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Floating Production Storage and Offloading

Designing valves for harsh environmental and processes conditions.
#pressure-relief

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Over the past 40 years, floating production storage and offloading (FPSO) vessels have become a cost-effective source of oil and gas production in remote locations. These large vessels are self-sustaining storage and processing facilities on the ocean that generate their own power to prepare crude oil from wells into oil and gas for pipeline distribution and oil tank transfer. The processes alone are challenging, but on an FPSO, the equipment must also be designed to fit into compact areas and specified for the marine environment.

With the remote location comes the expectation that equipment must last as long as possible and work safely and consistently. Suppliers are expected to design their products with all these factors in mind. Control valves aboard FPSOs are considered a small but critical piece of the process as they are necessary to control flow accurately from one area of the process to another.

If a critical control valve is not working properly, the entire system can be put at risk — but when functioning properly, the processes will run optimally with maximum output. Sizing a critical control valve for offshore applications has multiple challenges: high pressure, corrosion, erosion, cavitation and noise, all the likely culprits in standard flow control challenges. In addition, precautions must be taken for the corrosive environment and remote location if repairs are needed. It is imperative that when selecting control valves, you find the safest solutions that have extended life cycles, mitigate damage and control processes preventing unplanned down time and maximizing product output.

FPSOs are huge vessels custom built or retrofitted from existing oil tankers. They are advantageous because they can hold both process products and store saleable products. The upfront cost can be high, but savings is typically realized over the life of a project. Some of the advantages are: typically up to 50% less set up time than an offshore platform; no permanent piping or infrastructure required, and most importantly, when oil wells are depleted the FPSO can be moved to a new location and reused.

From the bottom side of the FPSOs (see below) there is a network of connections that are anchored to the bottom of the ocean floor by a mooring system which allows for rotation or movement of the vessel as needed. The primary connections are umbilicals and flowlines. The ‘umbilical’ connections are essentially electrical conductors providing power, control and communication for production. There are also production lines knows as flowlines that carry crude oil from the well to risers connecting to the vessel. The flow lines can be used also for injection of seawater to extract oil when reserves are getting low in the well.

Rendering of the Barossa FPSO.
Source: MODEC

On the top side of the FPSO, units are modularized to save space. In the very simplified process shown below, wellhead fluid or raw crude is separated for multistage processing. The raw products are processed as a liquid or gas before being transformed into a final product and stored. Note that ocean water is also used in this process and is either put back into the ocean, treated for use or injected into the oil well.

Source: SBM


Critical Control Valves in FPSO

Hundreds of valves are required for production and process control aboard FPSOs. Each has a specific purpose, some more critical than others. Our focus in this article is to highlight critical applications, understand the purpose of the valves, process challenges and show solutions that allow operations to function smoothly. While not specifically mentioned, in each application it is assumed that all external components of the valve should include stainless steel accessories, tubing and fittings, to combat the effects of the salty marine atmosphere. Using any materials other than Austenitic, Austenitic Ferritic (Duplex) Stainless steel or nickel alloy (CRA, Corrosion Resistant Alloy) could lead to premature corrosion, safety issues and production down time. Also, it’s important to note that with limited space, storing spares onboard for repairs is not feasible. With the nearest valve shop hundreds of miles away, sizing and selecting the right control valve for these applications is key.

Antisurge Control Valves in Gas Compression

The production of gas starts with isolating lighter hydrocarbons from crude taken from oil wells. Through several processes of separation using high- and low-pressure steps and cooling, the gas is then ready to be compressed. The process involves expensive compressors, that can only operate between 50%-100% of rated capacity. Due to fluctuations in the process, compressors can experience surges when the upstream pressure becomes lower than the outlet. When this happens, flow changes direction and can cause major damage to the compressor.

To avoid such occurrences compressor stations are equipped with anti-surge control valves to protect compressors. They are positioned downstream of the compressor so that when an upset occurs and there is risk for reverse flow, the control valve can open, and decrease the pressure downstream, maintaining flow in the desired direction. To work successfully the valve must open quickly (1-2 second by positioner and 0.5-1 second opening time by solenoid). Anti-surge valves must maintain an acceptable noise level (difficult with the high-pressure drops), be comprised of corrosion resistant materials, and consistently maintain a class V shutoff.

Compressor antisurge recycle valve. Source: Baker Hughes

Top: Example of antisurge valve. Bottom: 30” aluminum bronze ASME B16.34 Class 150 FF (flat face), 1 stage special trim for noise attenuation. Either raised or flat face can be used as end connection in this application depending on customer preference. Source: Baker Hughes

Cooling Water Valve

With plentiful amounts of seawater available, the FPSO requires large quantities of water for applications to assist in operations namely for cooling down equipment on the vessel and reinjection into the oil well for production. Sea water is pumped from the ocean via a sea water lifting pump followed by further treatment including removal of sulfates and air before going through the cooling water valve. The challenge in this application is the high capacity and noise levels. Low pressure globe valves are required to handle high capacities and special trim is required to attenuate noise.

As with every application aboard an FPSO, corrosion is a concern, therefore special materials for the body and trim must be used to mitigate damage. Commonly used body materials such as nickel aluminum bronze, duplex and Hastelloy C are highly effective and have excellent corrosion resistance. A combination of exotic alloys can be used for trim materials to help extend the life and functionality of the valve.

Sea Water Injection Systems

Sea water injection systems on an FPSO consist of overboard dump valves, pump recirculation and injection to well valves — each type critical to production. Each valve application requires high rangeability and the ability to function with high-pressure fluctuations, cavitation and entrained particles. For example, the overboard dump valve is used for start-up of the high-pressure injection pump. Even though it is only used at start- up, it is critical to the process and must function on demand. The conditions are harsh containing high pressures that drop to atmospheric pressure at the outlet. Cavitation and vibration are associated with this process and should be accounted for.

Cavitation is a phenomenon that occurs when the pressure of a fluid drops below its vapor pressure and then recovers above the vapor pressure. As a result, bubbles form and then implode causing damage to the closest metal boundary, severely shortening a valve’s performance and life expectancy. Similarly, vibration from high pressure drop will induce mechanical wear and fatigue on parts as well. A robust valve design must be selected with multistage trim to control the pressure drop in stages, eliminating cavitation.

One example is a valve used in high pressure applications that uses multiple stages to eliminate cavitation, taking the pressure drop in stages so the process fluid doesn’t drop below the vapor pressure at any point. In some cases, a downstream plate can be used to provide back pressure and alleviate some of the pressure drop from the valve. Valves can range from AMSE B16.34 pressure class 1500 to API 6A rated pressures 10,000 or 15, 000 psi and can be found in a number of materials with multiple trim stages depending on process conditions.

The valve pictured below is a good example of a robust design that can be specified up to 15,000 psi. It contains multiple stages to reduce pressure and is trash tolerant due to the axial flow. The trim is designed to never allow the pressure to drop below the vapor pressure, thus preventing cavitation.

Conclusion

FPSOs continue to be a popular solution for processing and storing crude in remote areas of the ocean. They are advantageous because they do not require any permanent underwater infrastructure, take 50% less time to commission than a traditional offshore platform, combine upstream and midstream processing into one site, and can be moved to another location when the oil wells are depleted. Control valves are vital components of the process and are integrated into all units of the FPSO, some more complex than others. Some of the most challenging control valves are antisurge, cooling water and sea lift system valves. They are used in the process to control temperature, flow and pressure with harsh service conditions and can highly impact production if they are not properly sized and selected. Each control valve should be carefully designed for a given application to overcome flow control challenges while mitigating damage from the environment. Control valves are critical to the offshore vessels, ultimately keeping the process running safely, protecting equipment, and maintaining quality product output.


Rebecca O’Donnell is a product manager for Masonielan control valves for Baker Hughes. She has more than 20 years experience in promoting, specifying and educating the  market on the value and technical benefits of control valves.

Teo Arcopinto is a valve solution engineer with Baker Hughes, working in Casavatore, Italy. He has been in the control valve industry for more than 20 years, focusing on power and oil and gas severe service valves. 

  
 

 


 

Editor’s note: An update was made to the online version of this story. The caption under the 30” aluminum bronze valve originally said it was a raised face flange but it is actually a flat-face flange connection.

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