When the location and purpose of the Independence Hub project was established, it guaranteed that records would be set along the way to its completion (set for mid-2007). Independence Hub, located in approximately 8,000 ft (2,440 m) of water at Mississippi
|Independence Hub is seen from DCV Balder as SCR installation progresses. (Photo by Don Francis)
Canyon Block 920 in the Gulf of Mexico, is set among 10 natural gas fields, which flow to the platform and then to shore through one 24-in. export pipeline. The export pipeline from the Hub (called Independence Trail) will carry gas and condensate approximately 135 miles (217 km) to a fixed junction platform located in West Delta Block 68 in 110 ft (34 m) of water. Gas and condensate will be brought to shore from the junction platform by tie-in to the Tennessee Gas pipeline for processing at Port Sulphur, La.
The scale of the project is remarkable. The field developments connecting to the Hub cover an area of about 1,800 sq miles (4,662 sq km). The umbilicals contain approximately 1,000 miles (1,609 km) of stainless-steel tubing, which is roughly the distance between Houston and Denver. A good measure of the project’s scale is its planned result — delivery of 1 Bcf/d to US consumers. This is an increase of approximately 2% to the US supply and a 10% increase in the natural gas supply from the Gulf of Mexico.
A catalog of records is associated with the project. The platform itself is the world’s deepest, and it is the Gulf of Mexico’s largest gas processing facility. It is held in position by the world’s longest mooring lines at 2.4 miles (3.8 km) each, which are connected to the world’s deepest suction pile installations. The project produced the world’s deepest subsea production tree, flowline installation, steel catenary riser (SCR) installation, export pipeline and subsea umbilical order.
Structures in extreme water depths, especially if they are connected to many pipelines, require special attention to the weight of the SCRs. And minimizing the fatigue of the risers caused by the platform’s motion is a critical design goal.
One of the project’s many challenges was to design a floating facility that could adequately manage the extreme weights that 8,000-ft water depths would impose on the platform through the SCRs. Modeling and tank tests were used to determine how the Hub would move. This information was then provided to the pipeline designers to determine fatigue life for the SCRs. Design fatigue life is 30 years, but a safety factor of roughly 10 makes the actual time to failure much longer than the expected life of the facility.
Initially, there is capacity for 19 SCRs to be hung off the platform, including the 20-in. for the export line, 12 10-in. flow line risers and four 8-in. flow line risers. More SCRs can be added if new flow lines are required for future discoveries; the platform has enough excess payload capacity to tie back up to 10 additional fields.
Mooring lines, flow lines and risers
Heerema Marine Contractors was awarded the contract for the installation of the Hub, mooring piles, mooring lines and SCR in April 2005 and began the work in March 2006. The company applied technology based on using remotely operated vehicles (ROVs) on which suction pile pumps are mounted and the Balder Mooring Line Deployment Winch (MLDW).
The mooring piles for the platform’s 12 mooring lines are the world’s deepest permanent mooring piles. The 18-ft by 88-ft (5.49-m by 27-m) suction piles, with a dry weight of 180 metric tons, were installed at a water depth of 8,011 ft (2,442 m) using the Deepwater Construction Vessel (DCV) Balder.
DCV Balder was used for several projects. One such project — SCR connections — is in progress as of this writing. There are eight in total; seven are flow lines and the eighth is the export SCR (Independence Trail).
The Balder is one of the world’s largest offshore construction vessels, measuring 449 ft (L) by 282 ft (W) by 137.8 ft (D) [137 m (L) by 86 m (W) by 42 m (D)]. It has a tandem lift capacity of 6,300 metric tons. The vessel has the world’s largest capacity J-lay tower for deepwater pipelaying.
SCR connections in this water depth posed unique challenges. “I’ve been working offshore all my adult life, and if you had asked me 10 years ago if [connecting SCRs in 8,000 ft of water] could be done, I would have said the technical challenges to accomplish that goal would be very significant and overwhelming,” said Mike Lemker, construction superintendent, deepwater Gulf of Mexico for Anadarko Petroleum.
Heerema proposed a modification to the Balder’s pipelay system at the onset of the project, which was agreed to and completed as the project was underway. The company also installed (and welded) two of the 8-in. SCRs that the vessel is picking up and hanging off. Yet another challenge was laying pipe in significant current, which ran about 2 to 2.5 knots during installation.
Weld quality for fatigue life at this depth was a major challenge. It took 3 months to qualify welding procedures for the project.
Balder performed another difficult operation by installing two flow lines from subsea manifolds to subsea satellite wells that were shorter than the water depth. The first was in the Spiderman field, where the vessel installed a flow line 3,270-ft (997-m) long in a water depth of 8,080 ft (2,464 m). The second was in the Jubilee field where a 6,440-ft (1,964-m) long flow line in a water depth of 8,730 ft (2,662 m) was installed.
In all, approximately 220 miles (353 km) of flow lines and SCRs were installed for this project. Each of the seven dynamic SCRs has a different combination of strakes and fairings. On average, each of these SCRs has approximately 3,600 ft (1,098 m) of fairings and 4,000 ft (1,200 m) of strakes. Installing these strakes and fairings in an efficient manner was also a major challenge that was successfully met.
“The symphonic cross-haul and hangoff execution of all the SCRs was a masterful piece of engineering in my mind,” Lemker said. The SCRs were laid out ahead of time, before the Independence Hub hull’s arrival. They had to be picked up and moved to the correct side of the platform to permanently connect them.
In the face of all these challenges, it was especially gratifying to all involved that it was done quickly. From beginning of contract negotiation to execution spanned only about 18 months. “To do this, to build all the hardware, with all the long lead-time items we supplied them, is something the whole team is proud of,” Lemker added.
To underscore the fact that nearly everything done to complete this project was an accomplishment, the subsea architecture — in 8,000 ft of water — was successfully laid using a 20-ft by 20-ft (6.1-m by 6.1-m) target box within 5° of level.
As with all records, these will eventually be broken, but for now, Independence Hub is a remarkable achievement of ingenuity and teamwork that makes an enormous contribution to the industry’s knowledge base and the nation’s energy supply.