Crude oil processing

Harald Tønnesen and Finn Harald Sandberg, Norwegian Petroleum Museum

Crude oil coming up from the reservoir must be separated from the associated gas and water, and have its pressure and temperature reduced through a multistage process. Only then can it be stored in the big cells in the gravity base structure (GBS) for each platform. Processing the oil is intended to stabilise it enough to allow tanker transport.

— Equipment floor for crude oil manifold in the utility shaft at Statfjord B. Photo: Shadé Barka Martins/Norwegian Petroleum Museum

Wellheads

The wellstream flows from the reservoir with a temperature of roughly 91°C. Initially as high as 228 bar, its pressure has declined steadily since the Statfjord late life project became operational in 2007. A wellhead is installed on each of the risers coming up through two of the shafts supporting the topside. This comprises a “Christmas tree” array of valves and attachments for casing and production tubing. Each wellhead has a choke valve which conducts the wellstream into the manifolds. There are three of these, allowing the wellstream from a given well to be directed to one of the production trains or a test separator.

Separation

The crude passes from the manifold into the inlet separator, a horizontally mounted pressure vessel equipped with internal deflector plates which help to separate water and gas from oil. From there, the wellstream flows through three further horizontal vessels connected in series to function as degassers. The pressure is reduced in each stage to liberate more gas.

Stage 1 Inlet separator    67.9 bar
Stage 2 Degasser no 1 21.7 bar
Stage 3 Degasser no 2      5.9 bar
Stage 4 Degasser no 3      0.7 bar

The oil takes just over two minutes to flow through each separator stage. From the final degasser, it passes to an electrostatic coalescer and then to oil coolers.

A horizontally mounted pressure vessel, the coalescer lies directly under degasser no 3. High voltage electricity is used there to enhance the formation of water droplets. Reducing the water content of the oil, these droplets sink to the bottom of the container and can be drawn off to a treatment plant before being discharged to the sea. Operated with circulating seawater, the coolers reduce the oil temperature to below 35°C prior to storage in the GBS cells. Since the late life project became operational, some of the compressor equipment has been modified to handle large volumes of oil at a lower pressure.

Well testing and test separation

A well is tested after maintenance and workover, and periodically as required. This measures its performance, capacity and oil/gas/water ratio under operating conditions identical with normal production. The test separator is run in parallel with the inlet separators, and allows the flow to be diverted to a train if required. It can also be operated in parallel with both the inlet separator and the first degasser stage.