1. Field of the Invention
The present invention relates to methods and to a system for controlling a subsea blowout preventer (BOP) system, in general, and specifically relates to such BOP control systems and methods having both electrical and hydraulic lines leading from the surface of the sea to the BOP system on the ocean floor.
2. Description of the Prior Art
Safety considerations in offshore drilling activities dictate that a subsea BOP must be able to rapidly close the well bore regardless of water depth at the drilling location. Conventional hydraulic BOP control systems experience unacceptable delays in operating subsea BOP functions in deep water applications because (1) the time required to send a hydraulic activation signal through an umbilical hose from the surface control station to the subsea pilot control valve becomes excessively long in deep water, and (2) delivery of sufficient quantities of pressurized operating fluid to the BOP function from the surface requires a substantial amount of time. These two elements of a complete BOP sequence time are usually referred to as signal time and fill-up time, respectively.
Methods used previously to reduce signal time have included increased hose sizing and higher operating pressure, while fill-up time has been minimized through the use of subsea fluid storage accumulators to effectively reduce the distance some of the fluid must flow before reaching the BOP. The adequacy of these methods has been challenged by the desire to drill in deep waters approaching 4,000 feet where conventional systems have drawbacks. Large diameter hose bundles in long lengths require substantial deck space for storage and pose running/retrieval handling difficulties. Also, the usable subsea accumulator volume diminishes with increasing water depth because of external hydrostatic pressure effects, thus forcing more accumulator bottles to be installed subsea as the water depth increases.
Although multiplex electric BOP control systems are known in the art, such systems are very expensive and complex. Prior to the systems described hereinafter in accordance with the present invention, the operators were faced with the prospect that, in order to drill in deeper water, their existing hydraulic control systems would need to be replaced with the more complex, more expensive multiplex electric BOP control systems.
Previous attempts to retrofit electrical controls to existing hydraulic control systems have involved substantial and complex equipment modifications with significant installation/check-out cost onboard the drilling rig.
It is therefore the primary object of the present invention to provide a new and improved BOP control system.
It is also an object of the present invention to provide a new and improved BOP control system which uses existing hydraulic lines to control non-critical functions and electrical lines to control critical functions.
It is yet another object of the prevent invention to provide a new and improved system for providing electrical signals through hydraulic lines.
It is still another object of the present invention to provide new and improved methods for controlling subsea BOP systems.