Abstract:
A method for testing an electric motor which is connected to a valve element by at least one component comprises starting the motor, measuring a movement parameter of at least one of the motor and the component without moving the valve element, and stopping the motor when the measured movement parameter indicates that the valve element is about to move.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for testing an electric motor which is used to, for example, actuate a subsea valve. More specifically, the invention relates to a method for testing the operability of such a motor without actuating the valve. 
     Subsea installations often include a number of valves for controlling the flow of fluids through certain of their components. These subsea valves are normally hydraulically actuated, since hydraulic actuators are traditionally regarded as being fairly reliable. However, current hydraulic actuators are practically limited in the depths at which they can be used. In contrast, electric actuators are not so limited. Thus, by replacing current hydraulic actuators with electric actuators, such as electric motors, subsea installations can be located at greater depths and can thus potentially experience large cost savings. However, while electric actuators are common in many industries, they are not often used in subsea installations. This is due mainly to reliability concerns, since a failure of a subsea valve can potentially lead to environmental disasters. 
     Hydraulically actuated subsea valves are normally tested by opening and closing the valves. To do this the subsea installation must be shut down; but since these valves are usually only tested once per year, this is not regarded as a problem. The hydraulic actuators themselves are not specifically tested since they are regarded as very reliable. 
     However, electric actuators do need to be tested since their electric motors are considered to be less reliable than hydraulic actuators. Due to these reliability concerns, an operator may desire to test the electric motors relatively frequently to ensure that they will perform as intended. Therefore, the testing of the electric motors should ideally not affect the valves to which they are connected to avoid having to shut down the subsea installation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, these and other limitations in the prior art are overcome by providing a method for testing an electric motor which is connected to a valve element by at least one component. The method comprises the steps of starting the motor and measuring a movement parameter of at least one of the motor and the component without moving the valve element. In a further embodiment, the method comprises the additional step of stopping the motor when the measured movement parameter indicates that the valve element is about to move. 
     In accordance with another embodiment of the invention, the method comprises determining a value for the movement parameter which corresponds to a movement of the valve element, comparing the measured movement parameter with the value, and stopping the motor when the measured movement parameter is approximately equal to this value. 
     In an exemplary embodiment of the invention, the movement parameter comprises a number of turns of the motor. In addition, the value comprises an approximate number of turns the motor can make before the valve element begins to move. Furthermore, the comparing and stopping steps of this embodiment comprise comparing the measured number of turns of the motor with the predetermined number of turns the motor can make before the valve element begins to move, and stopping the motor when the measured number of turns is approximately equal to the predetermined number of turns. 
     The method of the present invention utilizes a well-known feature of mechanical transmissions, namely, that a certain amount of play exists due to tolerance differences between the interacting mechanical parts of the transmissions. Thus, the motor can be run within this area of play without moving the valve element. Therefore, the motor can be tested without the need to shut down the subsea installation. 
     These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-sectional view of an exemplary electric actuator with which the method of the present invention may be used; 
         FIG. 2  is a schematic block diagram illustrating a first embodiment of the invention; 
         FIG. 3  is a schematic block diagram illustrating a second embodiment of the invention; 
         FIG. 4  is a schematic block diagram illustrating a third embodiment of the invention; 
         FIG. 5  is a graph illustrating certain parameters of the operation of the invention depicted in  FIG. 2 ; 
         FIG. 6  is a graph illustrating certain parameters of the operation of the invention depicted in  FIG. 3 ; and 
         FIG. 7  is a graph illustrating certain parameters of the operation of the invention depicted in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an exemplary electric actuator  10  is shown which includes an electric motor  12  that may be tested by the method of the present invention. The actuator  10  is normally placed in an oil-filled enclosure, but this enclosure has been omitted from  FIG. 1  for better clarity. The actuator  10  comprises the electric motor  12 , a gearbox  14  which is connected to an output shaft of the motor, and a drive shaft  16  which is rotated by the gearbox. In the exemplary actuator  10  shown in  FIG. 1 , the drive shaft  16  is connected to a flexible joint  18  which includes a part  20  that allows the actuator  10  to bend or flex somewhat under load. The distal end of the joint  18  includes a sleeve  22  which comprises an inner surface that is hexagonal in cross section. 
     The motor  12  comprises a housing which encloses the electric windings and is normally filled with a silicon oil or other suitable dielectric fluid. In one embodiment of the invention, the motor  12  is a brushless type DC motor and the gearbox is a planetary gear which has a relatively large gear ratio, such as 50:1 or 60:1. In addition, the actuator  10  also includes conventional drive electronics (not shown) for receiving control signals and electric power. 
     Referring still to  FIG. 1 , the actuator  10  is shown engaging an exemplary subsea valve  40 . The valve  40  is normally attached to a subsea installation  60 , such as a subsea Christmas tree, portions of which have been omitted for clarity. The subsea installation  60  may include an ROV panel  62  to facilitate the engagement of the actuator  10  with the valve  40 . 
     The valve  40  shown in  FIG. 1  is a linear-type valve, such as a conventional gate valve which includes a valve element  42  that is attached to a valve spindle  44 . In  FIG. 1 , portions of the valve  40  are split along its longitudinal axis  30  to show the valve in both its fully open and fully closed positions. The proximal end  46  of the valve spindle  44  is threaded over part of its length, and these threads are engaged by a rotary bushing  48  to thereby form a rotary-to-linear motion converter. Such converters are well known in the art and therefore need not be described further. The bushing  48  is attached to a hexagonal bolt  52  which is rotatably supported in a bearing  54  and is received in the sleeve  22 . Thus, when the motor  12  is activated, it will rotate the sleeve  22  and the bolt  52 , and the rotary bushing  48  will convert this rotation into linear movement of the valve spindle  44 , as is well known in the art. 
     The ROV panel  62  and the bolt  52  preferably comprise a standard API interface. In addition, the actuator  10  is housed in a removable unit so that, in an emergency, the actuator may be removed from the standard API interface and the valve  40  may be actuated directly with an ROV tool. 
     When the motor  12  is activated, the valve element  42  will not move immediately due to the play in the mechanical parts of the actuator  10  and the valve  40  which must first be overcome. Two main mechanical parts contribute to this play: the gearbox  14  and the rotary bushing  48 . The motor  12  will therefore rotate a number of times before the valve element  42  starts to move. In accordance with the present invention, this play is used to test the motor  12  without actuating the valve  40 . 
       FIG. 2  schematically illustrates a first embodiment of the invention. As discussed above, activation of the motor  12  will result in a rotational movement  13  of the drive shaft  16 . This rotational movement  13  in is turn converted to a linear movement  19  of the valve spindle  44  by a converter  17 , which includes the rotary bushing  48  and the threaded end  46  of the valve spindle. 
     In accordance with the present invention, a movement parameter is measured in order to provide an indication that the valve spindle  44  is about to move. In the embodiment of the invention which is illustrated in  FIG. 2 , for example, this movement parameter is provided by a conventional revolution counter  15 , which may be used to measure the number of turns of either the motor  12  or the drive shaft  16 . Thus, the invention ideally also includes a suitable test controller  11  which is designed to start the motor  12  in order to initiate the test and to stop the motor when the measured number of turns, as determined by the revolution counter  15 , exceeds a predetermined number of turns. The predetermined number of turns is preferably dependent on the gearbox ratio and may be determined in a laboratory test before installation of the actuator  10 . 
     A graph illustrating the number of turns of the motor  12  with respect to time is shown in  FIG. 5 . When the motor  12  is first activated, it will rotate an initial number of turns N to take up the play in the mechanical parts. This number of turns is completed in the time indicated by T 1 . From this point the valve element  42  will start to move and the motor  12  will continue to rotate until the valve  40  has reached its end position (open or closed). The high ratio gearbox  14  requires that the motor  12  make a relatively large number of turns to actuate the valve  40 , and hence a significant number of turns N before the valve spindle  44  will start to move. Before installing the motor  12 , it may be tested in a laboratory to determine the number of turns N which it can make before the valve element  42  begins to move. This information can be used during a test of the motor  12  by only running the motor the predetermined number of turns N or alternatively, the corresponding time T 1 . The number of turns which the motor  12  makes during this test can be measured using the revolution counter  15 . 
     A second embodiment of the invention is schematically illustrated in  FIG. 3 . This embodiment is similar to the embodiment described with reference to  FIG. 2 , except that the movement parameter is provided by a conventional position detector  21  rather than by the revolution counter  15 . The position detector  21  is arranged to measure the linear position of the valve spindle  44 . Furthermore, the test controller  11  is designed to stop the motor  12  when the position detector  21  indicates that the valve spindle  44  has moved a preset distance, which in turn indicates that the play has been taken up and the valve element  42  is about to move. 
     A graph illustrating the travel of the valve spindle  44  with respect to time is shown in  FIG. 6 . As discussed above, the valve spindle  44  will not start to move until the play in the actuator  10  has been taken up. The movement of the valve spindle  44  can be measured using the position detector  21 . As shown in  FIG. 6 , the position detector  21  will not register movement of the valve spindle  44  for the first few seconds, corresponding to an elapsed time T 2 , until the position D is reached. From that point it will register a steady movement of the valve spindle  44  until the valve  40  reaches its end position (open or closed). 
     A third embodiment of the invention is schematically illustrated in  FIG. 4 . This embodiment is similar to the embodiment described with reference to  FIG. 2 , except that the movement parameter is provided by a conventional moment probe  23  rather than by the revolution counter  15 . The moment probe  23  is arranged to measure the moment on the motor  12 . Furthermore, the test controller  11  is designed to stop the motor  12  when the moment probe  23  indicates that this moment exceeds a preset load, which in turn indicates that the play has been taken up and the valve element  42  is about to move. 
     A graph illustrating the measured moment on the motor  12  with respect to time is shown in  FIG. 7 . As explained above, the third embodiment of the invention uses a moment probe  23  to measure the moment on the motor  12 . When the motor  12  is first activated, a small moment load will initially be measured due to the motor moving the gearbox  14  and drive shaft  16 . When the valve  40  starts to move, a peak will occur in this load. After the valve  40  has started moving, the load will level off or even decrease until the valve reaches its end position (open or closed). The load will then increase again. This last increase in the load may be employed to signal an end position of the valve  40  and stop the motor  12  so that it does not burn out due to overload. When the moment exceeds the preset load L, corresponding to the time T 3 , this indicates that the play has been taken up and the valve element  42  is about to move. 
     The results of the movement parameters measured by the revolution counter  15 , the position detector  21  or the moment probe  23  may be transmitted to and stored in a central processor which is housed in a subsea control module. In this regard, the operation of the central processor corresponds to the test controller  11  which is schematically illustrated in  FIGS. 2 through 4 . 
     Furthermore, the test of the motor  12  can be conducted at regular intervals without human intervention since the procedure can be stored as a routine in the central processor or the test controller  11 . 
     It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.