Low load compliant mooring history and status update

The National Data Buoy Center (NDBC) has traditionally deployed three core mooring types in its Weather Buoy Program. These consist of an all-chain mooring, semi-taut mooring, and an inverse-catenary mooring. The all-chain moorings are comprised completely of chain and form a catenary in the water column. These moorings are used in shallow water only, due to the weight of the chain on the buoy, and derive their compliance from the access chain. The inverse-catenary moorings are comprised almost entirely of synthetic line with a buoyant section in the middle mooring to produce an "S" shape in the water column. The buoyant section is created using either a buoyant line such as polypropylene or through the use of mooring floats. These moorings derive their compliance from the excess line that forms the "S" shape and are used exclusively in deep water. The semi-taut moorings are similar to all-chain moorings in composition but use a section of nylon line in the middle mooring to reduce the dead weight of the mooring. These moorings are used in moderate depths. In 2004, NDBC conducted a study of the previous six years of mooring failures from these configurations (a total of 75) and determined that the majority of the known modes of failure could be distilled into three main categories; cyclic fatigue of connection hardware, chafe zone erosion, and fish bite/vandalism. From this information, a design effort was conducted to eliminate or mitigate these failures to improve overall network mooring performance. The result has been termed a low-load compliant mooring. The performance of this mooring is similar in a hydrodynamic manner to a traditional inverse-catenary mooring but with the capability to be deployed in all but the most shallow locations. The preliminary mooring model analysis indicated that the mooring loads would be significantly less than existing configurations, which would have a major impact on cyclic fatigue along with the selection of synthetic connection hardware. The chafe zone was completely removed via the design, and the fish-bite problem was mitigated through the design of a fish-bite-resistant line. Since the initial design, 32 stations have converted to the low load compliant design with only three failures to date, all which have been attributed to vandalism. This paper will discuss the design and modeling details more thoroughly and provide performance data for the deployed moorings, including observed sea conditions.