Horn Mountain Spar Risers - Evaluation of Tension and Installation Requirements for Deepwater Dry Tree Risers
Authors: O&apos;Sullivan E.J. , Shilling R.B. , Connaire A.D. , Smith F.W.A. 
Published: October 2008

The development of dry-tree vessel technology and the installation of several Spar systems in increasingly deep water in the Gulf of Mexico has led to the parallel evolution of several designs of dry-tree risers, most of which rely on air cans to provide top tension. These risers typically consist of multiple concentric pipes, with production risers configured as production tubing within a single or dual casing steel riser construction. The tension applied to the top of each pipe must be enough to prevent damage through buckling of either the riser or tubing under all installation, operation and workover conditions, while still providing a margin of safety various damaged conditions. The sum of these tensions determine the capacity of the tensioning system required for all relevant loading conditions, without overstressing the riser or tubing in design sea states.,

To evaluate the tension requirements and the distribution of tension between constituent pipes, a rational approach has been developed and successfully applied to BP's Horn Mountain Spar dual casing riser system, which at 5,423ft represents the deepest Spar riser system to date. The approach uses finite element multi-tube analysis to determine the relative elongation and load sharing between the tubing, inner riser and outer riser under all load conditions. Riser and tubing elongation, internal fluid effects, seawater & riser temperature distributions under installation and operating conditions are all considered as part of this approach. This approach implicitly considers the 3-D riser and wellbay geometry and the wellhead elevation at each of the subsea well locations.

As part of installation planning, this approach has been used to confirm the air can tensioning system capacity, chamber size and redundancy requirements; predict air can elevations and buoyancy requirements at each stage of riser installation; and establish riser and tubing tensions along with relative stretch requirements to land each string in the surface wellhead system to ensure the appropriate in-service distributions of tensions for all service conditions.

Horn Mountain Spar

The Horn Mountain field is located 84 miles from Venice, Louisiana offshore in Mississippi Canyon Block 126 and 127 in a water depth of 5,423ft. This represents the deepest dry tree production riser installation to-date. Leases for these blocks are held 67% by BP, and the remaining 33% by OXY USA, Inc. with BP as the Operator.

The Horn Mountain field is being developed with a Truss Spar production facility. The 106ft × 555ft Truss Spar accommodates a temporary completion rig that has robust sidetrack and workover capabilities, and lightweight production facilities that allow for 4,700ton single piece deck lift. A schematic and picture of the Spar are shown in Figure 1. The wellbay, which is 52ft × 52ft in overall dimension, is in a 4 × 4 type configuration and accommodates 14 production top tensioned riser (TTR) slots and 2 import steel catenary riser (SCR) well slots. The seafloor layout was optimized for pullover drilling as a mirror image of the wellbay with 50ft between adjacent wells as indicated in Figure 2.