Abstract:
A subsea control module has a casing inside which there is at least one pair of electrodes, there being electronic means connected with the electrodes of the or each pair for monitoring at least one electrical characteristic between the electrodes as a result of a fluid to which the electrodes are exposed. This can include, for example, a change due to an ingress of seawater and/or an ingress of hydraulic fluid into the interior of the casing. The or each pair of electrodes include an array in which each electrode of the pair has finger portions interleaved with finger portions of the other electrode.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation of and claims priority to and the benefit of U.S. patent application Ser. No. 12/816,734, filed Jun. 16, 2010, which claims priority to and the benefit of United Kingdom Application No. 0910426.6 filed Jun. 17, 2009, each incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to monitoring of undesirable fluid ingress into subsea control modules. 
         [0004]    2. Description of the Related Art 
         [0005]    The hydraulic and electronic components of a subsea well, such as a hydrocarbon extraction well, are typically housed in a sealed vessel termed a subsea control module (SCM), located on a Christmas tree which is mounted on the sea bed above the well bore. An SCM is, typically filled with electrically insulating oil, to alleviate the need to design it to withstand high pressures and provide a first line of defense for the control system against the sea water environment. Typically, an SCM houses hydraulic manifolds, directional control valves and a subsea electronics module (SEM) which is itself a sealed unit. Thus, ingress of sea water resulting from a leak in the SCM housing, or ingress of hydraulic fluid from a small leak in the hydraulic system, will not in itself cause a malfunction of the control system. However, well operators, historically, have needed to know if there is a sea water leak since this will result in corrosion of the components in the SCM and possible failure of the system earlier than expected. 
         [0006]    Existing arrangements consist of sets of metal electrode pairs mounted on an insulating panel or a metal frame with insulating inserts within the SCM, typically four, at equal intervals between the bottom and the top, each connected to an operational amplifier via a low voltage source and a series resistor, thus enabling the detection of the presence of the ingress of electrically conductive sea water, and, in a crude manner, the degree of displacement of the original oil filling. A typical application of the technique is illustrated diagrammatically in  FIG. 1  in which a casing  1  of an SCM is shown as a transparent outline to show an electrically insulating panel  2 , mounted on the base  3  of the SCM, with pairs  4  of electrodes mounted on it.  FIG. 2  shows circuitry  5  around an operational amplifier  6 , typically housed in the SEM within the SCM, to which the electrode pairs, are connected. Since the resistance across a pair of electrodes when in contact with sea water is very low, i.e. effectively a short circuit, they are shown in a block  7  as simple switch contacts  8 ,  9 ,  10  and  11 . One of the electrodes of each pair is connected to a voltage source V, typically 2 volts. The gain of the circuit  5  is the ratio of the resistance of a feedback resistor  12  across amplifier  6  and the effective resistance provided by input resistors  13 ,  14 ,  15  and  16 , each of which is in series with a respective one of the electrode pairs  4  and an input of amplifier  6 . Each of the input resistors is chosen to be of a resistance which is one quarter of that of the feedback resistor  12 . Thus, if there is water ingress into the casing  1  to the level of the lowest electrode pair, then the contacts  8  of the block  7  are effectively closed and the output  17  of the operational amplifier  6  will rise to ¼ of V. Likewise, further ingress of sea water reaching the remaining electrode pairs will result in the output  17  rising to ½ V, ¾ V and V respectively as the contacts  9 ,  10  and  11  become effectively closed. Thus, a crude indication of the sea water ingress level is obtained by the electronic circuitry of the SEM reading the output  17  of the circuit  5  and transmitting the information topside, as a digitised version of the analogue signal, to the well operator, typically via the well umbilical, as part of the well housekeeping/diagnostic telemetry. 
         [0007]    A problem with the existing technique is that it is unable to detect the ingress of hydraulic fluid into the SCM resulting from a leak in the hydraulic system in the SCM. Currently, a well operator has relied on hydraulic fluid leak detectors at the fluid source but these cannot confirm whether the leak is actually within the SCM. A further problem is that current flow through the electrode pairs results in their corrosion. 
         [0008]    This invention enables the detection of both the ingress of sea water and hydraulic fluid in the SCM and provides a better indication of the degree of ingress, and reduces corrosion of the sensing electrodes. 
         [0009]    Recent measurements have been made in the laboratory of the change in conductivity of the insulating oil in an SCM with contamination by the hydraulic fluid used for the well control system, which is a glycol based trans-aqua fluid. Results show that the conductivity of the contaminate in the oil is much less than that due to sea water and thus the existing contamination detection technique described before was not sensitive enough to be able to detect the ingress of trans-aqua fluids. Measurements have also shown that sea-water and trans-aqua hydraulic fluid (glycol) in insulating oil result in an immiscible fluid with both contaminants having a greater density than the oil. Thus, ingress of these contaminants displaces the transformer oil from the base of the SCM upwards. This invention provides an improved method for the monitoring of undesirable fluid ingress into an SCM to enable detection of the ingress of trans-aqua hydraulic fluid as well as sea water, whilst still providing a zoned measure of the degree of ingress and using a variety of methods of measurement which also reduces, substantially, corrosion of sensing electrode pairs. 
       SUMMARY OF THE INVENTION 
       [0010]    According to the present invention from one aspect, there is provided a subsea control module having a casing inside which there is at least one pair of electrodes, there being electronic means connected with the electrodes of the or each pair for monitoring at least one electrical characteristic between the electrodes as a result of a fluid to which the electrodes are exposed, wherein the or each pair of electrodes comprises an array in which each electrode of the pair has finger portions interleaved with finger portions of the other electrode. 
         [0011]    There could be at least one such array on a base of the casing. Preferably, there is a plurality of such arrays on the base of the casing, each of which; for example, is at or near a respective corner of the casing. 
         [0012]    Preferably, there is such an array disposed on a side wall of the casing and covering a zone of the side wall. In this case, said array disposed on a side wall could cover a lower zone of the side wall, which array, for example, also covers a portion of the base of the casing. Said portion of the base of the casing could be at or near a corner of the casing. 
         [0013]    Preferably, there is at least one further such array after said array disposed on a side wall of the casing, the or each further such array covering a respective zone up the side wall of the casing. 
         [0014]    The at least one electrical characteristic could comprise at least one of resistance and capacitance. 
         [0015]    Preferably, the or each array is disposed on a mat of electrically insulating material. Preferably, said electronic means is provided by a subsea electronics module of the control module. 
         [0016]    According to the present invention from another aspect, there is provided a subsea control module having a casing inside which there are a plurality of electrode pairs, there being electronic means connected with the electrodes of each pair for monitoring at least one electrical characteristic between the electrodes of the pair as a result of a fluid to which the electrodes are exposed, wherein said electrode pairs are disposed on a base of the casing. 
         [0017]    Preferably, there are also a plurality of further electrode pairs, each of which covers a respective zone up a side wall of the casing, said electronic control means being connected with the electrodes of each of the further electrode pairs. 
         [0018]    Preferably, for both aspects of the invention, the electronic means applies a signal between the electrodes of the or each pair of electrodes for selected periods of time at selected intervals. 
         [0019]    A module according to the invention could include pressure sensing means for sensing the pressure of fluid in the casing for use in providing an indication of an increase in pressure due to fluid leakage within the casing. 
         [0020]    A module according to the invention could include release means for releasing fluid from the casing in response to an increase in fluid pressure within the casing above a threshold. 
         [0021]    There could be a flowmeter coupled with said release means for providing an indication of the release of fluid. 
         [0022]    According to the present invention from a further aspect, there is provided a subsea control module having a casing inside which there is at least one electrode pair, there being electronic means connected with the electrodes of the or each pair for monitoring at least one electrical characteristic between the electrodes of the pair as a result of a fluid to which the electrodes are exposed, wherein said electronic means applies a signal between the electrodes of the or each pair of electrodes for selected periods of time at selected intervals. 
         [0023]    According to the present invention from yet a further aspect, there is provided a subsea control module including pressure sensing means for sensing the pressure of fluid in a casing of the module for use in providing an indication of an increase in pressure due to fluid leakage within the casing. 
         [0024]    According to the present invention from yet a further aspect, there is provided a subsea control module including release means for releasing fluid from a casing of the module in response to an increase in fluid pressure within the casing above a threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  illustrates a prior art system; 
           [0026]      FIG. 2  illustrates circuitry for use with the system of  FIG. 1 ; 
           [0027]      FIG. 3  illustrates a system according to an embodiment of the invention; 
           [0028]      FIGS. 4 ,  5  and  6  illustrate forms of circuitry for use with the system of  FIG. 3 ; and 
           [0029]      FIG. 7  shows how pressure release means can be provided. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Referring to  FIG. 3 , the individual electrode pairs of the system of  FIG. 1  are replaced with electrode pairs, in each of which each electrode of the pair has finger portions interleaved or interlaced with finger portions of the other electrode to form an electrode array. The arrays are on an electrically insulating mat  18 , typically mounted on the internal base  3  of the casing  1  of the SCM and extending up a side wall of the SCM. Typically, each array is printed in copper on to a flexible printed wiring board and then gold plated for protection against corrosion, the board being mounted on a frame. The mat is divided into four sections, a lower section with a first electrode array  19 , including a horizontal portion  19   a  on an electrically insulating mat on the base  3  at or near a corner of the casing  1 , and a vertical portion, with the latter rising to a quarter of the height of the SCM. The other three sections have respective electrode arrays  20 ,  21  and  22  and cover half, three quarters and the top quarter of the height of the SCM, resulting in four vertical detection zones. Three other small horizontal gold plated electrode arrays with interleaved electrode pairs in the form of arrays  23 ,  24  and  25  are also mounted on the SCM base  3  on electrically insulating mats, at or near each of the other three corners respectively. The electrode pairs of each of the detection zones and the three corner mats are connected to conditioning and detection circuitry housed in the SEM within the SCM. The vertical arrays  19  to  22  provide a measure of the quantity of ingress of undesirable fluid into the SCM, i.e. up to a quarter, half, three quarters and full displacement of the original insulating oil by sea-water or hydraulic fluid or both. The electrode array  19  by virtue of portion  19   a,  and the electrode arrays  19 ,  23 ,  24  and  25  provide early detection of small quantities of sea-water and/or hydraulic fluid ingress even when the installation of the SCM is not truly vertical and are typically connected in parallel to the conditioning and detection circuitry and treated as one array. 
         [0031]      FIG. 4  shows a block diagram of the conditioning and detection circuitry for the detection of fluid ingress using changes of resistance between the electrodes of the arrays. With the SCM filled with clean, uncontaminated oil, the resistance between the terminals of any of the electrode array  19  to  25  is very high, typically tens of megohms. With a small ingress of trans-aqua fluid, e.g. enough to cover arrays on the mats mounted on the SCM housing base, the resistance will fall typically to a few hundred kilohms. Ingress of sea water will also be detected by the system as only a small amount of ingress will cover at least one of the arrays of the mats on the SCM base and reduce the resistance between the electrodes of the array to only a few tens of ohms. The electrode pair on each mat, for example that of array  19 , is fed from a low voltage DC source  26 , typically 2 volts, via a current measuring resistor  27  and isolator switches  28  and  29 . The current flow through the electrode pair produces a voltage fed to a differential amplifier  30 , which produces an analogue output, converted to a digital message that is added to the well monitoring telemetry, fed topside, via the well umbilical. Although only one electrode array ( 19 ) is shown in  FIG. 4 , the other arrays  20 - 25  are selected in turn by isolator switches similar to  29  of  FIG. 4 . The connection of the low voltage source  26  to the arrays is controlled by the isolator switch  28 , which is operated by a digital control system also located typically in the SEM. The isolator switch  26  is closed only for a brief period, just long enough for the conditioning and detection circuitry to make a measurement, and repeated infrequently. Since the ingress of fluid is typically a slow process, measurement cycle time to measurement execution time ratios in excess of 10,000 to 1 are adequate. This reduces the corrosion of the electrode pairs on a mat due to electrolytic action to a negligible level. The process described above is repeated for the detection of sea-water ingress, by the same low voltage source  26 , connected to the arrays via a current measuring resistor  31  and an isolator switch  32 , producing an output to the well telemetry system from a differential amplifier  33 . Since the resistance between the electrode pairs on mats resulting from hydraulic fluid ingress is much greater (kilohms) than that resulting from sea water ingress (tens of ohms), the value of the measuring resistor  27  is much greater than the resistor  31  to produce a greater detection circuitry gain. Although the ingress of sea-water will swamp the hydraulic fluid ingress detection circuitry, this is of little concern since the detection of sea water ingress is itself sufficiently serious to warrant corrective action by the well operator. 
         [0032]      FIG. 5  shows a block diagram of alternative conditioning and detection circuitry for the detection of fluid ingress utilising the change of capacitance between the electrode pair on any of the mats. This method can also be employed as an addition to the resistance measuring method, to provide greater confidence to the well operator that the detection of hydraulic fluid ingress, in particular, is accurate. A low voltage AC source is connected across two of the arms of a bridge circuit comprising three capacitors  35 ,  36  and  37  and the electrode pair of array  19 , with two isolator switches  38  and  39 . The other two arms of the bridge are connected to AC amplification and detection circuitry  40 , to produce a DC output which is fed to the telemetry system of the well. The values of the capacitors  35 ,  36  and  37  are chosen to match the capacitance between the electrode pair of array  19  when immersed in the oil within the SCM, so that the bridge circuit is balanced and there is no output to the circuitry  40 . Ingress of hydraulic fluid into the SCM results in a change of capacitance between the electrode pair of array  19  and thus an AC output from the bridge and into the amplification and detection circuitry  40 , which in turn produces an output to the well telemetry system. A variation of this method is to electronically adjust the value of the capacitor  36 , to maintain the balance of the bridge, i.e. zero output from the amplification and detection circuitry  40 , and use a measure of the bridge balancing capacitance as the source to the well telemetry system. Again, the arrays  19 ,  23 ,  24  and  25  are typically connected in parallel to the conditioning and detection circuitry and treated as one array and monitoring of other zones achieved by selecting the arrays  20 ,  21  and  22  by additional isolator switches. 
         [0033]      FIG. 6  shows a block diagram of an alternative method of detecting a change of capacitance due to undesirable fluid ingress. This method utilises the change of capacitance between the electrodes of array  19 , resulting from the change of dielectric constant of the fluid it is immersed in, when there is ingress of sea-water and/or hydraulic fluid. The change of capacitance results in a change of frequency of an oscillator  41  which can be measured and translated into a DC output using a frequency measuring circuit  42 , such as a discriminator and detector, as an output to the well telemetry system. The array  19  is selected by isolator switches  38  and  39  and different zones can be monitored by further isolator switches selecting the arrays. Again, the arrays  19 ,  23 ,  24  and  25  are typically connected in parallel to the conditioning and detection circuitry and treated as one array, and monitoring of other zones achieved by selecting the arrays  20 ,  21  and  22  by additional isolator switches. 
         [0034]    Leakage of hydraulic fluid in the SCM results in an increase in pressure within the outer casing  1 . This can be monitored, typically, by a diaphragm type pressure sensor  43 , mounted on the wall of the SCM outer casing  1 , and connected to the SEM within the SCM and its output also fed into the well monitoring telemetry system. The detection of hydraulic fluid ingress by the methods described above can thus be supported by a change of pressure, giving greater confidence of the detection process to the well operator. 
         [0035]    An additional method of monitoring leakage of hydraulic fluid into the SCM, to provide even greater corroboration of the electrical detection method, is to fit a differential pressure release valve and a flowmeter to the SCM casing as illustrated in  FIG. 7 . The pressure release valve  44 , is set, typically, to open when the pressure in the SCM exceeds the external environmental pressure by 5 psi, resulting from a hydraulic fluid leak. When the valve  44  opens the flow of liquid from the SCM to the environment is detected by the flowmeter  45 , whose electrical output is connected to the well monitoring telemetry system, thus advising the well operator to a fluid leak. 
         [0036]    The key advantage is that the invention permits detection of hydraulic fluid leakage within the SCM in the absence of sea water ingress, and provides an indication of the degree of leakage, neither of which are possible with existing SCM ingress fluid ingress detection systems. Furthermore the system detects the ingress of sea water as well. Corrosion of the detection electrodes is also virtually eliminated. 
         [0037]    This application claims is a continuation of and claims priority to and the benefit of U.S. patent application Ser. No. 12/816,734, filed Jun. 16, 2010, which claims priority to and the benefit of United Kingdom Application No. 0910426.6 filed Jun. 17, 2009, each incorporated by reference in its entirety.