In 2014, one of the largest semisubmersible offshore platforms in the Gulf of Mexico began producing in water depths that exceeded 3,000 feet. To control scale, corrosion and hydrate formation, the platform utilized hydraulically actuated diaphragm (HAD) metering pumps for flow assurance. No metering pump failures have occurred despite the remote location and harsh environment. This success led the supermajor to select the same metering pumps for its next ultra-deepwater platform, which will be deployed in an even deeper part of the Gulf of Mexico later this year.

However, with the recent decline in oil prices, the operating environment has changed in the last few years. The downturn has taught offshore producers valuable lessons and they now seek cost efficiencies and technological innovations to simplify and streamline operations. Metering pumps manufactured with distinct features are proving to meet the requirements for today’s offshore operating environment.

Major deepwater production challenges

As new projects extend production operations to depths exceeding 5,000 feet, some processes — such as flow assurance — have become more important than ever because the cost of failure has increased exponentially.

Flow assurance focuses on the continuous delivery of hydrocarbons from the reservoir to the refinery.

One of the highest risk areas in ultra-deepwater environments is the “Christmas tree” that sits on the seabed. The back pressures funneling into the Christmas tree from below the seabed can measure 10 times greater than the pressures needed to move oil and gas through the pipeline to the platform. The Christmas tree acts as a choking mechanism that reduces the pressure from the subsea side of the well, which can exceed 15,000 psi, down to standard pipeline pressure at approximately 1,400 psi.

It is at this choke point where hydrates are most likely to form. Hydrates are ice-like structures that form when light hydrocarbons and water mix under high pressures and low temperatures. Hydrates restrict flow, and if left untreated, they could plug the wellhead, block production entirely and cause expensive remediation operations.

Figure 1. For high-pressure chemical injection, metering pump performance ratings may be required to reach up to 20,000 psi (1,379 Bar).

A metering pump’s role in offshore production

Hydrate formation is best mitigated with the help of chemical inhibitors like methanol. Inhibitors work by lowering hydrate equilibrium temperatures to a point at which hydrates cannot form.

Chemical inhibitors are delivered continuously (or intermittently) by metering pumps, which are positive-displacement chemical dosing devices that deliver a wide range of chemicals to provide flow assurance and equipment integrity and protection. Typically metering pumps must convey these fluids at high injection rates and at high pressures, which are needed to overcome friction losses and to move the chemicals through long subsea tiebacks that can stretch for miles.

Metering pumps in offshore environments must comply with API-675 standards, which stipulate requirements for design, construction and performance. The HAD metering pumps not only meet these requirements, but they can deliver the massive amounts of power and pressure required to pump flow assurance chemicals via umbilical cords to wellheads in ultra-deep environments.

Pressure, flow rate & configuration criteria

For the platform set to come online this year, all of the functional testing has been completed and the various stages of the platform are being assembled. Once deployed, the new site will be one of the largest ever deployed in the Gulf of Mexico, with up to 20 Christmas trees from six drill sites that each tie back to the topside platform.

The supermajor operator and its engineering, procurement and construction contractor selected the metering pumps based on specific pressure, flow rate and pump characteristics.

Much of the success this supermajor has achieved with its flow assurance operations comes from the strength and reliability of the metering pump’s metallic diaphragm, a technological advancement specifically designed to reach higher pressures. To deliver flow assurance chemicals like methanol to multiple wellheads in 5,000 feet of water, and then deliver them an additional 25,000 feet below the sea floor, this type of diaphragm conveys chemicals at high injection rates and pressures of up to 20,000 psi.

A metering pump’s diaphragm is usually the limiting factor for the pump’s performance. Since the early days of the ultra-deepwater play, few metering pump companies had any experience addressing pressures exceeding even 10,000 psi. It was generally understood that metal diaphragms were required to handle the extreme pressures, but pump manufacturers had yet to discover how to flex a metal diaphragm so it could withstand the pressure without failing.

Milton Roy, metering pumps, offshore, oil and gas

Figure 2. A crosssection of a metering pump fitted with a hydraulically actuated disc diaphragm liquid end

Through years of innovative engineering and through extensive trials with application-specific metal alloys, an HAD metering pump was developed that continues to set the standard today. The combination of the material in the diaphragm’s construction with the inherent design, which keeps the diaphragm balanced between the process fluid and the hydraulic fluid, enables the diaphragm to operate under low stress. Because none of the moving parts get stretched or compressed, the pump can deliver the required flow rates at the required pressures and operate for up to 96,000 hours with minimal maintenance.

The supermajor also based its selection criteria on two other critical areas: flexibility and reliability.

Having metering pumps with a flexible, configurable design is essential in today’s operating environment. Equipment housed on the platform needs to operate as effectively and cost-efficiently as possible. Metering pumps must be engineered to fit into vertical or horizontal motor configurations so they accommodate the footprint of different platform requirements. Metering pumps must be able to deliver a wide range of flow assurance chemicals, and they must be able to pump different corrosion inhibitors and antiscaling agents. This flexibility is best achieved by pumps that offer liquid end options that can be customized to different applications.

Metering pumps must also provide extensive turndown capabilities. Turndown is expressed as a ratio over which one can adjust the output of a metering pump. At any point during operations, pressure and temperature can vary widely, requiring different quantities of chemicals at different times over the life of the well. The metering pumps used should offer different stroke and drive adjustments that can achieve the flow rate required by the process.

When discussing offshore operations, equipment reliability remains one of the top requirements for a metering pump because the value of a single hour’s worth of production exceeds the cost of any given pump.

Equipment deployed on offshore platforms is difficult to reach and maintenance is expensive. Metering pumps used in these environments should be designed to run for 20 years without unplanned or emergency maintenance intervention. Predictive maintenance for wear items such as check valves, O-rings and seals should be performed on schedule, and all wear items should be able to be replaced in less than an hour without specialized tools. Because of these reasons, the reliability of the HAD pump design remains unmatched by other types of pumps.

Conclusion

Despite particular requirements and unique challenges, offshore oil and gas production still represents one of the industry’s best rewards, as hundreds of billions of barrels remain sitting in known, yet untapped offshore reserves around the globe.

The U.S. Energy Information Administration estimates that crude production in the Gulf of Mexico will reach record levels this year, with as many as six massive deepwater fields set to launch within the next 18 months.

HAD metering pumps are poised to enhance the efficiency of ultra-deepwater production as they strike a balance between the critical requirements of power, accuracy, safety, flexibility and reliability while fulfilling the short- and long-term cost benefits.

Kenny Louque is the southern regional manager for Milton Roy where he has served the oil and gas industry for the last 35 years. Louque can be reached at ken.louque@miltonroy.com or 337-754-7834. Milton Roy was the inventor of positive displacement metering pump technology and has serviced the needs of oil and gas producers and refineries since 1936. For more information, visit miltonroy.com.