Design and Implementation of the First Buoyed Steel Catenary Risers

Abstract

Abstract This paper presents the design and implementation of buoyed steel catenary risers (SCR). Two 6" steel catenary risers, installed in November-December 2001, were designed to include 50,000-pounds of buoyancy per riser in 3,300 feet of water for Mariner Energy, Inc.'s King Kong/Yosemite field development in the Green Canyon area of the Gulf of Mexico. The risers are suspended from the Agip Petroleum "Allegheny" tension leg platform (TLP) where reduced riser loads were required for additional process equipment and safe TLP operation. Introduction The King Kong/Yosemite development consists of three subsea gas wells located in Green Canyon Blocks 472, 473 and 516 in water depths of 3,800 - 3,900 feet. The subsea wells are commingled at a manifold located in Green Canyon block 472 into two 7" production flowlines. These 7" flowlines transit approximately 15 miles to Green Canyon Block 254, in 3,300-foot water depth, where they transition to 6" SCRs to board the Agip Petroleum "Allegheny" mini Tension Leg Platform, (TLP). The field layout is pictured in Figure A-1. The riser locations near the platform are illustrated in Figure A-2. During the course of the riser design, and after order of riser pipe and titanium stress joints, it was discovered that the weight of additional process equipment and the planned SCRs placed the Allegheny TLP beyond its allowable load capacity. In order to maintain a safe load budget for the TLP and to account for future process equipment addition it was decided to reduce the total TLP load by 100-kips via buoyancy on the risers. The challenge involved making a buoyed riser system work within the constraints of a non-buoyed, simple catenary SCR. The buoyed riser design involved an iterative approach to optimize the riser and buoyancy to satisfy the requirements for design codes, S-Lay installation and project cost and schedule constraints. The position and characteristics of the buoyancy were important to produce a riser catenary that met project requirements and code stress and fatigue criteria. In the final configuration, each riser utilizes 271 separate, surface installed buoyancy modules applied over a continuous length of 800-feet, starting below the VIV strakes, to provide 50-kips of net buoyancy plus a 5% safety factor. The resultant riser has an arc length of 3,532-feet with a horizontal offset of 1028-feet at a departure angle of 8°. Figure A-3 illustrates the final riser configuration. The final profile of the riser is a shallow "S" exhibiting a slight double catenary. Analysis shows that, compared to a conventional SCR, the buoyed riser exhibits increased bending and dynamic stress in the sagbend. This results from the "lighter" SCR motion characteristics and the departure from the second catenary, best illustrated as a shorter horizontal offset to touchdown than conventional simple catenary SCRs. Riser stresses are managed by placement, distribution and size of buoyancy modules. Execution of this project demonstrates the viability of the buoyed SCR for deepwater developments and how this concept can be used to expand or enhance the capacity of floating production systems. This paper presents the design and implementation of the buoyed steel catenary risers (SCR) for the Mariner Energy, Inc., King Kong/Yosemite field development. Included is an overall description of the riser key parameters; design including static, dynamic, vortex induced and wave fatigue analyses; and installation experience. In the description of the static analysis, comparing and contrasting results of the original simple catenary SCR, the final buoyed SCR configuration and one of the iterative buoyed SCR configurations examined before selec