An Interlock is a control permissive that exists to prevent or warn an operator against potentially undesired operator commands being issued to the subsea system. Depending on an operator’s access level, he / she may be able to override the interlock in order to perform the desired operation. Interlocks can be categorised into two types: process interlocks and product / system interlocks, though not all customers or SCV’s make this distinction.

Process interlocks are interlocks which are specific to a particular project dependent on field layout, tree functionality, etc. These are often defined by the customer’s process requirements or by regulatory agencies; e.g., prevention of opening the tree crossover valve if the production master valve and annulus master valve are open.

Interlocks defined by the SCV for the protection of the subsea system; for example, low hydraulic pressure inhibiting opening (pressurising) of a tree valve. These interlocks are typically not able to be overridden by an operator.