Rational Approach to Performance-Based Design for Mooring Loads on Marine Structures

Abstract: The ASCE COPRI committee on the Design of Piers and Wharves for Non-Seismic Loads is currently developing a new standard intended to address loads specific to marine structures that are not addressed in ASCE 7 or ASCE 61. Two of the most common load types that the other ASCE standards do not cover are the design for mooring and berthing loads, which are transferred to the structure via specialized hardware attachments. These loads depend on vessels calling at a facility as well as the environmental conditions under which those vessels may arrive or stay at the facility, making them complex loads to determine and parameterize.
One goal that the committee has set out to incorporate into the design for mooring and berthing anchorages and structures is the inclusion of a performance-based approach for these analysis methods. Performance-based evaluations have become common in seismic design and involve determining performance objectives that provide one or more combinations of design events and allowable damage levels. This paper intends to address the philosophy of how a performance-based approach may be applied to the design of marine structures for mooring loads, with the intent that this approach may be applied in the new standard.
Mooring loads are transferred from the vessel at berth to the marine structure as tension from the mooring lines onto hardware fixed on the structure. Various other criteria documents govern the vessel lines and their arrangement, but typically take an approach predicated on hierarchical component failure and calculated based on the Minimum Breaking Load (MBL) of onboard lines and/or loads determined from site-specific mooring analysis (“lean-on” loads). The mooring hardware is selected based on operational needs and typically has an associated Safe Working Load (SWL) capacity, which is permanently associated with the hardware. The paper will focus on rationalizing the design of the hardware anchorage and supporting structure to ensure that a desirable precedent of failure modes is built into the design, such that the final performance of the overall system meets the owner’s expectations. Additionally, the proposed methodology determines loads based on the SWL and/or mooring analysis lean-on loads, while upholding the failure hierarchy principle. Consideration is given to real-world damage observed from historic incidents, failure modes, energy and reaction absorption influences, repairability of system components, risk acceptance based on structural use/occupancy, and economic impacts at various damage levels for system components. The goal is to provide a generalized approach that may be further statistically evaluated for the determination of specific load factor determination to be incorporated into the new standard.

How does this project / topic contribute to the advancement of the industry and profession?: As part of the new Piers and Wharves standard, this paper will provide a philosophical approach for performance based design marine structures for mooring loads. To date there has not been a formal design method in ASCE documents for such an approach and this paper is intended to aid the working committee in selection of an appropriate design philosophy.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The topic introduces a novel approach to the design of mooring structures which incorporates a performance based design philosophy.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect is identifying the most applicable modes of failure, their associated risks and possible occurrence, and to thereby develop a rational precedence for desirable failure modes which can be incorporated into a performance based design approach.

Who is the target audience for this paper?: Designers, regulators, and owners