A specially developed subsea connector designed and built for harsh environment service is being used for the first time on BP's Thunder Horse project in the Gulf of Mexico.

These connector systems are based on Tronic's patented Controlled Environment (CE) technology and have been used for both in-well applications and throughout the subsea production infrastructure. Since 2001, Tronic has been working with BP's Upstream Technology Group to develop and qualify a connection system for use in vertical subsea X-mas trees (called a vertical feedthrough) that would allow the in-well instrumentation the option to choose between established electronic systems and the developing field of fiber-optic based sensors. The design specification for the project was to enable the technologies to be selected after the X-mas tree manufacture was completed with no modifications to the tree.

BP recognized that downhole data is critical to the development and optimization of its deepwater Thunder Horse field in a water depth of 6,050 ft (1,845 m) in Mississippi Canyon blocks 778 and 833 in the Gulf of Mexico. It was also recognized that the high temperature/high pressure bottomhole conditions present in this field were at the edge of the performance envelope of traditional electronic-based in-well instrumentation, and that developing technologies based on fiber-optic sensors may be better suited to this application. As a result, the ability to use the tried and tested electrical technologies during the early part of the development while allowing the development, qualification and later use of the fiber-optic technology was essential to this high profile project. Consequently the Thunder Horse development team selected the interchangeable tree feedthrough system conceived and proposed by Tronic.
The design specifications required a number of studies to be undertaken to develop the new product.

These studies included:

• Design of the connectors;
• Materials to be used at elevated temperatures and pressures (270°F or 135°C; 1,020 bar [15,000 psi] working pressure);
• Prototyping and qualification;
• Production, Quality Assurance and delivery of qualified systems; and
• Installation.

A single-pin electrical wellhead connector system was already well established within the industry for use with vertical subsea trees and was rated and qualified to 250° F (121°C) and 1,020 bar (15,000 psi). To meet the temperature rating of the Thunder Horse connector (270°F or 135°C) and to match the exceptionally high reliability and operability performance of the existing connector, it was decided that the new design would not be a carte blanche development, but would be based on the existing connector design.

The single electrical pin connector's primary seal integrity is obtained through metal-to-metal seals with secondary elastomeric back-up seals for optimum performance at the elevated pressures and temperatures experienced in the Thunder Horse wellhead. The conductor cable carrying the signal from the in-well sensors is terminated to a Tronic dry mate electrical connector beneath the wellhead, which is then mated with the bottom of the tubing hanger receptacle mandrel connector. At the top of the receptacle mandrel is the male wet mate receptacle connector. This, in turn, mates with the valve block female wet mate CE plug connector installed in the Xmas tree. The female wet mate mounted in the tree incorporates a long and flexible tube that allows the system a limited degree of compliance to work reliably within the misalignment envelope specified by the tree manufacturer as the tree is installed on the wellhead.

The wet mate CE connector is the key component and features a dual barrier for environmental isolation, ensuring extremely reliable connection and high-integrity mechanical containment of pressurized well fluid. The data transmission system is connected to the subsea infrastructure via a wellhead outlet which incorporates a standard wet mate DigiTRON plug/receptacle connector.

For Thunder Horse it was realized that the materials used in the connectors would need to be closely examined, and a development program to investigate this was undertaken. This program consisted of:

• System risk assessment;
• Compatibility testing of materials at elevated temperature using test solutions prepared to NORSOK standard MCR710;
• Material aging tests at elevated temperatures;
• Key properties measurements of materials over extended test periods (up to 168 days);
• Material life expectancy assessments using IEC216-2 (end of life criteria) and deterministically using industry standard methods (including Arrhenius); and
• Effect on oils and elastomers used for insulation and pressure compensation of extended exposure to elevated temperatures.

Following the study and design element of the project, the qualification program to prove the mechanical and electrical integrity of the new connector was key to establishing its suitability for Thunder Horse. The connector system was subject to verification in accordance with API6A Section F2.35 "Other End Connectors" PR1. In addition, and at BP's request, further testing was undertaken at Tronic's UK facility and also at the Southwest Research Institute (SWRI) in San Antonio, Texas. This additional, exhaustive program comprised:

• Hydrostatic proof cycling pressure test at 1.5 times working pressure (1,500 bar, 22,500 psi);
• Pressure and temperature cyclic testing using nitrogen at working pressure (1,020 bar, 15,000 psi) and high (275°F or 135°C,) and low (32°F, 0°C,) temperatures;
• High voltage insulation breakdown testing at 3,000V dc;
• Insulation resistance testing at 500V dc;
• Contact resistance testing;
• Connector stack-up testing;
• Multiple mate/de-mate testing in an agitated sand and silt environment at seabed hydrostatic pressure (220 bar, 3,250 psi);
• Misalignment testing in agitated sand and silt environment at seabed hydrostatic pressure; and
• Long-term thermoplastic creep testing

Extended testing was successfully completed giving BP confidence that the newly developed connector would meet the long-term reliability and performance required. In addition, the use, as a base, of the established connector system provides further confidence in the long-term performance of the connector.

To date, 26 electrical feedthrough systems have been supplied to Thunder Horse, of which 18 have been installed in the first nine subsea production trees (two in each). BP have specified that trees 10-26 - for delivery over the next 2 years - be prepared with penetrations for the connector system so that either a subsea electrical connector or an optical connector could be used. In addition, eight tubing hanger running tool systems have been delivered to the tree manufacturer (FMC) for use during the installation of the subsea Xmas trees.

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