Red Hawk Polyester Mooring System Design And Verification (2005)

Abstract This paper presents the design and testing of the polyester mooring system for Kerr-McGee's Red Hawk platform installed in 5300 ft water depth at Garden Banks. A new test plan was developed and used to evaluate polyester rope stiffness for extreme loop-current conditions. A relationship was observed between the final installed rope length and the loop-current stiffness behavior of the lines. The loop-current stiffness is dependent on the amount of "construction stretch" removed during installation. Different rope designs and installation procedures are evaluated. Introduction Platform offset is an important driver for design of the risers supported on the platform. For the Red Hawk cell spar, the "missing one line case" in the extreme loop current event is the governing design condition for both maximum mooring line tensions and maximum platform offset. The loop current is characterized by steady loads acting on the platform for up to several weeks. The platform offset calculation requires knowledge of the polyester stiffness characteristics during a loop current event. Unfortunately, present design guidelines do not define polyester stiffness values relevant for a deep draft floater exposed to loop current. A new test plan was developed for Red Hawk and used to evaluate different rope designs and installation procedures. Future revisions of design guidelines should include a test method to standardize the static stiffness values used for loop current offset calculation. The final installed rope length depends on the "construction stretch" removed during installation. Higher installation loads result in more permanent stretch removal and longer rope lengths. Similarly, the loop-current stiffness is highly dependent on the amount of permanent stretch removed during installation. Higher stiffness values resulting in smaller platform offsets can be achieved by exposing the rope to higher installation loads, thereby removing as much permanent stretch as practical. The ABS and API modulus test procedures, Ref. [1] and [2], both begin by loading the rope to 55% of minimum breaking load (MBL). For typical mooring installations, the mooring lines are exposed to lower peak loads than 55% MBL. Subsequent measurements made on the rope sample loaded to 55% MBL will then overpredict the loop-current stiffness and the installed rope length. For Red Hawk, stretch was removed using an anchor handling vessel and a rigging that amplified the bollard pull. The mooring requirements dictated by riser design necessitated the use of a stiffer polyester yarn previously not used in offshore mooring rope constructions. Mooring System Configuration An overview of the Red Hawk project is given in Ref. [3]. The Red Hawk cell spar is moored in 5,300 ft water depth with six evenly spread taut legs comprising of chain-polyester-chain. Figure 1 shows the hull configuration with mooring lines and topside. Figure 1: General overview of the Red Hawk cell spar (Available in full paper) The configuration of the cell spar is efficiently suited for the connection of each leg of a six leg mooring system at the junction between each of the six outer cells, see Figure 1.