SUBSEA PRODUCTION TECHNOLOGY: Controlling, distributing high voltages for seabed equipment

Connectability critical for pumps

Th Tron
Th Tron
Typical Speed 2B system layout.
Click here to enlarge image

In 1995, a joint industry project was formed to assess the feasibility of operating a control and distribution system on the seabed. The project, known as Speed (subsea power electrical equipment demonstrator) was led by Alstom and culminated in the development of a 1 MW system capable of operating at depths of up to 3,000 meters.

The system employed a Tronic 11 kV three-phase, wet-mate connector to make the connection between the umbilical and the distribution system. However, as the potential applications for subsea power distribution emerged, it became apparent that variable speed drives would be required capable of operating motors with outputs up to 2.5 MW. This has resulted in a considerable escalation in the total demand for subsea power to the extent that schemes have been proposed for which the total load is 30 MVA. It also became apparent that the deployment depth would need to be increased. Hence, the original Speed concept required enhancements.

In September 1999 Tronic (Ulverston, UK) embarked on another joint industry project. This ongoing program, led by Alstom Automation, and known as Speed 2, includes participation from Shell UK Exploration and Production, BP Amoco Exploration (UK), and Chevron Petro-leum Technology Company (USA). The objective of Speed 2 is to develop a transmission, distribution and control system operable in depths of up to 3,000 meters, with the capacity to deliver up to 30 MVA. A key feature is the development of a reliable, subsea mateable connector that can operate at system voltages up to 36 kV, but which has been tested to 76 kV.

Pump power

A typical Speed 2B system for the supply and control of five 2 MW pump motors weighs up to 250 tons with overall dimensions of 27 meters by 6 meters. It comprises a cable termination head (CTH), transformer module (T/X), load switching module (LSM) and five variable speed drive modules (VSMs), all of which are supported by guide posts mounted on a base plate. Modular systems have been devised to provide flexibility, ease of deployment and the opportunity to change components if necessary.

The main supply from the umbilical is terminated in the CTH. From there the supply is fed by jumper cables to the step-down transformer, where the three-phase transmission voltage (33 kV nominal) is reduced to a distribution voltage of 5.8 kV. Again using jumper cables, the transformer output is fed to the LSM, which is used to connect or isolate the supply and the load. Each load has its own variable speed drive module to regulate speed and power output according to the process demands.

Flexible cable assemblies are fitted with wet-mateable power connectors to allow a module to be disconnected from the system. This also enables the connectors and harnesses to be replaced if necessary. The drive modules, being mounted on guideposts, can also be removed and replaced independently. During mate and de-mate operations, the connectors are handled by an ROV, and a parking position is provided for each connector when it is not engaged. Main requirements for systems using Speed 2 concepts have been identified as follows:

  • Operational depth: down to 3,000 meters
  • Stepout distance : up to 50 km
  • Transmission voltage : 33 kV nominal (36 kV maximum) @ 500 A
  • Distribution voltage : up to 7.2 kV @ 2,500 A
  • Maximum rating : 30 MVA
  • Operating temperature : 0-20° C.

Connector specification

Single-phase, wet-mateable connectors are required on the jumper cables that link 33 kV, three-phase supply from the CTH with the step-down transformer. Performance requirements identified for this application include:

  • Continuous rating : 21kV (1-phase) / 36kV (3-phase) @ 500A
  • Operating frequency 10 Hz to 75 Hz
  • Operating depth : to 3,000 meters
  • Ambient temperature : 0-20 degrees C
  • Contact resistance : < 0.1 meters
  • Impulse level : 170 kV (peak)
  • Insulation resistance : >10 G _at 20° C
  • Short circuit rating : 25 kA (62.5 kA peak) for 1 sec.
  • Maximum temperature rise : 70° C
  • Partial discharge: <10 pC at 36 kV phase-earth
  • Design life: 20 years.

Connector reliability

Connector reliability has been the most important consideration. A review has therefore been undertaken of all mechanisms that could give rise to premature failure and in particular to the effects that the increase in voltage would have on performance. This would ultimately impact on the material selection. Consideration was given to possible electrical degredation mechanisms, including partial discharges, water treeing and tracking.

Compatibility of elastomeric components with balancing oils has been performed as has selection of metals with regard to corrosion. The Tronic Controlled Environment (CE) principle formed the basis of the design. This employs pressure balancing to virtually eliminate differential pressure across elastomeric sealing surfaces and thereby improve their reliability.

Dual sealing mechanisms, independent oil chambers, and built-in levels of redundancy within the insulation systems provide additional reliability. The advantages that a CE power connector has over other types are its compact design, low weight, use of few moving parts and the ease with which the connector halves can be mated without the need to use complex ROV intervention techniques.

An 11 kV, three-phase, wet-mateable CE connector had already been developed and tested by Tronic, but to make the leap to 36 kV would not be as simple as just increasing the quantity of electrical insulation employed in the connector. Instead, enhancement of the CE concept would be required to ensure that the insulation would provide reliable service without being susceptible to degradation by the electrical aging processes.

A decision was taken early in the development to treat the jumper cable and connector assemblies as three separate single-phase units, each rated to 21 kV, rather than attempting to produce a three-phase unit. This would save weight and avoid inter-phase electrical stresses, which are higher than those produced between phase and earth. Nevertheless, the insulation has been designed to withstand continuously the full system voltage of 36 kV. This is necessary to provide the high margins of safety required for such a demanding application.

ROV intervention

For ease of ROV intervention with the jumper cable assemblies, a right-angled cable entry was chosen. This introduced the need to perform a right-angled termination and to design a right-angled connector backshell that could be assembled on site.

The most involved part of the program so far has been the development of the cable termination. This is a result of a combination of critical functions that it must perform, namely:

  • Providing primary insulation for the cable to conductor contact
  • Stress grading the cable insulation
  • Acting as a secondary seal against water in the event of a failure of a primary seal
  • Maintaining compatibility with the pressure balancing fluid
  • Must be field installable.

The solution has been to use an elastomeric elbow that can be sealed onto the cable insulation first and then secured to the connector contact using fasteners. Electrical stress control is provided by a combination of field grading materials and geometrical techniques. Testing of the first prototype connector is due to commence in the spring of 2001. In addition, an extensive program of endurance and reliability tests has also been devised. Components critical to the reliable operation of the connector are being evaluated (primary seals, main insulation, cable termination and the dynamic sealing mechanism).

The connector is on schedule for completion by the middle of 2001 and will be available for use in advance of a real system deployment. The product should provide an increased margin of comfort to the industry because a significant part of the enabling technology will have become available.

More in Subsea