WO2015161892A1

WO2015161892A1 - Optical underwater navigation

Info

Publication number: WO2015161892A1
Application number: PCT/EP2014/058488
Authority: WO
Inventors: Paul Meldahl; Eiolf Vikhagen
Original assignee: Statoil Petroleum As
Priority date: 2014-04-25
Filing date: 2014-04-25
Publication date: 2015-10-29

Abstract

A method of navigating under water comprises: providing an underwater vehicle, carrying at least one interferometer, under water above a seabed; directing at least one laser beam from said underwater vehicle to said seabed so as to produce reflected light, being light reflected from the seabed; using said interferometer to combine said reflected light with a reference beam and thereby calculating the velocity of the underwater vehicle relative to the seabed; and using said calculated velocity to calculate the change in position in said underwater vehicle over time.

Description

Optical Underwater Navigation

FIELD OF THE INVENTION The invention relates to improvements in underwater navigation. BACKGROUND OF THE INVENTION

It is known to base underwater navigation on the following concepts and methods:

1 ) Taking pictures of the underwater terrain, but this method can be used only where a known terrain model exists;

2) Inertial instruments, such as accelerometers and gyroscopes, carried on underwater vehicles, but this method is limited to the accuracy of the instruments; and

3) Acoustic Doppler measurements, where acoustic signals may be reflected from the seabed or from objects or stations placed on the seabed, but this method is limited by the wavelength of the acoustic wave.

Known methods can be costly to implement, and also lack the accuracy required for certain applications and environments, such as AUV operations and survey operations carried out under ice.

SUMMARY OF THE INVENTION The invention provides a method of navigating under water and an underwater navigation arrangement as set out in the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a number of Autonomous Underwater Vehicles (AUVs) arranged to carry Temprei cameras;

Figure 2 shows the forward and rearward directed laser beams of each AUV; Figure 3 is a plan view from above one of the AUVs, showing the AUV 2 provided with two additional Temprei cameras which provide lateral laser beams; and

Figure 4 shows how the velocity is measured by the Temprei camera.

DESCRIPTION OF PREFERRED EMBODIMENTS

Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

The embodiments described use one or more interferometers, which may be in the form of a device which we refer to as a "Temprei camera", of the type described in the applicant's patents US 7,660,188; US 7,583,387; US 7,933,003; US 8,498,176; US 8,400,871 and US 201 1 -0102806. The content of each of these patent publications is incorporated by reference into the present description.

Some features of Temprei cameras will first be summarised below. Temprei cameras can be used to analyse the response to a seismic source by making "a movie of the particles on the sea bottom as they move in response to the returning P- and S- waves.

P-waves (compression waves) from a seismic source travel through rock and sea water, whereas S-waves (shear waves) travel only through rock and not through water (p 1 , lines 17-24). Before the Temprei camera, detection of S-waves therefore required geophones on the sea bed, but the moving of geophones affected reliability (p 2, line 23) and was costly (p 2, line 26). It is possible to use a seismic source and a number of mobile camera units which monitor the response at the sea bottom by applying light (not necessarily visible light) and recording the reflected light at a sampling rate of generally less than 1 ms. In effect this constitutes filming the bottom 13, and making a movie. Fixed and/or moving sources may be used. The camera units can, for example, be mounted on cables which are towed behind a vessel. The position of the cameras relative to the seabed can be determined by acoustic techniques, and the cables are steered by "wings" on the cables, to ensure that the camera remains close to the ocean bottom.

The cameras allow seismic imaging of subsurface layers using interferometry to examine speckle light reflected from the seabed.

Each Temprei camera comprises an interferometer. Interferometric techniques use a detector or detector array to detect laser light reflected from an object, together with a reference beam. Interference between the beams can be used to determine position and movement of the object. Laser light reflected from rough objects has a speckle nature, and a speckle pattern on the detector must be used for the measurement. Whilst moving through the water above the sea floor, the Temprei camera is able to measure relative movement of the sea floor along the direction of a laser beam which is emitted from the camera.

To remove unwanted signals due to movement of the instrument, phase modulation of the reference beam can be carried out, for example using an electro-optic modulator, to vary the phase. The phase velocity is controlled to fit the velocity of the instrument, so that most of the unwanted signals due to movement of the instrument are eliminated. Essentially, the phase velocity is adjusted (ie tuned) until the least noise / sharpest signal is obtained.

Figure 1 shows a number of Autonomous Underwater Vehicles (AUVs) 2 arranged to move close to the seabed 4 under an ocean 6. A surface vessel 8 may be provided with an active global navigation link and a partially active navigation link to the AUVs 2. The surface vessel 8 may navigate using navigation information from one or more of the Temprei cameras transmitted by an acoustic link between the surface vessel 8 and the or each AUV 2.

Each AUV 2 carries a number of interferometers, which project laser beams 10 onto the ocean floor 4. Figure 2 shows in more detail that each AUV 2 has a forward directed laser beam 12 and rearward directed laser beam 14.

The laser beam 12 is directed toward the sea floor 4 with an angle toward the moving direction 16 of the interferometer (not shown) carried by the AUV 2. The reference beam in the interferometer is modulated to compensate for longitudinal camera movement in the direction 16. The compensation signal is used to measure the sensor velocity component along the beam. The relation between the compensation and the sensor velocity component is linear and can be calibrated by independent velocity measurements. In this way optical velocity Doppler interferometry can be used to determine the velocity of the AUV 2 relative to the seabed 4.

Tilt and roll of the sensor (Temprei camera) may add an unwanted component to the velocity measurement. These unwanted components can be eliminated by summing output from symmetric pairs of sensors. In Figure 2 the forward and rearward directed laser beams 12 and 14 are symmetrically arranged an angle b on each side of vertical line 18 when the AUV 2 is horizontal. However, when the pitch of the AUV 2 changes by angle a, as shown in Figure 2, each of the laser beams 12 and 14 also moves through an angle a, as shown in Figure 2, so that the laser beams take up new positions 20 and 22 respectively.

In one embodiment the forward and rearward directed laser beams 12 and 14 are produced and analysed by two separate Temprei cameras and the signals from the two cameras are summed. The summed signal is independent of the pitch of the AUV 2, and the summed signal can therefore be used to determine the velocity of the AUV 2 without the need to know the actual pitch of the AUV 2. In an alternative embodiment the two laser beams 12 and 14 are produced and analysed by a single Temprei camera which is able to multiplex between the two laser beams 12 and 14. The velocity measurements can be integrated over time to determine the position of the AUV 2, thus allowing the Temprei camera to be used for navigation purposes.

Figure 3 is a plan view from above one of the AUVs 2, and shows the AUV 2 provided with two additional Temprei cameras which direct lateral laser beams 24 and 26 downwards to the ocean floor 4, to one side, in this case the left side, of the AUV 2. As the AUV 2 moves forward in the direction 16 it may rotate (clockwise or anticlockwise as viewed from above in Figure 3) generally about a rotation point 28. The point of rotation may vary, generally within a region, which can be regarded as a rotation region 29.

The two lateral laser beams 24 and 26 may be positioned so that one beam is in front of the rotation point 28 or region 29 and one beam is behind the rotation point 28 or region 29. However, embodiments are also possible in which this is not the case, and both lateral laser beams are on the same side of the rotation point 28 or region 29. Preferably there are at least two such lateral laser beams 24 and 26, which are preferably positioned with different offsets from the rotation point 28 or region 29. However the two lateral laser beams 24 and 26 do not have to be positioned symmetrically about the rotation point 28 or region 29. When the AUV 2 rotates through an angle, as shown in Figure 3, the lateral laser beams measure the rotation and translation of the AUV 2. Each of the lateral laser beams 24 and 26 can measure velocity along the direction of the laser beam, and the difference between velocity measured by the two lateral laser beams 24 and 26 can therefore be used to calculate rotation of the AUV 2. The AUV 2 may or may not also be provided with inertial instruments which allow measurement of acceleration and rotation. However, the lateral laser beams 24 and 26 allow rotation of the AUV 2 to be measured more accurately than measurements taken from inertial instruments.

Figure 4 shows further details relating to how the velocity is measured. Figure 4 shows a schematic representation of a Temprei camera 30, which is carried by one of the

AUVs 2 (not shown in Figure 4) moving in the forward direction 16 above the seabed 4.

A laser beam 32 is emitted from the camera 30 and reflected back to the camera 30 from the seabed 4. The laser beam 32 passes through a glass window 34 of the camera 30, and is refracted as a result of different refractive indices n1 and n2 on either side of the window 34. For example, if the camera 30 contains air, n1 is the refractive index of air inside the camera 30, and n2 is the refractive index of sea water outside the camera 30.

If the laser beam 32 leaves the camera 30 at an angle 02, as shown in Figure 4, the (optical) velocity v_m measured by the laser beam is given by: v_m = n2 . v_c . sin(02)

Although we have described Temprei cameras carried by AUVs 2, the Temprei cameras can be carried by any suitable underwater device, including for example Remotely Operated Vehicles (ROVs) or a towed underwater device.

In the method of underwater navigation proposed here any number of Temprei cameras can be used, depending on the application and requirements. If there is no pitch, roll or rotation of the underwater vehicle to worry about, then even a single Temprei camera can be mounted on the underwater vehicle in order to measure distance travelled by the underwater vehicle.

Two laser beams can be used to measure distance and compensate for pitch changes, as described above. In this case the underwater vehicle may carry two Temprei cameras, or a single camera can be used with multiplexing as described above.

Three laser beams can be used to provide more information. Four laser beams, with different orientations, can be used to provide full measurements of distance travelled, whilst compensating for changes in pitch and rotation.

Two and two symmetric camera beams can be summed to compensate for the effects of tilt and roll of the camera when camera velocity is measured. To find the total distance travelled, information of the full 3-dimensional orientation and movement of the camera is preferred. To detect camera translations in all directions as well as rotations of the camera around the x, y and z axes, several separate laser beams can be used, measuring in different directions. It is also preferred to use measurement axes (laser beam axes) which do not meet (ie. cross) in space, to improve the detection of rotations.

The method can be used with a number of underwater vehicles, such as a swarm of AUVs. In this case a Temprei camera or cameras can be fitted to each underwater vehicle, or to only one or some of the underwater vehicles, with the other underwater vehicles acting as "followers" which are directed based on the navigation information provided by the underwater vehicle(s) provided with a Temprei camera or cameras.

The use of the Temprei camera as described here increases the accuracy of navigation by many orders of magnitude compared to acoustic Doppler navigation. Preferably the underwater vehicle moves close to the ocean bottom, for example between 2 and 10 metres from the seabed, and the measured velocity is integrated over time. Underwater vehicles provided with the described optical navigation system can navigate over longer distances without contact with units on the ocean surface and without reference points at the ocean bottom.

The Temprei interferometer can be used to measure distances over which the camera is traveling. The applicant has calculated the accuracy in longitudinal distance measurements to be less than 1 mm deviation after more than 122 km of distance travelled. It has also been found that the interferometer is insensitive to variations over time of the refractive index in water.

The embodiments described provide increased efficiency and accuracy of underwater navigation, support robotization (for example swarms of underwater vehicles), and allow navigation under ice.

Although we have described the use of one or more Temprei cameras, the embodiments described may use one of more interferometers of any type.

Claims

CLAIMS:

1. A method of navigating under water, the method comprising:

providing an underwater vehicle, carrying at least one interferometer, under water above a seabed;

directing at least one laser beam from said underwater vehicle to said seabed so as to produce reflected light, being light reflected from the seabed;

using said interferometer to combine said reflected light with a reference beam and thereby calculating the velocity of the underwater vehicle relative to the seabed; and

using said calculated velocity to calculate the change in position in said underwater vehicle over time.

2. A method as claimed in claim 1 , which comprises:

causing said underwater vehicle to travel in a forward direction;

directing at least two laser beams from said underwater vehicle to said seabed, wherein at least one laser beam is a forward directed laser beam and at least one laser beam is a rearward directed laser beam;

using said at least one interferometer to detect reflected light from both said forward and rearward directed beams so as to compensate for pitch changes in said underwater vehicle relative to the seabed.

3. A method as claimed in claim 2, which comprises summing signals produced by said forward and rearward directed laser beams.

4. A method as claimed in any preceding claim, which further comprises:

directing at least one lateral laser beam from said underwater vehicle to said seabed, the or each lateral laser beam being directed to the left or right side of said underwater vehicle when viewed from above; and

using said at least one interferometer to detect reflected light from said at least one lateral laser beam in order to detect rotation of said underwater vehicle.

5 A method as claimed in claim 5, wherein said at least one lateral laser beam comprises at least a front lateral laser beam and a rear lateral laser beam, said front lateral laser beam is located in front of said rear lateral laser beam when said underwater vehicle is caused to move in a forward direction.

6. A method as claimed in claim 5, wherein said underwater vehicle has a rotation point, or rotation region, about which said underwater vehicle tends to rotate when caused to move in a forward direction, and wherein said front lateral laser beam originates in front of said rotation point, or rotation region, and said rear lateral laser beam originates behind said rotation point, or rotation region.

7. An underwater navigation arrangement for performing a method as claimed in any preceding claim, said underwater navigation arrangement comprising:

at least one underwater vehicle, carrying at least one interferometer, arranged to direct at least one laser beam from said underwater vehicle to the seabed.

8. An underwater navigation arrangement as claimed in claim 7, wherein said at least one underwater vehicle carries at least two interferometers.

9. An underwater navigation arrangement as claimed in claim 8, wherein said at least one underwater vehicle carries at least four interferometers.

10. An underwater navigation arrangement as claimed in any one of claims 7 to 9, wherein said at least one underwater vehicle is an Autonomous Underwater Vehicle, AUV, a Remotely Operated Vehicle, ROV, or a towed underwater vehicle.

1 1. An underwater navigation arrangement as claimed in any one of claims 7 to 10, which further comprises a surface vessel provided with a communication link to said at least one underwater vehicle.