Hydraulics, clamps and connectors crucial for successful system installation
Shell Expro's Auk Alpha platform in the UK North Sea.
In an age dominated by cost reduction initiatives, a market arose for the installation of advanced electrical submersible pumps (ESPs) in fields previously considered too hostile or of too low yield. Now through the introduction of coiled tubing as a deployment and production string, the use of ESPs has been taken a significant step further.
The coiled tubing deployed ESP system has been developed from proven technology used by Centrilift onshore in California. However in March 1994 Centrilift, a division of Baker Hughes based in Aberdeen, was awarded a contract from Shell/Esso for the Auk Alpha platform in the North Sea.
The resulting installation, completed successfully in March this year, is the first to be carried out in the North Sea and, it is believed, anywhere in the world offshore. It is also the first downhole ESP system purchased by Shell UK Exploration and Production, operator in the UK North Sea for Shell and Esso.
Located in 270 ft of water in the central North Sea, 155 miles east southeast of Aberdeen (block 30/16), Auk Alpha already has a 20-year production history, producing on average 17.5 MM bbl/year. The wells are extremely deviated and high in water content.
Due to lack of space on the Auk Alpha platform, the system would need to be installed using the existing drilling rig - although a further and major advantage of the system is that the rig can be eliminated. Economic benefits result from lower installation costs, reduced workover costs and fewer manpower requirements.
To harness these advantages for the hostile North Sea as opposed to a land environment, a number of major changes had to be faced and innovative solutions realized. It called for an atmosphere of close co-operation between Shell Expro and Centrilift.
Responsibilities were defined with Centrilift covering the supply of all equipment required for the downhole completion while Shell Expro retained the wellhead modifications. This enabled the number of contractors to be kept to a minimum as Centrilift assumed the position of main contractor.
Instrumental in achieving the set objectives of Shell Expro's conceptual design was the establishment of a Downhole Development Team with Centrilift's chosen suppliers represented at weekly meetings to finalize equipment specifications and requirements. Attention focused on:
- A hydraulic set/release packer, which at the time was not available on the market
- Development of secure, high strength cable clamps which eliminate slippage in the absence of tubular couplings
- Coiled tubing connectors with high tensile ratings equal to that of the tubing
- Right angled penetrator systems to ensure any wellhead modifications would be minimal.
Based on the proven technology of the Baker Twin-Seal Packer, the coiled tubing ESP version has been developed with a hydraulic set and hydraulic release mechanism which is activated by the use of a wireline plug.
For the highly deviated wells on Auk Alpha, hydraulic release became a pre-requisite because the tensile rating of the 2 7/8-in. coiled tubing would not allow a standard shear release system to be used. This particular completion called for a 9 5/8-in. packer to be deep set in 47 lb/ft casing.
With regard to the clamps, the main area of concern was potential slippage due to the smoothness of the coiled tubing and lack of tubing couplings. As a solution, manufacturer Auto Alloys, working with Centrilift, increased the slippage design criteria from the standard 3 ton requirement to a minimum of 5 ton. Prototype testing proved highly successful.
Concerns also existed about the rate at which, by conventional standard bolting, the cable clamps could be made up onto the coiled tubing satisfying the need for a good installation speed, but without creating misalignment. Auto Alloys and NNCL came up with a new design, known as the swing bolt, which met these requirements and also reduced the amount of waste on scrap clamps.
A further factor concerning the downhole equipment and its deployment was the use of coiled tubing connectors throughout the production string. Several sub-assemblies (including the packer and sub-surface safety valve sub-assemblies) would need to be connected directly onto the 2 7/8-in. coiled tubing as well as individual lengths of coiled tubing being connected to each other.
The design, courtesy of BJ Services, needed to take the loads expected throughout the completion and to equal or better the tensile rating of the tubing.
Final major design consideration were the penetrator systems, to be supplied by Remote Marine Systems (RMS). The packer penetrator system did not cause any problems as the Baker Oil Tools coiled tubing ESP-designed packer was suitable for the standard RMS design. Main design work focused on the wellhead area where Shell Expro wanted its existing Xmas tree to have the least number of modifications possible. The result was a right-angled, fit-for-purpose penetrator.
Prior to installation commencing offshore, Centrilift had to supply topsides electrical equipment which included a variable-speed controller, step-up transformer, junction box, surface cable and power generator.
A high level of engineering support was taken on to finalise equipment design with Brown & Root Energy Services that would enable the system to tie in to the platform's shutdown and monitoring requirements. Specific to Auk Alpha were modifications to meet the shut-down/fire and gas requirements for the diesel-driven generator, which has been plumbed in to the platform's existing system.
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