Question 1

SCENE: DP2 PSV holding 60m south of a fixed gas platform, Hs 1.8m from north, wind 18kt NNW. All thrusters online. DP auto, ASOG green. 0347: Without warning, the vessel begins accelerating slowly south. DP position readout shows growing offset. AZ1 shows 98% demand to the south — the same direction the vessel is moving. Environmental forces are from the north. What does this pattern indicate and what is the DPO's immediate conclusion?

Accepted Answer

This is a probable drive-off: AZ1 is producing maximum thrust away from the setpoint, opposite to the environmental force direction — instead of fighting the northerly forces, AZ1 is amplifying the southward movement. The DP system should be demanding northward thrust from AZ1; the 98% south demand confirms a runaway thruster, not a DP system corrective response The drive-off signature is: maximum thruster demand in the direction of movement, opposite to environmental force. With a 18kt NNW wind and 1.8m northerly swell, AZ1 should be demanding northward thrust to hold position — 98% southward demand is the textbook thruster runaway pattern. Every second spent waiting while AZ1 is at full runaway thrust is another metre toward open water south, away from the platform. The DPO must recognise this in the first 15 seconds.

Question 2

CONTINUING: Vessel is accelerating south with AZ1 at 98% south demand. The DPO has 20–25 seconds before the vessel's southward momentum becomes difficult to arrest without risking overshooting toward open water. What is the correct first physical action?

Accepted Answer

Manually stop AZ1 at the thruster control panel to remove the erroneous drive force, then assess remaining thruster status — only after AZ1 is stopped should joystick or manual mode be considered; switching to joystick with AZ1 still running south just adds counter-thrust to a fight you may not win fast enough Stopping the runaway thruster removes the erroneous force. Switching to joystick before stopping AZ1 means the DPO is now trying to overcome AZ1's full southward thrust with the remaining thrusters, potentially in a losing fight against the momentum already built. Motor protection on emergency stop is handled by the thruster controller — a manual stop is not a free-spin stop and will not mechanically damage the thruster in normal circumstances.

Question 3

CONTINUING: AZ1 has been stopped. Vessel continues southward drift by momentum — currently 80m south of setpoint (140m from platform). DPO has switched to joystick. Master and OIM have been alerted. What must the DPO assess BEFORE applying any counter-thrust in joystick mode?

Accepted Answer

Assess: (1) current vessel velocity and momentum — is the drift accelerating or decelerating? (2) platform clearance trajectory — will the vessel clear the platform on current drift without any further input? (3) available thrust from remaining thrusters (AZ2, BT1, BT2) — can they arrest the drift safely, or will aggressive counter-thrust cause the vessel to overshoot position toward the platform? With AZ1 stopped, the vessel has momentum in the southward direction. Aggressive counter-thrust risks stopping the vessel and immediately rebounding it northward — back toward the platform. At 140m from the platform, the margin is much greater than at 60m, so there is time to assess the trajectory before applying counter-thrust. Calm, assessed joystick is safer than emergency maximum thrust at this moment.