Matterhorn TLP Dry-Tree Production Risers
Authors: Jordan R. , Otten J. , Trent D. , Cao P. 
Published: October 2008

Abstract
This paper describes the design, configuration, and in-service operation of the Matterhorn top tensioned, direct vertical access (DVA) production risers. This riser system represents the first application of DVA risers on a mono-column tension leg platform (TLP). The system incorporates many new features, including being the first system to be run with preinstalled VIV suppression fairings designed for the "cold core eddy" (CCE) submerged current phenomenon.

Introduction
The Total Matterhorn project is located in the Gulf of Mexico in Mississippi Canyon block 243. The platform has nine slots for DVA risers and is located in 2,820 ft. of water. The project was planned to utilize seven (7) risers and they have all been installed. Completion operations are on going and the platform is currently producing. The platform has processing facilities for 33,000 BOPD, 55 MMSCFD natural gas, 20,000 BWPD produced water, and 30,000 BWPD injection water. Production from the platform is carried away by steel catenary risers (SCR) 8" oil, 10" gas, and 6" water. The platform also provides five (5) porches for future connections of subsea tiebacks. Additional information on the platform can be found in OTC 16611 (Hurel, et al, 2004).
The DVA riser system was designed, procured and constructed by Atlantia Offshore Limited. The risers were installed by Total with Atlantia's assistance. The subsea wellheads were pre-drilled from a semi-submersible drilling rig before platform installation.
The purpose of this paper is to discuss the design, configuration, and new hardware developments that have contributed to the successful execution of the Matterhorn project.

Well System Overview
The Matterhorn project was designed to provide risers for one high pressure gas well, one gas well, one water injection well, and four oil production wells. The oil wells were also to be designed for gas lift. As the Matterhorn platform is a monocolumn TLP the inside diameter of the moon pool affects the overall size, and consequently the cost of the TLP hull. It was therefore a design objective to provide a compact and efficient use of the production deck space. For Matterhorn a 10'x10' spacing with a 3 × 3 grid was selected for the nine wellbays. With this arrangement, a 36' diameter moonpool was used.
To optimize the riser design, it was decided to provide three different nominal riser sizes: 10 3/4", 9 7/8", and 9 5/8". The riser stack-up is shown in Figure 8, and is essentially the same for each riser size. Major components of the riser system are:Tieback connector (TBC)Tapered stress joint (TSJ)Lower cross-over jointAnode jointsStandard riser jointsFairing riser jointsUpper cross-over jointsKeel jointsSplash zone jointsTension jointsProduction Riser Tensioner (PRT)Top of hull and production deck roller assemblies During the design process, it was determined that common top and bottom joints could be used on each of the riser strings. This enabled quantities of the critical special joints to be increased, and therefore costs and spare parts interchangeability were improved.