Subsea valves pose specification challenge

As subsea oil exploration forges its way into more extreme environments, niche hardware expertise becomes critical to success.

Bent1 1301vsd

Enlisting the aid of experts smoothes path to success

John Drew
LB Bentley, Severn Glocon Group

As subsea oil exploration forges its way into more extreme environments, niche hardware expertise becomes critical to success.

Reliability is the watchword for subsea production hardware, and this goes beyond big-ticket equipment such as christmas trees and manifolds. Each component and subcomponent of a subsea production system needs to perform reliably to optimize overall safety and productivity.

Establishing a partnership approach between suppliers and operators could bring significant benefits. It could also accelerate the development of new technologies to enhance safety and productivity at an industry level.

Small-bore subsea valves, typically associated with the injection of well chemicals or the control of bigger valves, are a case in point. There can be as many as 30 of these valves on keystone equipment such as christmas trees, and these valves are vital to the performance of these assets. These small-bore subsea valves appear itemized on the "critical components list" and should be treated as an integral part of the hardware jigsaw. They are a solution, not a short lead time commodity, and they can help enhance oil recovery rates cost-effectively and safely.

Bent1 1301vsd
Subsea valves play a critical role in the successful operation of production systems.

There is no such thing as a "standard" subsea valve. The conditions encountered and the variations among individual wells mean off-the-shelf products are redundant. Temperature and pressure ranges are widening, and this impacts all valve components and materials.

It is possible to rationalize the overall design of valves to make them simpler, less vulnerable, and more robust. But there is an increasing need for better optimization in line with actual operating conditions. This trend will continue as exploration heads into deeper, more extreme environments. To truly maximize safety, performance, and longevity, it is best to focus on precisely specified requirements for each individual project and application.

Up-front optimization

Operators and engineering contractors generally talk with valve specialists during the execution phase of a project. In many cases, by the time the valve suppliers are involved, the specification has been predetermined by operator engineers. What's more, suppliers are often briefed by the procurement department - not the engineers themselves - making it difficult to have a direct, technical conversation up front.

Consequently, there can be a gap in niche small-bore expertise during the front-end engineering and design (FEED) phase, and this can impact delivery times and budget during the execution phase. Any issues identified by the supplier after the specification has been received need to be flagged and communicated to the operator engineers. If they are not in direct contact, this can add days, even weeks, to the lead time. A short conversation at the outset could prevent the issue from escalating.

In the longer term, valves that have not benefitted from up-front expert specification might not perform to the best of their ability. Over the decades that they are in service, even a small reduction in efficiency can have major implications for overall performance and recovery rates.

Keep it simple

When it comes to specification of valves, the user-friendly mantra "simple is best" should prevail. However, recent meetings of the Association of Well Head Equipment Manufacturers (AWHEM) have raised concerns about a growing tendency for over-specification and ambiguity in requirements.

Sometimes, unrealistic demands lead to production bottlenecks, increased costs, and longer lead times - with little or no real impact on well safety or productivity.

Everyone who works in subsea knows no two projects are the same. It stands to reason that it is preferable to define exact technical requirements at the outset of a project, using the help of niche expertise. This helps ensure that valves are specified, designed, and manufactured for optimum performance and reliability for the operating envelope.

Intelligent specification

Small-bore valves simply need to open and close to release fluids as required. They need to do this reliably and within the appropriate space available. However, the extreme conditions of subsea environments have an impact on the performance of this straightforward task. Depth, pressure, and temperature factors layer the process in complexity, and the situation is compounded by evolving industry standards and qualifications.

The best way to specify subsea valves is to move away from seeing them as commodity items and to look at the solution they provide.

Initially, it is important to consider the parameters of the application. This is where suppliers' niche expertise can impact reliability. Up-front awareness of application challenges and conditions - water depth, temperatures, pressures, and service media - can facilitate creative, open-ended engineering that leads to breakthrough design solutions.

Valve materials, generally determined by the operator, also can have a bearing on decisions surrounding valve type and bore size. Core factors influencing material selection are strength, corrosion resistance, and cost. In general, the main pressure-containing components of small bore subsea valves are made of duplex stainless steels, nickel-based alloys, and occasionally low-alloy steels clad with a weld overlay in an exotic material. Tungsten carbide, with its very high hardness and wear resistance, is often used either as a hard-facing material sprayed onto valve gates and seats, or as solid gate buttons in rotary valves.

Bent2 1301vsd
All the materials of construction in a small-bore rotary valve need to be considered when specifying for a particular use.

Subsea valves need to withstand all fluids that pass through them. These fluids can include fresh water, sea water, injection chemicals, and wellbore fluids. Even valves that will only see clean injection chemicals during normal service may be subjected to aggressive well fluids in a fault situation. Valves that could be exposed to hydrogen sulfide need to be specified for "sour service," and their materials selected accordingly.

The size of valve specified relates mainly to the volume of fluid that needs to flow through it. Injection chemicals vary considerably but usually include methanol, which can be supplied in large quantities, hence the use of 1-in. bore valves. Other injection chemicals such as corrosion inhibitors and scale preventers tend to be injected in smaller volumes, so a ½-in. bore valve may be sufficient.

Other, more superficial, requirements might include ROV interfaces and hydraulic connections. These may not have an impact on the internal valve function, but intelligent specification can still bring 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.

HP/HT challenges

Among the considerations for many contemporary subsea projects are the particular demands of high-pressure/high-temperature (HP/HT) applications. HP/HT wells currently are concentrated in the Gulf of Mexico and the Northern seas. However, there are pockets of HP/HT activity in other regions, such as Asia/Pacific.

It is common for valves destined for these regions to need to withstand pressures of 16,000 psi (110.3 MPa) and temperatures of +177°C (350°F). In fact, recent years have seen the spectrum of required temperature ranges for subsea extend significantly from P to U (-29°C to +121°C, or -20.2°F to +250°F) to low temperatures of -46°C (-51°F) and highs of up to +205°C (400°F). A few years ago, the maximum required pressure rating for a valve was 15,000 psi (103.4 MPa); whereas today orders increasingly demand up to 20,000 psi (138 MPa).

Valve technologies are rapidly evolving to meet these extreme conditions.

Material issues

Since the subsea journey began, valve developments have focused mainly on designing, streamlining, and optimizing the internal components. Metal-to-metal seals, pioneered in the 1980s and 1990s, bring many advantages and can withstand the requirements of today's subsea applications. But the elastomers/polymers used in the hydraulic actuators of these valves need to be considered, too, and are set to be a crucial area for research and development over the coming years.

While metal-to-metal seals might withstand a full range of operating temperatures, elastomers/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 that the fact that the integrity of elastomers/polymers can be affected by the media passing through the valve and you have a very complex scenario.

Call for collaboration

As subsea developments continue, the range of operating temperatures and pressures will continue to expand. The industry needs to look ahead and to consider how to ensure subsea hardware is poised to deal with new challenges.

A key focus of research and development is bringing simplicity to the complex needs of the subsea industry. Designing valves with minimal moving parts and uncomplicated operating mechanisms reduces the risk of failure, which maximizes safety and productivity. This philosophy can extend to individual christmas trees and manifolds as well as to processes such as specification and the overarching industry qualification standards.

To accelerate technological developments and keep pace with emerging requirements, hardware suppliers and operators collaborate at a micro and macro level.

A joint approach is necessary to really define applications and operating environments. An intelligence-led approach is needed to specify individual valves and other hardware components so they are designed to meet real needs safely and effectively. Let's identify the technology gaps that exist in this exciting industry, and facilitate breakthrough, creative engineering to help close them.

More in Production