Sub-Surface Safety Valve Systems
SkillUP Oil & Gas
World-class Training Solutions | Committed to Excellence | Driven by Industry Professionals
Sub-surface or downhole safety systems include all the equipment necessary to enable downhole closure to prevent injury to personnel and damage to surface facilities and environment due to the production of reservoir fluids. The ‘sub-surface safety system’ includes the downhole safety valve, any device(s) required for installation of the valve, the control conduit or cable, and surface controls.
Applications of Sub-Surface Safety Valves
The requirement for the use of SSSVs is dependent upon local operating company philosophy and governmental legislation, refer to BP’s DHSV Guidelines document produced in 2002.
The major function of a Sub-Surface Safety Valve System is to close a well in downhole in the event of an emergency occurring at surface. Although it is an emergency device it may be used when any, or combination, of the following incidents may occur causing potential danger to personnel, the environment, facilities and installation:
Although the prime function of an SSSV is for use in an emergency, it is also used for operational purposes, such as; leak testing or providing additional barrier protection when work is being carried out on the Xmas tree, wellhead or downstream equipment.
In general, the choice of the safety valve for an application must be considered on the well conditions, location, company philosophy or local regulations, depth requirements, life of the well, etc. The variety of systems available is wide and the choice may be difficult. It is often also based on past operational history and experience, and this would apply to safety systems on both tubing and annular bores.
Types of Sub-Surface Safety Valve Systems
The modern sub-surface safety valve was developed from early, much lower technological versions, produced in the 1930's. The initial demand from operators was for a downhole valve that would permit flow during normal flowing conditions but would isolate formation pressure from the wellhead to prevent damage or destruction in the event of an emergency occurring. This valve would be for installation downhole in the production tubing string.
The first valve that was developed was the Sub-Surface Controlled Safety Valve (SSCSV), which had a poppet type of closure mechanism (a mushroom shaped valve that sealed against a correspondingly shaped valve seat). Compared with today's valves, this simple type of valve had several disadvantages such as; restricted flow area, tortuous flow path, and low differential pressure rating and calibration difficulties. Despite these limitations, the valve was used successfully for many years before other versions were eventually developed such as the ball and flapper valve, which had less tortuous flow paths, and larger through bores.
From this simple beginning, the Surface Controlled Sub-Surface Safety Valve (SCSSV) was developed later in the 1950's. This design moved the point of control from downhole to surface (direct to remote control). This type of valve was a fail-safe valve, (i.e. it would close on loss of control pressure) and provided a much higher degree of safety with manual surface control of opening and closing, hence, higher responsiveness to a wide variety of abnormal surface conditions (fire, line rupture, etc.) and larger flow area. Initial demand for this valve was slow due to its higher cost and the problems associated in successfully installing the hydraulic control line; hence, its usage was low until the 1960s.
The modern SCSSV in use today is still normally controlled by hydraulic pressure supplied from a surface control system and is ideally suited to manual or automatic operation which pioneered the sophisticated emergency shut-down systems required today. The versatility of the valve allows it to be used in specialized applications as well as in conventional systems. Electronic or hydraulic/electronic controlled versions are currently under development and will be available in the near future.
A problem associated with SCSSVs as compared to SSCSVs was their operating depth, which was limited by the hydrostatic head of pressure of the hydraulic fluid in conjunction with the fluid friction in the control line. Continuing development of these valves now enables setting at depths over 2,000ft. SCSSVs are available in two variants; the Tubing Retrievable Safety Valve or the Wireline Retrievable Safety Valve. SCSSVs are available with, either, ball or flapper type closure mechanisms to enable through tubing interventions.
A more recent development has been sub-surface annular safety systems which were developed for a particular demand to protect offshore platforms against large gas escapes from annuli when they are used for gas lifting on artificial lift wells or when an annulus is used for production. These are also surface controlled systems which require some type of annular pack-off between the tubing and the casing, such as a packer, used in conjunction with a safety valve for shut-in. These systems usually employ a poppet valve arrangement that enables a pump through capability should the valve become inoperable in the closed position.
A report produced by a number of operators called the SINTEF report for which operators provide their data on safety valve performance, indicates that the TRSV with a flapper type closure is the most reliable valve in use today.
领英推荐
Single-Ball Safety Valve
The single-ball safety valve is a fully open downhole safety valve that is run in the open position and closes permanently when the disk is ruptured. The operator mandrel is balanced to internal pressure and is locked in the open position to prevent premature closure. Upon rupturing the disc, hydrostatic pressure is applied to the operator mandrel to close the valve.
The large differential pressure from hydrostatic to atmospheric and the 3 1/4-in2 operator area provide more than sufficient force to cut 7/32-in wireline cable, even in shallow wells. The ball valve operator locks in the closed position and prevents the tool from reopening until it is retrieved to surface. The lock can be reset without disassembling the tool to enable functional testing before running in the hole.
Pump-Through Safety Valve
Pump-Through Flapper Safety Valve
In case of a sudden increase in annulus pressure resulting from a tubing leak, the pump-through flapper safety valve provides reliable downhole shut-in. When placed above the recorder carrier, it also acts as a backup shut-in-in tool for the final shut-in, minimizing wellbore storage during the final pressure buildup. The valve also has the ability to pump into the formation, irrespective of tubing or annulus pressure integrity above the valve.
This fully opening downhole safety valve is run into the hole open and is closed permanently when annulus pressure rises about the rupture disc rating. The operator mandrel is biased to internal pressure and locked in the open position to prevent premature closure. Upon bursting the rupture disc, hydrostatic pressure moves the operator mandrel up against an atmospheric chamber, uncovering the spring-loaded flapper. Pumping down the tubing lifts the flapper off its seat and kills the well.
References:
Founder of tripwithindia.com
5 个月Thanks for sharing