TECHNOLOGY AT WORK

Practical innovations help
solve operating problems

Thomas Wallace, Pikotek,
Lakewood, Colorado, and Lee Bone,
Arco Production Research, Plano, Texas

Arco Production Research has developed an improved high-pressure flange sealing system which has been in service for more than 10 years, including failure-free application in severe-service North Slope applications. Saudi Arabian state oil company Saudi Aramco has approved the system, and the VCS (Very Critical Service) flange gasket is being tested in the field by BP Exploration U.K., Conoco U.K. and Shell Oil Co. (International). The system is manufactured by Pikotek of Lakewood, Colorado.

The problem. Most high-pressure oil field flange connections (system pressures >2,000 psi) use some type of metallic gasket to achieve fluid and/or gas seal in flowlines, injection lines and high-pressure pipelines. The most common flange/gasket connection in ANSI 900 class (API 2,000 lb) flanges and higher is the ring type joint (RTJ) connection, which seals by deforming a metal ring into concentric grooves machined into opposing flange faces. While this connection will usually yield a high-pressure positive seal if the flanges are in relatively good condition, it is vulnerable to various types of corrosion, particularly in contact with corrosive fluids.

Improved seal solution. The improved method of high pressure flange sealing developed by Arco continues to provide many benefits not previously found in conventional high-pressure flange connections. This VCS gasket uses a matrix-reinforced, high-density composite material, permanently laminated to a corrosion-resistant, metal alloy core (316 stainless or 2205 duplex). Together they comprise a rigid seal retainer, thus providing the "back-bone" for secondary sealing elements, and offering the same structural integrity/high-pressure sealing capabilities found in an RTJ all metal-to-metal ring gasket seal.

By using an advanced polymer resin compound in the reinforced composite material, the seal retainer actually has higher compressive strength than a mild-carbon-steel ring gasket, thereby improving its resistance to over-compression due to over-torqued stud bolts. Since the retainer never takes a permanent set, unlike conventional flange gaskets, the improved sealing device is reusable.

Measurable benefits over conventional gaskets are based on sealing off the entire flange face from corrosive media, while providing a structurally permanent, high-pressure fluid/gas seal. The seal retainer matches up exactly to the through-conduit bore of the flange, thereby eliminating any potential fluid trap. With no metal-to-metal contact between opposing flange faces, the process of localized corrosion is eliminated. And since all sealing is accomplished on the flange face, the design will seal with any flange type (RTJ, raised-face or flat-face), and it will mate dissimilar flange types as well.

The other major benefit is the ease of installation. Rather than having to spread the flanges and force a conventional ring gasket into the groove while bringing the flanges together, the VCS gasket simply drops into place, aligning itself on the bottom half of the bolt pattern while lining up perfectly with the flange bore. Consequently, flanges need only be spread enough to drop the gasket into place.

The seals are, typically, either pressure-energized elastomeric O-ring seals or spring-energized Teflon lip seals, depending on the service. Because they are system-energized, they do not rely entirely on bolt torque/seating stress. Therefore, gradual bolt relaxation from pressure and/or thermal cycling is not as big a problem as it is with conventional flange gaskets. By encapsulating the seals in dove-tail (trapezoid) shaped seal grooves, they cannot extrude or evacuate the grooves under even extreme operating pressure. They are, in fact, forced down into the metal retainer core more with greater system pressure, thus providing an enhanced seal.

Principal components of high-pressure flange seal for Very Critical Service (VCS).

Ten years of application. Applications include extensive use on the North Slope of Alaska, where the product is used for electrical isolation of wellheads connected by common flowlines, dissimilar and incompatible metal flange separation, i.e. carbon steel on one side mated to stainless or duplex, as well as high-temperature, high-pressure corrosive water service.

Over a 10-yr period, with thousands in service on the North Slope, including Prudhoe Bay, Kuparuk and Lisburne fields, there has not been a recorded in-service failure. Arco-Alaska, Inc. has used the design in high-pressure seawater injection, as well as gas/oil separation and reinjection projects, due to the corrosive nature of the high saline content, and to take advantage of reusability features. Likely, this product will continue to be used in all high-pressure connections where corrosion plagues conventional metallic gaskets. Most applications will continue to be in ANSI 300 class flanges and higher.

Other applications now include carbon dioxide (CO2) injection, high concentrations of hydrogen sulfide (H2S) content in the production stream (both crude and natural gas service), salt-water service, subsea and inland buried pipelines, and extensive use in all types of cathodic protection systems.

The manufacturer recently introduced the same design, available with a fire-resistant back-up seal, making it industry’s first corrosion and fire-resistant flange gasket/seal for critical service. The design has successfully passed the API 6 FB fire test for bolted end connections and full API Spec. 6A certification as a fire-safe/corrosion-resistant alternative to RTJ ring gaskets is being obtained. This will allow use in high-pressure, fire-sensitive RTJ flange connections, including wellheads, high-pressure flowlines, trees and BOP stacks. Due to fire safety requirements now being implemented offshore, this design will likely see extensive service in fire-sensitive areas where connections are prone to problems relating to piping corrosion.

As noted, other recent developments include full product approval by Saudi Aramco, and extensive testing and field trial evaluation by BP, Conoco and Shell. The product is now in service in most major oil field applications with working pressures ranging as high as 11,000 psi, wet H2S concentrations ranging as high as 30% and temperatures as high as 450°F. WO