IMCA DP Practitioner cheat sheet

DP definition: a vessel automatically maintains position and heading (or a fixed track) by using its own thrusters, controlled by a computer system, without anchors or mooring lines.

Degrees of freedom: surge, sway, yaw are the three horizontal-plane DOF actively controlled by DP; heave, roll and pitch are not controlled by a conventional DP system.

Position reference systems (PRS) commonly used: DGPS, hydroacoustic (HPR/USBL), taut wire, Artemis-type microwave, laser, and Fanbeam-type systems; at least three diverse PRS are typical for operations where loss of position has high consequences.

Heading reference: two independent gyrocompasses are standard; the DP controller uses a weighted or selected heading source, and a single-gyro setup is a single point of failure.

Environmental forces are measured or estimated; the DP model estimates the steady (low-frequency) wind, current and wave drift force, while the wave-frequency motion is filtered out to avoid thruster wear.

Controller structure: a Kalman-type or model-based observer estimates vessel state and unmeasured forces; the controller then allocates thrust to satisfy the demanded force and moment.

Thruster allocation: the allocation algorithm distributes the total force and moment demand across available thrusters, respecting individual thrust limits and forbidden zones.

Notation convention: a right-handed body-fixed frame (x forward, y starboard, z downward) with position and heading expressed in a fixed earth frame is the standard basis for DP calculations.

Consequence classification (IMCA/industry practice): worst-case single failure is assessed against the vessel's ability to hold position; loss of position can lead to drift-off, drive-off or a large excursion.

FMEA (Failure Mode and Effect Analysis) is the core document for DP assurance: it identifies single-point failures and their effects, and its findings drive the DP redundancy design and the operational limitations.

Worst-case failure (WCF) design: after any single failure, the remaining thrusters must be able to hold position against the design environment; the WCF is the largest credible single failure identified in the FMEA.

DP capability plots show the maximum wind or current a vessel can hold against, for various headings, with all thrusters available and with the worst-case failure - the two curves define the operational envelope.

Redundancy: an equipment class 2 vessel has redundancy such that a single failure of an active component will not cause loss of position; class 3 adds protection against fire and flooding in one compartment.

Power plant: DP redundancy depends on generator and switchboard configuration; a single busbar or a single generator can be the WCF even when thrusters are duplicated.

Thruster configuration: fixed pitch vs controllable pitch, azimuth vs tunnel, and forbidden zones (e.g. thruster wash on a dive bell or ROV) are all part of the DP operational assessment.

Consequence analysis (e.g. for a drilling or dive support vessel) considers excursion from the wellhead or worksite, riser angle limits, and the time available before the limit is exceeded.

DP trials and annual surveys verify that the as-built system matches the FMEA and capability documentation; the FMEA should be reviewed whenever equipment is changed.

Operational decision: when the predicted worst-case failure cannot be held, the operation must be suspended or the vessel moved to a heading where capability is adequate.

DGPS: accuracy depends on correction service and satellite geometry; failure modes include loss of differential corrections, multipath, and common-mode satellite errors - two DGPS receivers on one antenna are not diverse.

Hydroacoustic position reference (HPR/USBL): uses transponders on the seabed; accuracy degrades with water depth, sound velocity errors, and acoustic noise from thrusters; a single transponder array is a single point of failure.

Taut wire: a weighted wire from the vessel to the seabed; it measures angle and wire payout; it is accurate in shallow water but limited in depth and can be disturbed by current or by the wire fouling.

Microwave/laser systems (e.g. Artemis, Fanbeam, RADius): measure range and bearing to a target on a fixed structure; they need line of sight and a suitable reflector, and they fail if the target moves or is obscured.

Gyrocompass: heading reference; common failure modes are settling error, power loss, and drift; two independent gyros with a comparison alarm are the minimum for DP operations.

Wind sensor: provides feed-forward wind force; an anemometer icing up or a blocked sensor gives a false wind reading, which the DP model may interpret as a real force.

Motion reference unit (MRU): measures heave, roll and pitch to compensate the position reference for vessel motion; a failed MRU corrupts the position input even when the PRS itself is healthy.

Diversity rule: position references using different physical principles (e.g. DGPS plus taut wire plus HPR) are preferred over multiple units of the same type, because common-mode failures are avoided.

PRS comparison: the DP system monitors agreement between references; a persistent disagreement between two references is a warning to investigate, not a reason to simply deselect the outlier.

Drift-off: the vessel loses position because the thrust demand exceeds available thrust, typically after a failure that reduces capability; the vessel drifts with the environment.

Drive-off: the vessel moves away from the setpoint because the DP system commands thrust in the wrong direction or over-thrusts, often due to a reference or sensor fault feeding the controller.

Common root causes reported in DP incident analyses: human error in setup or operation, PRS failure, thruster or power failure, and software or sensor faults.

Alarm hierarchy: the DP system presents alarms by consequence (loss of position, degraded position, advisory); the operator must understand which alarms require immediate action.

Watchkeeping: a DP operator maintains continuous watch, monitors position, heading, thruster load and power margin, and records the operation; a change of watch includes a formal handover.

DP operations manual and checklists: pre-operation checks cover PRS, sensors, thrusters, power, and communications; the checklist is signed off before the operation begins.

Communication: the DP operator, the bridge team, the dive or ROV supervisor and the client representative agree on the operation, the limits and the abort criteria before starting.

Emergency procedures: on loss of position, the response is defined in advance (e.g. disconnect, move off, raise the ROV); the decision authority and the trigger limits are agreed before the operation.

Incident reporting: DP incidents are reported and analysed (IMCA-style station-keeping incident reporting) so that lessons learned feed back into the FMEA and the procedures.