Question 1

A DP2 anchor handler has only one functional position reference system (DGNSS) after a Fanbeam unit becomes unavailable and a second DGNSS unit stops updating. The vessel is approaching a mooring spread in 400m water. The DPO considers proceeding on the single DGNSS.

Why is proceeding with a single position reference a significant safety concern, regardless of its apparent accuracy?

Accepted Answer

With a single position reference, there is no way to detect if that system develops a fault or position jump — the DP control system will accept whatever position it provides with no comparison check, meaning a subtle position error could drive the vessel off station before any alarm is triggered Position reference systems require a minimum of two (ideally three) independent systems so the DP software can cross-check readings and detect a faulty or drifting reference. With only one system, there is no comparison — if that system drifts, jumps, or begins providing a false position, the DP control system has no way to detect the fault and will act on the erroneous data. This is a particularly dangerous failure mode because the system appears to be working normally right up until the excursion occurs. IMCA M140 recommends minimum reference system configurations for each operation type; the ASOG should specify the minimum required before operations commence.

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.