Glossary

The language of Dynamic Positioning

The DP vocabulary in plain seafarer's language. Every term is drawn from the library's entries and links back to the full treatment where one exists.

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Redundancy group: A set of interconnected power generation, distribution, auxiliary support, thrusters and controls that together can keep the vessel on station on its own. In a true 2-split, each group is self-contained and self-supporting — its own power, thrust and services — for at least 30 minutes after the other group fails.
Capability level: The DNV-ST-0111 scale for documenting station-keeping ability. Level 1 is a fully prescriptive calculation for verification and benchmarking; Level 2 adds quasi-static, project-specific adjustments; Level 3 uses time-domain simulation. Site variants substitute site-specific environmental data. Each step raises withstandable environmental force by 50% or more.
Centre of rotationCOR: The point about which the DP system holds the vessel's heading and position. When the COR is shifted away from the centre of gravity, the thrusters must also generate a yaw moment — COR ≠ COG is always a partially degraded state, and the longer the lever arm, the harder the response.
Centre of gravityCOG: The vessel's balance point and the default centre of rotation. Static capability plots are drawn with the COR at the COG; any other COR needs its own plots, intact and post worst-case failure (DNV-RP-E306).
Worst-case failureWCF: The single failure, determined by the FMEA, that costs the vessel the most capability. Capability plots must be drawn for this condition as well as intact; the redundancy concept is designed around surviving it.
Single-fault tolerance: The property that no single fault in any active component or system causes a loss of position. The defining requirement of DP Equipment Classes 2 and 3 under IMO MSC.1/Circ.1580, demonstrated by FMEA and proving trials.
FMEA: Failure Mode and Effects Analysis — the analysis that proves the redundancy design intent: independence of groups, fail-safe behaviour, identification of common points. Its quality is itself assured (OCIMF framework) and reviewed for gaps (MTS TECHOP).
Common point: A hidden connection between redundancy groups — cross-connected control power, shared UPS, combined supplies with diodes, common fuel or cooling auxiliaries — that lets one failure defeat both groups. Common points must be eliminated, not merely identified.
2-split: The honest two-group configuration: each group runs the vessel alone, and the modern test kills all power in one group — including stored energy — while the other holds position with no unexpected alarms.
Group redundancy testGRT: An annual performance test of the surviving redundancy group, not merely a failure test of the group shut down. The shutdown should include every energy source in the affected group, including UPS and battery-backed systems, to expose concealed crossovers.
DP equipment class: IMO's classification of DP systems: Class 1 — loss of position may occur after a single fault; Class 2 — no loss of position after a single fault in any active component; Class 3 — as Class 2, plus surviving fire or flooding in any one compartment.
DPVAD: DP Verification Acceptance Document — the record of surveys and testing required by IMO MSC.1/Circ.1580, with a model form in its appendix.
DP incident: A major system failure, environmental or human factor which has resulted in loss of DP capability. The most serious of the three station-keeping event reporting levels.
DP undesired event: A system failure, environmental or human factor which has caused a loss of redundancy and/or compromised DP capability — short of losing capability outright.
DP observation: An event that has not resulted in a loss of redundancy or compromised DP operational capability, but is still deemed worthy of sharing with the fleet.
Station keeping: Automatically maintaining a vessel's position and heading by its own propellers and thrusters, against wind, waves and current — the measure, calculate, correct loop run by the DP control computer.
Position reference system: The independent measurements of where the vessel actually is — GNSS/GPS, hydroacoustic beacons, laser-based systems. DP needs several, because the control computer is only as honest as its references.
Lever arm: The distance between the centre of rotation and the point where a force acts. Shifting the COR lengthens the lever arm for the environmental load, so the thrusters must supply a larger yaw moment for the same force.
Thruster utilisation: The share of a thruster's available thrust currently demanded by the DP system. Moving the COR off the COG raises utilisation unevenly between groups, eating the margin that redundancy is supposed to protect.
Vessel shielding: The hull acting as a breakwater: measured North Sea reductions of 66% and 40% in wave height on the sheltered side. A heading choice with a wave consequence — the working side goes in the lee — but it does not reduce the forces the thrusters must balance.
ASOG / WSOG: Activity Specific / Well Specific Operating Guidelines — the translation of the redundancy concept and capability limits into the operating limits the watch actually works to.
DPO: Dynamic Positioning Operator — certified under the Nautical Institute's four-phase scheme: Induction, Sea Time, Advanced course, Final Assessment.

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