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Pazflor’s world-first for subsea separation

Total is mooring its new floating production, storage and offloading (FPSO) vessel over the Pazflor field, in readiness for start-up of the field using the world’s first field-lifetime subsea-separation facilities

Total is mooring its new floating production, storage and offloading (FPSO) vessel over the Pazflor field, in readiness for start-up of the field using the world’s first field-lifetime subsea-separation facilities. Pazflor’s four reservoirs are due to start flowing in the second half of the year, at plateau adding 220,000 barrels a day (b/d) to the company’s existing 460,000 b/d of production from Angola’s Block 17 (PE 2/11 p12).

Pazflor’s successful start-up will be seen as a big step forward for subsea processing – a suite of technologies forecast to be used extensively as smaller and more difficult deep-water fields are developed. Moving processing to the seabed will reduce platform requirements and cut project costs.

At Pazflor, subsea gas/liquid separation units have been installed at a water depth of 800 metres on the field’s three heavy-crude reservoirs – the fourth holds light oil. The units separate natural gas from the crude and formation water, allowing the liquids to be pumped to the FPSO while the gas flows up through separate risers. Special hybrid pumps, combining multiphase and centrifugal stages, were developed for the units.

Seeing the benefits

Opting for subsea separation avoids the need to handle the wellstream in multiphase flow and cuts the risk of hydrate formation. Other benefits include: avoiding the need for heavy slug-catchers on the FPSO; reduced pumping requirements; a simplified seabed layout; and lesser insulation requirements for the flowlines.

The three units, designed for a 20-year life, were built by FMC at Kongsberg, Norway. The company’s $980 million contract also covered Pazflor’s 49 wellheads – 25 producers, 22 water injectors and two gas injectors – and the three manifolds, controls and other equipment.

FMC also built the subsea separation unit for Statoil’s Tordis field, in the Norwegian North Sea. The Tordis facility, the first commercial subsea separation unit, was retro-fitted at the mature field and started up in early 2008 – but it had to be shut down after a few months and is still out of use. Oil was seen in the sea and its source was traced to a leak of oily water from a seabed depression near the field.

While the Pazflor units will separate gas from the wellstream liquids, the Tordis facility was designed to separate the field’s large volume of formation water from the oil and gas, which were pumped together in multiphase flow to the platform. The formation water was reinjected underground from the separation unit.

According to investigations by Statoil and the Norwegian Petroleum Directorate, the problem occurred because the expected porous Utsira sand formation was not present at the location of the reinjection well. Consequently, the injected oily water fractured the overlying shales and leaked up to the seabed. 

Rapidly growing market

FMC forecasts a rapidly growing market for subsea separation and pumping facilities. The company says field benefits include: reducing back-pressure on the reservoir, to increase the production rate and ultimate recovery; extending plateau production; facilitating long tie-backs for outlying structures; allowing the development of low-energy, or low-permeability reservoirs; cutting capital expenditures; and reducing the need for infill drilling.

At Pazflor, subsea separation has made possible the development of the three low-energy reservoirs, Perpétua, Zinia and Hortensia, holding viscous crude of 17-22°API (while Acacia contains light crude of 35-38°API). The two grades of crude will be commingled on the FPSO.

Total has not given a reserves estimate, but says the four reservoirs cover an area of 600 square km – hence the need for 175 km of seabed flowlines. Interests in Block 17 are Total, 40.0%, Statoil, 23.33%, ExxonMobil, 20.0%, and BP, 16.67%.

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