Umbilical Design and Installation for Mustique and Shasta Tie-backs
Authors: Hibbs D.A. , Cunningham M.T. , German Norb 
Published: October 2008

Abstract
The design and installation of control umbilicals for the Mustique and Shasta subsea satellite well tiebacks are reviewed with respect to how cost-effective umbilical termination assembly (UTA) designs can be achieved while also providing fill control system sparing and good installation flexibility. The sparing philosophy of the elcctrohydraulic control system as a whole from the platforms to the tree control pods is discussed. This approach lcd to the selection of a bottom connection method for the umbilicals by atmospheric diving system (ADS) and thus made possible the use of a lay vessel which could be scheduled independently of the well completion and tree installation operations, The benefits and risks of requiring splice less umbilicals are discussed as well as their impact on manufacturing deliveries and installation schedules, The Mustique/Shasta umbilicals were an industry first with a single contractor responsible for the system planning, design, manufacture, installation, subsea connection and on-production performance guarantees. A key ingredient of this concept is the design of umbilical termination assemblies (UTAS) and ROV maintainable interconnections. This contracting approach offers significant benefits to the customer. The benefit of the sparing approach taken is illustrated by describing how two subsea failures were remedied without incurring operational downtime.

Introduction
The Mustique and Shasta subsea satellite wells are Iocated in the Gulf of Mexico. The Mustique well is located the 832 feet of water in Garden Banks 240 and is tied-back to tie Chevron platform in Block 236 over an offset distance of 11.5 miles, The well shut-in pressure is 7,500 psi, The Shasta wells are located in 860 and 1040 feet of water respectively in Green Canyon 136, These wells are tied-back to Texaco Platform No. 6. The offset distances of the wells are 9.5 and 11 miles from the platform (Fig 1). The engineering and installation project management of the work was organized by contractor's representatives seconded to a team working for Hardy Oil and Gas, the operator for the project. The installation method initially chosen for the umbilicals was the layaway technique. This method utilizes the umbilical lay vessel in combination with the drill rig used for well completion to install the tree with the control umbilical connected to the tree, This method is favored by some operators to ensure that the umbilical are correctly connected to the tree's control pod and that all the control functions arc confirmed operational.
Due the long offset distances for these wells, it was apparent that the control system of choice would be electro-hydraulic. Therefore it was important to choose an installation method that offered the maximum reliability in electrical connection, This was the key reason that initially the layaway installation method was preferred.
Several factors led to the consideration of an alternative installation method. These were based on the potential delivery constraints imposed by the umbilical vendor's manufacturing schedule and available installation equipment.