HP/HT Requirements Drive Subsea Sealing Technology
Andrew Longdon, UK technical manager, Trelleborg Sealing Solutions, discusses why unconventional oil well exploration and processing is driving subsea sealing technology.
As operators try to maximize the yield from offshore oilfields, exploration has moved to greater depths subsea. Unconventional wells, previously believed to be too deep for existing capabilities are now being reinvestigated – relying on subsea processing technology to meet new High Pressure / High Temperature (HP/HT) conditions.
Conventional subsea operating temperatures of up to +120 °C / +248 °F have increased to over +200 °C/ +392 °F in deep water locations, whereas typical pressures have on occasion trebled from a previous standard of 69 MPa / 10,000 psi to 207 MPa / 30,000 psi.
Seals are a critical element within subsea systems, in particular within subsea valves and downhole tools. They ensure optimum operation of oilfield equipment and are the primary barrier to the prevention of system fluid loss or system fluid contamination from external sources. Subsea seals are the main source of prevention of hydrocarbon leakage from oilfield completion or production equipment and as such, perform a vital role in meeting environmental and workforce safety concerns, in addition to enhancing performance of the tools themselves.
Application Specific Design
Standard sealing solutions of previous years are now unable to withstand the extreme operating temperatures and pressures of deep water locations. To enable exploration and production in unconventional wells, specialized solutions in both material and seal profile technology is required. Oilfield operators and equipment manufacturers increasingly require application-specific seal designs to provide high integrity sealing.
The Multi-Contact Turcon® Variseal® has been developed by Trelleborg Sealing Solutions to replace multiple chevron seals or V-stack type products in subsea valve stems requiring high integrity sealing capability. Comprising of a series of single acting spring-energized seals housed within a U-shaped Turcon® polytetrafluoroethylene (PTFE) based material seal profile; the unique single-piece design significantly improves sealing integrity / leak tightness in high pressure applications by providing higher contact force across multiple seal hardware interfaces.
Increased Service Life
When system pressure is released during cycling, particularly in high pressure applications, the flex life of PTFE based materials can be affected – shortening effective seal life and ultimately leading to system downtime. Next generation PTFE seals such as the Multi-Contact Turcon® Variseal® are designed to provide integrated redundancy, helping to avoid this – providing better performance and ultimately lowering overall operator costs. With the addition of anti-extrusion devices, the Multi-Contact Turcon® Variseal® can withstand pressures of up to 207 MPa / 30,000 psi.
NORSOK M-710 Qualification
HP/HT applications have led to a significant increase in demand for NORSOK M-710 – Qualification of non-metallic sealing materials and manufacturers (Rev. 2, Oct. 2001). Individual materials are rigorously tested and approved based on criteria including aging within sour and sweet gas service conditions and significantly for elastomeric seal manufacturers Rapid Gas Decompression (RGD).
Rapid Gas Decompression
Rapid Gas Decompression is a concern to oilfield OEMs and operators. It can occur when applied system pressure is quickly released, causing absorbed gas to expand and potentially damage elastomer seals – leading to system failure.
Inherently, elastomer seals contain voids. Gas or gas mixtures in contact with elastomer surfaces are absorbed and will saturate elastomer seals. At high-pressure this absorbed gas is in a compressed state. When external pressure is reduced, either rapidly or over a relatively short period of time, the compressed gas nucleates at the voids, expanding within the elastomer.
The voids inflate leading to high tensile stresses or strains in the void walls. Depending on the strength and hardness of the elastomer, this can cause the elastomer to break or crack.
Trelleborg Sealing Solutions have developed a range of elastomer compounds to satisfy the Norsok M710 specification for both RGD and sweet and sour well conditions.
Extreme Temperature Downhole Environments
Isolast® XploR™ J9513 from Trelleborg Sealing Solutions is the first perfluoroelastomer (FFKM) to achieve the NORSOK M-710 approval for Rapid Gas Decompression.
FFKM elastomers offer exceptional mechanical performance (high modulus / high strength), and offer outstanding long-term compression set – naturally increasing service life. Isolast® XploR™ J9513 can operate within downhole system temperatures of -20°C to +240°C/-4°F to +464°F and is chemically resistant to virtually all media, including aggressive combinations of hydrocarbons mixed with brines, corrosion inhibitors and completion fluids.
Unconventional well exploration and subsea processing technology is rapidly evolving in order to meet the demands of oilfield operators. HP/HT operating requirements are driving subsea sealing technology and only continued investment in both material and seal profile development activities will position seal manufacturers to deliver the next generation of sealing solutions.