API REPORT OSAPR 1
Orifice Coefficients for Two-Phase Flow Through Velocity Controlled Subsurface Safety Valves
|Publication Date:||1 September 1976|
Subsurface safety valves (SSSVs) are required by law in most offshore producing wells and are one of many devices available for well fluid containment. The purpose of the valves is to shut off well flow in the production tubing below the mudline in the event disasters, such as explosions or fires, disable surface shutdown devices. Several types of SSSVs are used, including those which are controlled from the surface by hydraulic fluids, pressure sensing valves, and differential pressure or fluid velocity actuated valves. This report deals only with velocity actuated SSSVs.
Actuation of the velocity-type SSSV is based on a simple force balance principle. Loss of pressure above a valve increases the flow rate through the valve and also the pressure loss across the valve. For subcritical flow, the pressure loss across a restriction, such as the choke or bean used in a safety valve, is proportional to the flow rate of fluids. The safety valve is held open by spring and seal gripping forces which together are greater than the opposing resultant well fluid forces generated by normal production rates. However, for higher than normal production rates corresponding to loss of tubinghead back-pressure, the net well fluid forces become great enough to overcome the spring and seal gripping forces and to actuate valve closure. The consequences of incorrect valve sizing are either premature closures which result in lost production and operator expense or loss of protection from using a valve which cannot be closed by well flow rates corresponding to disaster conditions.
Before recent API safety valve standards were written, functional testing procedures and selection of manufacturing tolerances on critical valve components were left to the discretion of valve manufacturers. New API standards and recommended practices have been written by the API Committee on Standardization of Offshore Safety and Anti-Pollution Equipment (OSAPE). These documents provide manufacturing tolerances and a formal procedure for functional and performance testing.
Current recommendations for valve type and spring and choke size for each well condition are made using technology based on single-phase flow theory. Since most valves operate under gas-liquid flow conditions, the development of improved multiphase flow prediction was recognized as a high potential area for safety valve improvement. As a result, the API Offshore Safety and Anti-Pollution Research Committee (OSAPR) awarded a research grant to The University of Tulsa to study multiphase flow through safety valves and chokes.
The purpose of this report is to present final results of The University of Tulsa research project. The remainder of this section will describe background work by the API OSAPE and OSAPR Committees which established the framework for this project, Future sections will describe previous studies in this area (Chapter 2), the experimental equipment designed and constructed and the testing procedure (Chapter 3), the experimental data obtained (Chapter 4), development and evaluation of improved correlations (Chapter 5), example calculations using one of the final correlations (Chapter 6), and conclusions and recommendations (Chapter 7).
API Subsurface Safety Valve Standards
Subsurface safety valve studies were given high priority by the API OSAPE Committee immediately after its formation in late 1972. The first action of the Committee was to develop standards and recommended practices for SSSVs. In March 1973 an OSAPE Committee SSSV Task Group set up meetings with offshore Gulf Coast region operators to determine numbers of each make and type of SSSV being used and gather statistics on their malfunction and failure. Members of the Task Group also visited companies operating in the Santa Barbara Channel. Cooperation from the operators and the USGS on releasing quarterly failure analysis reports made it possible to complete the survey. Table 1.1 shows that 84.9% of all SSSVs installed in offshore Gulf Coast wells are the velocity actuated type. Even though other types of valves are now required by law in most new wells and in most old wells in which tubing is pulled, there will be a substantial number of subsurface controlled SSSVs in use for many years. Table 1.2 shows a summary of specific failures of SSSVs uncovered in the survey. Notice that erosion was cited for 79.1% of all failures of velocity-type SSSVs. However, since the occurrence of emergency flow conditions which would actuate valve closure is infrequent, the survey cannot account for valves which would not have closed in an emergency because of incorrect spring or choke size.
The OSAPE SSSV Task Group wrote two documents, one defining specifications for manufacturers of SSSVs and another outlining recommended practices for their use and installation. The First Editions of both documents were published in October 1973. API Spec. 14A set minimum performance levels for SSSVs based on the producing environment. The required performance tests are conducted at Southwest Research Institute, San Antonio, Texas where a test facility was constructed under an API contract. The document also outlined functional tests to be conducted by the manufacturers on all SSSVs being put into service. Only API RP-14B is of specific interest to this study and is based on the following objectives:
1. Specify design parameters that should be considered by an engineer designing a SSSV system.
2. Provide a sizing computer program for subsurface controlled SSSVs which will be maintained by the API and made available to the oil industry.
3. Provide important considerations for the safe, efficient installation of a SSSV system.
4. Recommend minimum testing, inspection and maintenance procedures for SSSVs and related equipment.
The logic diagram of the computer program to be used for sizing velocity-type SSSVs is shown in Fig. 1.1. Steps 1-3 are based on measured well flow data and are used to predict the productivity of the reservoir. Since a safety valve is installed in the we'll, the calculation highlighted in Step 1 is included to account for pressure drop across the valve. Once reservoir productivity is defined, Steps 4-7 are used to determine desired valve bean (choke) size and spring force required to balance the fluid forces at closure rate conditions. Checks are also made in the program to ensure that closure rates and wellhead pressures are within acceptable ranges and that manufacturers' recommendations are followed.
API Subsurface Safety Valve Research
Results of the OSAPE Committee survey and discussions at meetings of the OSAPR Committee defined two high potential research areas in SSSV operation and sizing: multiphase flow prediction and sand erosion control. In June, 1973, the OSAPR Committee awarded a research grant to The University of Tulsa Petroleum Engineering Department to develop multiphase flow prediction methods for velocity type SSSVs. In September of that same year, Texas A & M University was awarded a grant to study sand erosion in oil and gas production equipment.
Pressure loss calculations currently used in the API valve sizing computer program are based on single-phase flow through the valve. The total predicted pressure drop across the valve is used both in the force balance calculation for determining spring forces and in determining reservoir productivity. Since most valves operate under multiphase flow conditions, the use of multiphase flow predictions methods for valve calculations is desirable.
The initial University of Tulsa research project is now completed and followed the timing chart shown in Fig. 1.2. The end result of the project consists of correlations for predicting pressure drop across the Otis J and Camco A-3 valves. The correlations should result in more confident selection of choke size for these valves.
Early emphasis was placed on compiling all industry data and ideas pertinent to The University of Tulsa research effort. A one day workshop was held at The University in September, 1973. The session was attended by thirteen representatives of producing companies and valve manufacturing companies, The agenda included the following specific presentations:
1. An overview of the total API effort on standardization and research on velocity-type SSSVs.
2. A review of The University of Tulsa research proposal and current experimental facilities.
3. Descriptions of recent flow tests conducted on velocity-type SSSVs by valve manufacturing and producing companies.
4. A summary of the OSAPE Committee Task Group survey on SSSV use and failure statistics.
Valuable suggestions were obtained from workshop participants on such items as pressure tap locations, choke sizes to be tested, ranges of flow rates, experimental pressure levels and order of conducting experimental tests. Many of the recommendations were implemented during the experimental stage. Included were recommendations to concentrate on 2 in. nominal Otis J and Camco A-3 valves and choke sizes between 20/64 and 32/64 inches.
To continue effective industry and API participation in the research program, the OSAPR Committee formed an advisory subcommittee of several engineers knowledgeable in fluid flow from major offshore producing companies. Membership on the advisory subcommittee was altered several times due to transfers, changes in employers, etc., but continuous contact with Tulsa University Principal Investigators was maintained throughout the project. The main method of contact was through numerous meetings between the University research staff and subcommittee members, and through periodic progress reports.
The experimental program to produce data of utmost utility in safety valve sizing became more clearly defined during the advisory subcommittee meetings. Ranges of fluid flow parameters such as gas and liquid flow rates, pressures, choke sizes and tubing size were established. It was determined that total pressure drop across the test fixtures should be measured. Also, it was recognized that measurement of pressure profiles through the flow channels of the safety valves might be needed in future studies to employ the force balance principle in calculating valve closing rates. This is because fluid forces entering the force balance are the result of pressures at specific locations in the valve acting over known surface areas. Therefore, several pressure taps along the length of each test fixture were included to measure pressure profiles.
Although the original research proposal confined most testing to simple choke shapes mounted in 2 in. tubing, it was recognized that the safety valves had additional diameter variations in the flow channels that could result in additional pressure loss and different pressure profiles. Test fixtures simulating two commonly used SSSV designs, the Otis J and Camco A-3 valves, were requested from the manufacturing companies. The companies promptly donated the needed fixtures, thus saving the research project considerable time and expense. Initial exploratory tests were planned to evaluate the importance of valve flow channel design on pressure profiles. Results would indicate if subsequent testing could be confined to simple choke shapes. If testing with simple choke shapes proved adequate, then the data and resulting correlations could be applied with more confidence in sizing other types of valves. The test fixtures used are described in detail in Chapter 3.
Analysis of Available Data
Data on pressure drops through SSSVs were furnished to the University research staff from private industry sources. These data and a number of other published articles were reviewed to determine their applicability to closing rate prediction and the data to be gathered in the research project. While these furnished data were helpful in planning the research program, they were judged to be insufficient for development of generalized correlations to size SSSVs because of their limited range of flow variables and choke sizes. Also, the great majority of the available data was for single-phase flow, and, therefore, added no information for prediction of multiphase flow pressure drops.