Stephen Paul Armstrong

S-Lay Installation of Riser Fairings for the Independence Hub

One of the challenges of insuring adequate fatigue life of marine risers due to vortex-induced vibrations (“VIV”) is the installation of VIV suppression devices in a cost effective manner. In the past, short fairings for production risers have been restricted to J-lay or Reel-Lay installations, where the fairings are installed either before or during pipelay, or retrofit installations where the fairings are installed underwater, using specially designed tooling and Remotely-Operated Vehicles, after the riser has already been installed. Both of these options can be expensive by either restricting the vessels that can install the risers (in the case of J-lay installation) or through expensive rental of ROV and associated equipment for retrofit installation. S-lay installation of fairings is particularly challenging due to the shape of the fairings and the need to install thrust collars to keep the fairings in their axial position once they are installed. Additional complexity is added by the requirement that the fairings be able to withstand mud forces from the temporary placement of the riser on the seabed. These challenges were addressed for the Independence Hub project, which required fairings for both adequate fatigue life due to VIV and for thermal heat transfer considerations. An S-lay testing machine was developed for testing fairings experiencing roller loads up to 115 kips. In addition, wave tank tests were performed to insure that the fairings would stay in proper position during S-lay while experiencing wave forces. Upon successful test results, the S-lay fairings were installed on the Spiderman East, Spiderman West, Jubilee East, Jubilee West, and Merganser flowline risers for the Independence Hub project. These flowline installations are the deepest so far in the world. This required close collaboration by all personnel involved to meet the challenges of insuring success for this type of installation. This paper discusses the methodology used to produce successful S-lay fairing installations for the Independence Hub risers. Presented are test results from the roller and wave tank tests, and a discussion on how the final design was derived. Finally, field experiences from the offshore installations are discussed. This paper should be of interest to engineers involved in the design and installation of deepwater risers.Copyright