DP2 Vessel After Thruster Drive Overtemperature
A 92 m platform supply vessel is holding DP2 station 500 m from a dive support site in 85 m water depth. Thrust is provided by two 2.5 MW aft azimuths, two 1.2 MW bow tunnel thrusters and one 1.0 MW retractable. The No.1 aft azimuth variable-frequency drive cooling fan fails; the drive temperature reaches 85°C and the unit automatically derates to 60% torque. The DP capability plot for the present weather (wind 18 kn, wave 1.2 m, current 0.8 kn) was calculated with all thrusters at full rating. The DPO proposes to continue because “DP2 permits the loss of one thruster”. The dive supervisor is waiting for clearance. The vessel's FMEA notes that the two aft VFDs share a common cooling-water header but have separate fans. The annual DP trial is due in two months. The technical manager is ashore and has not been informed.
1. Assess whether DP2 redundancy is intact and whether the dive operation should continue.
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The DPO's statement is incomplete. DP2 requires that, after the worst-case single failure, the vessel can maintain position and heading within the operation's limits. Here the No.1 aft VFD has not failed completely, but its available thrust is derated to 60%; the existing capability plot is invalid because it credited full thrust. The capability must be recalculated with No.1 aft at 60% or zero, and then the worst-case single failure must be applied to the remaining system - for example loss of the No.2 aft azimuth or the aft switchboard. If the recalculated capability cannot hold the present 18 kn wind, 1.2 m wave and 0.8 kn current, the dive operation should stop.
The shared cooling-water header is a serious common-cause concern. If a header failure can disable both aft drives, simple thruster redundancy does not cover it. The FMEA must demonstrate isolation between the two drives or the arrangement is not fully DP2-compliant for this failure mode. Pending that demonstration, the vessel should be treated as degraded.
2. Describe the immediate operational and technical actions on board.
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Inform the bridge team, dive supervisor and technical manager immediately; do not resume diving until the assessment is complete. Reduce thruster load and monitor the drive temperature. Attempt to repair or replace the cooling fan, or provide a temporary independent cooling arrangement approved by the drive manufacturer. Do not bypass the temperature protection or force the drive to full torque. Record the event, temperatures and derating in the DP log and the planned maintenance system.
Recalculate the DP capability and worst-case failure using the actual thrust available, and compare the result with the site-specific DP footprint. If the drive can only run at 60%, treat it as unavailable for redundancy purposes. The dive supervisor should be given a written clearance before operations restart.
3. State the follow-up with class, flag and the DP assurance process after the repair.
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After repair, conduct a DP trial or failure-mode test to verify the VFD, fan and cooling-header redundancy. Review the FMEA for the common cooling-water header and revise the DP operations manual and capability plots if the arrangement is changed. The annual DP trial may need to be brought forward because the vessel has experienced a significant thruster-drive failure.
Notify class and flag if the DP system or its redundancy is affected. The ISM system should record the temporary derangement, the operational restriction and the corrective action. The dive contractor must be informed in writing of the outcome before the vessel resumes DP diving operations.