Offshore/subsea systems

How Will Subsea-Processing Technologies Enable Deepwater-Field Developments?

This study examines how subsea processing (SSP) can develop into an important enabling technology for future ultradeepwater-field developments and long-distance tiebacks.

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This study examines how subsea processing (SSP) can develop into an important enabling technology for future ultradeepwater-field developments and long-distance tiebacks. The authors identify the gaps that need to be closed and describe the decision-making process during the field-development life cycle by considering the technical and economic constraints of various SSP technologies.

Introduction

A generalized definition of SSP is any active treatment of the produced fluids at or below the seabed to improve recovery factor of reservoirs. SSP technologies include multiphase pumping, subsea separation, gas compression, and raw-seawater injection.

Subsea separation coupled with liquid boosting is effective in enabling production at very low flowing tubinghead pressures, even in deep water. This method also is well-suited for use where heavy, viscous oil or low reservoir pressure is the rule. Gas fields often are developed with subsea wells and multiphase transport to onshore facilities or to offshore processing platforms. Separation allows decreasing boosting-power requirements. Subsea-separation technology is progressing quickly because of its huge potential in minimizing topside water-handling requirements and separation of gas, oil, and water from the production fluid. Subsea gas-compression technology is one of the faster-growing technologies for large fields requiring pressure boosting (e.g., where subsea-to-beach development solutions result in long tieback distances). It improves the production and recovery from the reservoir by reducing backpressure on the wells.

As of this writing, more than 25 subsea boosting systems and six major subsea separation systems have been installed or awarded throughout the world. Given the growing number of greenfield and brownfield applications, some analysts anticipate the number of SSP systems installed globally to double by 2020.

The complete paper contains a detailed discussion of the development of these technologies, from their origins to their current incarnations.

Why SSP?

SSP is typically considered for systems with a tieback to a host structure and can influence all phases of project life (startup, plateau production, late life, and tail end). SSP can consist of the following:

  • Use of single- or multiphase pumping systems to enhance the fluid-driving energy (subsea boosting)
  • Subsea separation and disposal of the produced water (two-phase separation)
  • Subsea separation of oil, gas, and water (three-phase separation)

Appropriately applied SSP technology can bring benefits such as the following:

  • Increase in recovery factors by reducing backpressure on wells
  • Enabling of long-distance tiebacks to host facilities
  • Mitigation of flow-assurance risks with bulk separation of phases
  • Reduction of topside infrastructure and payloads
  • Debottlenecking of existing topside water-handling/treatment facilities
  • Lowering of energy requirements by discharging unwanted water at seabed or injecting back to reservoir
  • Potential cost savings over use of conventional technology
  • Fewer offshore personnel, materials, emissions, and decommissioning tasks

Multiphase boosting can be applied at the topside facilities, riser base, wellhead or subsea manifold, or downhole. The applications at the surface wellhead, where the technology has been proved, raise confidence in operators for subsea applications. The use of subsea pumping can boost the production rate during startup and prolong the plateau production phase, reduce operating life, and increase ultimate recovery, therefore maximizing the net present value of a field development.
Technology gaps, however, include operational issues, electric-power systems, pump-control systems, topside control systems, and subsea pump stations.

Key factors affecting SSP-system-­concept selection include reservoir characteristics and fluids, cost, reliability, operability, and the potential for required intervention. These factors are discussed in detail in the complete paper.

Status, Challenges, and Limitations

The current status of subsea developments has been changing rapidly, with water depths reaching 10,000 ft, tieback distances reaching 100 miles, and seabed pressures of 15 ksi. Subsea-production systems in the market demand 20-ksi systems and beyond because the new developments often involve high-pressure/high-temperature (HP/HT) fields.

The key factors that have slowed down SSP growth include overall installed-­project cost, risk, and complexity in ­designing the complicated systems and technology-qualification programs. Many operators are focused on low project capital expenditure (CAPEX). Variable and nonstandardized requirements can lead to high risk, and technology-qualification programs cannot eliminate risk. But new lower-cost technologies and system architectures have allowed for reducing or even eliminating the need for much costly equipment. Better qualification programs are being implemented for deeper water and tougher environments. Industry is finding solutions to reduce complexity and decrease risk. Companies are streamlining their technical staff, and suppliers are combining to provide broad system-level expertise.

On the basis of expert reservoir analysis, there are hundreds of wells that could benefit from SSP in the current economic environment. It is necessary to move past pure CAPEX concerns to fully appreciate SSP’s tremendous potential in deepwater, long-distance-tieback, and Arctic-environment developments. Certainly there are opportunities for improvement, and subsea systems must be customized. Handling high-voltage power requirements of an SSP system, particularly subsea pumps, is a major challenge. Industry is moving quickly toward efficient transport of conditioned well streams over long distances (up to 300 miles) by use of separation, boosting, and wet-gas compression. Industry is aiming for 6-MW pumps and 15-MW compressors as a major part of increased hydrocarbon-recovery initiatives. High-voltage power will be more efficient than hydraulic power, particularly over long distances. Subsea high-voltage power requires efficient subsea power-transmission systems and reliable subsea power-distribution systems.

SSP-Concept-Selection Effect on Deepwater Development Projects

The future of deepwater developments is certainly susceptible to market instability, yet the forecast for growth in the deepwater sector’s CAPEX is estimated to be in the hundreds of billions of USD. Production expenditure is expected to be concentrated in West Africa, Brazil, and the US Gulf of Mexico because of strong deepwater sectors.

The need for SSP is established at an early phase of the project (identification and assessment). Because SSP is a high-cost, high-risk, high-technical-content, and high-impact element in major development projects, once the decision is made, the best opportunity to make a positive impact on the life cycle of a major capital project is during the early conceptual and planning stages (the front-end-loading phase), well before the financial-investment decision is made. The early stage in a project’s life cycle is the time when the ability to influence changes in design is relatively high and the cost to make those changes is relatively low. An initial feasibility study is made to verify whether the project is viable, both technically and commercially, followed by front-end engineering design to determine the basic specifications for the facility to be constructed for the project.

Cost-Effective SSP

To overcome the challenges outlined previously, focus must be maintained on technologies in flow assurance, thermal management, multiphase boosting, subsea separation and compression, subsea produced-water purification and monitoring, reinjection of carbon dioxide, and low-cost well-workover procedures. Longer tieback distances, demands for improved recovery and flow assurance, and reduced topside-processing requirements will all contribute to the future growth of SSP solutions.

Future SSP facilities will grow to include several electrically driven pumps and gas compressors to transport oil and gas over very long distances. There is a need for robust subsea power grids for variable-speed drives and other electrical subsea loads supplied from an onshore power plant or platform.

Cost-effective processing solutions depend on three different factors:

  • Optimized subsea-field architecture
  • Reliable and robust equipment design
  • Enhanced system development

Industry focus to develop an improved design for most subsea hardware is shifting to concentrate on enhanced subsea separation systems, raw-water injection, and multiphase pumping systems. This includes efforts to further develop multiphase pumps, injection pumps, gas compression, integrated umbilicals, compact separation systems, subsea cooling systems, and power-distribution systems.

Summary

Over the next decade, industry will relocate separation equipment, water-­injection systems, power-distribution and -drive systems, and many other modules to the seabed, thus minimizing the size and cost of floating structures and eliminating topside costs. Materials-technology developments are needed in two areas: HP/HT-compatible materials and lightweight structures manageable even by nonspecial vessels or rigs. The key areas in the development phase for deep­water solutions are high-voltage power and fiber optics for long-distance tiebacks. Advanced process monitoring and control is a growing technology that allows operators to monitor continuously and take appropriate ­actions in real time.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 27661, “How Will Subsea Processing and Pumping Technologies Enable Future Deepwater-Field Developments?,” by Phaneendra B. Kondapi, Texas A&M University, and Y. Doreen Chin, Ashesh Srivastava, and Zuying F. Yang, Subsea Engineering Technologies, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.