Capability Plot With One Generator Offline
A DP2 vessel is conducting a pipe route survey in 95 m of water. The vessel has three diesel generators; No. 3 generator is offline for maintenance, leaving two online. The approved capability plot for two-generator operation with the worst-case loss of one generator shows a maximum wind speed of 14 m/s in the prevailing direction. The actual conditions are 18 m/s of wind from the bow quarter, 1.6 kt of current, and 2.5 m significant wave height. The vessel is holding position but one thruster is at 82% pitch and the generator loading is fluctuating around 85%. The OIM wants to continue for another 12 hours to finish the survey line. The forecast shows the wind increasing to 20 m/s within three hours. The vessel's ASOG is currently green.
1. Assess whether the vessel is operating within its DP capability. Use the numbers and explain the worst-case logic.
Read the worked answer
The vessel is not operating within its approved worst-case capability. With No. 3 generator offline, two generators are online. The relevant capability plot is not 'two generators online' but 'two generators online with the worst-case loss of one generator', because DP2 requires the vessel to survive the single worst failure. That plot gives 14 m/s, while the actual wind is already 18 m/s and forecast to reach 20 m/s. The vessel therefore cannot demonstrate position holding after the worst-case generator loss.
The thruster at 82% pitch and generator loading around 85% show that the system is already working hard. These are not comfortable margins; they indicate that a gust, current increase, or loss of one generator could quickly exhaust the remaining power. The 2.5 m wave height and 1.6 kt current add load and may not be fully represented in a simple wind-limit plot. The correct conclusion is that the current configuration is outside the approved capability, even though the vessel is presently holding position. Present holding is not the same as worst-case capability.
2. What should the DP practitioner recommend to the OIM, and what immediate operational actions are needed?
Read the worked answer
The practitioner should recommend suspending the survey and moving to a safe standby location or restoring No. 3 generator. The ASOG should be changed from green to red or amber as appropriate, and the OIM should be informed that the vessel cannot meet DP2 worst-case requirements in the current configuration. The decision should be based on the 14 m/s worst-case limit versus 18 m/s actual and 20 m/s forecast, not on the fact that the vessel is still holding.
Immediate actions include: inform the bridge and engine room, reduce thruster demand where possible, stop the survey line, and prepare a controlled move to a safer heading or location. If the vessel must remain in position briefly while recovering equipment, the DP operator should monitor thruster pitch, generator loading, and position reference quality continuously. Do not wait for a generator trip to occur. If No. 3 generator can be restored safely, do so and update the capability plot before resuming. Any continuation should require a formal management decision with a documented risk assessment, not an informal instruction.
3. What margin and documentation should be applied before resuming DP operations?
Read the worked answer
Before resuming, the vessel should be restored to the configuration assumed by the approved capability plot, or a new plot should be issued for the actual configuration. The actual environmental conditions should be below the worst-case limit with a practical margin, not exactly at the limit. A common approach is to require a margin of at least 10–20% in environmental load or to use the next worst-case plot; the exact margin should be defined in the vessel's DP procedures. The forecast trend should also be considered, not just the current reading.
Document the suspension, the ASOG change, the capability plot used, the actual weather, the generator and thruster status, and the OIM notification. Record the reason for resuming and the conditions at that time. If No. 3 generator is restored, complete the necessary checks and update the DP configuration. If the vessel resumes with two generators, the revised ASOG and any operational limitations should be signed by the master and communicated at handover. Retain the records for the DP incident/near-miss file if the vessel operated outside the approved envelope.