Extending floating production into ultra-deepwater

Operators are increasingly acquiring leases with water depths that are challenging the industry's technological limitations.

Operators are increasingly acquiring leases with water depths that are challenging the industry’s technological limitations. In theUS Gulf of Mexico, there are 88 active leases in water depths of 3,000 m (9,800 ft) or greater, and more acreage in this range will be up for bid next year in the GoM. For floating production systems, Shell’s Stones FPSO will set a new benchmark when it comes online in the GoM in about 2,900 m (9,500 ft) of water. Semisubmersible and spar-type production platforms are common in deepwater as well, but these designs are currently limited to about 2,500 m (8,200 ft) of water. The deepest water depth for a TLP is about 1,400 m (4,600 ft), and the conventional tendon design is a limiting factor.

The water depth progression for floating production facilities has somewhat plateaued since around 2007, signaling the need for a step-change in the technology. Pending the industry’s appetite for drilling ultra-deep wells in this market, is it possible to extend floating production facilities into water depths beyond 3,000 m?

New studies

Recently,Granherne conducted a study on the impact of extending wet-tree semisubmersible and truss spar production platform designs to 4,500-m (14,800-ft) water depths. The results are summarized inside this issue by Richard D’Souza and Rahul Subramanian of Granherne and Rajiv Aggarwal. The study involved using DNV GL’s Sesam DeepC suite of software to conduct fully coupled nonlinear time domain global performance analyses for both semisubmersible and truss spar designs, for 2,500-m and 4,500-m water depths. The authors suggest that the main challenges relate to the technical feasibility and installation of risers and mooring systems and their impact on the size and global performance of the hulls. The full analysis begins on page 60.

Meanwhile, a paper delivered last month atPennWell’s Deep Offshore Technology International Conference & Exhibition presented the results of a concept screening for a new floating platform for ultra-deepwater GoM subsalt Paleogene reservoirs. The paper, “Low Cost Flexible Production Systems for Remote Ultra-Deepwater Gulf of Mexico Development,” was presented by Jelena Vidic-Perunovic with Doris Inc. It summarized a RPSEA-funded study of an alternative to the existing ship-shaped hull solution, aka FPSO.

The key design parameters for the hypothetical field host platform include capacity for 1 MMbbl of storage and 60,000 b/d of production for 20 years in 2,000-3,000 m of water. Each well is equipped with wet trees, and processed oil is directly offloaded via shuttle tankers. Following concept screening and model testing, a round hull FPSO was selected with a “more cost-efficient” steel catenary riser configuration. The author suggests that the concept would not be prohibited by any of the major classification societies. But, as it would be novel for the GoM, it is likely that the US authorities would require additional reviews and approvals.

Another paper delivered at DOT, “The Low Motion FPSO (LM-FPSO); A Novel SCR and TTR Friendly Floater,” presented an additional alternative to the traditional ship-shaped FPSO. The concept is configured with a conventional box-shaped hull, free-hanging solid ballast tank, conventional mooring, and steel catenary risers. The author, Alaa M. Mansour with INTECSEA, suggests that the LM-FPSO concept maintains the advantages of the conventional FPSO, while “offering superior motions and stability performance.” A case study was presented with the LM-FPSO supporting a dry-tree application in 500-m (1,600-ft) water depth in the South China Sea. Included in the study was a cost comparison with a conventional turret-moored FPSO with a wellhead dry-tree TLP. The author estimates a potential costs savings with the LM-FPSO to be $750 million.

David Paganie

More in Deepwater