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Self-installing buoyant tower offers production solution

Written by  Clyde Crochet Sunday, 01 December 2013 00:00

GMC Inc.’s Clyde Crochet discusses a self-installing tower, currently deployed off Peru, which offers a new substructure solution compared to conventional jackets/piles.

The buoyant tower concept represents an innovative bridging design where floating (deepwater) and fixed (shallow water) substructure technologies merge to yield a new substructure scope for mid-water depth applications–50m to 260m. GMC Ltd. and subcontractor Horton Wison Deepwater (HWD) developed the design and markets the concept under the joint venture company, HortonGMC.

The tower was recently utilized by BPZ Energy to achieve production at its Corvina field offshore Peru. The CX15-1D well achieved an initial flow rate in November of 500 b/d, naturally flowing with no water production and normal gas oil ratio. A second development well was spud in early November with completion expected to follow in January.

The buoyant tower resembles a cell spar hull configuration with an attached suction can at its base that allows the tower to pivot about its imbedded base in a compliant manner while restraining the base from movement in vertical, lateral, and torsional directions. The topsides payloads are flexible with cell quantity configuration and sizing.

The tower is configured with upper void tanks for buoyancy, lower air over water ballast tanks for ballasting operations, and fixed ballast in the bottom tank sections. A net small downward force is maintained with ballasting operations applicable in response to topsides weight changes of significance.

Paralleling the design aspects of deep water spars, the buoyant tower configuration provides a center of buoyancy above its center of gravity and yields a hull scope unconditionally stable.

A key enabling attribute for the tower substructure is buoyancy and its availability for a mid-water depth scope. Compared to a conventional jacket/pile scope, buoyancy offers unique project execution flexibility.

The overall configuration and geometry for a four-cell tower scope under construction is illustrated in Fig. 1.

Challenge – CAPEX for Conventional Jacket Substructure

BPZ Energy assumed operator responsibility for Z-1 Block offshore Peru with its existing assets in 2005. BPZ’s initial block activities represented reservoir assessments and modifications to existing scopes versus new facility construction/ installation.

BPZ defined and pursued budgetary pricing for new facility scopes under a conventional jacket/piles/topsides model. BPZ’s budgeting efforts quickly identified two project challenges specific to an offshore Peru installation site:

  • Budget pricing revealed significant transportation and installation costs attributed to site distance from established fabrication/marine infrastructures
  • Early jacket and pile sizing seismic conditions (Fig. 2) represented a significant cost risk; offshore Peru was known as an active earthquake location on the Pacific Ring of Fire. In May 1970, an earthquake measured 7.8 Ms, causing 41,000 deaths and 100,000 injuries.

Solution – Reducing CAPEX

Collectively, CAPEX costing, risk assessments, and revenue forecasts combined to challenge BPZ’s economics for project sanctioning. Embracing “necessity is the mother of invention,” HortonGMC responded offering the buoyant tower with its unique features to address sanctioning challenges:

  • A very compact substructure design enabling overall scope transport to be limited to a single marine asset
  • A buoyant substructure design enabling the same transport marine asset to also perform the overall installation scope given a buoyant substructure and favorable site conditions for a topsides skidover mating – Fig. 3.
  • A compliant versus a rigid substructure design for favorably addressing an active, higher level seismic zone and eliminating a need for large pile/skirt pile scopes with accompanying jacket scopes.

Solution –Transport and install execution steps

Under the buoyant tower concept, all sanction challenges were addressed. A single heavy lift vessel transported the overall scope and accomplished most installation scopes. In proximity to the final installation site, tower float-off and topsides mating operations were performed with the assistance of regional tugs for tower movements following the below execution steps:

  • Float-off and up-ending of the tower
  • Installation of buoyant tower fixed ballast
  • Return/adjacent positioning of the buoyant tower within the topsides support frame
  • Topsides skid-over above the positioned buoyant tower
  • Buoyant tower de-ballasting operation for mating tower and topsides
  • Vertical tow of mated tower/topsides to install site
  • At install site, two-hour ballasting effort for achieving 8m soil penetration of suction can

The transport arrangement of tower and topsides along with the topsides skid-over methodology with an elevated, cantilevered skidding frame are shown in Fig. 4 and 5. Substructure benchmarking – Buoyant tower versus jacket

Fig. 6 benchmarks quantity of marine assets and respective durations for a buoyant tower versus a jacket scope. The transport and install economy of effort is obvious. Most notably, the requirements and expense for a derrick barge for jacket positioning, pile driving, and topsides installation are absent from the buoyant tower configuration.

Fig. 7 commercially addresses the buoyant tower marine asset advantage and benchmarks the transport and install costs. CX15 buoyant tower CAPEX costs are compared to budget estimates for a jacket/pile substructure execution. The buoyant tower transport and install advantage is evident.

Fig. 8 benchmarks buoyant tower and jacket earthquake bending moments. The buoyant tower compliant advantage is evident.

Underlying features enable buoyant tower to address other project drivers

As presented above, the buoyant tower concept was uniquely configured to address BPZ’s project drivers – transport and install costs for a remote location.

Supporting the buoyant tower capacity’s to address/satisfy BPZ’s project drivers were a set of unique underlying features inherent in its design. These underlying features are not limited to a single configuration/execution scenario. They offer new project execution flexibility and can be uniquely configured to address project drivers/challenges in any phase of the project execution – design, procurement, fabrication, transportation, and installation.

Fig. 9 identifies unique buoyant tower features and provides a listing of feature impacts that can offer new solutions for unique project drivers/challenges.

Each of the above “Feature Impacts” can offer a new solution. Fig. 10 and 11 and accompanying descriptions represent examples of “high-lighted” features offering new solutions to specific project drivers/challenges:

  • Jacket versus buoyant tower sizing efforts for two (2) substructures, 226m and 235m water depth, yields a projected total steel savings in excess of 11,500mt as graphed below. (Topsides weight assumption: 4,500mt in each instance.)
  • The buoyant tower may offer new solutions for marginal field development where minimal topsides and short design life prevail. A tower could potentially be configured for onshore topsides/hull integration and ease of relocation/reuse following its initial service life.

Fig. 11 depicts the transport of a topsides integrated with the buoyant tower.

What is the Risk?

While the buoyant tower does represent a new substructure concept, its core elements are established and do not possess “first of a kind” risks to design, build, install, operate, and decommission.

  • The buoyant tower resembles a cell spar with its design and operational requirements well understood from floating deepwater scopes.
  • As outlined in Fig. 9, the buoyant tower can offer additional flexibility/options in fabrication, loadout, transportation and installation phases with some options inherently safer and lower risk than a conventional jacket execution.

The risk level assessment for a buoyant tower is very low. However, there is a distinction of some note: the buoyant tower is not a fixed structure and does require ballasting operations for topsides load changes of significance.

A requirement for ballast operator training exists and is analogous to the requirement for operator training for production operations. Beyond this, the overall buoyant tower risk levels are believed to be on par or more favorable than a conventional jacket substructure.

Arguably, the overall buoyant tower represents a flexible concept that can uniquely facilitate maximizing the overall safety, quality, cost, schedule, and risk drivers for a project where a conventional execution may be challenged in design, procurement, fabrication, loadout, transportation, and/or installation. OE

Clyde Crochet is a project manager for Houston-based GMC Inc., a wholly-owned subsidiary of UK-based GMC Ltd. His most recent project was the CX15 installed offshore Peru on BPZ Energy’s Corvina Field. His career spans 40 years of worldwide offshore design, fabrication, and installation scopes. www.gmcltd.net

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