Design and Manufacture of Couplings for the Hutton TLP Mooring System

ABSTRACT The components of the Hutton TLP tension legs are assembled using integral rotary shouldered couplings. This taper threaded coupling was purposed designed to withstand the severe fatigue and high static loads imposed by the environmental and pretension requirements. As no previous work had been undertaken on the design of high strength taper threaded couplings for this application a carefully coordinated design and manufacturing programmer had to be developed. The paper outlines the design and analysis techniques used, together with the testing performed and the results obtained. In addition, the task of ensuring that design requirements were met during manufacture and installation wi11 al so be discussed and solutions, in particular the purpose designed electronic gauging system, described. INTROUCTION Installation of the TLP at the Hutton Field was completed on the 15th July, 1984 with the mooring system's sixteen tension leg assemblies, four at each platform corner, connected to the seabed foundations. The two days of tension leg deployment was the final test for coupling computability when components from three different manufacturing sites were connected together for the first time. A total of 256 joints were "made up" to the prescribed design limits required to ensure satisfactory performance throughout the platform operating life. Proceeding this short installation period was a design process which lasted four years and encompassed extensive analysis and testing. During this time material requirements were pushed to extremes, new attitudes were required in production and new technology introduced into thread gauging practices to enable the manufacture of couplings in accordance with stringent performance requirements. In addition the assembly and make up requirements for this coupling were more demanding than that required for conventional drill collars. MOORING SYSTEM DEFINITION Each tension leg assembly (1) comprises seventeen components as shown in Figure 1. At the upper end of each leg is a tension adjusting assembly which transfers the leg load to the hull structure via the load block assembly. The load path extends down three tension 1eg elements to the crossload bearing which is located at the base of the tension leg shrouds. A further eleven tension leg elements extend below the crossload bearing terminating at the anchor connector which locks into the foundation template. A 75mm bore is provided down the centre of each tension leg to allow hydraulic operation of the anchor connector, this is filled with a corrosion inhibitor which also acts as a couplant for the in service NDE' system. Angular displacement of the tension legs due to the horizontal motion of the platform is accommodated by elastomeric flex joints in the anchor connector and cross load bearing. The seventeen components are connected by integral rotary shouldered screwed couplings as shown in Figure 2.