight of independent evidence, and the reason should be logged.
3. What should happen after the vessel is brought under control, including reporting and follow-up?
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Once the vessel is safe, the DPO should keep the DGPS deselected until the cause is understood, maintain the laser or another independent reference, and complete a DP incident/near-miss report. The report should state that both DGPS jumped simultaneously, that the DP controller's best-position logic favoured them, and that the laser/visual/radar evidence contradicted them. Include the time, position, weather, PRS selections, alarm history, and the action taken.
Notify the company DP technical authority, the FPSO, and the vessel's manager. Investigate the DGPS correction service, antenna placement, multipath from the FPSO, and ionospheric conditions. Review the DP operating procedures for common-mode GPS failure and the ASOG for loss of a position reference. Consider adding a diverse PRS or changing the weighting logic, and conduct a DP drill on rejecting a false PRS. The hose connection should not resume until the DP configuration is restored to an approved state and the FPSO is satisfied that the vessel can safely hold position.