European operator Perenco and its partners have successfully completed installation of a new platform concept for the D1B development off West Africa.

The platform, located offshore Cameroon in 164 ft (50 m) of water, has two production wells and one water injection well, with produced oil being transported via a pipeline to a nearby platform. Impressively, the field was brought on stream within 9 months of project approval, partly due to rapid completion of offshore installation work activities.

Commercial viability of smaller offshore hydrocarbon accumulations, marginal fields, largely depends on the costs of building and installing the required production facility. The pros and cons of conventional small platforms versus subsea production systems are well known. However, in many cases it is the cost of a dedicated crane vessel during the installation phase that renders the field uneconomic.
Perenco faced this dilemma with D1B, but successfully solved the low cost build and installation challenges with the collaboration of two companies.

Innovative

Perenco is noted for its innovative solutions in marginal field developments. The "rig assembled jacket" arrangement derived by the engineering division in Paris was no exception. The answer was surprisingly simple and elegant, involving assembly of the platform at the offshore location using the same jackup rig required for drilling.

The jacket comprises a mid-water frame - a micro jacket - and a top frame, both connected to a set of vertical conductors, which also act as the main legs of the jacket. Since the field development required three wells, the resultant structure configuration is essentially a tripod tower. Finally, to provide support against environmental loadings, the top frame is tied back to three suction piles by guy wires (Figure 1).
To minimize the jacket installation time, Perenco took advantage of currently available connection technology: Conductor joints were connected together during driving operations using Merlin HD (heavy-duty drivable) connectors supplied by Oil States Industries (UK) Ltd. in Aberdeen, Scotland. The micro jacket and top frame were both locked to the driven conductors using the Hydra-Lok swaged connection system, provided by Oil States MCS Ltd. in Barrow-in-Furness. The performance of these connections proved to be a primary factor in completing installation of the jacket cost efficiently.

Operations

Offshore operations were carried out from the GlobalSantaFe jackup Glomar High Island V. Assembly of the structure commenced with the micro jacket being lowered to the seabed to act as a template for conductor driving. After driving the 30-in. by 1.5-in. Grade X52 conductors, the top frame was located in position and the micro jacket lifted to its mid-water elevation and temporarily suspended from the top frame. The conductors were then locked to the micro jacket and top frame using the Hydra-Lok cold-forging swaging system to expand the conductors into grooved sleeves located at the top and bottom corners of each frame, involving 12 swages in total.
Conductor make-up, driving and swaging operations were all completed in less than 10 days. Installation of the suction piles and guy wires was completed shortly after.

Design

The novel design and construction of the "rig assembled jacket" fully utilizes the jackup drilling rig by allowing it to remain on location. This eliminates the need for mudline suspension equipment, rig move out, a separate installation vessel, rig re-approach and mudline suspension reconnection. The Merlin and Hydra-Lok connection systems enable the concept aims to be achieved, namely, reduced field development time and realized cost savings.

Assembled

The Merlin "make and break" connector has a unique series of parallel grooved teeth rather than the more traditional helical thread form. This arrangement ensures that the assembled connector cannot "back off" when subject to cyclic fatigue loading. The Merlin is assembled using a hydraulic make-up tool, which elastically deforms male and female components of the connector to provide a mating clearance and then pulls the two halves together, axially. After releasing hydraulic pressure, the pin expands and the box contracts, causing the teeth to mesh, creating a metal-to-metal seal within the interface (Figure 2).
Including handling, the time required to add a conductor joint to the string during driving operations is typically 15 minutes, with the Merlin connector taking around 30 seconds to make up. The connector can also be disassembled by reversing the make-up procedure.

Swaging

The Hydra-Lok swaging system has a 20-year track record of use around the world for structural pile connections

on offshore structures. The connection arrangement has Lloyd's Register of Shipping and DNV "type approval." The system has also been approved for use by certifying authorities Bureau Veritas and ABS.
Perenco's choice of the swaging system for the conductor to frame connections was based on its field proven speed and reliability, and the fact that the connections have immediate full strength capability when completed. Subsequent driving or other construction activities can be carried out without compromising structural integrity.

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