Development of a Multi-Function Fail-Safe Hydraulic Control System for Underwater Christmas Trees
Authors: Bush R.E. , Drouin A.H. , Fowler J.H. 
Published: October 2008

ABSTRACT
Hydraulically actuated Christmas tree valves have been designed to be inherently fail-safe to the normal position of the valve. Well pressure in the tubing provides the energy that normally returns the-valve and actuator to the normal, or fail-safe position, and in the absence of the well pressure this function is accomplished by spring force. Thus, where there is an accidental failure in the hydraulic actuating pressure source, the presence of well pressure in the valve body will automatically shut in a well.
This important characteristic inherent to the valve design, should be retained in the control system chosen for remote operation. A direct hydraulic line will control valve actuation, while retaining fail-safety, but a single function control offers little flexibility and additional direct lines are expensive in multiple function applications. The development of a multi-function hydraulically actuated control unit is described and the procedures used to test performance reliability are reviewed.
The control system developed, employs periodic hydraulic pressure pulses as a means of advancing a position selector through a cycle of nine positions. A maximum of eight hydraulic lines may be coupled to this selector unit leading to individual valve operators on a single tree or to a series of valve operators on a maximum of eight lines. The control unit, known as Pulsed Valve Selector (PVS) is inherently fail-safe, since the removal of hydraulic pressure automatically returns the selector piston to the safe position, thus removing actuating pressure from all valve operators coupled to the PVS unit. Typical applications for the hydraulically controlled PVS unit are given. The results of field applications now in progress will be reviewed in subsequent reports.

INTRODUCTION
Two principal methods have been used to control valves on a subsea wellhead: hydraulics and electricity. Most pure hydraulic systems have involved connecting a pipe directly to each hydraulic operator or a group of operators to be operated together. This system has the advantages of simplicity and safety. However, in order to have much flexibility, a large number of expensive hydraulic lines must be laid to the wellhead.
Pure electrical systems have the advantage of control versatility. However, the " fail-safe" characteristic of the hydraulic operator becomes difficult and expensive imitate.
Hybrid systems, making use of the fail-safe hydraulic operator and the versatility of electrical control, have been tried. These use electric motor-driven pumps to generate hydraulic power. The extra energy transformation makes these units relatively inefficient and bulky. The fact that these systems are used at all is a testimony to the fail-safe hydraulic operator.
This paper discloses a method whereby a purely hydraulic system may have some of the control versatility of an electrical system. One hydraulic line is used both as a power source and as a signal carrier. Control of the pressure in the line results in the operation of the desired valve or combination of valves.
This system takes advantage of the small size, simplicity, and the inherent fail safe characteristic of the hydraulic operator while requiring a hydraulic line that may be laid with the well flow lines.