Question 1

A DP2 vessel on a wind turbine installation project has two Motion Reference Units (MRUs) fitted. A junior DPO asks what the MRUs actually contribute to the DP system and whether they matter for station keeping.

What is the primary function of MRUs in a DP system?

Accepted Answer

MRUs measure the vessel's roll, pitch, and heave motions and feed this data into the DP control model — without accurate MRU data, the DP system cannot correctly translate vessel motion into thruster commands or properly filter position reference noise caused by wave-induced vessel movement MRUs (also called VRUs — Vertical Reference Units) measure the dynamic motion of the vessel in all six degrees of freedom, primarily roll, pitch, and heave. The DP mathematical model uses this data to filter out wave-induced oscillations from position reference signals — without MRU input, position data from GPS or Fanbeam includes wave-motion noise that the DP system would try to correct with thrusters, causing unnecessary thrust activity. MRUs also enable the DP system to correctly resolve forces given the vessel's instantaneous angle. A failed MRU degrades the accuracy of the DP control model and should be flagged.

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.