Question 1

A DP2 construction vessel has three gyrocompasses fitted. During a pre-operation check, the DPO notices that Gyro 2 is reading 2.1° higher than Gyros 1 and 3. The vessel's DP system has a three-gyro median voting algorithm.

What action should the DPO take, and why is the three-gyro arrangement important?

Accepted Answer

The DPO should flag the discrepancy, reject Gyro 2 from the DP system until the fault is investigated, and understand that with three gyros the median voting algorithm can identify and reject a faulty reading — with only two gyros remaining, if those two diverge there is no way to determine which is correct Three gyrocompasses allow the DP system to use a median or voting algorithm: if one gyro deviates significantly from the other two, it is identified as the outlier and rejected. With only two gyros, the DP system cannot determine which is correct if they diverge — and heading error directly commands the DP system to apply incorrect thruster forces. A 2.1° heading error in a sensitive DP operation near a structure can result in a significant position excursion. The discrepancy should be investigated and Gyro 2 excluded until the fault is resolved.

Question 2

A DP Class 2 vessel is operating on a subsea construction project. The chief engineer asks whether the vessel can continue operating in the event of a single active component failure.

What does DP Class 2 guarantee in this situation?

Accepted Answer

The vessel can maintain position after a single failure of any active component or system, provided redundant systems are in place and operational IMO MSC/Circ.645 and IMCA M103 define DP Class 2 as: no loss of position following any single failure of an active component or system. Redundancy in active components is required — thrusters, generators, UPS, switchboards. Fire/flood protection requires DP Class 3. Static components (cables, pipes) are not covered by the single failure criterion.

Question 3

A semi-submersible drilling rig is classed DP Class 3 and operates in deep water on a well with well control implications. A trainee DPO asks what the key distinction is between DP2 and DP3.

Which statement correctly describes the DP Class 3 requirement?

Accepted Answer

DP3 requires redundant systems to be located in separate watertight compartments with physical separation, such that a single fire or flood cannot affect both systems simultaneously DP Class 3 extends the single failure criterion to include fire and flooding in any one compartment. This demands physical separation — redundant power systems, DP control systems, and thrusters must be arranged in separate fire and watertight subdivisions. A cable fire that destroys one entire DP system must leave the other fully capable of maintaining position. This is the "worst credible failure" standard.