Taking FPSO offshore processing and moving it subsea has many attractions – including, for example, smaller surface vessels (if any) with the associated cost savings and production that is less prone to adverse weather conditions. Packing more processing equipment into trees and manifolds while avoiding significant weight increase is where recent developments in small-bore rotary valves are set to play a key role in the next generation of subsea processing systems.

Subsea processing trends

The trend toward greater subsea processing has been gathering pace for some time. This is due in part to recognition by operators that some deepwater discoveries such as heavy oil, ultra-deepwater, and arctic fields will rely on the development of new enabling subsea processing technologies capable of handling HP/HT conditions, water depths up to 3,000 m (9,843 ft), and longer step-outs.

But there also is a changing perception of what is now possible subsea. Statoil has introduced the concept of a subsea factory – a process plant on the seabed – in the belief that compact separation facilities on the seabed will be needed in arctic and deepwater areas like the Gulf of Mexico (GoM) and Brazil.

Statoil already has taken the first technological steps with the world’s first complete subsea solution for separation and injection of water from the Tordis wellstream and the first subsea facility for injection of raw seawater on Tyrihans. Next year the company plans to realize sub-sea gas compression in the ?sgard field.

Rotary valve – subsea enabling technology

Reliability is the watchword of subsea production and processing hardware. Every component and subcomponent of these assets needs to perform reliably to optimize overall safety and productivity. Processing at greater water depths brings with it the additional challenge of minimizing the equipment size and weight for ease of deployment subsea.

Valve technology has kept pace with the growing demands of subsea production.

The need for reliability and simplicity of valve operation in the face of difficult operating conditions, together with increasing demand for smaller and lighter valve technologies, has seen the emergence of the rotary gate valve as a key enabling technology for the next generation of subsea processing systems. Small-bore rotary valves already are widely used on important subsea equipment such as christmas trees, where they play a vital role in the performance of these assets.

Small-bore rotary gate valves, measuring typically 3/ 8 in. to/ 4 in., are commonly used for handling control fluids, well fluids, and chemical injection at pressures up to 16,500 psi and temperatures in the range of -29°C to 155°C (-20°F to 311°F). It is the simplicity of their operation that sets rotary gate valves apart from other sub-sea valve technologies such as through-conduit and needle valves. Operation is through a simple 90° turn. The valve internals cannot be damaged by over-torque.

The metal-to-metal seals of the rotary gate valve are able to withstand a full range of operating temperatures. Elastomers and polymers are much more vulnerable to extremes – some perform well in heat, others in cold. Finding one that can do both is not so straightforward. Add to this the fact that the integrity of elastomers and polymers can be undermined by the type of media passing through the valve, and the effectiveness of metal-to-metal sealing becomes all too apparent.

Subsea valves, in general, must be capable of withstanding any fluids that may pass through them. These could include fresh water, seawater, injection chemicals, or wellbore fluids. Even valves that will only see clean injection chemicals during normal service could be subjected to aggressive well fluids in a fault situation. Valves that could potentially be exposed to HS are specified for “sour service,” and their material classes are selected accordingly. By using the minimum of moving parts, the rotary valve becomes more reliable. It is not susceptible to problems arising from contaminated fluids and is water-depth insensitive.

The valve bore size is determined by the volume of fluid that needs to flow through it.

Until now, smaller rotary gate valves have usually been limited to corrosion inhibitors and scale preventers injected in small volumes. With the development of a 1-in. rotary gate valve, however, larger volumes can be handled. This makes them suitable for injection of methanol for preventing hydrate buildup, enabling the replacement of larger and heavier through-conduit and needle valves.

Fresh thinking needed

It almost goes without saying that there is no such thing as a “standard” subsea valve.

It is common for valves destined for HP/HT regions to need to withstand pressures of 16,500 psi and temperatures of 177°C (351°F). In fact, recent years have seen the spectrum of required temperature ranges for subsea extend significantly from -29°C to 121°C (250°F) to low temperatures of -46°C (-51°F) and highs of up to 205°C (401°F). A few years ago, the maximum required pressure rating for a valve was 15,000 psi, whereas today orders increasingly demand 20,000 psi.

Valve technologies are rapidly evolving to meet these extreme conditions. A key focus of rotary valve R&D has been designing valves that are smaller and lighter and have minimal moving parts and uncomplicated operating mechanisms to significantly reduce the risk of failure.

It follows that if design engineers are to meet the increased demands for subsea processing, there has to be recognition that small-bore valves are part of the solution to achieving a range of design and performance objectives. This in turn will encourage greater collaboration between valve makers and designers working on FEED studies.

Optimized, compact valve design

For the next generation of subsea processing systems compact design will mean valve size, shape, and interfaces are more important than ever. Rotary gate valves are smaller than other valves. What is less well-known is that the shape of the valve can be customized to fit a particular space within the manifold without affecting valve performance.

ROV interfaces and hydraulic connections may not have an impact on the internal valve functionality, but intelligent specification can still bring real operational benefits. There might be alternative ways to position interfaces and mountings to make better use of space, or it might be advisable to select a valve that cannot be damaged in the event of over-torque by an ROV. Another consideration is double-block or mono-block structures for multiple valves, which reduce potential leak paths to further enhance safety and reliability credentials.

Using small-bore rotary valves presents the design engineer with significantly more options than other types of valves when designing subsea processing systems. Taking an intelligence-led approach and working with the valve manufacturer when specifying individual valves and other hardware components will ensure that the industry is well placed to deliver the next generation of subsea processing systems.