The development of dry-tree vessel technology and therninstallation of several Spar systems in increasingly deep waterrnin the Gulf of Mexico has led to the parallel evolution ofrnseveral designs of dry-tree risers, most of which rely on airrncans to provide top tension. These risers typically consist ofrnmultiple concentric pipes, with production risers configured asrnproduction tubing within a single or dual casing steel riserrnconstruction. The tension applied to the top of each pipe mustrnbe enough to prevent damage through buckling of either thernriser or tubing under all installation, operation and workoverrnconditions, while still providing a margin of safety variousrndamaged conditions. The sum of these tensions determine therncapacity of the tensioning system required for all relevantrnloading conditions, without overstressing the riser or tubing inrndesign sea states.rnTo evaluate the tension requirements and the distributionrnof tension between constituent pipes, a rational approach hasrnbeen developed and successfully applied to BP’s HornrnMountain Spar dual casing riser system, which at 5,423ftrnrepresents the deepest Spar riser system to date. The approachrnuses finite element multi-tube analysis to determine thernrelative elongation and load sharing between the tubing, innerrnriser and outer riser under all load conditions. Riser andrntubing elongation, internal fluid effects, seawater & riserrntemperature distributions under installation and operatingrnconditions are all considered as part of this approach. Thisrnapproach implicitly considers the 3-D riser and wellbayrngeometry and the wellhead elevation at each of the subsearnwell locations.rnAs part of installation planning, this approach has beenrnused to confirm the air can tensioning system capacity,rnchamber size and redundancy requirements; predict air canrnelevations and buoyancy requirements at each stage of riserrninstallation; and establish riser and tubing tensions along withrnrelative stretch requirements to land each string in the surfacernwellhead system to ensure the appropriate in-servicerndistributions of tensions for all service conditions.
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