Question 1

A DP2 vessel is transiting to a worksite and the chief engineer proposes running in closed bus configuration to improve fuel efficiency during the approach. The DPO is about to begin DP operations.

Why is closed bus operation a significant concern for DP2 redundancy?

Accepted Answer

Closed bus ties two otherwise independent power systems together — a fault on one bus can cascade to the other, causing a total blackout and loss of all DP capability simultaneously On a DP2 vessel, the power system is split into two or more independent sections (open bus) so that a fault, short circuit, or overload on one section cannot spread to the other. Closing the bus tie creates a single integrated power network — the very common cause failure that DP2 redundancy is designed to prevent. IMCA DPE 01/23 specifically documented incidents where closed bus operations during DP led to total blackout and loss of position. Closed bus may be permitted in transit or with specific risk assessment, but must be opened before entering a risk zone.

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.