Assembly of the next-generation subsea separation, boosting and injection system, designed and built by FMC Technologies for Statoil’s Tordis field, was completed in early April, 2007. The Tordis system, the industry’s first full-scale application of subsea separation of oil, water and sand, was delivered in July to the Statoil, who will bring it onstream Oct. 4.

Increased oil recovery project
The system is being installed as part of an increased oil recovery project at the Tordis field in

Figure 1. Illustration of the subsea separation, boosting and injection (SSBI) station. Separator and desander vessels are shown in orange. The multiphase and water injection pumps and leakage detectors are shown in white. (Protection structure had been removed for better visibility.) At distance the water injection well (upper left) and pipeline inline manifold (right) can be seen. (All figures courtesy FMC Technologies)
656 ft (200 m) of water. Discovered in 1987, production started in 1994, but oil production from the field had been declining due to an increasing amount of excess water coming up from the reservoir. The system and other upgrades are expected to improve Tordis’ recovery factor from 49 to 55%, or roughly 35 million extra bbl of oil. Of that amount, FMC’s subsea processing equipment is expected to contribute 19 million bbl of the increased recovery.

This will be achieved by removing the water and sand from the well stream subsea and then injecting the water and sand into a separate subsea disposal well, thus reducing the back pressure toward the Tordis field and allowing more hydrocarbons to be produced. The oil and gas will then be boosted through a multiphase pump back to the field’s Gullfaks C offshore platform. This eliminates the problem with limited capacity for water treatment topside.

The first major phase of testing took place prior to the assembly of the complete station (subsea platform). This was system operation testing, a 3-month period of control system and rotating equipment functionality testing. To make the testing more realistic, a field dynamic simulator was used to provide feedback on the effects of control operations.

Assembly at Tønsberg
The system was assembled in March at Tønsberg in southern Norway where the 500-ton foundation base structure and some of the modules were built. The base structure was then rolled quayside, and the two heaviest modules — the 230-ton manifold and 170-ton separator vessel — were lifted onto it by a floating crane-vessel. The partly assembled station was then rolled back into the construction hall for installation of the desander, multiphase and water flow modules, and the two 2.3 megawatt pumps.

Extensive testing continued until June, including mechanical interface testing to ensure that all parts fit mechanically and intervention can be carried out as planned. The system has been designed in a modular fashion to allow individual modules and components to be separately retrieved.

Simultaneously, the desander module was put through its paces in a full test loop. With an estimated maximum of 500 kg of sand a day accumulating at the bottom of the separator, sand disposal is an important function. By flushing the sand into the desander module and then into the disposal well mixed with separated water, we save considerable wear-and-tear on the injection pump caused by the abrasive effect of sand passing through it.

Personnel from Statoil’s Gullfaks C operations team, who will have the responsibility for controlling the system, were familiarized with the desanding function and other system features. Desanding is semi-automated, so operators need only press a button to initiate each set of operations.

Subsea separation required
The Tordis field is linked to the Gullfaks C platform via two 6.8 miles (11 km) pipelines. Tordis
Figure 2. Illustration of the Tampen area, with Tordis connected to Gullfaks C via two 6.8 miles (11 km) pipelines.
is a mature field, which produces more and more water and less oil. There is not enough capacity on the platform to separate the vast amount of water needed to keep Tordis in production. Installation of a subsea separation, boosting and injection (SSBI) system removes the water subsea and enables lowering the wellhead pressure. This is the main driver for increased oil production even if the reservoir pressures start to drop. This allows Tordis to remain operational for an additional 15 to 17 years.

The well stream is routed to the SSBI system, where water, sand and hydrocarbons are separated. Oil and gas are pumped through a 2.3 MW multiphase pump to the Gullfaks C platform.

The ability to handle sand is important in a mature field IOR project. The desanding process for Tordis is designed to handle up to 500 kg of sand per day. The separated sand is ejected into the water stream downstream of a 2.3 Mw water injection pump and injected into the dump reservoir in the Utsira formation through a water injection subsea tree.

Control and operation
In order to provide stable conditions for the wells, the pressure in the subsea separator is controlled by adjusting the multiphase pump speed. The liquid level of the separator is self-controlled with an overflow drain. The level of the interface between the water and oil layers in the separator is controlled by adjusting the water injection pump speed. For each pump there is also minimum flow protection with recirculation loop. It can be noted that even if the process resides subsea the main part of the control system is situated at the platform. Both the controllers and the variable speed drives that actually determine the pump speed, are located on the platform.

The subsea separation, boosting and injection station is design for the maximum shut-in
Figure 3. Separator module.
pressure from the production wells, however due to the presence of pumps on the station an overpressure protection system is included for protection of the subsea system, pipelines and risers. This system is a subsea shutdown system classified according to safety integrity level (SIL) of 2, which shuts down both pumps.

The system is designed for flexible operation. The production can either be routed directly to the platform like today or via the SSBI. In the latter case the flow on its way to the separator can run through the multiphase meters or in bypass. A similar bypass possibility also exists for the multiphase pump.

Startup of the system will be to first start a small production from a limited number of wells through the separator and in bypass of the multiphase pump. Thereafter the pumps will be started one at a time. Startup and shutdown of the pumps will be performed with automatic control sequences.

Large quantities of sand can be expected and most of this will separate out in the separator. The sand removal system is batch operated and will run a couple of times each week. Then the sand will be jetted and sucked down to a desander vessel. The underpressure in the desander compared to the separator is obtained with an ejector driven by high pressure water taken from the discharge of the water injection pump. After all the sand is removed from the separator it is emptied from the desander and into the water injection system downstream of the pump. This sand handling system is designed such that the subsea pumps only will see a very small fraction of the produced sand. The sand removal operations are also run by semi-automatic control sequences.

Combining cultures
One of the biggest challenges faced by the project was to bring two very different cultures together. The subsea culture, as we know it today, aims to achieve high reliability by keeping things simple. But a next-generation subsea system, like the separation station for Tordis, is a complex processing system that must be capable of flexible, real-time control and fine-tuning, just like a topside system. Achieving the right balance between simplicity and complexity is no easy task. However, tackling the challenges was made easier due to the good cooperation between the different parties involved.

Future developments
The industry is moving into deepwater applications where subsea processing has a considerable potential to increase recovery and reduce development costs. The application of subsea processing can increase oil from brownfields like Tordis and assist greenfield developments such as Tyrihans, where the installation of raw seawater subsea pumps 35 km away from the topside processing facility will minimize topside equipment on the Kristin semisubmersible platform. The future challenge is to devise greenfield solutions where subsea processing equipment is used to facilitate wellstream transport over considerable distances to onshore processing facilities. If successful, this will revolutionize the nature of field development solutions.

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