3.4 Wind Sensor, MRU & UPS

Closing the Sensor Picture

With the controller, model, filter, and gyro covered, three more elements complete the everyday DP sensor story: the wind sensor (anemometer), the motion reference unit (MRU), and the uninterruptible power supply (UPS). They are different physically but are tested together because each supports the controller’s ability to keep a clean, continuous loop when the environment or the power plant misbehaves.

Wind Sensor: Feed-Forward, Not Position

Wind acts continuously on superstructure and hull. If the DP system waited until the vessel was already blown off position before thrusting, footprint would grow and thrusters would work harder. Feed-forward uses measured wind speed and direction, plus model wind coefficients, to estimate wind force/moment and apply compensating thrust before large position error accumulates.

Placement and error sources are classic exam and watchstanding topics:

• Shadowing by the accommodation block, crane, flare tower, or helicopter hangar can under-read true wind from certain relative directions.
• Turbulence / updraughts near funnels and vents create noisy direction.
• Mast vibration and icing (cold climates) degrade quality.
• Relative wind is what the sensor sees; the system accounts for vessel motion and heading when forming earth-referenced wind used in force estimates.

If one sensor is shadowed while another is free-stream, voting or manual selection of the healthy unit prevents the controller from “believing” a falsely low wind and under-compensating. Conversely, a noisy sensor can inject false feed-forward if not rejected — thrusters may busy themselves without a real weather change.

Operator tip: when changing heading relative to weather, watch whether wind sensors still make sense versus the visual sea state and other instruments. A sudden drop in reported wind with seas unchanged often means shadowing, not a miracle calm.

MRU: Geometry Compensation for References

Position-reference antennas and transducers are almost never at the vessel’s common reference point (CRP). They sit high on masts or deep on hulls. When the vessel rolls, pitches, or heaves, those sensors move through arcs even if the CRP holds station. Without correction, the PRS would report false horizontal motion.

The Motion Reference Unit (MRU) (sometimes VRU — vertical reference unit — in older language) measures:

• Roll
• Pitch
• Heave (vertical motion)

and the DP system uses those angles/accelerations with known lever arms to compensate PRS data. Result: thrusters respond to true CRP motion, not mast-top circular motion in a seaway.

Failure effects: a failed or biased MRU can inject false position corrections — the vessel may start “correcting” for motion that is not real at the CRP, increasing thruster activity and footprint. Voting among MRUs and correlation with seastate helps the DPO judge whether an MRU alarm is critical.

Note the control distinction again: DP thrusters do not actively control heave/roll/pitch like a motion-compensated gangway system might; the MRU’s job for classic DP is primarily measurement for compensation, not closed-loop vertical station-keeping.

UPS: Keeping the Brain Alive

Thrusters need megawatts from the power plant. DP computers, operator stations, reference systems, gyros, MRUs, and network switches need clean, continuous electrical power at a tiny fraction of that energy. An Uninterruptible Power Supply (UPS) bridges short main-supply failures and brownouts so the control system does not reboot mid-operation.

If main power fails, thrusters stop producing force (unless separate emergency arrangements exist), so the vessel will drift eventually — UPS does not mean “DP continues forever on batteries.” What UPS does mean is that when power returns, the controller, references, and sensors are still alive, so blackout recovery of the DP system is faster and safer than a cold reboot of every computer and gyro warm-up cycle during a crisis.

Exam trap: options that say the UPS “drives the thrusters for station-keeping” are wrong. Options that say UPS is optional for Class 2/3 control loads conflict with the functional intent of MSC.1/Circ.1580 endurance requirements.

Combined Fault Scenarios (How Exams Mix Topics)

Real incidents and good MCQs combine sensors. Practice the diagnostic map:

Practical Setup and Watchstanding Checklist

1. Wind: Confirm which anemometer is selected; compare sensors; note known shadow sectors for your vessel.
2. MRU: Confirm online status and that PRS lever-arm corrections are active for the references in use.
3. UPS: Know which panels are UPS-backed; treat UPS alarms as critical; never assume thrusters run on UPS.
4. After power events: Verify DP computers, references, gyros, wind, and MRUs are healthy before resuming critical activity — not only that generators are back online.
5. Change isolation: When troubleshooting, change one variable at a time and allow filter/control settling.

How This Section Ties the Chapter Together

• The controller + model need clean inputs.
• The Kalman filter needs trustworthy measurements and good residual force estimates (wind feed-forward improves those residuals).
• The gyro closes yaw; the MRU keeps PRS honest in a seaway; wind anticipates weather force; UPS keeps the electronics alive when the plant blinks.

If you can assign each symptom to the correct box — controller, filter, gyro, wind, MRU, UPS, PRS, thruster, power — you are thinking like a DPO under assessment and under pressure.

Quick Comparison Table for Revision

Master the roles, not the brand labels, and sensor questions become systematic rather than guesswork.