Question 1

A DP2 diving support vessel is operating in DP using USBL (Ultra Short Baseline) as one of three position references, with divers at 60m depth. The DP system is also receiving acoustic signals from a taut wire. The vessel's aft azimuth thruster is at 80% demand.

What risk does high thruster demand pose specifically to the USBL position reference?

Accepted Answer

Thruster cavitation and noise at high thrust levels generates acoustic interference that can degrade or blank the USBL signal — this is a known common cause where the same event (high sea state requiring high thrust) can both stress the DP system and degrade the acoustic reference USBL and other acoustic position references (HPR) are vulnerable to noise generated by thrusters, particularly azimuth thrusters at high pitch angles that cause cavitation. This is a significant common cause failure: in the worst weather conditions (when thruster demand is highest and position is hardest to maintain), acoustic reference quality degrades precisely because the thrusters are working hardest. DPOs must monitor USBL quality indicators during high thrust periods and have contingency plans when acoustic references degrade in heavy weather with divers in the water.

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.