P.3 ~ DeepStar study assesses cost-effective dry tree solutions

DeepStar evaluated a large number of novel concepts for a common design basis, focusing on identification and characterization of key technical performance parameters, constructability, and economic factors. The economic factors are most important for these designs to be considered attractive for development of marginal fields with short field life and fewer wells. The authors present the alternative development themes for obtaining dry tree and direct vertical access capability.

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Floating platform alternatives

A total of 11 hull and four riser designs were selected for five development themes with DVA of wells from the platforms. Of these, two were benchmark designs (TLP, truss spar), eight were novel hull designs, one was a conventional semisub design, and one was a novel DVA riser system under various qualification stages and TRLs. One hull was evaluated for three design cases (C1, C3, C4), thus a total of 13 design cases were undertaken. The three other riser designs included proven TTR, SCR, and SLWR.

Eight engineering companies undertook sizing, weight and displacement estimates, global performance analysis, and cost estimation for 13 designs for the common design basis and provided the results for evaluation and comparative assessment in this project.

For comparative assessment, the alternative novel hull and riser systems were considered under three Groups (I, II, and III) as given in the table to enable useful comparison and identify specific (or distinct) design ranges for various parameters for novel designs under each group.

Further details on the key features of these designs can be found in the longer and more detailed paper presented at the recent Deep Offshore Technology conference, and additional references can be found in that document as well.

Technology readiness review

Vortex-induced motion (VIM) susceptibility review was performed for all designs. The unique design features of columns and pontoons in the following novel hulls were identified by concept owners with potential to impact vortices formation and help obtain low-amplitude VIM:

  • 5-sided columns in OPTI hull designs (OPTI-DRI-C1, OPTI-C3; OPTI-C4)
  • Columns rotated at 120 degrees in case of 3-column MiniFloat-V hull
  • HVS design with special measures, such as column steps at pontoon level suppress VIM
  • OctaBuoy with unique hull design and closer spacing of columns, which reflect the vortices.

The TRL of the overall designs was addressed in the project by a review of designs of key sub-systems and their development stages. They were allocated TRL from 0 to 7 with "0" for unproven concept and "7" for field proven concept. The following were considered in technology readiness assessment of various sub-systems of each novel design:

  • All novel designs in theme C1 require design development and qualification of well bay and deck structure to support up to six RAM tensioners with long strokes and the impact load at bottom out in the robustness criteria. Such work was started and further analysis, design, and testing effort may be required for the designs presented here
  • One novel hull design in Group I (OPTI) is in-service in Central GoM with subsea tiebacks
  • One design (OctaBuoy) in Group II has been constructed for wet tree application but not installed
  • Two novel designs (HVS, E-Semi) have gone through Class approval-in-principle and model tests
  • Two novel designs (DCC, FHSB) are in concept development stage and need Class approval-in-principle and model tests in wave basins
  • The hull designs for themes C2 (Semi w/CVAR) to C4 (OPTI) require minimum qualification as they have been used with subsea tieback
  • The CVAR design uses proven hardware, and requires qualification of larger non-structural ancillary units (buoyancy modules), further analysis and optimization of its configuration and design, and qualification of its installation and operations through the riser
  • The MiniFloat-V design concept is in-service for small scale wind energy applications, and requires full design development and qualification of the larger size hull required for oil and gas applications.

Conclusions

The work undertaken in the DeepStar project confirmed the feasibility of semisub shaped and hybrid semisub novel hull designs (theme C1) to provide low-cost DT solutions for the development of deepwater marginal oil and gas fields in three regions. The novel semisubmersible hull designs evaluated in theme C1 overcame the limitations of benchmark TLP and truss spar designs, met all design constraints, and are feasible for quayside or floatover integration of topsides.

The alternative development themes (C2, C3, C4) with wet trees and DVA of wells from the platform were also shown to be feasible with a semisub-shaped hull, but will have increasing cost of subsea units from theme C2 to theme C4. The DVA theme C2 with CVAR, an offset riser, disconnect the drilling and workover operations, and provides opportunity for development of deeper reservoir (>15,000 ft below seabed) without much impact on the platform design. The potential of three-column MiniFloat-V hull design (C4) to support a modular drilling rig, a drilling TTR with RAM tensioners, and SCRs and umbilicals is of high value for cost-effective development of fields in alternative ways and to provide DVA to wells and controls to subsea units.

In 3,000 ft water depth, benchmark TLP designs provide low-cost development solution. In the OWA region, the TLP foundation design would require qualification work; thus a semisub based design with suction anchors could provide a competitive solution. Comparative assessment of these designs showed that the value from novel semisub and hybrid designs increases for ultra-deepwater (> 5,000 ft water depth) and harsh metocean regions such as Central GoM and OWA. In such regions, the novel semisub hull designs would become enabling solutions for development of marginal oil and gas fields.

The alternative hull designs are available with varying TRLs, thus requiring minimum to significant technology qualification efforts. Their use in regions with moderate to benign seastates would require lower tensioner stroke and enable use of some Group I designs. The existing semisub designs in deepwater GoM are proven with wet tree applications and thus would require less qualification effort from addition of DT and DVA operations.

Due to moderate increase in the total payload on hull with water depth for all development themes evaluated, there is a potential for regional standardization of novel hull designs to obtain the "Design-One-Build-Many" capex and reuse benefits. These features would be desirable and enable development of marginal fields with short field life, assumed 10-years in this project, when the semisub based platforms are considered to develop multiple fields. The movement of semisub to another field would be a simpler operation for semisub designs than for a TLP and a truss spar.

The estimates for eight novel hull designs and one novel riser design generated through collaborative industry effort with in-kind participation by eight engineering companies (the concept owners) enabled establishment of a significant database, which is organized in three groups of designs for a useful comparison and assessment. The database provides clarity on impact of regions and water depths on alternative designs, variations among three groups, and comparisons with benchmark TLP and truss spar designs. This database would be valuable for worldwide fields.

Acknowledgments

The authors wish to express their gratitude to DeepStar Management and to their 12 operating companies: Anadarko, BG Group, BP, Chevron, ConocoPhillips, Maersk Oil, Marathon Oil, Nexen, Petrobras, Statoil, TOTAL, Woodside. In addition technical support was provided by the DeepStar X400 Floating Systems technical committee throughout the project and in support of this publication. The authors wish to thank the eight participating concept owners: Aker Solutions, Exmar Offshore, FloaTEC, Granherne, INTECSEA, Marine Innovations & Technology, Moss Maritime, and Technip, for performing in-kind evaluations of their novel concepts for the common design basis and providing detailed reports and comparative assessment metrics. The first two authors also wish to thank Granherne project team at Houston and Perth for the work done for the design basis, benchmark theme C0 and theme C2 design cases. This article is based on a paper presented at the Deep Offshore Technology International Conference & Exhibition held in Aberdeen, Scotland, October 14-16, 2014. The full report with supplementary appendices (about ~2,000 pages) is available to the DeepStar operating companies atDeepStar.org.

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