DP Mini Guide: Position Reference System Failure

Crew Connect DP Academy · Mini Guide 01

Position Reference System Failure

Topic: DP Systems  ·  Sub-topic: Position Reference Systems  ·  Level: DPO / Advanced Trainee  ·  Read time: 8–12 min

1. What is it?

A Position Reference System (PRS) provides position information to the DP control system — DGNSS, taut wire, hydroacoustic, laser or radar-based systems depending on the vessel and operation. The important question isn't just naming the system in use. It's: what information is this system giving me, how reliable is it, and what happens if I can no longer trust it?

2. Why does it matter?

DP decisions are only as good as the information feeding them. A PRS can fail in two very different ways: an obvious failure (it drops out and alarms), or a degraded failure (it remains available but is quietly feeding unreliable data). The second is often more dangerous, because it doesn't announce itself.

3. Recognition

What might tell you something is wrong, beyond a clean alarm?

• PRS alarm or dropout
• Sudden position jump
• Disagreement between references
• Intermittent, non-alarming deviations
• Unexpected vessel movement or controller behaviour
• Increasing position error that doesn't match the environment

4. Risk

The immediate risk isn't just “one system has failed.” Trace the chain:

PRS degradation → reduced redundancy → reduced confidence in position → potential incorrect DP response → position/heading degradation → reduced operational margin → potential loss of position → consequence to the operation.

5. But…

Don't stop at the obvious answer. You've lost one PRS — but:

• Is another PRS actually independent, or does it share a common antenna, correction source or failure mode?
• Is the remaining PRS genuinely healthy, or just not yet alarming?
• Are you in a critical operation, and what does your ASOG require right now?
• How close are you already to the capability limit, and is the weather trending the wrong way?

6. Immediate action

The immediate priority is rarely “fix the PRS.” It's establishing the current condition of the whole DP system: confirm the failure, check remaining references and their independence, check redundancy and capability margin, check ASOG status, and communicate as required — before deciding whether to continue.

7. Decision point

With independent, healthy references remaining and the vessel within position limits, the correct decision depends on the full picture — ASOG, FMEA, redundancy, environment and mission criticality — not a fixed universal answer. Holding position right now does not, by itself, prove you remain within the required risk envelope.

8. Consequence & reassessment

Say you choose to continue with increased monitoring. Five minutes later, wind picks up and your remaining reference starts showing intermittent deviations too. That changes the picture — new information has to trigger a fresh decision, not a re-confirmation of the old one. This is the difference between a one-off answer and real DP watchkeeping.

9. Recovery & verification

The trap: an alarm clearing is not the same as the problem being resolved. Before resuming normal operations, actively verify — PRS data agrees with other references, required redundancy is restored, capability has been reassessed, ASOG status reviewed, relevant personnel informed. Don't infer recovery from the absence of a flashing light.

Sources: IMO MSC.1/Circ.1580 §3.4.8, IMCA M252 (PRS/sensor guidance), IMCA M220 Rev.3, IMCA DPE 04/20. This guide is educational content, not a substitute for accredited DP training, vessel-specific FMEA/ASOG, or full-mission simulator assessment.