Annual DP Trial Deficiency Before Field Departure
A DP2 vessel has just completed its annual DP trial at a shipyard. During the worst-case failure test, one of two online generators is tripped. The power management system does not open the bus tie and does not shed non-essential loads. The second generator overloads and trips after about 12 seconds, leaving the vessel without power until the emergency generator starts. Separately, No. 2 thruster shows a 6-second delay in pitch response during a step test. The yard says the events are 'software glitches' and proposes to reset the system and sail in three days. The class surveyor has already left the vessel, and the DP verifier's report is unsigned. The vessel is scheduled for DP operations in the 500 m zone on arrival.
1. Can the DP practitioner accept the trial as satisfactory and allow the vessel to sail? Explain the reasoning.
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No. The trial did not demonstrate the worst-case failure behaviour required for a DP2 vessel. The critical failure was not the generator trip itself but the power management system's failure to isolate the fault and shed load; the result was a cascade to a blackout. A DP2 vessel must be able to survive the worst-case single failure and continue to hold position. A 12-second loss of all power is a loss of position capability, not a software inconvenience. Resetting the system does not prove that the logic has been corrected.
The 6-second pitch delay on No. 2 thruster is a separate concern. It may be within an acceptable response time depending on the thruster type and the DP capability study, but it must be assessed against the FMEA and the trial programme, not dismissed. Combined with the PMS failure, the vessel has not provided evidence of redundancy.
The practitioner should refuse to sign the trial report as satisfactory, inform the master and company DP authority, and require corrective action and retesting before departure. Commercial pressure to sail is not a technical justification.
2. What corrective actions and retesting should be required before the vessel can be considered DP-ready?
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First, the PMS logic must be corrected and proven: on loss of one generator, the bus tie should open as designed or the system should otherwise prevent cascade, and non-essential loads should be shed within the specified time. The FMEA must be reviewed to confirm that the bus tie arrangement does not introduce a single point of failure. Second, the No. 2 thruster response should be investigated and, if necessary, repaired; the accepted response time should be documented against the manufacturer's data and the DP capability study.
Third, a repeat DP trial should be witnessed by a competent DP verifier and, where required, class and flag. The trial should include the same worst-case generator trip, verification of bus tie operation and load shedding, thruster step tests, and a position-holding check after the failure. The results should be recorded with time stamps and trend data.
Until retesting is complete, the vessel should not be treated as DP-ready for 500 m zone operations. If a temporary concession is considered, it must be formal, time-limited, and supported by a revised ASOG with reduced capability; it must not be a verbal acceptance of a known blackout risk.
3. How should the ASOG and DP operations be managed if the vessel is allowed to sail before full retesting?
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If management decides to sail before full retesting, the DP practitioner should require a documented risk assessment and a revised ASOG. The ASOG should reflect the actual degraded condition: for example, red for 500 m zone operations and for any DP critical activity, with a requirement for additional generator capacity online, manual load shedding, and a trained engineer standing by the switchboard. The vessel should not enter the 500 m zone until the PMS and thruster issues are cleared.
The ASOG change must be communicated to the OIM, the DP operators, and the engine room, and it must be displayed at the DP console. The DP watch should include enhanced monitoring of generator loading and bus tie status. Any further abnormality should trigger an immediate abort. The concession should have an expiry date and a clear closure action: retest, update the FMEA, and restore the approved ASOG.
This is not a substitute for correcting the defect. It is a temporary control to manage a known limitation while the vessel is in transit or in non-critical operations.