SCSSVs are specially designed wellhead shutdown valves that are held open by the maintenance of a constant hydraulic pressure. These valves are usually located hundreds of feet below the bottom of the sea or surface of the land. Sometimes more than one valve is required per well.
The valve is installed in the wellhead tubing and the hydraulic control tubing is run between the tubing and the intermediate casing. One type of SCSSV is shown in Figure 1700-10.
The number of wells being controlled by a hydraulic system depends upon local preference. Sometimes individual hydraulic systems are preferred. Usually the wells are grouped in logical “blocks” that enable good operator access and control in case of a problem. Normally, a limit of no more than 10 to 20 wells per hydraulic system will allow all the SCSSVs to be reset in under 5 minutes.
Hydraulic pressure is supplied by a pneumatically driven pump with a second pump as a backup. The backup pump can be another pneumatically driven pump with a manual operator option or just a manually operated pump.
A low-pressure sensor can be installed to monitor the hydraulic pressure and alert the operator. A relief valve is provided on the discharge of the pump to relieve excess back pressure back to the supply tank. The main pump is driven by approximately regulated 100-psig air or natural gas.
Regulations such as API RP 14B and OCS Order No. 5 govern the requirements for these systems.