Beginnings of a Unified Approach to Mooring Design: Equipment SWL Selection. Review & Observations

Abstract: The design of mooring systems for vessels at piers and wharves involves calculating tension forces in lines and compression forces on fenders caused by external loads on the vessel under set environmental conditions. The results of a mooring analysis are used to ensure compliance with performance limits: fender compression forces below rated capacity and mooring line tensions below reduced breaking strength. Various organizations provide guidelines for equipment selection, but often, project-specific design requirements are necessary.

Certain design premises used in the selection of mooring equipment capacities, like the global failure hierarchy and local failure hierarchy, are widely accepted but need further evaluation. The global failure hierarchy suggests a progressive order of system failures, starting with winch brake render and ending with structural damage. The local failure hierarchy specifies that the hook's Safe Working Load (SWL) should exceed the line's Minimum Breaking Load (MBL) to ensure the line breaks before the hook fails.

Some guidelines have noted that the ideal global failure hierarchy can lead to overly conservative designs, as equipment capacities tend to exceed force demands obtained from analysis, especially with multiple-hook assemblies. Additionally, selecting equipment with SWL larger than the line's MBL may be overly conservative for vessels that carry unnecessarily large lines, with higher breaking loads than otherwise required by analysis.

The comparison between the line's MBL and the hook's SWL adopted by the local failure hierarchy may be overly conservative if the goal is to ensure progressive damage in accidental overload situations. A more reasonable approach, consistent with most structural design practices, would be to compare the hook and line ultimate capacities, or alternatively, compare their allowable capacities. Setting the hook's SWL equal to or larger than a portion of the line's MBL, representative of its working capacity, would remove some of the conservatism and continue to ensure progressive failure, as well as align with the design philosophy permitting damage in accidents while avoiding loss of life, structural integrity, and disproportionate consequences. This proposed approach aligns with guidelines from organizations like OCIMF, MOTEMs, and PIANC, which advocate for working load limits around 50% of MBL for mooring lines.

Although guidelines exist for equipment selection, project-specific design requirements are often necessary. While certain design premises like the global and local failure hierarchies are widely accepted, they may lead to conservative designs. Adjusting the SWL selection approach to align with the current design philosophy can ensure progressive failure while preventing severe consequences, as intended by current design guidelines.

How does this project / topic contribute to the advancement of the industry and profession?: The existing methods for mooring equipment selection vary significantly across the industry and are not necessarily consistent with all acceptable approaches to mooring design. The inconsistencies can be resolved and this paper provides guidelines for reassessing the equipment selection methodology so that practitioners and code authors can evaluate and/or adopt in the future

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The topic provides a new and innovative approach to an old problem by examining the objectives and previously assumed considerations around the mooring equipment selection rules

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: Understanding the origin of the seemingly contradictory requirements in current mooring equipment selection methods was the hardest part to overcome. Going back to first principles, reassessing goals and objectives, and looking for a wholesome approach to mooring design were the tools used to develop a new proposal.

Who is the target audience for this paper?: marine design practitioners, code and guidelines authors.