Operators have long been attracted to the cost savings and production increases offered by subsea separation. The technology challenges, however, have so far prevented the concept from becoming reality. But hope is on the horizon.
In a paper presented recently at the Deep Offshore Technology conference in New Orleans, the authors discuss the benefits, technical challenges, and possible solutions for subsea separation. Rune Fantoft of FMC Kongsberg Subsea AS and Toine Hendriks of CDS Engineering BV presented the paper, “Deepwater subsea separation - technical challenges and solutions.”
One example they presented was for subsea water separation followed by water re-injection back to the reservoir. Although it has great potential, the authors said high added cost calculated for technical risk, some missing qualified components, and general conservatism has held back this market. But, since the cost savings potential is tempting and as the technical uncertainties are reduced as a result of extensive R&D work, the subsea processing option becomes more and more interesting for the operators.
As the authors point out, FMC Kongsberg Subsea and CDS Engineering have worked together to develop and qualify technology for compact subsea separation systems.
In most applications where subsea water separation is considered for near-term installation, the main driver is that the subsea option is more cost efficient than the alternatives. In mature fields with increasing water production, such production to a surface facility must be choked back, or the liquid handling capacity of the system has to be increased.
Removing the water from the well stream at the seabed reduces the total flow to be lifted to the surface. This dramatically reduces the static head of the fluids in risers, which facilitates higher production rates from the reservoir and increased revenue for the operator. The effect of the reduced static head becomes especially interesting for deepwater application.
In addition, they note, subsea water separation is seen as a competitive tool for artificial lifting in deep waters compared to gas lift, subsea multiphase pumping, and ESPs.
In addition to the direct economical arguments, the separation of the liquid phases at the seabed can be carried out at more favorable process conditions, they said. Separation problems can be reduced and potentially eliminated by performing this process at the seabed. Effects that can contribute to enhanced separation efficiency compared to surface processing include:
Subsea water separation can be beneficial as a field development element both for technical and economical reasons.
• Higher separator pressure res-ults in lower hydrocarbon density and an easier separable oil phase.Higher pressure can alsogive fewer emulsion-stabilizing agents that can cause separation difficulties
•Higher temperature (as the separator is upstream of the well-stream cooling in pipeline / riser) creates easier oil-water separation conditions due to lower viscosities
• Separation closer to the reservoir and at higher pressure gives a shear history of the oil-water mixture that can result in less emulsification and easier separation conditions than if this process is performed further downstream
• Potential reduction in fluid mixing from different subsea reservoirs, as often found in separators at surface facilities, which can cause separation difficulties
• Less risk of slugging and flow instabilities at the inlet of the subsea separator, as the separation is carried out upstream of long pipelines and risers.
Technical challenges
For any subsea water separation application there are some key challenges, and the authors pointed out these:
•Water re-injection quality with respect to the allowable content of oil-in-water and solids-in-water as well as the maximum allowable solids particle sizes in the re-injection water
• Solids separation from the well stream when solids are produced at a higher rate than is allowed in the water to be re-injected
• Removal of separated solids to a suitable location to avoid any accumulation of solids in the system over the operational lifetime
• System flexibility for varying operational conditions all through the system lifetime. In this context, the control of the key parameters of the separator is essential, such as active level monitoring and control in gravity separators
• High overall system availability requires a robust and reliable design of all components to ensure good cash flow for the operators
• Retrievability of key components, including the separator unit, to facilitate modification in case of potential failure due to any unforeseen events.
For deepwater applications the separation process needs to be in compact separation units to facilitate deepwater installation and retrieval. It can also be difficult to manufacture large units for the fields due to the excessive external pressure. Hence, the authors say, the dimension of the separator becomes an important parameter.
With respect to separation technology for subsea water separation processes the authors see the main focus on the design and solution for:
• Ensuring effective water separation from the oil and gas stream
• Handling associated sand produced with the well stream along with the fluids.
Two attractive building blocks suitable for performing the separation are:
• Gravity-based separation, which is the conventional way of performing the separation in topside processes, but further optimized for subsea applications
• Separation technology that uses high G-forces generated by swirling flow to facilitate ultra-compact phase separation.
Subsea water separation technology is becoming available with great economical potential and technical benefits for many applications, and is increasingly beneficial for deepwater and applications with challenging separation conditions.
Extensive qualification work being carried out to meet the requirements for the subsea water separation applications includes both new separation technology as well as auxiliary equipment needed for the complete subsea separation processes.
A new compact gravity based separation concept with integrated solids handling has been qualified through a completed test program. As a result, subsea water separation system is now considered to be available for near-term installation.
New separation technologies including new ultra-compact inline separation technology are emerging, which should be especially interesting for deepwater subsea applications.
