Shell gambles on ESP for long Gannet step-out

Gannet E and F developement showing downhole electrical submersible pump at Gannel E.

• Gannet E ESP test set-up [26,523 bytes]

Production has started from Shell UK Expro's Gannet E subsea development located 14 km from the Gannet host platform in the central North Sea. Reservoir characteristics necessitated some form of artificial lift, and given the uncertainties surrounding fluid and reservoir properties, electric submersible pumps (ESPs) were considered most suitable.

Subsea ESPs have been used before by REDA on Petrobras' Carapeba and Amoco's Liuhua fields, but never at such large step-out distances from the host platform. This created some unique technical challenges with regard to performance and reliability of the ESPs.

To mitigate against worst-case scenarios, Shell awarded an incentivized ESP performance contract to Aberdeen-based REDA Production Services for the design, supply, test, installation, and long-term monitoring of the complete ESP system. This comprised not just the ESP, but the downhole instrumentation, the topsides electrical scope and the control and monitoring package.

The system started up successfully in January of this year, and following an extended commissioning and ramp-up period, has now settled in to average daily production of around 18,000 b/d.

Development drawbacks

The heavy and viscous flow characteristics of Gannet E's oil (20! API, 17 centipoise viscosity, and a gas-oil ratio of 110 cf/bbl), combined with the low reservoir pressure, made artificial lift essential.

Various methods were considered by Shell early on in the project, including gas lift, subsea multiphase pumps. However, given the reservoir's uncertain characteristics, Shell opted for the ESP solution, as this provided most flexibility for fluid composition and rate.

ESP reliability was another major consideration as the high cost of potentially frequent workovers on a subsea well could have jeopardized the project's viability. From past performance, it was clear that the ESP equipment could be relied on to run well, provided it was properly designed, operated and managed. The best chance of success would come from stressing management of all interfaces impacting the ESP's performance - reservoir, completion, installation, subsea, power control and operation.

REDA supplied and installed over 20 ESPs for Carapeba and Liuhua - however, step-out distances in those cases was around 500 meters to the wellheads, followed by a well depth of some 1,500 meters. The main challenge for Gannet was to transfer the required power "cleanly"at variable frequencies over a 14.5 km submarine cable and down a further 2,000 meters to the ESP motor.

The additional cable can excite a resonant effect leading to damaging high circulating currents. This occurs when a motor is operated from a VSD which generates harmonic distortion frequencies. These frequencies can result in extra motor losses, increased heating and insulation damage - all potentially leading to premature motor failure. The level of distortion is quantified as the THD (total harmonic distortion) - the cumulative sum of the amplitudes of all the resonating frequencies/amplitude of the base frequency.

REDA selected the Halmar Robicon VSD for this project due to the low harmonic content. Computer predictions had been conducted to model the system's electrical performance, which was later confirmed following extensive tests. Testing was designed to replicate actual field conditions where possible. The pump was installed in a test well and driven from the Halmar Robicon drive at various loads and on cable lengths ranging from 5 km up to 20 km.

The conclusion of the tests was that each system is different and the combination of components in the entire electrical system will define the level of THD present. All the test data has now been used to refine the computer model to avoid the expense of full integration testing in future. Surprisingly, results indicated that cable lengths accepted as normal can in fact cause high levels of THD. To minimize the impact of THD, certain frequencies can be avoided, filters can be added and motors re-rated to compensate for potential heating increased.

Installation, operation

Following successful testing, the ESP was installed in July 1997 and suspended until the subsea tree was installed from a dynamically positioned monohull vessel this past January. The system started up early this year and, following commissioning, has been running continuously ever since.

The Gannet E ESP is operated via a dedicated control system. This package incorporates a remote telemetry unit to process hardwired/serial links from the downhole monitoring package, the drive and topsides process and a PC-based system in the control room for operator intervention. All information recorded on the system is trended and available in the company's offices onshore on a real time and historical basis.

The system is the central facility to start/stop and increase/decrease the frequency of the pump. In order to maintain uptime, the system is configured with alarm and trips which are adjustable by the operator to suit conditions. The system also operates on fallback scenarios - should key data be lost, alarm parameters will reset to a safer value based on the historical interpolation of corresponding data. Introduction of this system, backed by operator training, has smoothed the operation and maintenance, and, in turn the expected run life of the pump system.

Implementation of this technology has opened up many opportunities for satellite developments previously considered marginal. REDA is now using this experience for deployment and retrieval of subsea ESPs on coil tubing from a DP monohull.

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