6 Gas Compression and Subsea Boosting
6.1 General
Gas compression is a field development tool which can be of high interest for gas field, which pressure depletes regularly over the time; to maintain satisfactory production rate during lifetime. The main expected benefits from a subsea gas compression system are:
To be able to position the unit at close distance from the well to efficiently boost the gas (especially for long transportation and tied-back distance),
To increase reservoir’s productive life and then recovery percentage of the reservoir,
Avoid requirement for a topside compression platform while allowing development of reservoirs that would otherwise not be deemed as economical, by decreasing the CAPEX.
The main components of a gas compression system are well known from surface equipment, but surface compression facilities are complex systems.
A compression module would typically be combined with a gas / liquid separation system to provide safe operational conditions for the compressor. A typical compression unit can be composed of the following equipment:
A gas / liquid separation system (optional)
A cooling system (optional), which allows increasing the compression efficiency (reduced inlet temperature).
The compression module (compressor and motor)
A liquid pump (optional) connected with the separator and allowing liquid re-injection in the system downstream the compressor.
Subsea compression systems are developed by Technip FMC, Aker Solutions, OneSubsea, MAN Diesel & Turbo, BHGE.
6.2 Gas Compressor
The compression module is the core equipment of the system and shall be able to handle large flow-rates. The state-of-the-art technology of rotating centrifugal compressor is generally considered due to its efficiency and high flow capacity e.g. FMC Kongsberg / Siemens system depicted on Figure 6.2, “Siemens ECOII Compressor (Siemens / FMC Kongsberg)”.
The centrifugal compressor main components are:
Casing, which consists of a cylinder closed by end cover(s)
Diaphragms. Suction, intermediate and discharge diaphragms create the gas flow path within the stationary components.
The rotor mainly composed of a shaft and impellers.
Seals and bearings
6.3 Projects with gas compression
Subsea Gas Compression is a technology considered as field proven.
Currently, gas compression systems have been installed on three following fields:
Ormen Lange Gas Compression Pilot (GE compressor and AkerSolutions pump) is operated since 2011. Shell took the decision to stop the project in 2014. It was intended to use subsea compression for ORMEN LANGE phase 2 but it was later cancelled. Beginning of 2019, Shell has chosen subsea compression as a concept for the third phase of the Ormen Lange field development.
Shell, and its partners Equinor, Petoro, ExxonMobil and Ineos will now choose between two remaining options for subsea compression; one for wet gas developed by OneSubsea and built on technology installed on the Gullfaks field, and a wet gas-tolerant system from TFMC, which is based on experiences from the Åsgard field. Both of these options will need power from land.
Asgard is the longest subsea tieback in the world with subsea gas compression (MAN compressor and AkerSolutions pump). The system has been started in September 2015. It is also the subsea compression system with the longest track record of successful operation (50 000 operating hours with nearly 100% availability achieved in December 2018)
Commissioning of the Gullfaks Subsea Wet Gas Compression System (OneSubsea) started in 2015. The system ran for one month but had to be taken out of operation for almost two years due to umbilical leakage. The system was restarted in 2017 and has been running successfully ever since, boosting wet gas and increasing the production from several wells.
The advantage of a wet gas compression facility is that it does not require any treatment of the well stream before compression.
TOTAL is running studies (Depth Gas studies) to evaluate architecture schemes and technology requirement for subsea to shore development of gas fields. This put in evidence the requirement to address scrubber performance and liquid tolerance of compressor as well as qualification of dehydration unit (if concept is proven). Scrubber performance and compressor capacity to handle liquid have been crossed check showing good compatibility and unlocking schemes with dry gas compressor downstream scrubber. Dehydration feasibility will be addressed over 2019-2020.
Chevron has contracted Aker Solutions to support delivery of a subsea compression system for the Jansz-Io gas field, 200 km (124 mi) offshore the northwest coast of Western Australia.
Initially the contractor will perform front-end engineering and design (FEED) of a subsea compression station to boost gas recovery.
The field control station will send power from the shore to the subsea compression station. Compression should help maintain plateau gas output over time as reservoir pressure declines. Placing compressors on the seafloor, close to the wellheads, improves recovery rates and reduces capital and operating costs.
Jansz-Io, in water depths of around 1,350 m (4,429 ft), is part of the Chevron-operated Gorgon project.
