11.1 FMEA & Annual DP Trials

Why FMEA sits at the heart of Class 2/3 DP

Dynamic Positioning Class 2 and Class 3 are not marketing labels. They are engineering claims: after a defined single failure (and for Class 3, with additional fire/flood compartment thinking), the vessel should retain enough thruster and power capability to hold position in the analysed environment. Failure Modes and Effects Analysis (FMEA) — sometimes titled DP FMEA, redundancy analysis, or failure-mode study — is the structured document that proves or disproves that claim before the vessel is trusted for critical work.

On the NI Induction and Simulator path you are not expected to write an FMEA. You are expected to know what it is for, how trials relate to it, why modifications matter, and which conclusions a DPO must carry into everyday operations. Without that knowledge, capability plots, consequence analysis, open/closed bus decisions, and ASOG colours become disconnected guesswork.

What an FMEA actually does

An FMEA walks the DP-relevant plant failure by failure and asks:

1. What can fail? (generator, bus section, thruster drive, DP process station, reference common power, cooling, fuel common rail, control network, protection mis-trip, etc.)
2. What is the effect on power, thrusters, control, sensors, and station-keeping?
3. Is the effect contained so residual capability remains after that single failure?
4. Are there hidden common-mode links that make two “independent” items fail together?
5. What residual thrusters, power, and control paths survive?

The product of a good FMEA is not only a thick binder. It is a failure graph of the vessel: partitions, residual capability, and the worst-case failure (WCF) used later for capability plots and online consequence analysis.

[!IMPORTANT] FMEA proves single-failure tolerance for Class 2/3 design. It does not claim the vessel can survive every combination of simultaneous independent failures. Exam stems that pile three unrelated failures into one event are testing whether you confuse FMEA/WCF with multi-fault fantasy scenarios.

FMEA and equipment class — the design link

FMEA is therefore the evidence trail that the redundancy you see on drawings is real in failure physics. A vessel can have two bus sections on paper and still fail FMEA if a common cooling pump, common UPS feed, or protection discrimination gap blacks both sides from one initiating event.

Proving trials: first real-world proof

After design analysis, the vessel must be tested. Proving trials (DP proving trials, FMEA proving trials, or equivalent shipyard/class trials language) exercise selected failure modes on the real plant to confirm:

• residual thrusters actually receive power after section loss,
• bus-tie and protection behaviour matches analysis,
• thruster enable/disable and allocation respond as modelled,
• control redundancy (process station failover, network paths) works,
• sensors and UPS behaviour under partial power loss is acceptable,
• no unexpected common-mode collapse appears.

Proving trials convert theory into measured residual capability. If a simulated bus failure also trips the “healthy” side thrusters because of a wiring error, the FMEA must be revised and the defect fixed — you do not “paper over” the trial with a note that ignores reality.

Annual DP trials: assumptions age

Time, maintenance, software updates, and small modifications erode confidence that last year’s plant is this year’s plant. Annual DP trials (and similar periodic trials required by company, class, or charterer) re-verify critical FMEA assumptions:

• generators still start and take load as expected,
• preferential trip and load-shed paths still work,
• thrusters still respond and can be isolated cleanly,
• changeover of DP computers / networks still succeeds,
• open-bus residual configuration still leaves a usable thruster set,
• alarms and changeovers that operators rely on still appear correctly.

Annual trials are also a training event: bridge and engine room practise the failure signatures they may only see once in a career. For the exam, treat annual trials as validation that FMEA remains true, not as a random survey checklist item.

FMEA is a living document

Any material change can invalidate previous conclusions:

• new thruster, drive, or generator,
• switchboard or bus-tie protection upgrade,
• DP system software major revision,
• closed-bus protection package added or removed,
• PRS suite change that affects common power or interfaces,
• conversion work that re-routes cooling, fuel, or control air.

Operational rule: if the plant no longer matches the FMEA, the vessel is no longer operating on proven redundancy. Capability plots and online consequence analysis that assume the old residual set become optimistic lies. Company procedures usually require management of change (MOC), FMEA update, and trials before returning to critical DP work.

What the operator must know (without being the author)

You will not memorise every failure table. You should know, for your vessel and for exam scenarios:

1. What is the WCF in the normal open-bus (or proven closed-bus) configuration?
2. Which thrusters and how much power remain after WCF?
3. Does FMEA allow closed bus for the activity you are doing, and under what protection conditions?
4. What configuration does CAM demand so that residual capability is real (open bus, thrusters enabled both sides, generators online both sections)?
5. Where are key conclusions summarised for the bridge (DP operations manual, FMEA summary, ASOG notes)?

Worked bridge scenario

A DP construction vessel arrives on a new charter. The master hands the DPO a capability plot set and says “we are fine up to 40 knots.” The DPO asks for the FMEA summary and annual trials date. Trials are thirteen months overdue; a thruster drive was replaced six months ago with a different type and no FMEA addendum exists. The DPO refuses to treat the old post-WCF plots as authoritative for diving support until engineering and class/company MOC restore a proven residual picture. Lesson: plots without a living FMEA and current trials are decoration, not proof.

Exam traps for FMEA and trials

Bottom line: FMEA documents how single failures propagate and proves Class 2/3 design intent; proving and annual DP trials keep those assumptions honest on the real ship; modifications demand reassessment; and the operator must know the vessel WCF and the FMEA conclusions that drive configuration, capability, and stop-work decisions.