Control valves designed/manufactured to withstand demanding offshore conditions
The days of drilling in shallow water to build company reserves are fading rapidly. In today’s offshore environment, extraction requires drilling deeper wells at greater water depths, in more remote locations. As wells get deeper, their pressures are increasing steadily to almost 15,000 psi. These conditions pose problems when specifying components, including control valves, Fig. 1. There are many challenging service control valve applications on an offshore platform, due to high temperatures, high pressures, massive pressure drops, severe erosion and nasty corrosion. To ensure continuous production and safe operations, producers require upgraded control equipment and optimized designs to address these issues.
WATER INJECTION VALVES
Water injection is not unique to offshore oil and gas, as it is used in onshore production as well, but the application is much more prominent in offshore.
In typical applications, produced water (water that is removed from oil) is separated and cleaned on the topside of the platform, and it is then injected into the lowest portion of the reservoir via separate injection wells. With the difference in densities between oil and water, this raises the level of the oil, moving it toward the producing areas. The need for water injection increases over time, as the reservoir becomes depleted. This method, known as secondary recovery, helps improve yield rates. Injection valves are required to handle higher pressures, as the depth of the well increases, Fig. 2.
The control of all valves in this system is done by the platform’s control system. The booster pump recirculation valve controls the flow of recirculated water from the booster pump output to its input. Recirculating the fluid around the pump increases the inlet pressure to the main pump recirculation valve.
The main pump recirculation valve controls the flow of recirculated water through the main pump to attain and maintain the required pressure. This valve’s job is to increase pressures to either inject water into the well and/or discharge overboard. The discharge valve sends high pressure water to injection wells or dumps water into the sea.
The injection valve system is typically flagged as one of the most severe service applications on a platform, and for good reason. With historical inlet pressures up to 8,000 psig and future requirements exceeding 15,000 psig, it is under extreme stress, Fig. 3. A valve failure in the water injection system could cost a facility 3,000 bbl of lost production per day, so the valve must be designed for high reliability and minimal required maintenance.
Typically, multi-stage globe valves are chosen for their ability to handle high pressure drops and achieve precise travel requirements. These valves come in a wide variety of materials to best meet the demands of each specific application.
Overboard valves simply dump produced water directly into the sea. The low pressure overboard valve circumvents the high-pressure pumps and goes directly overboard. Preference for these valves are globe constructions that can help mitigate cavitation, Fig. 4. With flow rates increasing, it is not uncommon to see large rotary valves being utilized to properly control these higher flow rates. Care must be taken when using a rotary valve to ensure proper cavitation mitigation trim is selected.
Cavitation is the formation and collapse of vapor bubbles in liquid flow-streams. They are caused by changes in pressure and velocity. These cavities form when liquid pressure falls to near the vapor pressure as fluids passes through the control valve. When bubbles implode, cavitation occurs. Cavitation is one of the results of choked flow. It is the point at which increasing the pressure drop while maintaining a constant inlet pressure yields no further increase in flowrate. Physical damage to valve trim is usually characterized by a pitted, rough appearance.
Some designers might evaluate these applications and assume a butterfly or ball valve can be used in this service, but it is not uncommon for this application to pull a vacuum due to waves crashing over the walls of an offshore drilling ship or platform. When the outlet pipe of the overboard valve is submerged in a wave, a vacuum is created in the pipe, increasing the severity of the application.
This is a major concern because malfunction of these valves can stop production. If the platform cannot jettison its produced water, the operation cannot function as designed. The water overboard valve is also part of a larger water flood system, which works in tandem with the water injection valves. Waterflood is a blanket term for a secondary method of enhanced oil recovery. Both valves work in tandem to keep proper injection pressures to the reservoir.
In refining, gas treatment or onshore oil and gas production applications, standard-wall steel pipe is often used. However, offshore producers have additional issues, such as corrosion and vibration. These added problems create the need for more stringent piping strategies.
Seawater poses ultra-severe corrosion concerns. Materials like Duplex and Super Duplex are increasingly becoming insufficient for these applications, so higher grades of material are being used, such as titanium. Thin-walled titanium is a fairly standard offshore piping material, and it is becoming increasingly common to see fiber reinforced piping (FRP). The main reason for this trend is to decrease the weight of the overall system.
The valve industry uses a common data set to determine noise and vibration outputs from standard pipe. What is still in its infancy is the industry’s understanding of how to handle these newer pipe thicknesses and materials that are not guided by industry-accepted IEC noise predictions.
A standard-wall thick steel pipe provides substantial vibration dampening, but thin-wall metal or fiberglass piping provides much less dampening. High-vacuum applications, coupled with thin-walled titanium or fiberglass pipe, can therefore create excessive vibration. The effects of cavitation are also magnified with these types of piping systems, because they don’t provide the same dampening effects that standard or thick-walled piping does. The thickness provides additional vibration dampening and helps attenuate some noise from cavitation.
In an effort to address these issues, a designer may select a brute force valve or a cavitation isolation valve. A brute force valve is a stout valve designed to weather process-related damage and hold up without any specialized cavitation elimination techniques, like pressure staging. Using erosion-resistant materials in a brute force valve can be sufficient, if the pressure drops are low enough, but high-pressure drops often force consideration of different techniques, such as staging.
Isolation allows cavitation bubbles to collide inside a pipe and implode, thus protecting the valve and the downstream piping. The downside to isolation is that these implosions can induce a severe amount of vibration. As mentioned above, this is detrimental in a thin-walled or FRP piping system with inherently low damping. The solution is to reduce cavitation and its accompanying vibration with elimination techniques.
Cavitation causing heavy vibration can lead operations to shut down a process, to prevent equipment damage and harm to personnel. Another consideration with heavy vibration and cavitation is noise. Personnel aboard an offshore vessel are never very far from the process, and noise travels well over water, even if crew cabins are isolated from the process. Cavitation will lead to noise levels well above the OSHA recommended 85-dBA level. When a brute force or isolation approach doesn’t work, cavitation elimination is needed, which is accomplished by reducing pressure drop across two or more stages. Cavitation elimination comes with the added benefit of vibration reduction.
Butterfly and ball valves typically do not perform well in these types of high-vibration applications. It is not uncommon for a butterfly valve to fluctuate throughout its full range of travel, causing disrupted downstream flow and excessive vibration. Globe valves that are designed to eliminate cavitation are best-suited for this application
Although a multi-stage, cavitation elimination solution will work well in most applications, it can drive up costs to an unacceptable level. For best performance and longevity with lower costs, many valve vendors recommend the use of multi-stage, cavitation elimination technology, coupled with standard-wall thickness steel piping. This reduces the number of required stages and accompanying costs, due to the vibration dampening effects of the steel piping.
When decisions are made late in the process to change equipment, it can pose significant problems. The engineer responsible for sizing and selecting valves is typically different than the one responsible for piping design, but coordination between the valve and piping design is extremely important.
For example, multiple stages increase costs, and capacity is reduced when moving to multi-stage valve technology. Multiple stages reduce the flow area and flow efficiency of a valve. It requires more space to accommodate fluids and reduce pressures, accordingly, to remove cavitation. If a line has been sized, based on a valve with no cavitation mitigation trim, line sizes may need to be adjusted to compensate for the adjustment in capacity, typically involving a valve going up a line size or more. A better approach is to specify a valve with cavitation mitigation trim to minimize required line sizes, Fig. 5.
Newer offshore expansions and greenfield developments are trending toward higher-capacity lines rather than many smaller parallel lines. It is thus not uncommon to see a single, large, 20-in. ball valve required for one large line, whereas smaller globe valves may have been used in the past on multiple smaller lines. For these large valves fabricated with materials, such as carbon steel and 316 stainless steel, anti-cavitation trim can increase costs significantly. When using higher-alloy grades of material, like duplex, super duplex (et al.), costs can increase exponentially. Nonetheless, it is still more cost-effective in most cases to utilize anti-cavitation trim and smaller line sizes and valves, as opposed to upsizing lines and valves.
It can be tempting for a process engineer to specify a standard-grade solution for overboard applications because of the low temperatures and low pressure drops. However, when considering the potential for a vacuum, coupled with the cavitation potential due to a reduction in piping weight, extreme care needs to be taken for valve selection.
Best practice is for the engineer specifying the control valves to work in close coordination with the piping designer to ensure the entire system works together to minimize total cost while ensuring sufficient performance and reliability.
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