Question 1

A DPO is setting up position references before commencing a sensitive DP2 pipe-lay operation. The vessel has three GPS receivers fitted, but only two are receiving a valid DGNSS differential correction signal. The third is showing raw GPS only.

Why does the absence of differential correction on one unit matter for DP operations?

Accepted Answer

Without differential correction, raw GPS position accuracy may degrade to ±10–100m depending on satellite geometry and atmospheric conditions, making it unreliable as a DP reference in close-proximity operations Raw (uncorrected) GPS carries position errors of 10–100m due to atmospheric delays, satellite geometry, and multipath effects. Differential correction (DGNSS) removes much of this error, typically achieving 1–3m accuracy. For DP2 operations near a fixed structure, a reference system reading 50m off true position is not just useless — it is dangerous, as the DP control system may use it to drive the vessel toward the structure. The uncorrected unit should be flagged and its weighting reduced or removed in the DP reference system configuration.

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.