API REPORT OSAPR 5
Study of Pressure Drop and Closure Forces in Velocity-Type Subsurface Safety Valves
|Publication Date:||1 July 1977|
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 loss across the valve. For subcritical flow, the pressure loss across a restriction, such as the bean (or choke) 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 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.
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 predictions 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 research was 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. The study was performed specifically for 2-3/8 in. nominal Otis J and Camco A-3 valves. The results of this study, which was OSAPR Project No. 1, were reported in September, 1976.3
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 close at the pressure drop calculated in Step 6. This is illustrated in Step 7 and involves selecting the spring force which balances the pressure force tending to close the SSSV at the selected flow rate. The results of Project No. 1 can be used to perform the calculations highlighted in Steps 1 and 6.
A very important step in the total design procedure, Step 7, was not considered in Project No. 1. This involves predicting the pressure drop across the valve at which closure will occur when the valve is equipped with a particular bean and spring combination. Valve closure occurs when the fluid forces are sufficient to overcome spring forces and seal gripping forces or friction. Spring forces can be estimated from a knowledge of the spring constant and as a function of the number of spacers used to impart an initial compression in the spring. Prior to this study very little was known about the nature of the seal gripping and other frictional forces.
Previous closure test data were obtained using single-phase liquid or gas as the flowing fluid. Consequently, in order to improve this step in the design procedure a proposal was submitted by The University of Tulsa to the API OSAPR committee in November, 1975. This proposal was funded in February, 1976 and the existing experimental facility was used to obtain multiphase flow valve closure data to solve the problem. The SSSVs selected for testing were the nominal 2-3/8 in. Otis J. and Camco A-3 valves. The valves used were furnished by the manufacturers. Several combinations of bean size and spring force were utilized for each valve. Since the actual valves were used, the pressure drop occurring across the valve and its locking mandrel was measured and used to develop the design equations.
As a consequence of the procedure used to conduct a closure test, many additional multiphase flowing pressure drop data points were obtained for each bean size for both valves. These data points were combined with those obtained in Project No. 1 to develop improved pressure drop correlations. Since these data were obtained using actual valves, it is felt that the improved pressure drop correlations combined with the closure equations developed in this study represent a significant improvement in the overall SSSV design procedure.
The purpose of this report is to present find results of the OSAPR Project No. 5 research project. Future sections will describe the experimental equipment and testing procedure (Section II), the experimental data obtained (Section III), the development and evaluation of the closure correlation (Section IV), the revised pressure drop correlations (Section V), and the conclusions and recommendations (Section VI). Example design problems to illustrate the application of the results are presented as an appendix.