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API REPORT 10 Document Information:
Title
Development of Methods to Predict Pressure Drop and Closure Conditions for Velocity-Type Subsurface Safety Valves
American Petroleum Institute
Publication Date:
Feb 1, 1980
Scope:
1. INTRODUCTION
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.
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