Roll out the barrels

An extra 35 million barrels of oil could be extracted from the Tordis Field in the North Sea thanks to a new subsea separation system which has been installed in the seabed and is due to start operating next month.

The Gullfaks C platform, owned by Norwegian energy company StatoilHydro, produces oil from the Tordis Field. The company will attempt to improve yields with the first full-field subsea separation boosting and injection system.

The overall system was designed by FMC Technologies, a manufacturer and supplier of subsea production systems. Tracerco, a UK specialist in separator measurement, provided equipment to go into the separator.

The technology will remove water from Statoil's underwater oil and gas (hydrocarbon) field and reinject the water into a non-hydrocarbon reservoir. Hydrocarbons will then be sent through a multi-phase pump for processing at Gullfaks C.

'Removing the water and reinjecting it into a non-hydrocarbon well will reduce the back pressure in the pump and allow more hydrocarbons to be processed at Gullfaks C,' explained Simon Lambert, Tracerco's specialist measurement manager.

Six of Tracerco's instruments will help separate the mixture of oil, gas, water and sand that lies under the seabed. When the mixed fluid is pumped out, it goes into a separation vessel tank, where an inlet cyclone separator divides most of the gas from the fluid, and the gas is pumped to a separate pipe outside the tank.

The remaining mix in the tank separates through the gravity principle. The bottom layer is sand, and the following layers rise in order of water, oil and gas. The water is pumped into a special reservoir.

This introduces water into the tank to disturb the sand and pumps it into a special 'de-sander vessel.' The contents of that are then injected into the reservoir with the discarded water. The remaining oil and gas then flows through a series of pumps to Gullfaks C.

Tracerco's instruments are installed in the separator. These radioisotope devices, developed with FMC, are based on a re-design of Tracerco's Profiler unit.

The Profiler gives a density profile of a cross section of the internal subsea separator vessel and relays an image of the data in the same way a MRI scanner provides an image of the human body. The scan of the vessel provides operators with a range of data that can be interpreted to identify the oil and water interface and any emulsion or foaming layers that may be forming.

'These are important things for the operator because if there is too much emulsion, or foam, he has to add chemicals to reduce their amount,' said Ken James, Tracerco's technology development manager. 'The chemicals are expensive, but the device will allow him to optimise the way they are injected by understanding what's going on inside the separator.'

Tracerco engineers had to design the device so it could operate without maintenance in a subsea environment for at least 25 years. They did that by using materials that would not develop build-up in a subsea environment and designing each component to be separately retrievable if damaged or broken.

'The system has sensors in a dual configuration so if one sensor fails, we can remove it using a remotely operated vehicle,' said James.

The demand for these kinds of boosting and injecting systems are likely to increase. 'There is especially a need now with the increasing price of oil and depleting fields,' said James. 'Oil and gas fields are coming towards the end of their lives and companies are looking to get every last bit that they can at the best possible price.'

There has been talk for years about boosting and injection systems, but it wasn't until 2002 that FMC asked Tracerco to help create one for Statoil's Tordis site.

While the project likely brought on significant upfront costs for Statoil, James predicts that their successful results in the coming years will encourage other companies to use similar systems.

'For us it's quite a demanding project, but it has taken our technology forward,' he said. 'We believe more and more oil companies will invest in similar subsea separation systems.'