Alternative Direct Vertical Access Solutions for Development of Marginal Assets in Deepwater - a DeepStar Study

Aggarwal, R. (Granherne, a KBR Company) | Barton, D. (Granherne, a KBR Company) | Seah, R. (Chevron) | Kusinski, G. (DeepStar-Chevron)



This paper is based on a DeepStar study undertaken to evaluate innovative deepwater floating platforms and riser systems, with dry tree (DT) or direct vertical access (DVA) capability, for development of marginal fields and benchmarking with proven dry tree platform concepts [13]. The spread moored semi-sub shaped and hybrid novel hull designs with top tension risers (TTRs) and quayside integration of topsides overcome key concerns with both TLP and Truss Spar with TTRs and provide cost effective solutions for ultra-deepwater marginal fields.

The designs were developed for three regions (Gulf of Mexico, West of Africa, and Offshore Western Australia) and three water depths (3,000 ft, 6,000 ft, and 8,000 ft) for a common design basis with defined topsides and riser payloads, and specified design constraints. The estimates for novel designs were obtained from eight Concept Owners, and a comparative assessment was performed for 13 designs in three groups for nine combinations of regions and water depths. Coupled analysis was performed for most novel designs for GoM 6,000 and WoA 6,000.

The five field development themes with DT benchmarks, novel hull, novel riser, drilling or process will be described. The key design, global performance, and constructability features of alternative concepts will be discussed. The variability in key estimates for three groups will be identified. The novel hull designs evaluated enable integration of topsides at quayside or by floatover and provide ability to support DTs and long stroke RAM tensioners. For designs evaluated with a modular drilling rig and upto six TTRs, lower CAPEX is estimated.

The ranges of key design data, tensioner stroke, and global performance for each group have distinct variations. The novel riser design considered is Compliant Vertical Access Riser (CVAR), which is an offset riser with no tensioners, thus it enables use of semi-sub design with lesser draft. The development themes with wet trees and a drilling riser reduce the payload on hull.

Technology readiness level (TRL) varies for the novel concepts. The detailed comparative assessment undertaken showed high potential for standardization of hull designs. In ultra-deepwater regions with harsh metocean conditions, several of these novel hull designs provide enabling solutions.

This is the first major effort undertaken with evaluation of 13 alternative design cases with DT or DVA capability. The evaluations done for three regions would also be applicable in many other regions. The documentation of estimates in three groups identifies benefits from novel designs over benchmarks designs in each group, and distinct variations among groups. These results would help in selection of specific concepts for a region for further qualification and project readiness for development of worldwide marginal deepwater and ultra-deepwater fields.