Separating gas, oil, water, and sand at the seabed has great economic potential for many subsea field developments. A subsea separator can avoid or simplify costly surface platforms or floating vessels, as well as being an efficient tool to enhance hydrocarbon production. One solution of interest is the separation and re-injection of water at the seabed to avoid bringing the water up to the surface facility. A new subsea separation concept for such a system has been demonstrated through an extensive qualification programme that has included performance testing at full-scale and is now available for installation subsea.

The economic potential of subsea separation has been known for some time. Yet, while there are a substantial number of subsea pump systems in operation, there are only two subsea separation stations installed. One is the Troll Pilot for water separation and reinjection, operated by Norsk Hydro in the North Sea. The second installation is the VASP system for gas-liquid separation and boosting operated by Petrobras in Brazil. It has been identified that new applications require further optimised technical solutions to meet future requirements. For example, the Troll Pilot separation system does not include a sand handling system as well as it is a large system, similar in size and weight as conventional surface separators. Especially, the handling of associated sand is often considered as a key feature of subsea separation systems.

There are interrelated reasons why the development of subsea separation applications has lagged. First, some components of such a system would have to be newly designed and qualified. Second, a system incorporating new technology carries a higher potential risk. In addition to undemonstrated performance, both durability and ease of maintenance would need to be addressed to reduce the unknown aspect of the risk. Accordingly, plans for any new design must include qualification to reduce the unquantified risk of new technology to a minimum.

As a result of this, CDS Engineering BV, a specialised company providing state-of-the-art separation technology, and FMC Kongsberg Subsea AS, being a market leader in subsea production systems, joined forces in 2002 to develop robust, reliable and cost efficient solutions for subsea processing. The collaboration has since then resulted in several new technical solutions for subsea processing and extensive qualification work has been performed in order to make these solutions ready for use subsea.

One of the main initiatives is a programme for qualification of a new compact water separation concept with integrated sand management solution that now has been completed. This work has included extensive full scale testing at realistic conditions with respect to a real subsea separation application.

A New System Design
The new, compact water separation concept was developed to meet the overall goal to devise robust hardware that is able to operate successfully though the field life. The separator itself had to be compact and modularised to minimise cost and to allow for relatively easy and inexpensive retrieval in case of unforeseen problems. Separator compactness is increasingly important when designing systems for deeper water as conventionally designed vessels can be difficult or impossible to manufacture for installation in deep water due to the large required wall thickness.

The first step was to select components proven to be robust, reliable and cost-efficient. Insofar as possible, the system uses state-of-the-art components with track record from topside applications. It benefits from CDS Engineering’s separation technology experience. Secondly, the overall subsea separation system is based on designs successfully used by FMC Kongsberg Subsea in subsea production systems for many years.

One of the core new components is a patented inlet section that removes gas before the well stream enters the separator and routes the gas through a bypass line. This is designed with the inlet cyclone positioned partly above the separator vessel. In this location it can easily be sized for a very high gas flow rate compared to a similar inlet cyclone positioned inside the vessel, which is the usual case. This frees the almost the entire separator volume for oil-watersolids separation. It is a novel concept that allows the separator volume to be reduced by approximately 50 percent compared to state-of-the-art designs with equal flow rate capacities and separation performance. This is a very important improvement as it is critical to design subsea separators that fall within the lifting weight capacities of intervention ships.

Separator with gas bypass arrangement (top), and system for sand handling illustrated on the separator bottom

With respect to separation performance the requirement considered is typically that the water to be re-injected must contain less than 1000 ppm oil. Based on similarly designed separators with the same CDS internals the outlet concentration is normally in the range of 100 to 200 ppm. Hence, there is extensive experience from fields in operation showing that the required separation performance for the subsea separator will be met.

Another of the core new developments is a sand management system that removes the sand from the separator vessel smoothly with low disturbance of the water phase. Such systems are common in use in topside separators but special adjustments have been necessary to meet the requirements for subsea applications. Several concepts have been shown suitable for the application and have been extensively tested.

New compact separator concept compared to conventional separator designs with equal capacity Separator as part of a subsea system

A major goal of solids management is to separate as much sand as possible in the gravity separator in parallel with the oil and water separation. It has been shown through testing that most of the incoming sand separates relatively quickly, and the quantities of sand entrained with the outlet water flow can be within the requirements set for typical subsea applications. The sand that is removed from the separator is then dumped to an appropriate discharge location. In order to make the subsea system as simple as possible it is often considered to recombine the sand with the oil stream that is routed to the topside facility where the sand can be further handled.

Qualification Program
The objective of the qualification work has been to demonstrate the novel separation system for subsea application, and to characterise its functional capabilities and performance. This work is done in collaboration with Statoil ASA and partially funded by Statoil ASA and the Norwegian Demo 2000 programme. Qualification work has included the following activities:

• Detailed design of separation and sand management system of the subsea separator, including performance evaluation work by detailed CFD simulations of the separator and the key components.
• Small-scale testing of the separator and the key separator components to prove that these are suitable for use in this application.
• Small-scale testing of the technologies for sand removal and sand transportation to optimise the design and the operational procedure.
• Test the separation and sand system in full scale with respect to a real subsea separator with model fluids and solids at representative conditions to monitor its behaviour under the various conditions that could be expected for a subsea system.

Full-Scale Tests
After the smallscale separation and sand system testing was completed with satisfactory results a fullscale test setup was built. It was decided to test in a full scale system as it is not possible to downscale all effects of a separator and sand system properly. This has been an important reason for unsuccessful installation of topside separation and sand system technologies by many vendors in the past. The full-scale test system is therefore built with the real geometrical dimensions and for the same capacities as considered for a real subsea application.

The large scale test system has the following characteristics:

• Gas capacity of 3000 Am3/hr at a maximum density of 50 kg/m3, equivalent to 160 MMSCFD natural gas at 60 barg.
• Total liquid capacity of 800 m3/h, equivalent to 120,000 BPD, and the ability to circulate both oil and water at 0 to 800 m3/h.
• Sand injection capacity up to 150 kg/h.
• The ability to use a high molecular weight gas to represent high-pressure gas at a relatively low operating pressure.

The test system includes a separator vessel of 12-meter length (tan/tan) and 2-meter inner diameter.

Full-scale test arrangement 3D overview

The full scale testing has been completed, and has resulted in the following successful observations:

• The gravity based separator design based on the patented gas bypass principle has been demonstrated to work satisfactory and has been proven for subsea applications.
• A design for all separation components has been established and found suitable and proven for subsea use.
• Several sand removal technologies has been demonstrated as suitable and available for subsea use. • Concept for transportation of the sand to a suitable discharge location has been demonstrated. Based on this all key components in the system are demonstrated as suitable for subsea use. Hence, the new separation and sand management solution is now ready for implementation in subsea processing systems.