10.2 MSC.1/Circ.1580 vs MSC/Circ.645

Two generations of IMO DP guidelines

DP equipment classes did not appear in a vacuum. They sit inside IMO guidelines that describe how DP systems should be designed, surveyed, and verified. Two circulars dominate DPO teaching and industry conversation:

You are not sitting a naval architecture exam. You are expected to know that 1580 superseded the 645-era baseline for new construction, when it applies, and what kinds of expectations rose — especially around redundancy integrity, analysis, and control power.

MSC/Circ.645 — the older baseline

MSC/Circ.645 was the long-standing international reference many existing DP fleets were designed against. It established the familiar equipment class ideas (Class 1 / 2 / 3 philosophies) and industry practice for FMEA-style verification that classification societies expanded in their rules.

For a DPO, “645 vessel” is often shorthand for: designed and classed against the older guideline set. Those vessels remain in service worldwide. They are not “illegal” or automatically unsafe. They must still meet their class notation, annual trials, and operational procedures. The point of learning 645 versus 1580 is generational design expectation, not disrespect for older tonnage.

MSC.1/Circ.1580 — updated guidelines for vessels and units

MSC.1/Circ.1580 is the IMO Guidelines for vessels and units with dynamic positioning (DP) systems. It updates and replaces the older circular’s role as the primary IMO guideline text for modern application. Important conceptual points:

1. It addresses vessels and units with DP systems (not only a narrow ship stereotype — MODUs and other units are in scope of the guideline framing).
2. It retains the equipment class philosophy while reflecting industry experience since 645.
3. It is the circular industry cites for new builds after the applicability date below.

[!NOTE] Do not invent or inventively renumber “clause 3.2.1(a)” style references on the induction exam unless you have verified them from the circular. Assessors care that you know principles and applicability, not fake precision.

Applicability date — keel laid on or after 9 June 2017

The operational fact every DPO candidate should memorise:

MSC.1/Circ.1580 applies to new vessels and units with the keel laid on or after 9 June 2017, and to conversions as specified in the circular’s application provisions.

Exam trap: quoting 9 June 2017 incorrectly, or saying 1580 applies to all vessels immediately on that calendar day regardless of build status. The key phrase is keel laid on or after (plus conversion rules as specified).

What “higher expectations” means at conceptual DPO level

Without inventing clause numbers, DPO training and industry summaries emphasise that 1580-era application generally reflects tighter integrity thinking than many 645-era implementations. Themes you should be able to discuss:

1) Performance and station-keeping demonstration

Designs are expected to show that the DP system can hold position and heading under defined conditions, and that post-failure capability is understood. Capability plots, FMEA conclusions, and sea trials are not paperwork ornaments — they prove the class philosophy in weather and configuration envelopes.

2) FMEA quality and verification

Failure Modes and Effects Analysis (and proving trials) remain central. Modern expectations stress that the FMEA must be living and credible: identify single failures that matter, show residual capability, and link to annual DP trials. For Class 2/3, the FMEA is how “no single active failure loses position” becomes a testable claim.

3) Consequence analysis

Online consequence analysis (or equivalent continuous assessment of whether the vessel can still hold after the design worst-case failure in present weather) is a major operational and design theme. The DPO uses it as a live traffic light for whether critical work may continue after configuration or weather changes — not only as a yard-delivery feature.

4) Control power, UPS, and independence

Control computers, networks, and critical sensors need reliable control power. Uninterruptible power supplies (UPS), battery autonomy ideas, and independence of power feeds support the claim that a single power fault does not instantly kill the whole DP brain. At conceptual level: thruster power and control power are different problems — UPS typically protects control/sensor loads for a limited time; it does not replace main generation for thrusters.

5) Redundancy with fewer hidden common-mode holes

Industry experience after 645 highlighted common-mode failures: shared cooling, shared fuel, shared cable routes, shared software configuration errors, shared sensor suites. 1580-era thinking pushes designers and operators to treat “two of everything” as insufficient if both sides share one hidden dependency.

645 vs 1580 — DPO comparison table

How this links to equipment class (Section 10.1)

Equipment class definitions are used inside the guideline framework. Whether a vessel is Class 2 or Class 3 is a class/design outcome; whether it was delivered against 645 or 1580 is a generation of guidance question. A Class 2 vessel may be 645-era or 1580-era. Do not say “Class 2 means 1580” or “Class 3 means 645.” Say: class = failure philosophy; circular generation = which IMO guideline set informed design application for that build/conversion.

Operational scenarios (exam style)

What not to do on the exam

• Do not claim 1580 abolished Class 1.
• Do not claim every vessel worldwide re-classed overnight on 9 June 2017.
• Do not quote unverifiable paragraph numbers.
• Do not confuse IMO guidelines with IMCA good practice documents (IMCA M103 etc. are industry guidance, not the IMO circular itself).
• Do not confuse equipment class with the Nautical Institute operator certificate class (Limited/Unlimited).

DPO takeaway checklist

1. 645 = older IMO DP guidelines generation.
2. 1580 = updated Guidelines for vessels and units with DP systems.
3. Applicability for new builds/units: keel laid on or after 9 June 2017 (+ conversions as specified).
4. Conceptual uplift: performance, FMEA, consequence analysis, UPS/control-power integrity, common-mode thinking.
5. Onboard truth source: vessel class documents, FMEA, and SMS — circular knowledge supports understanding those documents.

Bottom line: MSC/Circ.645 shaped the classic DP class world; MSC.1/Circ.1580 is the updated IMO guideline set for modern new builds and specified conversions from the 9 June 2017 keel-laid threshold. Know the date, know the principle-level uplift, and keep class philosophy separate from circular generation.