Improving subsurface safety valve reliability: a problem/solution approach

In this safety valve design, the lock-open sleeve is removed and in its place is a radial punch communications system, providing a smooth ID. [16,430 bytes] Conventionally, wireline conveyed shifting tools seek a matching profile when going into the well and will often activate the lock-open sleeve and cause the downhole valve to fail. [7,678 bytes]

Safety reliability requires ongoing improvements

Don Aldridge
Baker Oil Tools
Surface controlled, subsurface safety valves (SCSSVs) are becoming more and more common in onshore as well as offshore operations throughout the world because of their ability to protect lives, the environment, and operators' investments if the wellhead becomes damaged. The US Government now requires safety valves for most Gulf of Mexico operations, and other countries' governments are following suit.

In addition to being necessary to prevent unwanted flow during catastrophic events, safety valve reliability can lead to longer completion life and curb unwanted workovers, thus reducing lifecycle costs for wells and fields. The importance of this issue was emphasized several years ago in an independent study on SCSSV safety and reliability, conducted by the Foundation for Scientific and Industrial Research (SINTEF) at the Norwegian Institute of Technology.

In suggesting that purchasing departments look more toward the life cycle cost for wells rather than focusing strictly on trial purchasing cost, SINTEF addressed the importance of considering SCSSV reliability, for both safety and production regularity. At the time of the report, the reliability of all valve classes had improved significantly since the foundation's previous report. However, opportunities for further improvement existed.

Why valves fail

While some downhole equipment remains static throughout the life of the completion, safety valves are routinely opened, closed, and tested to ensure that they are ready and working properly in the event of a catastrophe. However, the valves sometimes fail during normal production operations due to elastomer failure, failure of press fit plugs, or premature activation of lockout features.

If a valve fails during one of these procedures, it generally necessitates an unscheduled workover which interrupts production and, particularly in deep waters, can be very expensive. Or, it may require the installation of an insert safety valve with restrictive internal diameters.

Failure of the control fluid communication system has been shown to be the primary cause of safety valve failures. In fact, the SINTEF study revealed that more than 25% of failures in conventional, normally closed, tubing-retrievable subsurface safety valves were linked directly to leakage between the hydraulic control line chamber and the tubing wellbore, through the wireline insert valve communication feature. In most failures, the leakage was caused by premature or inadvertent activation of the lock-open sleeve in the ID of the valve.

A new safety valve designed around a patented control fluid communication feature provides an uninterrupted control fluid path to the piston and prevents accidental activation of lock-open sleeves.

Failure analysis

Independent testing and field experience confirm that accidental activation of subsurface safety valves is linked to linear motion sleeve designs that are integral to many safety valves. When routine intervention methods are performed - such as using a shifting tool to operate a sliding sleeve and bring a new zone on production - the wireline or coiled tubing that conveys the tool must pass through the ID of the valve.

Unfortunately, the profile of the valve's linear-motion lock-open sleeve is very similar to that of other tubing-mounted accessories, such as nipples and sliding sleeves. The wireline-or coiled tubing-conveyed tools automatically seek a matching profile when they are going in the well. Thus, if they hit on any of the shoulders or profiles in the lock-open sleeve, they will likely activate it, causing the control fluid to leak and the valve to fail. This calls for expensive, corrective well re-entry procedures.

Eliminating vulnerability

The previously described, newly designed tubing - retrievable safety valve removes the lock-open sleeve from the valve and substitutes linear lock-open motion with a patented, radial punch communication system. These design changes leave the safety valve with a smooth ID, free of sleeves or other devices that move linearly and can hang up wireline or coiled tubing conveyed tools.

Control fluid communication to the lock-open sleeve is achieved only by radial motion of a one-trip, jar-down wireline tool. As a result, there is no possibility for other tools to accidentally activate the lock-open feature while passing through the ID of the valve. In the event that if becomes necessary to puncture the valve, only the cylinder sub is punctured. The rest of the valve remains intact.

The new radial punch design has compiled a field history and has displayed a level of dependability that has gained operator acceptance. After 2300 installations, there have been no valve failures due to premature lock-open.

Performance improvers

In addition to radial punch communication, other safely valve design feature can further enhance performance integrity.

  • Outer housing thread seals:The seals used in the outer housings of safety valves are critical to safety system reliability. Failure in these seals can lead to a tubing-to-annulus leak, and possibly a workover. Metal-to-metal housing connections can eliminate explosive decompression, chemical and thermal degradation, and other problems associated with elastomeric sealing systems. A two-step thread design can further extend the valve's service life by distributing loading on the thread more efficiently, and providing significantly greater strength for a given section thickness than a single-step design.
  • Closure mechanism: A strong closure mechanism can protect the valve seat from stress-induced damage to ensure continuously reliable sealing. A thick, wedge-shaped flapper closure mechanism has been proven to be particularly effective for this purpose. The wedge design moves the first point of contact with the flow tube away from the hinge pin to minimize loading on the hinge and flapper pin. A special hinge design vitally eliminates flapper pin bending.
  • Self-equalizing mech anism: An integral equalizing mechanism eliminates the need to equalize differential pressure across the flapper. By venting shut-in pressure from below the safety valve to the tubing above the flapper, a valve can be equalized and can return the well to production quicker, safer, and more efficiently than when pumps and fluids must be used. Particularly effective is a through-the-flapper, metal-to-metal self-equalizing system that eliminates valve failure caused by erosion problems or buildup of fluid debris in the valve annulus.

Slam testing

Gas slam tests are an excellent indication of the reliability of a subsurface safety valve. To effectively evaluate the valve, these tests must duplicate or exceed real-life downhole conditions present when the well is flowing, the control line is bled off, and the safety valve closes with the flow against the flapper. The following list represents suggested minimum requirements:

Twenty slams, of which 15 are: 200 ft/sec (41 MMscfd) for 2-3/8-in. valves; 200 ft/sec (58.5 MMscfd) for 2-7/8-in. valves; 200 ft/sec (102.7 MMscfd) for 3- l/2-in. valves; 150 ft/sec (146.0 MMscfd) for 4-l/2-in. valves; 125 ft/sec (120.7 MMscfd) for 5-1/2-in. valves; and 80 ft/sec (146 MMscfd) for 7-in. valves. These flow rates are as high as 10 times those required by the API.

Continued technological advancements, such as those described here, which can substantially improve safety valve reliability, will be key to meeting the combined economic, environmental, and human safety standards that will drive 21st Century energy development and utilization.

References
"Reliability of Surface Controlled Subsurface Safely Valves - Phase IV", Foundation for Scientific and Industrial Research, Norwegian Institute of Tech nology, Report No. STF75 R91038, 1992.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.

More in Production