Downhole tractor with redundant motor drives with independent circuit breakers

The invention is a downhole petroleum well tractor having a main body (0) connectable to an electrical conductor logging cable (5) from a surface high voltage DC power supply (50), characterized by said main body (0) comprising a common HV DC power line (4) provided with high-voltage DC power supplied from said electrical cable (5); two or more HV DC branch power lines (40) from said common HV DC power line (4), each said HV DC branch power line (40) feeding power to a HV motor drive electronic unit (21) for a drive motor (2); each said drive motors (2) driving one or more drive devices (6) for running on and along a wall in a well for moving said tractor; separate HV DC circuit breaker units (8) on each said HV DC branch power line (40), each said HVDC circuit breaker (8) unit comprising control means (82, 86, 88) arranged for monitoring a current (I) on said HV DC branch power line (40) and controlling a HV DC power switch (84) on said HV DC branch power line (40) to break said current (I) if said current (I) exceeds said set current level (Imax). Each said HV circuit breaker unit (8) is arranged to disconnect its associated motor (2) in case of said associated motor (2) fails, by detecting an increased current above a set current level, in order to prevent shorting said HV power line (4), thus maintaining operation of the other motors (2) of the tractor.

INTRODUCTION

The present invention relates to a downhole petroleum well tractor for conveying logging or intervention tools in the well. More specifically, the invention relates to an electrically driven downhole petroleum well tractor connected to a surface DC high voltage supply via an electrically conducting logging cable.

BRIEF SUMMARY OF THE INVENTION

The invention is a downhole petroleum well tractor having a main body (0) connectable to an electrically conducting logging cable (5) from a surface high voltage DC power supply (50), wherein said main body (0) comprises a common power line (4) provided with energy supplied from said electrical cable (5), two or more drive motors (2) supplied with power from said common power line (4) each via a branch power line (40), each said drive motors (2) driving one or more drive devices (6) for running on and along a wall in a well for moving said tractor, and separate circuit breaker units (8) for each said drive motors (2) on said branch power line (40), said circuit breaker unit (8) arranged for monitoring a current (I) to said motor (2) and breaking said current (I) in case said current (I) exceeds a set current level (Imax).

The invention is also a method for preventing downhole petroleum well tractor failure, said tractor having a main body (0) connected to an electrically conducting logging cable (5) from a surface high voltage DC power supply (50), said main body (0) comprising a common power line (4) provided with energy supplied from said electrical cable (5), and two or more drive motors (2) supplied with power from said common power line (4) each via a branch power line (40), each said drive motors (2) driving one or more drive devices (6) running on and along a wall in a well and moving said tractor; wherein said method comprises continuously monitoring a current (I) on each said branch power line (40), and in case said current (I) on any one of said branch power line (40) exceeds a set current level (Imax), breaking said current (I) separately on said branch power line (40) by means of a circuit breaker unit (8) on said branch power line (40).

Further embodiments of the invention are defined in attached dependent claims.

An advantage of the invention is that by disconnecting the failed motor and motor drive it allows the remaining motors to function as normal and the whole tool can continue at reduced performance.

EMBODIMENTS OF THE INVENTION

FIG. 1is a simplified illustration of a tractor according to the invention with two or more wheel drive sections. Each drive section comprises two or more wheels for driving the tractor along a wall in the well, the wall being the borehole wall or an inner wall of a tubing or casing. Each wheel drive section is provided with drive wheels driven by brushless DC motors. Each brushless DC motor is powered through a DC electronic motor drive connected to a common high voltage power line in the tractor. The common high voltage power line of the tractor is connected to a DC line of the logging cable extending from the surface high voltage DC power supply. The preferred operating voltage on the common high voltage power line and the electronic motor drives is 600 V. Due to resistivity in the logging cable, which may be of a length at least the depth of the well, the surface power supply may have to operate up to 1200 V in order to try and maintain the desired operating voltage of 600 V and current up to 6 A or more on the common high voltage power line. If the current on the common high voltage power line suddenly drops, the voltage drop over the cable between the surface power supply and the tractor becomes lower. Thus the common high voltage power line may experience peaks or excursions up to 1200 V if the surface high voltage DC power supply does not adjust the voltage down or does not do so fast enough.

If one of the motors fails there is a risk that it may short the common high voltage power line unless the local current to the motor is broken. If the common high voltage power line is shorted then the tractor would fail and have to be pulled out of the well. Pulling the tractor would cause interruption of the logging procedure and cost additional time for retrieving the tractor and the conveyed logging string from the well, replacement or repair of the tractor and additionally the time for assembling and resuming running the tractor in the well.

The invention illustrated inFIGS. 1 and 2is downhole petroleum well tractor having a main tractor body, a main body (0) connectable to an electrically conducting logging cable (5) from a surface high voltage DC power supply (50). The logging cable (5) is here used as a broad term which may comprise a wireline or intervention cable or a composite rod-like cable with a conductor directly or indirectly connected to the main body's (0) common power line (4) provided with energy supplied from said electrical cable (5). The common power line (4) extends through all the wheel drive sections (9) of the tractor. In general the tractor has a connector (01) in its opposite end for mechanically and electrically connecting to conveyed tools. The tractor has two or more electrical drive motors (2) supplied with electrical power from said common power line (4) each via a branch power line (40). Each said drive motors (2) drives via a drivetrain (62) one or more mechanical drive devices (6) such as drive wheels (6w) or drive belts (6b) for running on and along a wall in a well, such as a borehole wall, a pipe wall, through a valve, etc., for moving said tractor along in the well. The tractor is provided with separate circuit breaker units (8) for each said drive motor (2) on each said branch power line (40), please seeFIGS. 2 and 3. The circuit breaker unit (8) is arranged for monitoring a current (I) to said motor (2) and breaking said current (I) in case said current (I) exceeds a set current level (Imax).

Thus, each said circuit breaker unit (8) is made for disconnecting its associated motor (2) in case said associated motor (2) fails, by detecting an increased current above the set current level, in order to prevent shorting said local HV branch power line (40) thus shorting said HV power line (4). Preventing such a short circuit of power maintains operation of the other motors (2) of the tractor in case one motor fails, resulting in continued operability of the tractor.

In a preferred embodiment of the invention, the drive motors (2) are high voltage brushless DC motors. In an embodiment of the invention, each said drive motor (2) comprises a motor drive electronic unit (21) connected to said separate local HV branch power line (40). The motor drive electronic unit (21) is a HV DC motor drive electronic unit (21). Such brushless motors are provided with an electronic motor drive unit (21) which shapes pulses for driving the motor in a desired direction and at a desired speed.

The electrical conductor logging cable (5) provides high voltage DC directly or indirectly to the common power line (4) in the tool. The surface high voltage DC power supply (50) may provide a voltage between on 300 V to 1800 V, but in an embodiment it provides up to 1200 V to the upper end of the logging cable (5) in order to provide a controlled working voltage of 600 V at the cable head to the common high voltage power line (4) at the tool, including its branch power lines (40). The surface DC power supply (50) must be adjusted for its voltage depending on the actually consumed current in the tractor so as for the voltage at the common high voltage power line (4) to be stable at 600V, but a sudden decrease in the consumed current may cause excursions of up to 1200 V at the electrical conductor logging cable (5). It is not desirable to use AC surface power supply because it would incur a considerable inductive resistance in the AC circuit comprising the very long cable in the well.

The motor drive units are provided with a 30 V input separate from the common high voltage power line (4), for control electronics.

In an embodiment of the invention, the circuit breaker (8) unit comprises control means (82,86,88) for monitoring said current (I) on said branch power line (40), and arranged for commanding a power switch (84) on said branch power line (40) to break if said current (I) exceeds said set current level (Imax). In the illustrated embodiment inFIG. 4, the control means (82,86,88) comprising a current meter unit (82) for measuring the current (I) on the high voltage branch power line (40), a threshold comparator unit (86) for comparing said measured current (I) and said set current level (Imax), and a control logic unit (88) for steering said power switch (84) on said high voltage branch power line (40). If the threshold comparator (86) finds that said current (I) exceeds said set current level (Imax), the control logic unit (88) commands high voltage power switch (84) to break the current on branch power line (40). Latch means (881) in said control logic unit (88) hold said power switch open if the current on branch power line (40) first has been broken in order to prevent further reconnection of the failed motor (2) and motor drive (21). In an embodiment of the invention, the control logic unit (88) and its latch means (881) may be reset by an operator when the tractor is at the surface so as for enabling testing and reactivation. In an embodiment of the invention it is arranged so as if the power is turned off from the surface completely it will release the latch means (881). This would allow the operator to restart the tool remotely. Should the failure persist the circuit breaker (8) will trip again.

In an embodiment of the invention illustrated inFIG. 4the current meter unit (82) operates based on measuring a voltage drop over a resistance (823) on said branch power line (40). In an embodiment, the resistance (823) is low-Ohmic in order to have low power consumption and low heat development of the resistance. The current meter unit (82) may comprise a sensor resistance bridge (822) having said sensor resistance (823) connected serially on said local HV power line (40), the differential voltage over said sensor resistance (823) connected to a differential amplifier (824) with an output voltage connected to a first input (861) of said threshold comparator (86), please seeFIG. 4. The resistance (823) for being measured for a voltage drop may be constituted by a length of the branch power line (40) itself.

In an embodiment of the invention, the control means (82,86,88) is a low-voltage circuit operating on the high voltage side of said high voltage branch power line (40), such as having an internal low voltage level V e.g. 12 V below said high voltage of said HV branch power line (40), please seeFIG. 4. The local low-voltage power supply (89) may comprise a control circuit (891) and connected between apparatus ground (AGND) and said local HV DC power line (40) for controlling said low voltage to said local HV DC circuit breaker and keeping it stable if said HV DC power line (40) varies in voltage, in order to provide stable low voltage to the high voltage circuit breaker (8).

In an embodiment of the invention, said HV current meter unit (82) may comprise a magnetic sensor current meter (82m) such as Hall effect sensor or a fluxgate magnetometer based device which measures the magnetic field about the conductor and thus indirectly measures the current on branch HV power line (40). Such a magnetic sensor current meter may operate without galvanic contact with the HV power line (40).

In a preferred embodiment of the invention, each motor (2) is connected via a transmission drivetrain (62) to said drive device (6). Each drive device (6) is preferably arranged on a drive arm (61) which may be hydraulically controlled to be forced against or retracted from the wall onto which the wheel drives. The transmission drivetrain may comprise pinion gears and a reduction gear arranged in said arm (61) as sketched inFIG. 2in order to reduce the motor's high rotational speed of 1500 to 10000 rpm and relatively low torque of 0.5 to 1.5 Nm, geared down 1:50 to 1:150 to a lower desired rotational speed of the wheel and to increase the wheel's torque.

The well tractor of the invention including its electric motors and said circuit breaker (8) is arranged for operating at well temperatures up to 180 degrees C. ambient temperature.

Stated more specifically, the invention may be defined as a downhole petroleum well tractor having a main body (0) connectable to an electrical conductor logging cable (5) from a surface high voltage DC power supply (50), characterized by said main body (0) comprising a common HV DC power line (4) provided with high-voltage DC power supplied from said electrical cable (5); two or more HV DC branch power lines (40) from said common HV DC power line (4), each said HV DC branch power line (40) feeding power to a HV motor drive electronic unit (21) for a drive motor (2); each said drive motors (2) driving one or more drive devices (6) such as wheels (6w) drive belts (6b) for running on and along a wall in a well for moving said tractor; separate HV DC circuit breaker units (8) on each said HV DC branch power line (40), each said HVDC circuit breaker (8) unit comprising control means (82,86,88) arranged for monitoring a current (I) on said HV DC branch power line (40) and controlling a HV DC power switch (84) on said HV DC branch power line (40) to break said current (I) if said current (I) exceeds said set current level (Imax). Each said HV circuit breaker unit (8) is arranged to disconnect its associated motor (2) in case of said associated motor (2) fails, by detecting an increased current above a set current level, in order to prevent shorting said HV power line (4), thus maintaining operation of the other motors (2) of the tractor. The electrical conductor logging cable (5) may be connected directly to said common power line (4).