MEG-4 did not create mooring confusion. It exposed it. This analysis explains why SDMBL, WLL, LDBF and brake render must all trace back to the ship’s design basis, and why SIRE 2.0 is now testing crew understanding, not just document availability.
When I was sailing as Chief Officer, mooring brake testing was understood in simpler terms. The brake was tested, marked, and the working idea onboard was that the brake should render at about 60% of the mooring line’s MBL. We did not speak in terms of SDMBL, LDBF, TDBF and WLL in daily deck language. The rope certificate carried the familiar number, and the brake was adjusted around it.
MEG-4 changed the reference point. The question was no longer only, “What is the breaking strength of the rope?” The better question became, “What was the ship designed for?”
For several years, many vessels treated this as a documentation update. The LMP was prepared, certificates were filed, brake tests were recorded, and new terms were added to company procedures. From 1 January 2024, amended SOLAS regulation II-1/3-8 and MSC.1/Circ.1620 pushed mooring inspection and maintenance into sharper focus for ships in operation. SIRE 2.0 then added another layer by moving inspection closer to crew understanding and not only to document availability
It is no longer enough for the ship to have a Line Management Plan (LMP) in a binder. Officers must be able to explain why the rope certificate, WLL, brake rendering value and ship design basis all return to the same number.
RightShip’s Mooring Safety Insights recorded 542 mooring line incidents between January 2015 and March 2025. Its RISQ inspection data recorded 11,390 mooring-related findings from January 2022 to January 2025.
Those numbers explain why this subject keeps appearing in audits.
The Ship Comes First
The starting point is Ship Design MBL, also called SDMBL. This is the ship’s mooring design reference. It is the base value against which the system is understood: lines, tails, WLL, brake rendering, fittings, structure and the LMP.
For new ships, this value is usually clear because the vessel is documented under newer mooring requirements. On older ships, their drawings may show winch rating, brake holding capacity, rope size, bollard strength or Equipment Number without clearly stating which value is now to be treated as SDMBL.
This is where the audit room becomes interesting. The LMP may show one value. The rope certificate may show another. The brake test report may show a third. Each document may be correct in isolation, but the ship still has to prove which number governs the system.
The Older Ship Problem Has Changed
Eight years after MEG-4, most serious operators have already assigned a Ship Design MBL to their older vessels. The issue is whether the ship can explain how that number was established. Their original drawings may not say SDMBL. Some may not show a clear mooring MBL in modern terminology. In many cases, the company has had to reconstruct the value from class records, Equipment Number data, mooring arrangement plans, SWL of fittings, winch information or structural verification.
That reconstruction may be valid, but the number cannot stand alone during inspection. If an inspector asks, “Where did this SDMBL come from?” the answer cannot simply be, “Company provided it.” The vessel should be able to point to the basis: class confirmation, approved calculation, old yard record, EN-based derivation or documented technical review.
The Equipment Number, or EN, is one route that may help older ships trace the original mooring basis. EN is a class design index connected to the vessel’s size, displacement and exposure, historically used to determine the required number and strength of mooring lines.
The ship is not simply looking for a number. It is trying to prove the design basis of its mooring system. That is the modern inspection gap: traceability of SDMBL.
From Number to Chain
Once the SDMBL is established, it becomes the anchor for every other mooring value. LDBF, TDBF, WLL and brake rendering are not separate figures to be picked from different documents. They are percentages of the same reference point.
If SDMBL is wrong, uncertain or not traceable, the rest of the mooring plan is built on weak ground. If SDMBL is clear, the chain should not be negotiated document by document.
| Component | MEG-4 baseline | Operational role |
| SDMBL / Ship Design MBL | 100% | The ship’s design anchor point. |
| WLL, wire / synthetic | 55% / 50% of SDMBL | Normal operating ceiling. |
| Brake render setting | 60% of SDMBL | Calibrated safety valve. |
| Main line LDBF | 100% to 105% of SDMBL | Main line strength matched to the ship. |
| Tail TDBF | 125% to 130% of SDMBL | Energy absorber, not the weak link. |
| Deck fittings / structure | Above operational load chain | Must not become the first failure point. |
WLL Is Not a Moving Number
Once SDMBL is established, the Working Load Limit (WLL) should remain fixed against that ship’s design basis. It should not change every time a replacement rope arrives onboard with a slightly different certificate value.
If the ship’s SDMBL is 100 tonnes, the WLL for a steel wire is 55 tonnes. If the new wire certificate shows an LDBF of 105 tonnes, the WLL does not become 57.75 tonnes. The ship has not changed. The deck fittings have not changed. The brake setting has not changed. The mooring analysis has not changed.
The ship did not become stronger because a new rope certificate arrived onboard.
The certificate confirms that the rope is suitable for the ship. Once companies or vessels start deriving WLL from each new certificate, the LMP becomes unstable and the safety margin becomes unclear.
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The Brake Is Not a Working Limit
The brake setting is another point where questions arise during inspections.
If WLL is 50% or 55% of SDMBL, and the brake renders at 60%, does the line exceed WLL before the brake slips?
The answer is … Yes.
It simply means WLL and brake rendering serve different purposes. WLL is the normal operating ceiling. The brake render point is the protection layer above it.
A vessel should not be routinely working above WLL. If line loads reach that region, the ship is already outside normal mooring limits. The brake does not render immediately at WLL because the system needs a small margin before the line starts paying out. If the brake rendered too early, the vessel could begin moving in conditions the mooring system is still expected to restrain.
So the space between WLL and 60% is more of a warning margin.
Why This Matters in Inspection and on Deck
Under the older inspection culture, a vessel could often rely on the presence of a complete file. Under the newer approach, especially with the human element focus of SIRE 2.0, the officer may have to explain the link between SDMBL, LDBF, TDBF, WLL and brake rendering. OCIMF notes that some SIRE 2.0 questions require inspectors to interview officers and crew on aspects of their duties, even when those duties are not being performed during the inspection.
A simple question is enough to expose the weakness: how was this WLL calculated?
This is the same document-versus-competence problem DeepDraft examined in ECDIS Certification Is Not the Same as ECDIS Competence: the certificate may exist, but the operational understanding still has to be proven.
If the answer comes from the latest rope certificate, the system has already lost its design basis. If the officer cannot explain how the SDMBL was established, the number in the LMP becomes weak. If the brake test value cannot be connected back to the same design basis, the ship may have documents, but not a defensible mooring chain.
The practical consequence appears on deck.
Once the vessel is alongside and the brakes are on, most ships do not have live tension readings for every mooring line. The deck crew cannot see whether one spring is carrying 25 tonnes, 45 tonnes or approaching the render point. They judge by catenary, lead, sound, movement, weather and experience.
If the paper chain is wrong, the crew is operating inside a false margin. The brake may be marked, but against the wrong reference. The LMP may exist, but the deck operation may not reflect the assumptions behind it.
The MAIB investigation into the LNG carrier Zarga shows why this distinction matters. The case was not simply about a parted line. The investigation identified that the vessel’s mooring lines were not fit for purpose, did not have the specified minimum breaking strength, were not compatible with the vessel’s mooring deck fittings, and that the required WLL was too high.
It was a system problem before it became a line failure.
DeepDraft View
MEG-4 exposed what the old use of MBL had allowed ships to carry for years: one word serving too many purposes.
For older vessels, the difficulty is real. The original drawings may not say SDMBL. The EN may have to be traced. The company may have reconstructed the value from class records or structural review. None of that is a problem if the route is documented and understood. The problem begins when the number appears in the LMP without anyone onboard being able to explain its origin.
The ship must establish one governing design basis and connect every mooring number back to it. The line certificate confirms suitability. The WLL defines the operating ceiling. The brake render point provides the safety valve. The LMP should connect all of it into a usable deck document.
If the Master, superintendent, inspector and deck officer cannot arrive at the same number, the mooring system is not properly understood. When that happens, the system is only waiting for load to expose the weakness.
This is the same operating-culture problem raised in Standing Watch vs Seated Bridge: design intent only matters when shipboard practice actually follows it.
The line does not care what term is written in the file. It only responds to tension.
Media Section
Sources
- OCIMF, Mooring Equipment Guidelines, Fourth Edition (MEG-4)
- IMO, SOLAS regulation II-1/3-8 and MSC.1/Circ.1620, inspection and maintenance of mooring equipment including lines
- OCIMF, SIRE 2.0 programme guidance
- RightShip, Mooring Safety Insights
- MAIB, Failure of mooring line on board LNG carrier Zarga
- Relevant class guidance on mooring design basis, Equipment Number and Ship Design MBL traceability
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