Practical guide
DP Watchkeeping
Four things a DP watchkeeper must always have in sight: the heading, the thruster groups, the proof of redundancy, and the quiet estimate underneath it all. Built from the library's Dynamic Positioning entries — every claim has a source passage behind it.
01
Heading — the first thing the DP watch owns
Position errors announce themselves on the plot; heading errors announce themselves on the capability plot, the power budget and the weather deck — often all three at once.
Heading is a resource decision, not just a setpoint
The heading that shelters the working deck may not be the heading that minimises thruster load. Every degree off the weather-optimal heading is paid for in thrust and power, and the capability plot — not the instinct — says how much is left in reserve.
Watch the heading window, not only the heading
Most DP controllers allow a permitted heading deviation. Know it, and know what the vessel does when it is exceeded: alarm, mode change, or a silent increase in sway demand as the weather vector swings broadside.
Shielding is a heading decision with a wave consequence
Putting the working side in the lee can cut the wave height the work site sees by half — but the environmental force the DP system must balance does not fall with it. Take the operational gain with the capability plot in front of you.
Heading and COR are married
Where an offset centre of rotation is in use, the yaw moment demanded depends on the weather angle relative to the lever arm. A heading change re-points the lever arm against the force; what was comfortable can become limiting without any new fault.
On watch: Before accepting any heading: what does the capability plot say at this heading, at this weather, post worst-case failure?
02
Thruster groups — where the force actually comes from
The DP computer holds position; the thruster groups make it true. The watchkeeper's job is to keep every group inside its envelope with margin to spare.
Utilisation is a group question, not an average
An average utilisation of 60% can hide one group at 95%. Static capability plots assume the centre of rotation at the midpoint; offset the COR and one group carries the yaw moment — the limiting group, not the mean, decides when the vessel is degraded.
Post-failure, the diagonal pair must do the job alone
After the worst-case failure the surviving redundancy group must still hold position and heading within the operating limits. On watch, the question is never 'are we holding?' but 'would we still be holding if this group tripped now?'
Thrust allocation can conceal saturation
The allocator spreads demand across available units. A group creeping toward its limit may look calm in total thrust terms. Watch per-group utilisation and azimuth rates, not only the power meter.
Drive-off is the unforgiving failure
A thruster failing to full thrust is worse than one failing to zero. FMEA categories that could create a drive-off — thruster controls, DP controls, position references, sensors — are the ones that get the traffic-light red in a gap analysis.
On watch: Name the limiting group out loud on every watch handover, and state its margin.
03
Redundancy tests — proof, not paperwork
A redundancy group is only real when it has been killed and the surviving group kept position. Everything else is a diagram.
The 30-minute rule
Each redundancy group must be self-contained and self-supporting — its own power, thrust and services — for at least 30 minutes, whether the plant is direct diesel, diesel-electric or pure electric.
A group redundancy test is a performance test of the survivor
Shut down the complete selected group — including stored-energy sources such as UPS and battery-backed systems — and prove the surviving group maintains position and heading independently. Anything left running unexamined may be a concealed crossover.
Critical tests do not go on a rolling cycle
Tests that prove performance, protection or detection on which single-fault tolerance depends must be proven annually. Rolling trials are reserved for demonstrably non-critical redundancy above single-fault tolerance.
Bad tests hide the truth
Battery endurance tests with inactive loads, or tests that leave crossovers energised, pass while the concept fails. The honest test kills all power in one group and watches the other hold position with no unexpected alarms.
On watch: Know which group is which on the switchboard before the test starts — not after the first bus goes dark.
04
The EKF — why the heading you see is an estimate
The Extended Kalman Filter continuously argues between the vessel model and the sensors. The heading the controller acts on is the outcome of that argument — not the raw gyro.
Prediction and correction
The EKF carries a mathematical model of the vessel's motion in surge, sway and yaw, predicts where the vessel should be, then corrects the model against every new measurement from gyros, MRUs and position references.
Wave filtering is the EKF's quiet work
The controller must not chase every wave. The filter separates the low-frequency drift — which the thrusters should correct — from wave-frequency motion they should ride through. Heading corrections are made against the filtered estimate, not the oscillation.
Weighting decides who is trusted
The filter weights the model against each sensor. With a single, slow position reference, model uncertainty rises between updates and the estimate leans harder on the vessel model — and on the gyro for heading. Lose the gyro then, and heading correction degrades fast.
Heading corruption is the insidious case
An uncompensated MRU or a biased gyro feeds the filter a lie it cannot see. The EKF will faithfully correct the vessel toward a wrong heading. Cross-check the displayed heading against the compasses and the reference-system geometry — especially after any sensor changeover.
On watch: When position and heading disagree with your eyes, ask which side of the filter — model or measurement — is winning, and why.
Read
The full DP entry
Capability levels, redundancy groups, COG vs COR and the rest of the source material.
Look up
DP glossary
Redundancy group, capability level, centre of rotation and the rest of the vocabulary.
Ask
Ask the librarian
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