Rotary Valves developments are creating added benefits, increased subsea production.
This article focuses on small bore valves for the subsea oil and gas sector. We explain how and why developments in subsea production are driving innovations in valve technology. Subsea valves need to operate reliably in extreme conditions, minimising their weight and footprint is essential.
Enhancing production efficiency is an ongoing focus for subsea oil and gas operators. A current trend is to take processing carried out by FPSO (Floating Production Storage and Offloading) vessels and relocate it on the seabed. Benefits include reduced vulnerability to adverse weather conditions, resulting in less downtime. However the approach brings its own challenges since it involves packing more processing equipment into subsea Christmas trees and manifolds without causing significant weight increase. In this context, ongoing developments in small bore valves are set to play a key role in the next generation of subsea processing systems.
Subsea processing trends
There is a changing perception of what is 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 artic and deep-water areas like the Gulf of Mexico, Brazil and the Beaufort Sea. Statoil has already taken initial technology 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 it plans to realise sub-sea gas compression in the Asgard field.
Rotary valve – subsea enabling technology
Reliability is the watchword of subsea production and processing hardware. Every component and subcomponent needs to perform reliably to optimise safety and productivity and processing. Processing at greater water depths brings the additional challenge of minimising equipment size and weight for ease of deployment subsea.
The need for reliable and simple valve operation in the face of difficult conditions, together with increasing demand for smaller and lighter technologies, has seen the emergence of the rotary gate valve as a key enabling technology for next generation subsea processing systems. Small bore rotary valves have field proven success and are already widely used on the important subsea equipment such as Christmas trees and manifolds, where they play a vital role.
Small bore rotary gate valves, typically measuring 3/8 inch to 1/2 inch, 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). The simplicity of their operation sets rotary gate valves apart from other subsea valves apart from other subsea valve technologies such as through conduit and needle valves. Operation is through a simple 90°C turn. The valve internals cannot be damaged by over torque. The true metal-to-metal seals of the rotary gate valves are able to withstand a wide 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 apparent. Also, when a valve is stroking the metal-to-metal seals maintain full surface contact and have a ‘wipe clean’ action. This ensures the contact surface of the valve is kept residue free, optimising reliability. Constant metal-to-metal stem back seating is another key feature of this design.
Subsea valves, in general, must be capable of withstanding any fluids that may pass through them. These could include freshwater, 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 hydrogen sulphide are specified for ‘sour service’, and their material classes are selected accordingly. By using minimal moving parts, the rotary valve becomes more reliable. It is not susceptible to problems arising from contaminated fluids and is water depth insensitive. Until now, small rotary gate valves have usually been limited to corrosion inhibitors and scale preventers injected small volumes. Going forward, with the development of a 1 inch version, it will be possible for larger volumes to be handled. This will make rotary gate valves suitable for injection of methanol for preventing hydrate build-up, enabling the replacement of larger and heavier through conduit and needle valves.
Fresh thinking needed
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). Recent years have seen the spectrum of required temperature ranges for subsea extend significantly, to lows of -46°C (-51°F) and highs of up to 205°C (401°F). Traditionally, the maximum required pressure rating for a valve was 15,000psi, whereas future designs will demand 20,000psi.
Valve technologies are rapidly evolving to meet these extreme conditions. A key focus of rotary valve R&D has been developing smaller, lighter designs, with minimal moving parts and uncomplicated operating mechanisms to significantly reduce risk of failure. It follows that if design engineers are to meet the increased demands for subsea processing, there has to be a recognition that small bore valves are part of the solution to achieve a range of design and performance objectives. This in turn will encourage greater collaboration between valve makers and designers working on FEED studies.
Optimised, compact valve design
For the next generation subsea processing systems, compact design will mean valve size, shape and interfaces are more important than ever. As well as being smaller than other valves, the shape of rotary gate valves can be customised to fit a particular space within the Christmas tree or manifold without compromising performance.
ROV interfaces and hydraulic connections may not have an impact on 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.
Small bore rotary valves present significantly more options than other types of valves. Collaboration between design engineers and valve manufacturers will help ensure the oil and gas industry is well placed to deliver the next generation of subsea processing systems.