API REPORT OSAPR 10
Development of Methods to Predict Pressure Drop and Closure Conditions for Velocity-Type Subsurface Safety Valves
|Publication Date:||1 February 1980|
Subsurface safety valves (SSSVs) are required by law in most offshore producing wells. 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 drop 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 through the restriction. 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 standards1 and recommended practices2 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.
The design procedure to be followed when selecting a velocity-type SSSV for a particular well is illustrated in Fig. 1.1, which is a logic diagram of the computer program described in API RP-14B.2 Steps 1-3 are based on measured well flow data and are required to predict the productivity or inflow performance of the well. Steps 4-6 are used to determine the bean size required to produce the desired pressure drop across the valve for the selected closure flow rate. Once the bean size and pressure drop are determined, it is then necessary to select the valve components which will allow the SSSV to (1) remain open during normal production and (2) close at the pressure drop calculated in Step 6. This is illustrated in Step 7 and involves selecting the spring force which properly balances the pressure force tending to close the SSSV at a variety of flow rates.
Prior to the completion of the API studies outlined below, recommendations for valve type and spring choke size for closure at a particular well condition were made using technology based on single-phase flow theory. Since most valves operate under gas-liquid flow conditions, the development of improved multiphase flow predictions was recognized as a high potential area for safety valve improvements. The purpose of this research would be to develop correlations for predicting pressure drop across A SSSV occurring during multiphase flow as a function of variables such as gas and liquid flow rates, bean or choke size, gas-liquid ratio and average pressure.
To date, the API Offshore Safety and Anti-Pollution Research (OSAPR) Committee has funded three projects at The University of Tulsa dealing with the determination of SSSV behavior in the presence of multiphase fluid flow:
(1) OSAPR Project No. 1--The final report for this project3 contains an extensive literature search into previous work in the area of multiphase flow through restrictions. The report also devotes considerable space to the evaluation of several mathematical models that had been proposed to calculate pressure drops through a restriction. Test data for this study was collected on both solitary beans and simulated SSSVs. The data are particularly valuable in that it presents pressure profiles through a restriction or valve and illustrates experimentally the phenomenon of pressure recovery. Also included is an analysis of the onset of critical flow. Finally, empirical correlations are presented to determine discharge coefficients for the SSSVs tested, 2 3/8 in. nominal Otis J and Camco A-3.
(2) OSAPR Project No. 5--The experimental fixtures in the second study4 were changed from mock valves to actual SSSVs (still only 2 3/8 in. nominal Otis J and Camco A-3 valves were considered). This alteration allowed the determination of closure conditions for various combinations of bean size, spring constant and number of spacers. In addition, by using a stepped procedure in the closure tests, a large volume of additional data on pressure drop prior to closure was also obtained.
(3) OSAPR Project No. 10--Results of the final SSSV study are contained herein. The additions resulting from Project No. 10 include an extension of pressure drop and closure data to 2 7/8 in. nominal Otis J and Camco A-3 valves and 2 3/8 in. nominal Otis F valves. Time was also devoted to revising all previous empirical correlations to include the more recently collected test data.
The current report contains results from all three OSAPR projects used to determine:
(1) Spring force opposing closure
(2) Forces caused by fluids flowing through the valve.
(3) Pressure drop occurring across the valve for particular flow conditions.
In addition, all data collected in the API work has been summarized in the event that the reader may wish to access the results of these studies for his own purposes.
One final, important point is worth mentioning. Monetary and time constraints did not permit the study of all commercially available SSSVs. The valves chosen for study were selected totally because they represented the large majority of valves in use at the time this study was conducted. However, this is in no way intended to be interpreted as a recommendation of a particular company's product, either by the API or by The University of Tulsa.