SPECIAL REPORT: Independence Hub sees record SCR installation

April 23, 2007
The 20-in. steel catenary riser, connecting the Independence Hub (IHUB) production platform to the Independence Trail natural gas export pipeline, ranks as the largest and deepest such riser ever installed.

The 20-in. steel catenary riser, connecting the Independence Hub (IHUB) production platform to the Independence Trail natural gas export pipeline, ranks as the largest and deepest such riser ever installed. Beginning at the IHUB, on Mississippi Canyon Block 920 in water 8,000 ft deep, Independence Trail is also the deepest export pipeline installed to date.

Enterprise hired Heerema Marine Contractors Nederland BV for installation of the IHUB floating production facility, including all risers.

Both the export riser and IHUB’s production risers were in temporary storage on the seabed near IHUB when Heerema began work in first-quarter 2007, having previously been installed by Allseas’ Solitaire. The export riser lay parallel to one of the production risers but on top of four others and under two. Production risers are 8 and 10-in. OD.

This article outlines the Independence Hub project and details installation of the export SCR.

IHUB

Enterprise announced Mar. 8, 2007, that IHUB had been successfully installed and had started earning demand revenues. Independence Hub LLC, a venture of Enterprise Field Services LLC (80%) and Helix Energy Solutions (20%), owns IHUB. Anadarko Petroleum Corp. will operate processing on the hub on behalf of the gas field owners that include Anadarko, Dominion Exploration & Production, Devon Energy Corp., Hydro Gulf of Mexico, and Murphy Oil Corp. Production from the fields served by IHUB is to begin in second-half 2007.

IHUB is the deepest offshore platform ever installed and, at 1 bcfd, also has the largest production capacity, representing a 10% increase in current natural gas deliveries from the Gulf, according to Heerema.

The Independence Trail Natural Gas Pipeline is a wholly owned affiliate of Enterprise Products Partners LP. The 134 mile, 24-in. pipeline connects Independence Hub to an interconnect with Tennessee Gas Pipeline on West Delta Block 68 (Fig. 1).

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Allseas Solitaire completed S-lay of the pipeline in August 2006.

The pipeline has a maximum operating pressure of 3,640 psi and a capacity of 1 bcfd. It has two subsea dual tees of 16-in. and 12-in. in water depths of about 6,500 and 4,500 ft, respectively, for future tie ins.

The 24-in. line consists of API 5L X-65 DSAW (double submerged arc welded) pipe coated with 14-16 mils thin film fusion-bonded epoxy (FBE) and an additional 2-3 mils rough coat FBE (OGJ, Nov. 27, 2006, p. 43).

The export SCR has a 1.21-in. WT, 9,000 ft of vortex induced vibration strakes, a dry recovery weight of 460 tonnes, and a wet recovery weight of 900 tonnes.

Equipment

Heerema used its Deepwater Construction Vessel (DCV) Balder for the IHUB installation. This section focuses on equipment Balder used while installing the export SCR.

Built as semisubmersible crane vessel in 1978, Balder completed conversion into a Class III dynamic positioning system DCV in 2001.

Heerema Marine Contractors used DCV Balder’s starboard crane (right) during cross haul and retrieval of Independence Hub’s 20-in. steel catenary export riser. The starboard crane includes two traction systems, each with 19 km of steel wire, allowing Balder to manipulate heavy structures at great water depths. Heerema used Balder’s 310-tonne capacity mooring line deployment winch (MDLW) for spooling the SCR’s cross-haul wire. The yellow piping and structural components in the background are on the topsides of the Independence Hub platform (Fig. 2). Photo by Christopher E. Smith.
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Balder has an overall length of 522 ft, overall breadth of 371 ft, and gross tonnage of 48,511 tons. Its Class III DPS produces 350 tonnes thrust using nine thrusters (2 × 4,400 kw, controllable pitch; 7 × 3,500 kw, 360° azimuth) with seven independent engine rooms. The system can be operated manually, via joystick, autopilot, or in full dynamic-positiong mode. Special DP functions include:

  • Track follow.
  • Heavy lift.
  • Follow floating object.
  • External force compensation.
  • Pipelay.

Balder also features:

  • Two heavy-lift cranes with a combined capacity of 6,300 tonnes at 110-ft radius. The starboard crane includes two traction systems, each with 19 km of steel wire, allowing Balder to manipulate heavy structures at great water depths (e.g., 500-tonne hook load in water 2,200 m deep, 250-tonne hook load in water 3,000 m deep).
  • Mooring line deployment winch (MLDW) with a capacity of 310 tonnes (Fig. 2).
  • Abandonment and recovery (A&R) winch system with a hoisting capacity of 650 tonnes. The MLDW winch capacity can be added to this if needed. Part of the vessel’s J-lay tower on its port side, the A&R winch uses 3,150 m of 138-mm wire (Fig. 3).

SCR installation

Balder’s work installing the export SCR involved four steps (Fig. 4):

1. Pickup. Balder used its A&R winch to recover the export SCR from the seabed. The A&R wire attached to a deepwater buoy on the SCR’s head, with Balder on the south side of IHUB.

After attachment, Balder moved to a 2,200-m distance from IHUB, creating a catenary which raised the SCR’s head 25 m above the seafloor. Balder then returned to station next to IHUB; stopping at a distance sufficient to execute cross haul of the SCR under IHUB, while staying within the weight limits of the A&R winch system.

Balder’s abandonment and recovery (A&R) winch system, incorporating the ship’s J-lay tower (right center), picked up the export SCR from the seabed and also participated in cross haul and retrieval. During retrieval, Balder’s portside crane (left center) slewed over the J-lay tower, lowering its 1,000-tonne auxiliary block to the SCR head for connection by ROV before moving the riser head to a work deck at the vessel’s stern (Fig. 3). Photo by Christopher E. Smith.
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2. Cross haul. Balder’s starboard crane picked up the cross-haul wire (previously installed and attached to IHUB using the MLDW) from the A&R winch in water 532 m deep and then transferred the SCR underneath IHUB’s hull from its south side to its north side. The A&R winch first paid out enough wire to make the transfer and then released after all load had been transferred to the cross-haul wire.

3. Retrieval. After cross haul, Balder reconnected the A&R winch to the SCR head for retrieval. Paying in the A&R winch returned the load to it, with the release of a hydraulic shackle swinging the SCR toward IHUB.

Before retrieving the cross-haul wire from IHUB with its starboard crane, Balder returned to a 450-m standoff distance to avoid interfering with mooring lines. During this process, the A&R winch paid in until the SCR reached water 135 m deep.

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While retrieval was under way, the portside crane positioned itself over the J-lay tower for connection to the SCR. Once Balder slewed the portside crane to its stern and verified connection, the A&R connection was paid out and disconnected, transferring the full load to the portside crane.

After moving to a point 625 m from IHUB, while at the same time raising the SCR to deck level, Balder prepared the export riser for installation on a work station at its stern. Preparation included:

  • Removal of flex joint protection cover.
  • Removal of hang-off collar protection.
  • Removal of flex joint laydown tool.
  • Installation of pull-in wire forerunner.

4. Installation. Once preparation work was completed, the Balder positioned itself on IHUB’s starboard side for final installation of the export SCR, using its starboard crane to pick up the pull-in wire from IHUB and transfer it to the on-board work station.

Lowering the SCR head and tensioning the IHUB’s pull-in wire moved the IHUB toward Balder, with the follow-target mode of the DCV’s DPS maintaining a safe distance between the two.

Balder’s portside crane lowered the SCR’s flex joint into its porch located underwater on one of IHUB’s 38 × 26-ft pontoons, transferring the load from the crane to IHUB where final connection of the SCR spool piece took place.

Acknowledgments

The author acknowledges the help of Heerema Marine Contractors Nederland BV and John Bouwman, senior project engineer.