High-power ESP tubing connector systems for deepwater applications

The design, installation, and interventions of down-hole electric submersible pumps (ESPs) in deepwater subsea wells in the Lower Wilcox developments in the Gulf of Mexico (GoM) were the focus of a joint development project between Chevron and Teledyne Oil and Gas. The success of this project in developing a reliable downhole artificial-lift system for use on the Lower Wilcox deepwater subsea wells could improve Chevron’s ability to exploit the vast resources of the Lower Wilcox trend in the GoM.

System design, development stages

The system design encompasses the ESP completion, subsea wellhead, tree, tubing hanger, and power and control system. The system design criteria are for 2,438 m (8,000 ft) water depth and a 23-km (14-mile) tieback as illustrated in Figure 1. The subsea wellhead, tree, and tubing hanger should allow installation of two pump power cables and 11 electrical and hydraulic lines. Each tubing hanger power cable will have the capability to power up a three-phase 7 kilovolt alternating current 220-Amp high-power ceramic-based ESP tubing hanger connector system for a 1,200-horsepower ESP.

FIGURE 1. Overall layout of the ESP system includes the ESP completion, subsea wellhead, tree, tubing hanger, and power and control system. (Images courtesy of Teledyne Oil and Gas and Chevron Energy Technology Co. Used with permission from OTC)

Project development follows Chevron’s technology development stages (TDS) process and uses existing technology where applicable as a basis for new designs and qualifications. Of the nine stages in the process, this project has just completed TDS 6, the prototype validation in relevant environment stage. The project has commenced TDS 7, the connector system qualification program stage, with multiple TDS 8 requirements (system integrations tests) also scheduled as part of the technology development program.

Connector system overview

The supplier of the connector system, Teledyne, uses its reliability program as a framework to drive the product development process. It is the company’s philosophy that the reliability process should be integrated in the early stages of research of materials and technologies and remain through development and final product integration in Chevron’s system. The reliability culture is derived from an understanding that the reliability of the system is a result of the reliability of the sum of all individual components and subsystems. Common reliability tools, including reliability block diagrams, fault tree analysis, and failure mode effects and criticality analysis (FMECA), are implemented to help better understand the relationship and sensitivity between subsystems reliability.

The ESP high-power (medium-voltage) connection system comprises a wet-mate penetrator connector pair, tubing hanger feed-through hose, and a dry-mate splice for ESP-cable field termination. The design is based on the existing high-power connector technology, with each phase of the electrical system electrically shielded for long-term reliability. The design employs various insulation materials including ceramic, polyether ether ketone, and multilayer elastomeric boot seals to achieve phase isolation in addition to fluid sealing.

Based on Chevron’s statement of requirements, in operation the tubing hanger wet-mate electrical penetrator is subjected to HP/HT conditions. Exposures include completion/brine fluids (calcium bromide and calcium chloride) at temperatures ranging from 4°C to 121°C (39°F to 250°F) and an internal (annulus) pressure of 15,000 psi (22,500-psi body-test pressure). The penetrator also forms a primary pressure barrier to environments including exposureto monoethylene glycol hydrate inhibitors and a test pressure of 15,000 psi.

FIGURE 2. The tubing hanger HP/HT wet-mate ceramic penetrator is a three-way wet-mate plug assembly.

The tubing hanger, which contains the wet-mate penetrator, is intended as a secondary pressure barrier from annulus to environment (ocean) following the packer seal (primary seal). The design is based on the use of metalized ceramic insulation that is electron-beam-welded to a copper conductor. The ceramic insulation has been selected based on its ability to support high pressures at high temperatures with minimal creep and/or degradation over time. Phase isolation is accomplished by applying a metallization coat on specific areas of the ceramic on the outer surface. The copper pins are hermetically sealed to the ceramic insulation through an electron beam-joining process. Figure 2 illustrates the design concept for the three-way penetrator wet-mate plug assembly. The pressure barrier includes a primary metal-to-metal seal and a secondary dissimilar seal on each pathway from annulus to environment. The connector system is field-terminated to a 2/0 American Wire Gauge ESP cable using an oil-filled pressure-balanced dry-mate splice located in the annulus below the tubing hanger. The dry-mate splice is pressure-balanced to annulus pressure by use of hermetic metallic bellows accumulators.

The wet-mate connection at the top of the tubing hanger is achieved via an oil-filled and pressure-balanced receptacle module using a phase-shielded dual-bladder mechanism. The receptacle is mounted to the subsea equipment by a compliance mechanism designed to accommodate misalignments resulting from stack-up radial and/or axial tolerances as well as load deflections.

Prototype testing, next steps

Since the technology is relatively new, comprehensive prototype validation testing was necessary for benchtopconfigured components and subassemblies in a relevant environment. More than 50 prototype tests were performed to mitigate high technical risks before proceeding with the qualification program. The risks were ranked via a design FMECA process, and the tests were categorized under the following TDS categories:

• TDS Level 4: Involved critical tests such as the high pressure for the electrical penetrator, materials compatibility, and accelerated life tests and electrical validation;
• TDS Level 5: Involved medium-risk tests such as pressure compensation ESP-cable dry-mate splice; and
• TDS Level 6: Involved integration fit test of the entire connection system into a prototype tubing hanger, double dry-mate splice, field-termination training, and landing tests using compliance mounts on subsea equipment.

The scope of the qualification program (TDS 7) is currently in progress and is planned over a period of 18 months using three preproduction connector sets. Through 28 major qualification tests, the program satisfies the Statoil TR 2313 standards and additional application-based tests. All pressure-barrier seals are independently qualified per API 6A F1.11 and F1.13 PR2 (temperature and pressure cycles) standard, API 17D (pressure cycles), NORSOK Standard M-710 (resistant elastomeric seal testing) for elastomeric seals, and API TR 6J1 (life estimation testing).

A complete system integration test (SIT) in TDS 8 is planned in 2014 for the high-power connector system mounted in subsea equipment. The SIT’s scope is to use full-scale production units and a test setup simulating operating conditions, including extreme operating pressure and temperature while running full electrical power.

Takeaways for well-established reliability system

The establishment of the reliability system for the project provided numerous lessons for designers. A few of these “takeaways” for a well-established reliability system include:

• Reliability consideration from the start of the project (TDS 1) using tools such as FMECA, fault tree, etc.;
• Comprehensive materials compatibility and aging program;
• Extensive prototyping effort and component validation;
• Integration fit testing using subsea prototype equipment;
• Full qualification testing (short term and long term) that encompasses different industry standards as well as application-based tests;
• Early SIT tests using subsea equipment products; and
• Coordinating a high-power connection system reliability program with other reliability systems by using the same matrices to align the development programs to serve the common goal of improving the performance and reliability of the entire system.

Editor’s note: This article is an excerpt from Offshore Technology Conference (OTC) Paper 24138 and is used with permission from OTC.