Thruster Maintenance and Beam Wind in the 500 m Zone
A 78 m DP2 platform supply vessel is scheduled to enter the 500 m zone of a semi-submersible rig to transfer cargo. The vessel has four thrusters: two bow tunnels and two stern azimuth thrusters. No. 3 stern thruster is isolated for planned maintenance, and the isolation certificate is valid until 18:00. The forecast is 22 kt of wind gusting to 30 kt on the port beam, with a 0.9 kt current. The vessel's approved capability plot for all thrusters shows a beam-wind limit of 30 kt; a separate plot for one stern thruster out shows 17 kt. Two DGPS and one acoustic position reference are selected. The OIM says the cargo is urgent and asks the master to enter the zone now. The DP operator notes that the acoustic system has been noisy since the thruster was isolated. The vessel is currently outside the 500 m zone in open water.
1. Using DP redundancy and capability reasoning, should the vessel enter the 500 m zone? Justify your decision with the numbers given.
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No. The decision must be based on the configuration actually available, not on the all-thrusters capability plot. With No. 3 stern thruster isolated, the vessel is already in a degraded configuration, so the worst-case failure is the loss of another thruster or a generator. The approved plot for one stern thruster out gives a beam-wind limit of 17 kt, while the forecast is 22 kt gusting 30 kt. The vessel therefore cannot demonstrate that it would hold position after the worst-case failure, which is the core DP2 requirement.
The all-thrusters 30 kt plot is irrelevant because No. 3 is not available. Gusting to 30 kt also means the instantaneous load may exceed the steady-state limit even before a failure. The noisy acoustic reference adds further uncertainty: thruster wash and reflections can degrade acoustic quality, and two DGPS are not fully diverse against common-mode GPS errors.
The correct judgement is to remain outside the 500 m zone, advise the OIM that entry is not acceptable under the current DP configuration, and either restore the thruster or wait for the wind to fall comfortably below the 17 kt degraded limit with margin.
2. If the OIM insists on proceeding, what specific actions and limitations should the DP practitioner recommend before any entry?
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The DP practitioner should not simply accept commercial pressure. First, escalate the technical basis to the master and the company DP technical authority: the degraded capability limit is 17 kt, actual conditions are 22 kt gusting 30 kt, and the worst-case failure has not been covered. Second, if anyone proposes a temporary concession, it must be documented and must include a revised ASOG with amber/red limits, a defined abort criterion, and a safe escape heading. Third, restore No. 3 thruster if possible and retest it, or obtain written confirmation from the technical authority that the configuration is acceptable; a verbal instruction from the OIM is not sufficient.
Operationally, the vessel should not enter until the wind is forecast to remain below the degraded limit with a margin, the acoustic reference is stable, and a second diverse position reference is available. If entry is still considered, reduce exposure: minimum personnel on the rig, no simultaneous cargo and bunkering, continuous DP watch, and a ready main-engine and thruster configuration. The master retains override authority. If the OIM continues to insist, record the disagreement and seek a management decision. Proceeding on the basis that 'the gust is temporary' is not a valid DP risk assessment.
3. What records and communications should be completed after the decision?
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Record the decision in the DP log and the vessel's official log, including the configuration (No. 3 isolated), the actual and forecast weather, the applicable capability plots, the PRS status, and the OIM's request. Note the specific reason for refusal or for any restricted operation: the 17 kt degraded limit versus 22 kt gusting 30 kt. If the operation is delayed, record the time, the authority contacted, and the agreed review point.
Communicate the outcome to the OIM, the rig DP operator, the master, and the company marine/DP authority. The communication should be factual and non-confrontational: 'Vessel is not entering the 500 m zone because the worst-case failure cannot be covered in the current thruster configuration.' If a near miss or disagreement occurs, raise it through the vessel's safety reporting system. Retain the capability plots and weather screenshots with the DP watch handover. After the thruster is restored, complete a DP checksheet and confirm that the ASOG is returned to the normal green configuration before the next entry.