IMCA DP Practitioner key terms

A computer-controlled system that automatically holds a vessel on a fixed position or heading, or along a predefined track, by using its own thrusters to counteract wind, wave and current forces.

The integrated hardware and software that compares measured vessel position and heading with the demanded values and calculates thruster commands to reduce the difference, including filtering, allocation and operator interface functions.

Any sensor or combination of sensors, such as DGPS, acoustic, laser or taut wire, that supplies the DP control system with measurements of vessel position relative to a fixed or moving reference.

A gyrocompass or equivalent sensor that provides continuous vessel heading to the DP control system; loss of reliable heading normally degrades or trips the DP system.

An instrument such as an anemometer or motion reference unit that measures wind, wave or current conditions and feeds the DP controller for feed-forward compensation and consequence analysis.

A control technique in which measured wind speed and direction are used to predict the environmental force on the vessel and pre-emptively adjust thrust, reducing position excursions before feedback correction acts.

A sensor, typically combining accelerometers and gyros, that measures vessel roll, pitch and heave so the DP system can compensate position references and environmental inputs for vessel motion.

Differential Global Positioning System: a satellite positioning method corrected by a shore or offshore reference station to give decimetre-level accuracy, widely used as a DP position reference.

A position reference that measures travel time of acoustic signals between a vessel-mounted transducer and subsea or seabed transponders to compute vessel position relative to the acoustic array.

A position reference in which a weighted wire is lowered to the seabed and the wire angle is measured to derive vessel offset from the anchor point; accuracy degrades with depth and current.

An optical position reference that tracks a target on a fixed structure, such as a platform, using a rotating laser beam to give high-accuracy relative position at short to medium range.

The logic by which the DP controller compares inputs from multiple position references and rejects or flags outliers, so that a single faulty reference does not corrupt the position solution.

The operator and automatic function of displaying and assessing differences between position references and the DP model estimate, used to detect drift, jumps or systematic errors in any one reference.

A navigation method that estimates present position by advancing a known previous position using heading and speed or thruster-derived velocity, used by the DP model between reference updates.

A propulsion device, such as a tunnel, azimuth or retractable thruster, that produces the forces and moments the DP system commands to hold position and heading.

The algorithm that distributes the total required force and moment among available thrusters, respecting individual limits, forbidden zones and minimum power consumption.

A classification society designation, commonly DP 1, DP 2 or DP 3, defining redundancy and fault tolerance requirements for the vessel's DP system and its consequences for operations.

The provision of duplicate or backup components and systems, such as extra thrusters, power sources or references, so that a single failure does not cause loss of position capability.

The specified single failure, or set of failures, that the DP system is designed to survive without exceeding defined position limits, forming the basis for class and operational planning.

An event in which the vessel exceeds the operational position or heading limits, ranging from a drive-off to a drift-off or slow drift, and triggering emergency response procedures.

A loss-of-position event in which thrusters produce unintended force, for example from a control or reference failure, causing the vessel to move away rapidly under power.

A loss-of-position event in which the vessel cannot generate enough thrust to resist environmental forces, so it slowly moves off station without active driving.

The ability of the vessel to maintain position and heading within specified limits under given environmental conditions, assessed against thrust, power and reference availability.

A graph showing the environmental conditions, typically wind speed against direction, in which the vessel can hold position with a defined set of thrusters available.

A certified person who controls the DP system, monitors position references and alarms, and executes the DP operations manual and contingency procedures during a DP watch.

A period of duty during which a qualified operator continuously monitors the DP system, position references, alarms and environmental conditions and remains ready to respond to failures.

The vessel-specific document specifying DP procedures, limits, checklists, contingency actions and responsibilities that the DP operator follows during operations.

Failure Mode and Effects Analysis: a structured study of DP system components and failure modes used to verify redundancy, identify single points of failure and support class notation.

A documented test programme, including FMEA proving trials, that verifies the DP system, thrusters, references and redundancy function as designed before operations begin.

A periodic verification, required by class or company practice, that checks the DP system, position references, alarms and redundancy arrangements remain functional and unchanged since the last full trial.

A real-time DP function that simulates the effect of the worst-case failure on position keeping and warns the operator if the vessel could not remain within limits.

A visual and audible warning generated when a monitored parameter, such as position error, reference discrepancy, power availability or thruster status, exceeds its configured limit.

A backup manual control system, independent of the main DP controller, that lets the operator hold position and heading using a joystick and separate sensors after a DP computer failure.

The DP operating mode in which the controller automatically commands thrusters to hold a set position and heading, with the operator monitoring rather than manually steering.

A DP operating mode in which the operator directly commands thrust or joystick input while the system may still provide position and heading feedback information.

A gradual, undetected error in a position reference that slowly moves the apparent position away from truth, often hard to spot without comparison against other references.