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ABS was selected as the classification society of record for Independence Hub because of its extensive experience in the Gulf of Mexico, its ability to act on behalf of the U.S. Coast Guard and its experience with polyester moorings

Describing the project as “massive,” Bob Major, ABS Project Manager, says the Independence Hub represents the third application of polyester/synthetic moorings in the Gulf of Mexico. The deep-draft semisubmersible hull with integrated topsides is classed by ABS as an A1 Floating Offshore Installation (FOI).

According to Major, in the case of the Independence Hub areas of emphasis included: ballast control redundancy, structural integrity and buoyancy after damage while also considering the unit’s compliance and readiness in terms of fire-protection, fire-fighting and life-saving equipment.

Mooring system design is one element that ABS reviews in the overall classification or certification process.  The Independence Hub features a 12-line taut polyester/chain mooring system connecting to 12 suction pilings. “Our challenge is to help operators find the safest, most effective and, in this case, lightest mooring system to match the range of environmental loads the installation will face,” he says.   

“Synthetic mooring lines are lighter weight than steel moorings in sea water, offering the deepwater market neutral buoyancy and a much lighter load on the overall platform system. As a result, operators can dedicate more space to payload, such as production equipment, and less to supporting the weight of the heavy steel strand,” added Major. 

The key considerations in choosing a mooring system are the environmental loads resulting from wind, waves and current. These factors pose increasing risks and technical challenges as activity moves farther offshore and into more hostile environments.  In the case of the Independence Hub, the unit has an 8,000 ft. water depth at platform, making it both the deepest offshore platform installed to date, and the deepest project using polyester moorings.   

For ABS reviewing this mooring configuration and its material was not new having gained the regulatory approval from the Minerals Management Service (MMS) and U.S. Coast Guard (USCG) for the first approval for the permanent use of polyester moorings in the Gulf of Mexico for BP’s Mad Dog Truss Spar.  However, because of the ultra deepwater depth and in light of recent hurricanes, a great deal of scrutiny was given to the mooring system.  ABS worked closely with the owner and designers on the acceptance of this for such deep waters. 

Prior to bringing acceptance of this technology into the Gulf of Mexico, ABS worked with offshore pioneer Petrobras in the use of synthetic rope for its taut-leg mooring system installed on the P-27 semisubmersible offshore Brazil in the Campos Basin in 1998.  One year later ABS was the first class society to issue the industry’s first Guidance Notes on the Application of Synthetic Ropes for Offshore Mooring.

The Guidance Notes address abrasion and wear and other fundamental industry concerns, including non-linear stiffness or behavior, minimum tension requirements, creep phenomenon and effective handling and storage of rope which is more than seven inches in diameter for current typical deepwater applications.