Abstract

The Banjo oil and Seahawk gas export risers are the first dynamic SCRs to be installed on truss spars and hosted by Jtubes. Each of the two truss spars, located at the Boomvang and Nansen fields in 3,440 ft and 3,670 ft of water depth, respectively, in the Gulf of Mexico, has one oil and one gas export SCR. This paper presents the detailed, post-lay, SCR installation method evaluations for these field developments, from vessel requirement, SCR pick-up, keelhaul under the spar mooring, positioning to J-tube bell-mouth, and pull-in to its hang-off position.

Introduction

The export systems of Boomvang and Nansen fields in East Breaks Block EB643 at 3,440 ft of water depth and Block EB602 at 3,670 ft of water depth, respectively, consist of one oil export line and one gas export line for each field. A truss spars at each field serves as the surface floating production system. SCRs (Steel Catenary Risers) connect the subsea pipelines to the topsides. Cantilevered J-tubes for the SCRs serve the function commonly accomplished by a flexjoint or stress joint, mitigating the bending moment at the support of the SCRs. All four SCRs employ such cantilevered J-tubes that extend from the bottom of the hard tank at elevation (-) 160 ft to the keel-tank with a bell-mouth exit at elevation (-) 507 ft. The SCRs, extended from the bottom of the hard tank where the J-tube ended and up through the moon-pool, hangoff at elevation (+) 51 ft with anchor flanges supported by split collars. All SCRs have a nominal departure angle of 14 degrees as their host J-tubes. For the Boomvang spar, the oil and gas export SCRs use 16-inch nominal diameter pipe with 0.812-inch wall thickness and 18-inch nominal diameter pipe with 1-inch wall thickness, respectively. For the Nansen spar, both the oil and gas export SCRs employ 12-inch nominal diameter pipe with 0.562-inch wall thickness. The wall thicknesses for the SCRs are controlled by the reel-lay method not mechanical integrity. The reel-lay method is selected for the pipe-laying of all four export pipelines and their SCRs. All SCR pipes are grade X65 seamless. The J-tube insider diameters are 19-inch, 23-inch, and 25-inch for the 12-inch, 16-inch, and 18-inch SCRs, respectively.

Due to the field development schedule, all four export SCRs will be laid prior to the arrival of the Boomvang and Nansen spars. After the spars are placed in position, mooring lines attached, the SCRs are then picked up and installed utilizing construction vessels. Two technical challenges exist in the installation of these SCRs. First, each SCR will need to be picked up by the support vessel, keelhauled through three spar mooring lines to reach the SCR hang-off side of the spar. Second, the SCRs on the truss spars are hosted by J-tubes, which extends from the bottom of the hard tank to the keel tank.

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