Ship Design Minimum Breaking Load (SDMBL)
SHIP DESIGN MINIMUM BREAKING LOAD (SDMBL)
The Ship Design Minimum Breaking Load (SDMBL) is one of the cornerstone concepts introduced in OCIMF’s Mooring Equipment Guidelines, Fourth Edition (MEG4). It establishes the minimum strength value that forms the foundation for designing a vessel’s entire mooring system – including winches, bollards, fairleads, and mooring lines – to ensure that all components function as a balanced and safe system.
In simple terms, SDMBL represents the load capacity around which the ship’s mooring arrangement is designed. Every piece of equipment connected to the mooring line – from the drum end to the shore bollard – is expected to withstand stresses proportionate to the SDMBL. This creates a harmonized system that can absorb energy and distribute loads effectively during mooring operations, minimizing the risk of parting or equipment damage.
However, practical implementation of SDMBL has led to significant discussion and confusion within the maritime industry, particularly around the distinction between “Assigned” and “Determined” SDMBL.
* Assigned SDMBL refers to the value originally allocated at the ship design stage by the naval architect or shipyard. It is typically based on the vessel’s size, displacement, and intended service conditions, and is used as a fixed design parameter throughout the ship’s construction and class approval process.
* Determined SDMBL, on the other hand, refers to a value established later in the vessel’s life, usually when ship operators or owners conduct a reassessment in line with MEG4 requirements. This determination may be necessary when changing the type or construction of mooring lines (e.g., from wire to HMPE), after significant modifications affecting the mooring layout, or when verifying compliance during mooring system audits.
The distinction is critical. While the *assigned* SDMBL may serve as a theoretical baseline, the *determined* SDMBL reflects the vessel’s actual operational capability and ensures that the strength of the mooring lines corresponds correctly with the design capacity of shipboard fittings. Using ropes with higher or lower MBL than the equipment’s SDMBL can lead to imbalance – either overstressing the equipment or creating unsafe energy release conditions during mooring failures.
Thus, SDMBL is not merely a number on paper – it is a safety and compliance parameter that directly influences the integrity of the mooring arrangement, the longevity of equipment, and the safety of personnel involved in mooring operations.
In practice, achieving proper alignment between the *Design SDMBL*, *Line MBL*, and *Working Load Limits (WLL)* of fittings is one of the biggest challenges for operators aiming for full MEG4 compliance. It demands careful recordkeeping, documentation of equipment test certificates, and periodic reassessment of mooring line properties to ensure that the mooring system continues to perform as designed throughout the vessel’s service life.
By understanding and correctly applying the principles of SDMBL – whether assigned or determined – vessel operators can move beyond mere documentation and achieve a truly engineered approach to safe mooring operations.