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 be low 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 arc 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.