For the Ormen Lange field, a 12.5MW subsea gas compressor station pilot is being built. It will be operated in a water-filled pit on land for two years and, if successful, then deployed subsea at 1000m depth along with three more identical stations. The paper describes the plant, the developed control system, the all-electric actuators, the subsea power distribution and the system for condition monitoring.

The pilot comprises several novel parts for subsea applications, e.g. magnetic bearings for the compressor, all-electric valve actuators of different types, a subsea high voltage (HV) circuit breaker module, subsea variable speed drives (VSDs) and subsea uninterruptible power supply (UPS) modules.

The control and safety systems are split into completely separated subsystems, and there is also a separate system for condition monitoring, which collected approximately 1 million data per second for the two years of trial run (with 250 Tbytes of data obtained after two years operation). The condition monitoring (CM) data are used for design verification, troubleshooting and monitoring gradual deterioration of the equipment. For the final subsea station, the aim is to use the CM data for planning maintenance in advance, termed "condition based maintenance".

Ormen Lange Compression Station Overview

A qualification prototype 12.5MW subsea gas compression boosting plant is being built for the Ormen Lange field, located 100km offshore Norway. The pilot will first be operated onshore in a wet test pit for two years at Nyhamna (landfall gas plant for the Ormen Lange field) before being deployed subsea. The subsea station will be arranged with four compression trains in parallel. This section briefly describes the pilot process plant. See Figure 1 for a simplified sketch.

Power is fed at 132kV to a subsea transformer, where the voltage is stepped down to 22kV, and then further fed to other consumers via a subsea circuit breaker module. Subsea UPS modules are used mainly to be able to power the subsea magnetic bearing unit, in case of power outage while the compressor rolls out. Subsea VSDs are utilised so that the speed of the subsea compressor and pump can be adjusted according to process needs; however due to the long step-out at Ormen Lange, it was not possible to use onshore VSDs.

The process medium consists of mainly gas with some condensate present. The gas is separated from the liquid in an inlet scrubber and is compressed by a 12.5MW gas compressor, while the liquid is pumped by a 400kW centrifugal pump. The gas and liquid are then recombined and sent in a common flowline to shore.

A sketch showing the pilot station in the test pit is shown in Figure 2. A standard size bus is shown along with it for size comparison. The subsea compressor station will use up to four compressor trains, arranged in parallel, as shown in Figure 3.

Main Modules

The plant is subdivided into smaller modules for intervention purposes and because the number of available service vessels with crane capabilities above 200t is limited.

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