The move from avoidance to risk management
Before DeepStar, about 14 years ago, the oil and gas industry identified issues related to ensuring hydrocarbon flow through pipelines and risers, but even the terminology “flow assurance” had not been pinned down. In fact, DeepStar derived the name “flow assurance” from the terms used by Petrobras, the Brazilian deepwater pioneer.
DeepStar has spurred a change in the art of flow assurance. Dendy Sloan, director of hydrate research in chemical engineering at the Colorado School of Mines, recently analyzed DeepStar’s effect on the deepwater industry and drew a conceptual picture of the way flow assurance is changing.
In the beginning
A 1999 survey Welling and Associates conducted of 110 energy companies, listed flow assurance as the major technical problem in offshore development. Opening the Sept. 24, 2003, Flow Assurance Forum in Galveston, Texas, Professor James Brill of the University of Tulsa discussed the need for a new academic discipline called “flow assurance.” Such a suggestion, presented to an audience of 289 flow assurance engineers, would not have been conceivable a decade ago, when the flow assurance community totaled a few dozen people, Sloan says. Yet Brill’s question indicates the importance of flow assurance, particularly as it relates to hydrates, waxes, scales, corrosion, and asphaltenes. The industry considers hydrates to be the largest problem in the Gulf of Mexico by an order of magnitude relative to other production concerns.
The history of flow assurance
Before DeepStar took on the flow assurance challenge, industry operated with anecdotal evidence of blockages that could cause weeks or months of lost production and sometimes resulted in field abandonment. The thermodynamic knowledge of the pressure and temperature conditions of plug formation enabled flow assurance to move from apprehension to avoidance over the decade between 1992 and 2002, Sloan explains. This knowledge provided estimations for quantities of thermodynamic inhibitors, such as methanol or ethylene glycol, injected at the wellhead for hydrate plug prevention.
The engineering of “avoidance” is defined as staying outside the pressure and temperature range where hydrates form. “This range can be displaced to lower temperatures and higher pressures by adding inhibitors,” Sloan says.
Avoidance, however, causes problems of its own. “Operators, in some cases, have injected so much inhibitor that it has jeopardized the economics of the projects,” Sloan says. This, of course, motivated the search for alternative means of addressing flow assurance issues.”
Expanding solutions
The decade’s experience base, aided by joint industry projects like DeepStar, TUWax, WaxAttack, and CSMHydrate, can reduce some flow assurance design safety factors. DeepStar flow assurance studies, in particular, have addressed three major flow assurance challenges: hydrates, waxes, and asphaltenes.
The successes of DeepStar are many, Sloan says. Research carried out through the JIP has advanced the deepwater flow assurance art, in three ways:
- It provided a global forum to leverage flow assurance R&D, allowing both pioneers and early entrants into deepwater to operate more efficiently
- It increased industry’s toolbox in the three areas of flow assurance: blockage safety, prevention, and remediation, resulting in more tools at the industry’s disposal as well as some understanding of their limits and the consequences of failure
- It provided industrial “best practices” in such activities as production fluid sampling, wax appearance temperature, and the use of coiled tubing.
All of the offshore production options for deepwater fields must contend with flow assurance issues.
A conceptual picture of deepwater flow assurance emerges from these lessons learned, Sloan says. With few exceptions, industry has engineered flow assurance very successfully over the last decade of deepwater production. The experience base has enabled engineers to be more cost-effective in design and operation. Industry has placed particular emphasis on past learning from the North Sea, with current lessons from the GoM, to provide future technology for offshore West Africa.
With production in ultra-deepwater (> 7,000 ft), time-independent thermodynamic avoidance might not be economic, according to Sloan. In fact, Sloan says, DeepStar studies yielded field examples where providing flow assurance using thermodynamic inhibitors reached its limit.
In very deepwater, the cost of methanol injection is very expensive and sometimes can jeopardize project economics. Deepwater flow assurance has changed the paradigm from anecdotal knowledge and apprehension to thermodynamic knowledge that allows hydrate avoidance, but flow assurance is proving very expensive in deeper water, Sloan says.
The path forward
Driven by economics, the flow assurance paradigm currently is shifting from avoidance to risk management. In ultra-deepwater, engineers now design via economic risk-management, which entails such kinetic flow assurance tools as anti-agglomerants, slurry flow, subsea separation, kinetic inhibitors, electrical heating, and “out-running” hydrates (a transient flowline procedure to prevent hydrate formation on startup).
Efficient use of such risk-management tools requires the transition from thermodynamic (time-independent) knowledge of blockage pressure-temperature conditions, to the time-dependent kinetic knowledge of how blockages form. DeepStar has current projects to combine hydrate kinetics with multiphase flow simulators to assess where and approximately when hydrate plugs will form on emergency shutdown and start-up.
In the past, industry thought flowline chemistry - in the produced fluid and chemicals that were injected - determined flow assurance.
Jefferson Creek, a senior flow assurance advisor at Chevron, says, “More recently, flow assurance has come to mean the coupling of multiphase flow and production chemistry to manage the interface between the reservoir and topsides processing. Whereas before, a flow assurance evaluation would have discussed hydrates, wax, scale, asphaltenes, and corrosion, flow assurance now includes network modeling and transient multiphase simulation to determine the pressure, temperature, shear - more basically the operability of the designed facility.”
In current and future research (Phase VII and potentially Phase VIII) DeepStar plans to systematically address these challenges to provide the deepwater industry with a reliable transportation technology to ensure reliable supplies of offshore energy.
E. Dendy Sloan Jr., PhD, PE, is the Weaver Distinguished Professor of Chemical Engineering and director of the Center for Hydrate Research at the Colorado School of Mines, where he heads a group of 23 researchers on natural gas hydrates. The second edition of his monograph and software Clathrate Hydrates of Natural Gases, was published 1998. A second book, Hydrate Engineering was published in April 2000. [email protected]
DeepStar has contributed many firsts to the flow assurance community. Progress continues today through hydrate slurry flow and asphaltene deposition studies. DeepStar is advancing the understanding of fluid systems that are “self inhibited” with regard to hydrate formation. DeepStar’s flow assurance advancements are numerous, including:
- Demonstrating first hydrate blockage formation and decomposition by one-sided depressurization
- Conducting comparison studies to determine best practices for chemical selection to manage wax deposition
- Developing and testing pressure and temperature fiber-optic sensors for coiled tubing blockage remediation
- Comparing simulators for asphaltene precipitation, hydrate formation, and wax deposition as well as testing chemical compatibility and initiating studies for novel deposition management studies like coating testing to avoid wax deposition
- Sponsoring one of the first industry flow assurance forums and acting as the driver to include the first flow assurance session in the Offshore Technology Conference program in 1995
- Serving as a launching pad for other joint industry projects (JIPs), including TUWax, Wax Attack, and GPRI effect of water on wax deposition.
Flow assurance remains one of the key technical challenges operators face in deepwater. DeepStar, through its collaboration of industry expertise, is well positioned to continue playing a leading role in advancing flow assurance science and technologies.
