Establishing Rational Load Factors for Mooring Loads

Abstract: Current codes and standards for maritime works in the US take a simple, yet confusing, approach to assigning load factors for mooring loads. A common approach is to assign a single mooring load factor, while mooring loads are comprised of several different, often independent loads. The most common individual loads are wind loads, current loads, wave loads, and passing vessel loads. Similarly, there is often little or no guidance on what magnitude of the mooring loads the load factor should be applied to. As an example, MOTEMS in California specifies a Departure wind speed with a 25-year return period for design of mooring points, but offers little guidance for selection of a design current, or how the two loads should be combined.

The development of LRFD in the United States and the most recent versions of ASCE 7 offer clear guidelines for how to combine simultaneous loads. The common practice is to combine a 50-year return period load with what is effectively an average point-in-time load, resulting in a higher load factor for the primary load, and a much lower load factor for a contributing load.

For wind loads, which often can be the dominating contributor to mooring loads, a load factor of 1.6 was in earlier versions of ASCE 7 applied to a 50-year return period wind. Additional factors would account for the Importance Category and the Exposure Category at the site in question. Also included is an additional factor of 0.85 which is applied to the loads to account for the fact that the design wind speed will not always come from the worst direction. Accounting for the directionality factor, the effective load factor for wind loads is, therefore, between 1.3 and 1.4.

One can easily assume that a ship’s mooring is a much more directional system than a building, which would lead to a directionality factor of less than 0.85. If a mooring analysis is run with a sweep of wind speeds from all directions, the assumption is already made that the worst wind comes from the worst direction.

This paper will, therefore, attempt to explore what an appropriate load factor is for wind loads on a moored vessel., and how this load should be combined with loads from currents, waves, and passing vessels.

How does this project / topic contribute to the advancement of the industry and profession?: This research contributes to current work by the Mooring and Berthing Subcommittee developing LRFD load factors in the upcoming ASCE/COPRI Piers and Wharves Design Standard. This research will:

• Give clear guidance on appropriate load factors for the different components of mooring loads
• Give a better understanding on how the wind component of mooring loads is affected by the highly directional nature of the loads

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: This topic implements the authors understanding of the underlying statistics behind wind, current, and wave loads. The combination of these has, as far as the authors know, not been implemented in codes and standards yet.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The greatest challenge was to understand how the directionality factor was developed for buildings and then try to implement this knowledge to mooring loads. It was also very challenging to derive how all the load factors should be adjusted when considering operational loads vs extreme loads, especially with respect to directionality.

Who is the target audience for this paper?: The target audience of this paper includes both developers of codes and standards, as well as practitioners who design mooring structures. The methodology proposed in this paper may also apply to adjacent structural disciplines that use LRFD load factors, especially related to wind loads.