GB2596568A

GB2596568A - Subsea power unit

Info

Publication number: GB2596568A
Application number: GB2010061.6A
Authority: GB
Inventors: Rodgers John
Original assignee: Impaq Tech Ltd
Current assignee: Impaq Tech Ltd
Priority date: 2020-07-01
Filing date: 2020-07-01
Publication date: 2022-01-05
Also published as: GB202010061D0; WO2022003311A1

Abstract

A power unit for subsea use comprises an electric motor 2 driving a generator 5 inside an external pressure vessel 1 containing a gaseous atmosphere. The atmosphere may comprise air and may be at a pressure less than the ambient pressure. The motor may be an induction motor. The motor may be a DC motor and may be connected to a rectifier. The generator may be an alternator. An output converter stage that may be located within the pressure vessel 1 may provide a DC output. A remotely operated vehicle, or ROV, (10, Figure 2) with electric drive motors may be supplied with electricity by a power unit (15), which may be mounted on or in a tether management system (TMS) (11) connected to the ROV via a tether (12). The power unit may alternatively be located on or in the ROV.

Description

SUBSEA POWER UNIT

Field of the Invention

This invention relates to subsea power units.

Background to the Invention

Power units for subsea use, for example for driving a remotely operated vehicle (ROV), typically use hydraulic motors because of their high power density. The hydraulic motors are supplied with pressurised hydraulic fluid from pumps within the ROV which are in turn driven by electric motors supplied with electricity from the surface via the tether.

The use of hydraulic power subsea is undesirable, however, because of the environmental damage that can be caused by escape of hydraulic fluid into the sea, and there is therefore a wish to be able to use only electrical power. However, this needs to be achieved without increasing the size and weight of the equipment. The investment into the development of electric cars has creat- ed suitable motors and drives that could be adapted to the underwater environ-ment, but for one problem: these products and components are designed to operate at a few hundred volts, whereas the power supplied to the ROV will be at a few thousand volts. At the standard supply frequency of 50 or 60 Hz a conventional step-down voltage transformer in the ROV would be too big and heavy. An option used on certain small electric ROVs (<50kW) is to supply the vehicle with electricity at a higher frequency (400 Hz or 800 Hz), because transformer mass decreases with increasing frequency.

However, there are technical and commercial reasons why the higher frequency supply cannot readily be used for higher-powered ROVs (>50 kW). In particular, high frequency transmission can lead to high common mode voltag-es, these have been linked to failure of motors due to the electromagnetic interference (EMI) from capacitive effects of the high switching frequencies which can be conducted through the motors stator and rotors which can overcome the bearing grease breakdown voltage leading to premature failure of the bearings.

In some applications where there is a change in impedance, waves can com-bine upon reflection leading to a rise in the voltage at the load which can be up -2 - to 2.5 times the DC link voltage. This can destroy the motors in a very short period of time due to degradation of the motor winding isolation, leading to phase to phase or phase to stator short circuit. There is also a skin effect in conductors at higher frequencies, whereby the electric current flows in larger conduc-tors mainly in the "skin" of the conductor, between the outer surface and a level within the conductor called the skin depth. At higher frequencies, the skin depth is smaller, effectively reducing the current-carrying capacity of the conductor. Another possibility is the use of a direct current (DC) supply to the vehicle, for example at 3000V with a power-converter (also known as a solid-state transformer) using power electronic components (e.g. IGBT devices) to convert an input voltage to a different output voltage. While DC:DC converters are readily available for use at lower voltages (up to a few hundred volts), scaling them up for reliable use at higher voltages has proved difficult.

What is needed is a simple solution that can be applied equally to con-verting existing hydraulic ROVs to electric as well as to building completely new ones.

Summary of the Invention

The invention provides a power unit for subsea use, comprising an electric motor driving a generator inside an external pressure vessel containing a 20 gaseous atmosphere, which may comprise air and which can be at a pressure less than ambient In one embodiment, an induction motor driven generator has AC current going in at one voltage and coming out at a different voltage. An output converter stage can be added (either within the pressure housing or external to it) to provide a DC source rather than AC. Similarly an input rectifier can be added (within or without the housing) to enable a DC motor to be used instead of an AC motor. If a suitable DC motor were used the input supply could be DC rather than AC.

While known DC systems are limited to a 3000V supply, the input motor in the power unit of the invention can be wound for other voltages (e.g. 4160Vac) and frequencies. -3 -

In a conventional hydraulic power unit (HPU), the motor is filled with oil and connected to a depth-pressure compensating device so that the pressure inside the motor housing is the same (although usually kept a few psi higher) as the surrounding seawater pressure, which could be several hundred times the atmospheric pressure at sea-level. The downside to this is that, oil being some 800 times denser than air, considerably more power (10% or more) is required to turn the rotor in oil compared to in air. This power consumed, or drag, is dependent on a number of factors of which the surface speed of the rotor (a factor of the rotor diameter and rotational speed) is the major one. The rotational speed of an induction motor is essentially constant and is a function of the sup- ply frequency (Hz) and the number of poles in the winding (this being based on pairs so the most basic motor would have 2 poles, then, 4, 6, 8 and so on with 2 & 4 pole being the most common). At a supply frequency of 50Hz (UK standard mains supply) a 2-pole motor will have a nominal speed of 3000rpm whilst a 4- is pole motor's nominal speed will be 1500rpm. At 60Hz (the standard marine fre-quency) the equivalent speeds will be 20% higher (i.e. in the ratio 6:5). The power developed by an electric motor is a function of speed so that a 2-pole motor will deliver up to twice the power of a similar size 4-pole motor. However the power developed is also a function of the torque (turning moment) that can be created and that will increase with an increase in rotor diameter. So more power requires a higher speed and larger diameter rotor, the combination of which creates drag. For this reason most HPUs use a 4-pole motor with 2-pole being restricted to lower powers (typically less than 100kW).

By contrast, in the power unit of the present invention is a closed system with the motor and generator inside a sealed enclosure with only electricity go-ing in and coming out. Because drag is no longer of importance, any size 2-pole motor can be used. Similarly the generator can run at the same speed as the motor or be geared to run at a higher speed, thereby reducing the size of the generator.

An alternative arrangement for ROVs would be to mount the electric power unit (EPU) of the invention on the TMS (Tether Management system or "garage"). This is a device that allows the ROV to be detached from the main lift -4 -umbilical and detached from any movement from the surface vessel (e.g. due to wave action, etc).The TMS contains a winch with a lightweight tether (which can be kms or more in length but is usually only a few hundred metres) connected to the ROV conventionally supplying medium voltage AC to the HPU on board the ROV. Mounting the EPU on the TMS would allow the size and weight of the ROV to be reduced and, by transmitting DC instead of AC, the tether size could probably be reduced even though the current would be increased in line with the reduced voltage between TMS and ROV.

Another application of the EPU would be as a seabed power-station.

There is increased interest in powering subsea equipment from onshore (possi-bly several hundred km away) rather from a surface platform. Presently this requires transmitting medium or high voltage AC to large step-down transformers inside oil-filled enclosures. To reduce transmission cable sizes DC would be preferred but this will not work with the seabed transformers. However it could work with an EPU according to the invention, fitted with a medium/high voltage DC motor.

Brief Description of the Drawings

In the drawings, which illustrate embodiments of the invention: Figure 1 is a diagrammatic representation of a power unit; and Figure 2 is a diagrammatic representation of a subsea ROV and associ-ated Tether Management System with a power unit attached.

Detailed Description of the Illustrated Embodiment

Referring to Figure 1, a power unit comprises a sealed pressure vessel 1 containing air at atmospheric (i.e. normal sea surface atmospheric) pressure.

Mounted within the pressure vessel 1 is a motor 2 supplied with electrical power by a cable 3 entering the vessel 1 through a pressure-resistant penetrator or gland 4. The electricity supply may be AC at 4160V and 60Hz, and the motor 2 is then suitably an induction motor.

The motor 2 is coupled to drive a generator 5, for example by way of a common shaft 6, although it will be appreciated that the generator could be driven through a gear drive, particularly if the generator is to be driven at a different speed to the motor. The generator 5 outputs a lower voltage AC supply -5 -via an output cable 7 for powering drive motors in a ROV for example. For this application, a voltage of 600-700V DC would be suitable, although some smaller vehicles might use a lower voltage, for example 48V DC. The output cable 7 exits the pressure vessel through another pressure-resistant penetrator or gland 8.

Figure 2 shows one application for the power unit of Figure 1. A subsea Remotely Operated Vehicle (ROV) 10 is connected to a tether management system (TMS) 11 by way of a tether 12 connected to a winch inside the system 11, so that the ROV can be returned to the TMS for winching back to the sur- face, for example. The TMS 11 is supported from the surface by a main lift um-bilical 13 through which an electricity power supply cable 14 supplies power to the TMS and thence to the ROV 10. The power unit 15 is mounted on the TMS and is connected to the cable 14. The output of the power unit 15 is passed to the ROV 10 via the tether 12, this arrangement reducing the size and weight of the ROV for the given power requirement of the ROV.

It will be appreciated that the power unit could also be mounted on the ROV 10. In an alternative arrangement, the power unit could be part of a power supply pod on the sea bed supplying a plurality of different items of subsea equipment.

In another embodiment, the power unit is mounted on or in the TMS or located on the seabed and acts as a docking station/charging point for a battery powered ROV, which either has no tether or a very simple data cable not providing power. -6 -

Claims

CLAIMS1. A power unit for subsea use, comprising an electric motor driving a generator inside an external pressure vessel containing a gaseous atmosphere.
2. A power unit according to Claim 1, wherein the gaseous atmos-phere comprises air.
3. A power unit according to Claim 1 or 2, wherein the gaseous at-mosphere is at a pressure less than ambient pressure.
4. A power unit according to Claim 1, 2 or 3, wherein the electric mo-tor is an induction motor.
5. A power unit according to any of Claims 1 to 3, wherein the elec-tric motor is a DC motor, and further comprising a rectifier connected to the motor and arranged to rectify an alternating current (AC) supply to the unit.
6. A power unit according to any preceding claim, wherein the gen-erator is an alternator.
7. A power unit according to Claim 6, further comprising an output converter stage to provide a DC output.
8. A power unit according to Claim 7, wherein the converter stage is located within the pressure vessel.
9. A subsea remotely operated vehicle having electric drive motors supplied with electric power from a power unit according to any preceding claim.
10. A subsea remotely operated vehicle according to Claim 9, con-nected to a separate tether management system via a tether, wherein the power unit is mounted on or in the tether management system.
11. A subsea remotely operated vehicle according to Claim 9, wherein the power unit is located on or in the vehicle.