By Jerry Greenberg

As the industry continues to discover and develop reserves in deep and ultra-deep water, service companies and operators are working to develop subsea processing systems, including subsea separation and compression systems as well as evaluating the reliability of unproven subsea compression equipment technology. This was the focus of discussion at the subsea processing technical session held the morning of Thursday, May 8.

World’s first full-field subsea separation system. “The dream of separating a wellstream on the seabed and boosting is now fulfilled,” said Gro Mogspeth with FMC Technologies.

The world’s first full-field subsea separation system, TIORA, was delivered by FMC Technologies to StatoilHydro’s Tordis field in the North Sea. The field has been in operation since 1994 and consists of nine wells producing through a pair of pipelines to the Gullfaks C platform. Water injection is used to support reservoir pressure. The field has been experiencing increased water cut and decreased wellhead pressure, resulting in lower production levels.

StatoilHydro awarded FMC Technologies an EPC contract in October 2005 to build the separation system, which was installed and brought onstream in 2007. It consists of a pipeline inline manifold (PLIM), subsea separation, boosting and injection (SSBI) station with topside control and power systems, one high voltage power umbilical, one control umbilical and a water disposal Christmas tree.

Separated water is injected into the Utsira reservoir, which is currently producing oil and gas to the Gullfaks C platform. StatoilHydro expects to increase the field’s life by up to 17 years, increasing oil recovery from 49% to 55% of reserves, equivalent to 35 million bbls of oil.

Compact electrostatic separation process. High water cuts increase process heat requirements, chemical requirements, overall vessel volumes, and equipment size and weight. Gary Sams with NATCO Research and Development said that if a significant quantity of water can be removed at lower temperatures then equipment size, weight and cost can be reduced or eliminated. To that end, NATCO is currently developing a subsea separation method.

The process includes a compact treatment vessel combining electrostatic coalescence and separation in either a series, for higher degrees of separation, or in a parallel process, for increased capacity. The high operating flux in the compact separator allows for pipe-sized vessels that can be placed subsea, and permit optimization of the performance with the smallest possible footprint.

During laboratory testing using a pipe vessel, NATCO processed up to 3,000 bbls of fluids and achieved an 85%-95% water removal from oil streams containing as much as 40% water. The current design has permitted up to 95% of wellhead water to be removed in a subsea electrostatic separator.

“We expect to increase from a 2,000-b/d capacity currently to 6,000 b/d by this November,” Sams said.

Technical challenges for subsea compression system. “Subsea compression technology is unproven so far,” said Bjorn Oiungen of Det Norske Veritas (DNV), “but the industry is learning.”

Oiungen presented a case study of a subsea production system where the gas tie-ins will face a minimum flow problem due to glycol accumulations in the pipelines. Subsea compression is one of the options being evaluated as a means to prevent reduced flow.

The technology is a novel but potential solution to reducing capital and operating expenditures compared with a topside solution, according to Oiungen.

Technology qualification analyses were required to evaluate the technical feasibility as well as the risks with compression alternatives. This resulted in a detailed failure mode identification and risk ranking of the new technology. Compared with a topside facility, the production forecast analyses showed that subsea processing has an equivalent potential to preserve high production flow.