Subsurface safety valves (SSSVs) are installed in the wellbore of hydrocarbon producing wells to shut off the production flow to the surface in case of an emergency. The importance of the correct installation of these valves to ensure well production flow is shut in during an emergency can not be over emphasised.
Improvements in valve design and reliability are of no avail if the valves are not correctly locked in the landing nipples (wire line run) and correctly function tested once installed. This article reviews the installation and testing procedures in order to prevent failure of the SSSV to operate as required. Reference should also be made to AP1 RP 14 B, Recommended practice for design, installation and operation of SSSV systems.
1. Workshop testing/assembly
Following complete redressing and servicing of surface controlled sub-surface safety valves (SCSSVs) in dedicated workshops, the following test/checks should be carried out:
The recommended procedure for preparation of sub-assemblies is as follows:
Where it is specified that joints should be made up with pipe locking compound (those below packer),the compound should be applied sparingly to pin ends only: after make up any excess needs to be cleaned off before it dries.
Separate drift runs should be made-down to and through no-go nipples. Connections made-up with pipe locking compound should not be drifted until the compound has hardened.
The test pressure should be equal to the maximum allowable pressure of the weakest link. The test consists of three parts:
The most important thing is to check for external leaks of connection and swivels.
-the position of such items as sliding sleeves;
-the serial numbers of the accessories;
-for gas lift valves and dummies, number each valve so that its exact location in the string is known.
It is recommended that accurate drawings are made of each sub-assembly for retention in the well files.
2. Installation of SCSSV
The procedures for installation of the valve will differ whether a tubing or wireline retrievable valve as discussed below. In addition the type of lock mandrel etc. will determine specific requirements which need to be referenced in vendor or company specific manuals/documents. In all cases the control line for SCSSVs will need to be run with the completion tubing.
The sub-assemblies including SCSSV should preferably be made up at the base workshop, as discussed above. However, on site it is recommended to re-perform the majority of the above checks. Reference should be made to API spec 4A Appendix F which describes the recommended procedure for field pressure testing. As proper testing facilities are not usually available on-site, appreciate safety precautions need to be ensured.
2.1 Tubing Retrievable (TR-) SCSSVs
Normally, during running-in of the completion string, the control line is pressurised to monitor its integrity. However, the control line must be depressurised at a given moment in order to install the Xmas tree at which stage the TR-SCSSV will close, unless a HOT is used.
Re-opening problems are particularly associated with ball-type valves due to the fact that the valve is designed for the ball to move down initially prior to rotating to the open position. In an unperforated or a well with a plugged tailpipe the fluid must be pressurised by this downward movement to permit valve re-opening. The re-opening operation is even more complicated when pressure exists below the closed TR-SCSSV i.e. trapped pressure between a tailpipe plug and TR-SCSSV. Generally, the exact pressure during these operations is not known which further complicates the re-opening operation. A further limiting factor is the working pressure of the control line.
Re-opening an otis type DLS with pressure below the valve
In general the following points should be considered
2.2 Wireline retrievable (WL-) SCSSVs
Prior to installation in the well the following checks should be made:
Cycle the valve using a hydraulic hand pump connected to the hydraulic inlet port. Check that the valve fully opens and closes.
Ensure that the mandrel and SCSSV packing stacks are undamaged and not bunched.
Check the dimensions and condition of the mandrel lock pin carefully. Note that the mandrel lock pin dimensions are critical, due to the small overall dimensions (0.38 in. long ´ 0.187 in. diameter). Any undersized or rounded-off lock pin shoulders could prevent locking of the fishing neck to the packing mandrel.
Check the alignment of the lock pin with the slot in the 'expander sleeve' to ensure that the fishing neck will be locked when the mandrel is completely closed.
Ensure that the indicator snapring is in the correct position.
Tighten the prong firmly to the core of the running tool.
Install a running tool on the SCSSV as per vendor procedures after applying hydraulic pressure to the SCSSV hydraulic inlet port to open the valve before insertion of the running prong. Bleed off the hydraulic pressure slowly so that the valve contacts the prong gently, thus preventing damage to both prong and valve.
Ensure that the hydraulic control fluid using during workshop/surface testing is identical to that used in sub-surface operations, furthermore check that this fluid is uncontaminated with other fluids (e.g. water) and is free of solids. Check that the hydraulic system in the valve is completely fluid filled.
Some Operating Companies choose to run the SCSSV with the prong detached from the running tool, this, of course, requires an additional wireline run to retrieve the prong;but this has the advantage that should a misrun occur with the SCSSV not being properly installed in the landing nipple, resulting in no hydraulic control of the safety valve, the prong prevents the ball or flapper from closing and makes retrieval of the SCSSV a standard operation. This practice is not recommended by some manufactures and could lead to damage of the valve. However, this type of prong could prove useful during the retrieval of SCSSVs.
Running WR SCSSV
1.Set zero on the depth indicator accurately so that the no-go or locator keys on the SCSSV are opposite the zero reference point on the wellhead. Ensure that the broken-out upper lubricator is lowered adjacent to the made-up position of the lower lubricator section, secured to the Xmas tree, before making this zero adjustment on the depth indicator.
2.Normally an SCSSV installation run is preceded by a recovery run. It is therefore recommended to accurately record the wireline depth of the SCSSV before its removal from the well.
3.Flush the control line with the correct hydraulic fluid during running-in operations.
4.Check the hanging weight of the tool string some 5 to 10 ft above the SCSSV landing nipple;continue pumping control line fluid and lower the SCSSV assembly into the nipple. Jar down gently, by hand if required, until a pressure increase is observed on the control line manifold. Stop pumping, mark the wireline at surface with a suitable marker.
The distance from when the inlet hydraulic port is straddled by the packing sets to the SCSSV hold-up depth is known. Continue jarring down gently until this known distance has been traversed by the SCSSV assembly.
5.Pump the control line to working pressure, shut off the air supply to the pump and observe for pressure integrity of the control line.
6.Jar down to shear the top shear pin and lock the SCSSV assembly in the landing nipple. Check the distance required to accomplish full locking using the wireline marker as a reference point.
7.Apply tension to the wireline, 200 lb* overpull (*minimum value, higher overpulls are recommended if the wire size and strength permit.)
8.Jar up lightly by hand two or three times, followed by a 200 lb* overpull.
9.Jar up to release the running tool from the SCSSV assembly.
10.Recover the wireline tool string, taking care not to set down the tool string during the recovery operation. Check the indicating lock feature, snap ring and shear pin for full locking of the mandrel.
3 Sub surface testing of the SCSSV
3.1 Directly following SCSSV installation
Following SCSSV installation
1.Close the SCSSV by depressurising the control line, note the time required for returns to cease and record the time on the SCSSV record chart. Also note the volume of returns from the control lone.
2.Depressurise the tubing above the SCSSV in four or five stages to zero or a predetermined pressure and observe for pressure integrity of the valve according to API RP 14 B (refer Section 13). During each stage check for leaks.
3.Equalise the pressure across the closed valve, preferably using pressure from another well, and re-open the SCSSV by hydraulically pressurising the control line.
4.Bring the well into production, allowing sufficient time for stabilisation of the flow (stable tubing head pressure/temperature) before conducting the SCSSV closure test under flowing conditions.
It is important to conduct this slam closure test under strictly controlled conditions.
5.Bleed off the control line pressure to zero. Note the time elapsed after triggering the control line pressure until a reduction in tubing head pressure is observed (Valve closure time). Record this information.
6.When the tubing head pressure has decreased to a predetermined pressure, close-in the wellhead flow-wing valve and observe for SCSSV integrity according to API RP 14 B.
7.Re-open the valve after equalisation and observe the hydraulic pressure required to achieve this. (i.e. pressure required to initiate valve opening and that required to operate the valve from closed to open position).
8.Note the control line operating pressure under flowing conditions.
The use of a pressure recorder will greatly assist during pressurising or equalisation and its use is therefore strongly recommended.
3.2 Routine testing of the SCSSV
The frequency of the routine testing of the SCSSV is based on valve performance in each operating area. Generally a valve in a new field will be tested more frequently than in an established area or field where a reliable valve performance indication has been established over the years. Obviously a valve should be cycled from time to time in order to avoid 'freezing' of the valve in the open position, which is the mode in which the valve is normally maintained when installed downhole. This fact should be borne in mind when establishing the interval of time between tests. Comparison of the data received will assist in determining the condition of the valve and estimation of the optimum period of time before valve reservicing, in the case of the WL-SCSSV, is required.
Once the Frequency of testing of SCSSVs has been established for a particular field/well, the interval of time between tests should be reviewed periodically. This test frequency review is considered necessary due to changing well conditions (increase in water cut, wax formation, change in gas composition, etc.) which could adversely affect valve performance. Legislation may influence the frequency of in-situ testing.
Recommended routine test procedure
1.Close in the well using the flow-wing valve.
2.Depressurise the control line pressure.
3.Conduct a pressure test across the shut SCSSV by bleeding off the pressure above the valve to zero.
4.Re-open the SCSSV after equalisation, preferably by pressurising with another well and return the well to production.
3.2.2 Record the test data
The following data should be accurately measured and recorded during routine SCSSV testing:
1.Minimum control line pressure - well flowing.
2.Pressure test data, SCSSV shut (well closed-in on flow-wing). Test procedure according to API RP 14B.
3.Control line pressure to re-open the SCSSV after equalisation.
4.After return of the well to production and stable conditions have been established record accurately the tubing head pressure and flow rate. Check the control line pressure and adjust it, if required. (Thermal effects could increase the control line pressure considerably).
The maximum pressure build-up allowed during SCSSV tests, both on installation and routinely, should be calculated according to the formula published in API RP 14B. These calculations should be calculated prior to commencing the work and included in the work program on an individual well basis.
3.3 Hydraulic fluids
Fluids that are commonly used in hydraulic systems can be classified in the following groups:
·water or water-based fluids;
In general the fluids used in hydraulic systems consist of a base fluid plus additives to obtain the required characteristics.
The control line fluid selected must be stable under all well conditions including temperature and pressure changes;and compatible with the elastomers and metal with which the control line fluid will be in contact. The use of water-based or water-diluted hydraulic fluids are not generally recommended, the acception being for deep well requirements.
The following fluid characteristics require specific consideration when selecting a hydraulic fluid for SCSSV operation:
·corrosion inhibiting characteristic;
·biological growth inhibiting characteristic;
·long-term fluid stability;
·fluid filterability and cleanliness.
1.Fluid viscosity: Viscosity is an important parameter since it influences the pressure loss in the conduits and consequently also the system response, it also, to a certain extent, affects the lubricating properties of the fluid required for the moving parts in the valves and actuators. Especially in deep wells, a highly viscous fluid could slow down the response time to an unacceptable level.
2.Corrosion inhibiting characteristics: Corrosion can occur internally in the hydraulic system due to the presence of oxygen in the hydraulic fluid and in the air on top of the fluid in the storage tank. To combat internal corrosion, inhibitors are normally added to the hydraulic fluids;the effectiveness of these additives in the long term will depend on conditions and precautionary measures such as:
-surface wetting characteristics;
-material compatibility with the corrosion inhibitor.
3.Biological growth inhibiting characteristics: Mineral oils and the water-soluble fluids may deteriorate due to biological growth. To prevent this, bactericide dopes are added to these fluids. these bactericides will generally be selected for types or growth that may be expected in the application for the hydraulic fluids.
4.Long-term fluid stability: Sub-sea systems may operate over extended time periods with little or no flushing of the hydraulic fluid taking place. Consequently the fluid stability is of paramount importance. To obtain the required stability the hydraulic fluid will contain additives like anti-oxidants and emulsifiers.
5.Seal material compatibility: In the hydraulic components, elastomer seals (static and dynamic) are exposed to the hydraulic fluid in the system. Polytetrafluorethylene (Teflon, Fluor),Fluorelastomer (Viton) and silicon rubber seal materials are not affected by the fluids irrespective of temperature and fluid type.
6.Fluid filterability and cleanliness: To meet the fluid cleanliness requirements, fluid filters are used in the system. Components in the hydraulic system that will generate contamination (or particles) include the hydraulic pump unit, the final control element and the fluid storage vessel.
By fitting filters at the pump suction, as well as the pump discharge, at the system fluid return line and at the storage vessel aeration opening, the fluid cleanliness can be maintained. The filter size must be adapted to the fluid flow rate, the allowable number of particles in the fluid and the expected interval between filter element changes
When filters with fine mesh sizes are required to meet the fluid cleanliness the selected fluid should not contain additives that will be filtered out. Synthetic or mineral fluid filterability may change considerably when contaminated with water or sea water.
Fluid cleanliness is expressed by the particle size distribution of a fluid sample (typically 100 ml). The distribution is determined either by optical means (counting particles on a standardised filter using a microscope) or by an automatic particle counter. A convenient method is to count the number of particles greater than e.g. 5 microns in 100 of fluid.
3.4 System flushing
To be able to obtain the fluid cleanliness with the fluid in the system, generally the system tubing and all hydraulic components in the system have to be flushed clean using a flushing fluid. Care should be taken that this fluid is compatible with the materials applied in the system and with the fluid that will ultimately be used in the system. When filling the system with hydraulic fluid, filters must be used on the filling connections to make sure that the fluid is of the correct cleanliness. It is recommended that detailed flushing and filling procedures are incorporated at the system design phase.
Containers for hydraulic fluid should be kept sealed until required for use, especially in very humid conditions, as some fluids are hydroscopic by nature and will absorb moisture from the atmosphere. The presence of even small quantities of water can initiate corrosion. In a dusty/sandy environment the fluid should be filtered into the equipment tank and every precaution taken to preclude entry of dirt into the tank.
3.5 System response
For the hydraulic system specification, the actuation speed is an important parameter, this speed determined by:
·the fluid volume;
·the distance between the actuator and power source;
·the diameter and volumetric expansion of the interconnecting conduits;
·the system's fluid supply capacity;
·the fluid viscosity.
Of these, only the viscosity is relevant when selecting the fluid;the other parameters are either fixed by the field lay-out or relevant to the selection of other system parts.
When deep setting hydraulically operated SCSSV, the use of synthetic or mineral oils may not be practical due to their relatively high viscosity when compared to water-based fluids, resulting in an excessively long response time.
4 Control lines
1/4in OD continuous steel tubing is generally standard for control lines, available in carbon steel, stainless steel and copper-nickel based alloys. The lines can be encapsulated in various jackets and supplied in dual form with/without a stress cable. The mechanical strength of the reflected material needs to be appropriate for the conditions during installation.
Prior to running the control line in a completion, it should be thoroughly flushed with clean control fluid and pressure tested. During running, a nominal pressure should be maintained on the control line and it should be kept under tension. The control line should be securely clamped at each tubing connection to both protect the line and retain it against the tubing. Hydraulic tensioning machines are available to assist in running the control line.
Control line or cable protectors that straddle the collar reduce the number of protectors required and give protection to the control line where they are most susceptible to damage.
'Swagelok' connections are commonly found to connect control lines, following gives the correct procedure for make up of these connections.
4.1 Installation instructions
Swagelok tube fittings come completely assembled, finger-tight. They are ready for immediate use. Disassembly before use can result in dirt or foreign material entering the fitting causing leaks. Swagelok tube fittings are installed in three easy steps:
Step 1: Insert the tube into the Swagelok tube fitting. Ensure that the tubing rests firmly on the shoulder of the fitting and that the nut is finger-tight.
Step 2: Before tightening the swagelok nut, scribe the nut at the 6.00 o'clock position.
Step 3: While holding the fitting body steady with a back-up wrench or vice turn the nut one-and -one-quarter turns*. (Watching the scribe mark, make one complete revolution and continue to the 9.00 o' clock position.
By using the scribe mark, there will be no doubt that the fitting has been tightened the one-and-one-quarter turns* required for a proper installation).
*For 1/16in, 1/8in and 3/16in size tubing fittings, only 3/4turn from finger-tight in necessary.
As a check on correct installation it is recommended that a Swagelok inspection gauge be used (Section 5.5.2).
4.2 Swagelok inspection gauge
The Swagelok inspection gauge is a device to determine when most fittings have been properly tightened. It is designed to assist personnel in checking an installation. It is not a necessary part of the Swagelok installation.
If the gauge fits between the Swagelok nut and the body hex of the fitting, it indicates that the fitting has not been sufficiently tightened.