Components of a four-phase subsea separation system.
While four-phase subsea separation faces a variety of technical hurdles, the chief obstacle may be the lack of unified vision and long-term commitment by the industry. Subsea separation offers almost panacean advantages for deepwater and marginal field production. Imagine eliminating water and sand at the mudline and piping export quality oil and gas to shore without the need for a surface facility.
There's more. By eliminating multiphase flow, many problems can be avoided, such as the potential for hydrate formation, liquid slugging, and corrosion. The reduced hydrostatic head on the well stream means less pressure on the reservoir, which enhances producible reserves. If an operator chooses to re-inject some of this clean water, those reserve numbers rise higher.
Baljit Singh, a Systems Engineer for FMC Energy Systems Group's Offshore Integrated Development Division, said the single greatest limiting factor to the advancement of subsea separation, until recently, was the lack of a need for the technology. As many as 15 designs have been proposed, advanced, and in some cases implemented, but developments have been made in divergent areas. There is little sharing of information between distinct projects. This forces every team working on a concept to start with virtually a clean slate.
As is the case with a variety of major technologies, including riserless drilling and intelligent completion, Singh said the industry is moving in several directions at once, sometimes replicating efforts and making little progress. While one group may make impressive inroads into water separation and another in oil and gas separation, there appears to be a lack of a unified vision so that the combination of these developments does not occur.
Three generations
To illustrate this point, Singh said of the 15 or more subsea separation systems he studied, few showed substantial "step-change" progress over those developed in the 1970s and early 1980s. Much like riserless drilling technology, subsea separation has lain dormant because there was no pressing industry need.
To offset the costs of research and development, subsea separation had to provide an essential piece to the development puzzle. Singh, among others, feels deepwater and the renewed interest in marginal field development will provide the impetus to advance subsea separation technology.
Evidence of this can be seen in the upcoming installation of two subsea separation systems, one on the Troll C field in the North Sea and the other offshore Brazil. The emphasis of these systems differs - one separates out water, while the other focuses on gas and liquids separation - but the result is a new, enabling technology.
In an effort to understand where subsea separation stands as a viable technology, Singh categorized known systems, dating back to the late 1960s, into three generations.
- First generation projects: Those that have been either temporarily or permanently shelved and never progressed very far include the Zakum, SPS, 3 SSTS, GA-SP, GLASS, KBS, and Petroboost systems.
- Second generation systems: Those in which a prototype has recently been developed or is in the process of being developed include the VASPS, AlphaPrime, and SUBSIS systems.
- Third generation systems: Those still in the concept stage include the DEEPSEP, CoSWaSS, CoSPP, and DIPSIS systems. Singh said the KOS/Aker system his company is developing also fits into this category.
What Singh found in an analysis of separation systems is that as many as 24 operators have participated in the development of these systems. Of the 24, 11 operators have participated in the development of more than one system.
When looking at these different efforts side by side, using a variety of criteria, Singh said some interesting trends emerge. Many of the first generation designs rivaled the second generation systems, indicating there may not have been any "step change" progress between the earlier, shelved designs and the building of following units. While there was limited detailed information on the third generation designs, because they are still in the concept phase and proprietary, Singh said many of the existing second generation systems being built and implemented now ranked as high or higher as these conceptual designs.
Regardless of how the technology is developing, Singh said there are some major tech nical hurdles to be address ed before subsea separation can become a widespread solution.
Technical limits
The key areas of technology where existing systems fall short, according to Singh, are in predicting and reacting to the changing quality of wellstream pro duction. This includes parameters such as flow rates, gas-oil ratios, and viscosity. The flexibility of a design to adapt to the flow characteristics of an individual wellstream is critical in separation and at the heart of these designs. Further, the number of phases being separated and the quality of the production required mean that system control and level sensing play an important role.
While there certainly remain technical limitations to the development of this technology, Singh said the primary factor is one of risk aversion. It will take some money, and a lot of guts, to put these systems in place. Singh said that newly emerged "super-major" producers may be in a good position to drive this technology and realize the benefits. These companies are large enough to fund such a project and also diversified enough to absorb the potential setbacks that come with innovation.
The greatest benefits of subsea separation will come, according to Singh, in marginal fields, where the reserves cannot justify expensive and heavy topsides and where there is a greater incentive to enhance reserves. These are fields often shed by the majors and picked up by independents that are able to make a profit through innovative technology and project management.
Subsea separation would be a welcome tool for the independents, but it will take some research and development capital to make the technology a reality. So there is an incentive for even the independents to participate in this development and reap some of the rewards.
Separated water
Singh points to a handful of questions that need to be answered before subsea separation can become a reality. First, what will be done with separated water? Conventional wisdom says there are two things that can be done with produced water, assuming that it cannot be stored onshore.
- Reinjection of some or all the water will help maintain reservoir pressure and increase overall production, but the water must be clean of oil or it may clog and damage the formation. If the water is to be expelled at the mudline, it must be free of contamination.
- Sand is another problem area. Currently, there are two prevailing approaches to dealing with the separated sand. It can be gathered in a vessel at the seafloor, which will be drawn to the surface periodically to be emptied, or it too can be cleaned of all hydrocarbons and released at the sea floor.
Singh said there needs to be more thought given to both these areas since it is difficult and expensive to get such associated materials clean enough to be released to the sea or reinjected. To properly address the immediate problems with subsea separation, Singh said that one method would be to adopt a unified approach with a target field.
Sharing of information will help the industry make large strides toward a comprehensive solution, and at the same time, dramatically reduce costs. Operators commitment during the early concept and design phase and willingness to implement the system will allow it to be successfully tested in the field. While any design is a long way from eliminating surface facilities, such a focused effort would be an important first step.
