IMCA DP Practitioner flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Dynamic positioning definition

A vessel automatically maintains its position and heading, or follows a predetermined track, using its own thrusters controlled by a computer system, without anchors or mooring lines.

Three controlled degrees of freedom

Surge, sway and yaw are the horizontal-plane motions a conventional DP system actively controls; heave, roll and pitch are not controlled by the DP thrusters.

Position reference system

A sensor or system that provides the DP controller with the vessel's position relative to a fixed reference, such as DGPS, hydroacoustic, taut wire, microwave or laser.

Heading reference redundancy

Two independent gyrocompasses are standard for DP; the controller selects or weights heading sources, and a single gyro is a single point of failure.

Wave-frequency filtering

The DP observer filters out first-order wave-induced motion so the controller responds only to low-frequency drift, avoiding continuous thruster wear from wave motion.

Thruster allocation

The algorithm that distributes the total demanded force and moment across available thrusters, respecting each thruster's thrust limit and any forbidden zones.

Body-fixed coordinate frame

A right-handed frame fixed to the vessel, typically x forward, y starboard and z downward, used with an earth-fixed frame to express position and heading.

Drift-off

Loss of position because the thrust demand exceeds the available thrust, so the vessel drifts with the environmental forces; often follows a failure reducing capability.

Drive-off

The vessel moves away from its setpoint because the DP system commands thrust in the wrong direction or over-thrusts, often caused by a faulty reference or sensor input.

DP alarm hierarchy

DP systems present alarms by consequence, such as loss of position, degraded position and advisory; the operator must know which alarms demand immediate action.

DP watchkeeping

Continuous monitoring of position, heading, thruster load and power margin, with formal handover at change of watch and a record of the operation.

Pre-operation checklist

A signed-off check of position references, sensors, thrusters, power and communications completed before a DP operation begins.

Abort criteria

Pre-agreed limits and triggers, such as excursion or position error, at which the operation is stopped or the vessel moves off, decided before the operation starts.

Incident reporting

Recording and analysing station-keeping incidents so that lessons learned feed back into the FMEA, procedures and training.

FMEA in DP

Failure Mode and Effect Analysis identifies single-point failures and their effects; its findings drive the redundancy design and the operational limitations of the vessel.

Worst-case failure

The largest credible single failure identified in the FMEA; after it, the remaining thrusters must still be able to hold position against the design environment.

DP capability plot

A plot of the maximum wind or current a vessel can hold against, for various headings, with all thrusters available and after the worst-case failure.

Equipment class 2

A DP redundancy class in which 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 single point

A single busbar or single generator can be the worst-case failure even when thrusters are duplicated, because all thrust depends on that power source.

Forbidden thruster zone

A thruster direction or operating zone that is prohibited because its wash would affect a dive bell, ROV, riser or other sensitive equipment.

Consequence analysis

Assessment of what happens if the vessel loses position, such as excursion from a wellhead or riser angle limits, and the time available before a limit is exceeded.

DP annual trials

Tests verifying that the as-built DP system matches the FMEA and capability documentation; the FMEA is reviewed whenever equipment is changed.

DGPS common-mode failure

Two DGPS receivers sharing one antenna or correction source are not diverse; loss of corrections or satellite geometry error affects both simultaneously.

Hydroacoustic position reference

A seabed transponder system measuring range and bearing; accuracy degrades with depth, sound velocity error and thruster noise, and one array is a single point of failure.

Taut wire system

A weighted wire to the seabed measuring angle and payout; accurate in shallow water but depth-limited and vulnerable to current and fouling.

Microwave position reference

A system measuring range and bearing to a target on a fixed structure; it needs line of sight and a suitable reflector and fails if the target moves.

Gyrocompass failure modes

Settling error, power loss and drift are typical; two independent gyros with a comparison alarm are the minimum for DP operations.

Wind sensor failure

An iced or blocked anemometer gives a false wind reading, which the DP model may treat as a real force and use to command incorrect thrust.

Motion reference unit

Measures heave, roll and pitch to compensate position references for vessel motion; a failed MRU corrupts position input even when the reference itself is healthy.

Reference diversity rule

Position references using different physical principles, such as DGPS plus taut wire plus hydroacoustic, are preferred over several units of one type to avoid common-mode failure.

PRS disagreement alarm

A persistent disagreement between position references is a warning to investigate the cause, not simply a reason to deselect the outlier.

DP incident root causes

Reported causes include human error in setup or operation, position reference failure, thruster or power failure, and software or sensor faults.