Mooring Design and Installation Considerations for the Mirage and Telemark Field Development

Abstract

The design and installation of the mooring system for the MinDOC deep draft floating platform for the Mirage and Telemark field development presented several unique challenges. This system was the first permanent mooring to be designed with post Hurricane Katrina and Rita metocean conditions. Another factor adding complexity and stringent performance requirements was the use of dry trees on the development. The combination of these requirements required an efficient, robust mooring design based on a polyester taut-leg system consisting of twelve moorings and driven piles. During the design stage, it was not known which vessel would install the mooring or perform the hookup to the hull. Therefore careful consideration was given to the type, size, and component weight, as well as the length of the polyester segments. This was done in an effort to maximize flexibility and allow the mooring to be installed by either an Anchor Handling Tug Supply (AHTS) vessel or a construction vessel. The mooring was designed to be installed in three (3) phases to increase flexibility during the offshore campaign. The advantages of this approach were that the project could install the mooring off the critical path, utilize smaller less expensive vessels for key phases, and reduce the time required for hookup. Over the last several years, the practice of prelaying polyester moorings on the seabed has been widely accepted and relatively common for MODUs in the Gulf of Mexico and around the world. However, approval for this practice was still unprecedented for a permanent mooring system. The approval process included a combination of field and laboratory testing to demonstrate that exposure to the seabed would not have a detrimental effect on the strength and fatigue characteristics of the polyester mooring ropes over the service life of the mooring. The ability to prelay the moorings on the seabed offered cost savings, risk reduction, and scheduling advantages to the project and allowed the project to maintain installation flexibility.