A hydraulic control system for controlling an external device ( 4 ) at a well installation includes a control module ( 2 ) for generating electrical and/or optical control signals. A control pod ( 8 ) receives the control signals, the control pod controlling the external device. A hydraulic line ( 10 ) links the control pod to the external device ( 4 ) for controlling it.

CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of United Kingdom Patent Application No. 0428001.2, filed on Dec. 22, 2004, which hereby is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a hydraulic control system and a well installation incorporating the control system. 
       BACKGROUND OF THE INVENTION 
       [0003]    In fluid extraction well installations there is a frequent requirement to control a small number of subsea hydraulic devices, typically valves for example, on a manifold or other structure from a well head tree, located typically 100 m distant from the manifold/structure. The traditional method of implementing this requirement is to install a hydraulic jumper between the tree and the manifold/structure hydraulic devices and use a tree ‘subsea control module’ (SCM) to control these devices. 
         [0004]      FIG. 1  illustrates a traditional arrangement for control of hydraulic devices, in this example valves on a remote manifold. A tree  1  houses an SCM  2 , which is connected to the manifold  3 . Each valve  4  on the manifold  3  is fed via a hydraulic control line  5  such that a directional control valve (DCV) in the SCM  2  controls the operation of one valve  4 . Each tree around the manifold would be connected similarly to a respective set of three valves. Historically, hose-type jumpers  5  have been employed to link the hydraulic control from the SCM to the manifold valves. However, with the current trend for subsea wells to be at greater depths, fluid well installation companies are specifying steel tube jumpers, which are extremely expensive, both to buy and to install. 
         [0005]    The requirement to operate hydraulic devices remote from the well head means that additional DCVs have to be integrated into the SCM. In general, SCMs are designed and manufactured as ‘common’ in that they contain sufficient DCVs to meet the requirement of a typical well. However, when further remote devices have to be operated, the ‘common’ SCM has to be modified which incurs substantial design costs. If, on the other hand, the ‘common’ SCM is designed to accommodate additional remote devices, then in many ‘straightforward’ applications the surplus capacity makes the SCM more expensive. 
         [0006]    Intelligent downhole systems are becoming more common and generally require three hydraulic functions, operating at high pressure (typically 10 k to 15 k psi), inside the SCM. Not all wells need an intelligent completion. It is usual to have a ‘common’ design of SCM, so in many cases these three functions are unused. Typically, an intelligent well system will also need an additional high pressure (HP) accumulator to ensure that operating the intelligent well does not adversely affect the ‘surface controlled sub-surface safety valve’ (SCSSV) which is also on the HP supply and vice versa. 
         [0007]      FIG. 2  illustrates a traditional arrangement for the control of downhole hydraulic devices, in this example valves  6 . The tree  1  carries an SCM  2 , which is connected to the downhole valves  6  via hydraulic feeds  7 . 
         [0008]    It should be noted that such systems are not the only systems available, for example British Patent Application No. GB 0319622.7 describes a decentralized control system which does not use an SCM. Likewise the system as described in British Patent No. GB 2264737 describes a further system in which the SCM is replaced by a multiplicity of integrated electronic and hydraulic functions in modules, such as smaller and dedicated electronic units and hydraulic units. In contrast to these two described systems, while this invention also employs modules that contain electrically operated hydraulic functions and perhaps electronic functions in some embodiments, in the present invention they are under the control of an SCM. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an aim of the present invention to obviate the need for steel tube jumpers and to allow standard minimum SCMs to be employed when there is a requirement to operate additional remote hydraulic devices. 
         [0010]    This aim is achieved by the removal of the hydraulic controls for remote hydraulic devices, e.g. DCVs, from the tree mounted SCM and housing them instead in a separate ‘pod’ which is then located external to the SCM and in some applications close to the remote devices. 
         [0011]    In accordance with a first aspect of the present invention, there is provided a hydraulic control system for controlling an external device at a well installation, comprising a control module for generating electrical and/or optical control signals, a control pod for receiving said control signals, the control pod comprising control means for controlling the external device, and a hydraulic line for linking the control means to said external device for the control thereof. 
         [0012]    The control signals may be transmitted from the module to the pod via an electrically conductive coupling, e.g. via a serial data link, or via optical fiber. 
         [0013]    A plurality of control means may be provided, linked to respective external devices by respective hydraulic lines. 
         [0014]    The or each control means may be a valve, for example a directional control valve. 
         [0015]    Preferably, the control pod is adapted to receive hydraulic fluid from a supply. 
         [0016]    According to a second aspect of the present invention, there is provided a well installation for location underwater, comprising a well tree, a well, an external device and the hydraulic control means according to the first aspect of the present invention, wherein the control module is located at the tree. 
         [0017]    The control pod may be located at a structure remote from the tree, for example a manifold. The external device may also be located at the structure. The pod may further receive low pressure hydraulic fluid from a supply located at the structure. 
         [0018]    Alternatively, the control pod may be located at the tree. The pod may receive hydraulic fluid from a high pressure supply via the control module. 
         [0019]    As a third alternative, the control pod may be mounted at or within the well. 
         [0020]    The external device may be located within the well. 
         [0021]    The external device may be a valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a schematic of a prior art arrangement for control of valves on a subsea manifold. 
           [0023]      FIG. 1  is a schematic of a prior art arrangement for control of downhole valves of a subsea well. 
           [0024]      FIG. 3  is a schematic of an arrangement in accordance with this invention for control of valves on a subsea manifold. 
           [0025]      FIG. 4  is a schematic of an arrangement in accordance with this invention for control of downhole valves of a subsea well. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 3  illustrates a first embodiment of the invention relating to the control of valves on a remote manifold/structure. In this embodiment, replacement of the hydraulic control lines from the tree with an electric or a fiber optic cable is achieved so that the need to modify or expand a minimal ‘common’ SCM is removed. An SCM  2  is housed on tree  1  and is connected either electrically or optically via a cable  9  to a pod  8 , which is mounted on the remote manifold/structure  3 . Each valve  4  on the manifold/structure  3  is fed via a hydraulic control line  10  from the pod  8 . Electrical or optical signals from the SCM  2  operate DCVs in the pod  8  which in turn control the hydraulic power from a local source, designated ‘LP (low pressure) supply’ in  FIG. 3 , to each valve  4  via hydraulic feeds  10  internal to the manifold/structure  3 . Thus the cost of steel hydraulic tubing from the SCM to the manifold/structure is obviated as is the need to add additional DCVs to the SCM. 
         [0027]      FIG. 4  illustrates a second embodiment of the invention relating to the control of downhole valves. In this embodiment, a pod can be located on the tree but external to the SCM thus avoiding the need to modify or expand a minimal standard SCM. An SCM  2  is housed on tree  1  and is connected either electrically or optically via cable  9  to the pod  8 . In this embodiment, the pod  8  is also mounted on the tree  1 . The pod  8  feeds downhole valves  4  via respective hydraulic control lines  7 . Electrical or optical signals from the SCM  2  operate DCVs in the pod  8 , which in turn control the hydraulic power from the SCM, designated ‘HP (high pressure) supply’ in  FIG. 4 , to each valve  4 , via the hydraulic control lines  7 . Thus the need to add additional DCVs to the SCM is obviated. 
         [0028]    As an alternative form of this embodiment, a pod may be located downhole and the hydraulic feeds, which could be several kilometers long, replaced by a much cheaper electric or fiber optic cable, similar to the arrangement used in the first embodiment of  FIG. 3 . 
         [0029]    In all these embodiments, the pod contains, as a minimum, electrically operated DCVs to provide hydraulic operation of the hydraulic devices at the location, powered from a local hydraulic source. When more than one device is to be operated it may be cost effective to replace the individual wires that provide electric control of each DCV with a serial data link, transmitting on its own separate pair of wires, or superimposed on the electric power, with decoding electronics incorporated in the pod. Alternatively the digital message could be transmitted to the pod via an optical fiber with a single pair of wires to provide electric power. 
         [0030]    It will be apparent that the described systems provide the following advantages over the prior art systems: 
         [0031]    1) Removal of both the need for long expensive steel hydraulic tubing, when used between a tree and a remote manifold/structure and the cost of installation which is expensive because of the need for special remotely operated vehicle (ROV) tools and facilities to install it. 
         [0032]    2) Removal of the need to modify a ‘common’ SCM when used to control hydraulic devices remote from the tree. Normally the pod would only be fitted to trees that need it. Although the consequence of this is that all trees would still need a mounting plate for it to be plugged into, these are relatively cheap. 
         [0033]    3) Enables replacement of the remote hydraulic device control i.e. a pod (e.g. by an ROV), without disrupting the operation of the SCM. 
         [0034]    4) Provides the opportunity, when applied to intelligent wells, of having just one pod and deploying it when needed and then recovering it afterwards, since an intelligent well operation is often only needed only a few times in the system's approximate 25 year life. 
         [0035]    5) For control of downhole hydraulic devices, the pod offers the opportunity to mount a small additional hydraulic accumulator inside the pod, although this may well have to sit on an auxiliary stab plate. Such an application may provide isolation of the SCM hydraulic fluid from the downhole hydraulic control system which, in terms of prevention of fluid contamination of the SCM hydraulics from the downhole hydraulics, is attractive to well installers.