Patent Description:
Structural failure in components of man-made assets that are located underwater in or on a submerged earth layer may have serious environmental and/or financial consequences. An example of a valuable asset is a subsea oil extraction system, which is arranged on a seafloor and typically includes wellheads, trees, production manifolds, interconnecting jumpers, and production risers. It is challenging but critical to monitor the structural integrity of such a system at all stages of its life cycle.

Displacement or deformation of asset components may have various causes. The geometry of the sea- or ocean floor may for instance change due to plate tectonic effects, volcanic activity, mining, and gas or oil extraction. In addition, asset components may subside into soft sediments due to gravitational pull, or other forces acting on the component (e.g. forces exerted by an anchor chain on a suction pile anchor). Also, temperature changes may give rise to stresses and positional changes between interconnected asset components. It may be desirable to monitor positional changes of the asset components and/or the surrounding submerged earth layer in time, to allow timely detection of excessive displacement of particular regions, so that necessary precautions may be taken to prevent potential damage to the asset components.

To be able to timely detect unwanted deformations within such submerged system, it is critical that deformations of and relative motions between structure components are observed in time. Measurement accuracies in the order of centimeters or less may be required to provide a timely indication of undesired positional changes.

Various underwater monitoring devices are known with sensors for observing spatial properties of underwater structures and their surroundings. Subsea monitoring devices should preferably be self-powered, energy efficient, and able to function autonomously for a prolonged time, to reduce the required deployment and collecting operations to a minimum. In deep sea environments with typical ocean floor depths in the order of several kilometers, the sensors must additionally be able to withstand pressures of several hundred bars.

International patent publication <CIT> describes underwater positioning systems configured to provide position information for a remotely operable vehicle (ROV). One system includes underwater beacons, each with an imaging device that observe light sources on a moving ROV and determines direction data representing a direction or change in direction of the ROV light sources with respect to the imaging device. Document <CIT> discloses an underwater observation unit which is able to obtain the distances and the position of the elements captured in an image.

It would be desirable to provide a versatile underwater observation unit, which can be deployed together with similar units to form a versatile system that enables various underwater observation and monitoring tasks with improved accuracy. Alternatively or in addition, it may be desirable to provide an underwater observation unit that can be deployed underwater for a prolonged time.

Therefore, according to a first aspect, there is provided an observation unit for underwater deployment on or in a submerged earth layer or a submerged structure. The observation unit comprises a housing, a light source, an underwater imaging device, a processor device, and a communication device. The housing is adapted for supporting the underwater observation unit relative to the submerged surface or structure. The light source is fixed to the housing, and is configured to emit light into the surroundings of the observation unit. The underwater imaging device is attached to the housing, and is configured to acquire image data of a second light source located within a wide field of view (FOV) of the imaging device, which covers the surroundings of the observation unit. The processor device is configured to receive the image data from the imaging device, and to determine positional data of the second light source relative to the imaging device. The communication device is configured to transmit the positional data to another underwater observation unit, and/or to an underwater vehicle or processing station.

The proposed underwater observation unit includes an imaging device, which is configured to image the surroundings of the observation unit over a wide range, e.g. by using a fisheye lens, and a light source for emitting light that can be observed by other nearby observation units. By using a plurality of such observation units, the light sources and imaging devices can cooperate to provide images and/or positional data of each other and objects in the vicinity. The proposed unit can be easily deployed in submerged (e.g. subsea) environments for a long term (e.g. years), and configured to acquire measurements at desired periods and at desired update rates.

According to the invention, the communication device comprises an optical signal transmitter, and the light source is part of the optical signal transmitter. This transmitter is configured to emit a data signal via the emitted light into the surroundings of the observation unit.

The small carrier wavelengths for optical data signals allows construction of small units and communication components with high resolution (e.g. a factor <NUM>,<NUM> compared to acoustics), low latency, and fast update rates. The light source can be efficiently used for data communication between this observation unit and another unit or nearby vehicle, as well as for imaging and position detection by imaging devices of observation units it the vicinity.

According to embodiments, the observation unit comprises an orientation sensor, which is configured to determine attitude data including at least a pitch and a roll of the underwater imaging device relative to the submerged surface or structure when the observation unit is in a deployed state.

The orientation sensor may for instance be an inclinometer, which is configured to acquire instantaneous attitude measurements (e.g. at least pitch and roll) for the observation unit. Angular measurement accuracy of <NUM>° may be achievable. The orientation sensor may comprise or be coupled to a memory unit configured to record the attitude data with time stamps.

According to embodiments, the underwater imaging device is formed by an omnidirectional photogrammetric camera with an ultra-wide FOV, preferably with an azimuthal coverage of <NUM>° and an elevational coverage of at least -<NUM>° to +<NUM>°, for instance of -<NUM>° to <NUM>°, or even of -<NUM>° to <NUM>°.

The term "omnidirectional" is used herein to refer to a camera that is configured to receive image data from any or all directions in an azimuthal plane that is orthogonal to a nominal axis onto which the camera is centered, i.e. has an azimuthal coverage of <NUM>°. The FOV of the camera may be limited along the elevational direction, and/or its reception sensitivity may change with increasing elevation angle above or below the azimuthal plane. For a displacement monitoring mode, an elevational coverage ranging at least from -<NUM>° to +<NUM>° may already suffice. A single image detector may be efficiently used in combination with a fisheye lens with an ultra-wide FOV of <NUM>° × <NUM>°, in order to acquire single photogrammetric images of a large portion of the surroundings of the observation unit.

According to a further embodiment, the housing is centered on a nominal axis, and the underwater imaging device is located on one distal portion of the housing, with its FOV facing away from the submerged surface or structure when the observation unit is in a deployed state. In particular, the underwater imaging device may be attached on a distal end of the housing that is upwards when the unit is in use, to provide optimal visual coverage of the surroundings.

In embodiments, the light source comprises a LED that is adapted for emitting light with wavelengths substantially in a range of <NUM> nanometers to <NUM> nanometers.

The use of active light sources (e.g. LEDs), also for inter-unit position monitoring, increases the maximum detection range significantly, as opposed to reflected light (e.g. by a factor of five or more in clear ocean water). Propagation of light underwater is nevertheless severely limited compared to propagation in air or free space. Preferably, the emission properties of the LED light sources are tailored to the optical transmission properties of the water in which the observation unit is deployed. In sea water, only green to blue light will propagate a substantial distance (tens to hundreds of meters) without being attenuated beyond practical use. For subsea operation, green or blue LEDs (e.g. light wavelengths ranging from <NUM> nanometers to <NUM> nanometers and an intensity maximum between <NUM> and <NUM> nanometers) may be most suitable. For coastal water with a considerable pollution level, longer LED emission wavelengths (e.g. tending towards the yellow part of the optical spectrum) may be preferred.

In embodiments, the underwater imaging device comprises a digital imaging sensor and refractor optics. The imaging sensor includes a two-dimensional (2D) array of pixels. The refractor optics may for instance be formed by a fish-eye lens, and has a focal length that slightly exceeds a distance to the imaging sensor. This focal length ensured that point-like light from a second light source that is located within the wide FOV of the underwater imaging device (e.g. a signal transmitter from another unit at a location remote from the observation unit), is imaged out of focus to generate a blob that extends over at least one or preferably multiple adjacent pixels of the array.

A remote point-like light source (i.e. LED) will illuminate less than one pixel of an image sensor array in a properly focused camera system, which renders determination of subpixel coordinates impossible. For the purpose of determining center coordinates of the remote light source, defocusing will cause the impinging light to cover several (e.g. at least two) pixels of the sensor array to enable subpixel detection, and hence a more accurate estimate of the direction of the light source.

In further embodiments wherein the underwater imaging device is configured to acquire more sophisticated image data (e.g. acquiring photographs, or laser striping data of the unit's surroundings), the refractor optics may be adapted to selectively adjust its focal length, in order to allow migration of the imaging focal point between a location slightly behind the digital imaging sensor, such that point-like light from the second light source is imaged out of focus (e.g. operation in positioning mode), and a location coinciding with the imaging sensor, such that the surrounding scene is imaged in focus (e.g. operation in photographic mode).

In embodiments, the observation unit comprising a memory unit for storing the positional data with timestamps, to form a dataset of time-dependent positional data.

According to embodiments, the underwater imaging device is configured to acquire further image data of a third light source located within the FOV of the underwater imaging device. The second light source and the third light source may be activated in a directly successive but temporally non-overlapping manner. The processor device may then be configured to receive from the imaging device the image data associated with the second light source, and the further image data associated with the third light source, and to subtract the image data from the further image data or the further image data from the image data, to generate distinguishable image intensity extrema for the second and third light sources while reducing or eliminating background information.

The image subtraction method increases the robustness of the positioning measurements, by removing (irrelevant) background image data and simultaneously transforming the light source signals into local regions with extreme values of opposite sign (relative to an average background value). This simplifies spatial detection and discrimination of the light source signals in the acquired images.

According to a second aspect, and in accordance with advantages and effects described herein above with reference to the first aspect, there is provided an underwater observation system including at least two observation units in accordance with the first aspect. The first and second observation units are adapted for underwater deployment at distinct first and second locations on or in a submerged earth layer or structure. A communication device of the first and/or second observation unit is configured to receive positional data of both the first and second observation units, and a processor device of the first and/or second observation unit is configured to merge received positional data with timestamps, to form a dataset of time-dependent positional profiles for both observation units.

A plurality of such units can be efficiently deployed in a meshed network arrangement, to provide quasi-static structure displacement monitoring functionality with measurement redundancy and considerable reliability (no single point of failure).

According to an embodiment, a memory unit of the first and/or second observation unit is configured to store the dataset of time-dependent positional profiles, and the communication device of the first and/or second observation unit is configured to transmit the dataset of time-dependent positional profiles to an underwater vehicle or an underwater processing station upon request.

A system with two (or more) observation units can be efficiently deployed in a network arrangement (e.g. a meshed network), which allows an underwater vehicle (e.g. ROV, UAV) to approach a single observation unit of the network and download network measurements via this single unit.

In embodiments, an underwater imaging device of at least one of the first and/or second observation unit is configured to acquire further image data of vehicle light sources provided on an underwater vehicle located within the wide FOV of the underwater imaging device. The processor device of the first and/or second observation unit may then be configured to receive the further image data from the imaging device, and to determine further positional data of the underwater vehicle relative to the imaging device. The communication device of the first and/or second observation unit may then then configured to transmit the further positional data to the underwater vehicle upon request.

The underwater observation system may be kept deployed in dormant mode in or on the submerged surface or structure for a long time, but may be woken up by an underwater vehicle entering the site, and ordered to start tracking and broadcasting the 6DOF position of the vehicle. This removes the need to install and calibrate a separate positioning system on site.

In embodiments, an underwater imaging device of at least one of the first and/or second observation unit is configured to acquire panoramic image data of surrounding objects in the wide FOV of the underwater imaging device. The processor device of the first and/or second observation unit may then be configured to receive the panoramic image data from the imaging device. Further, the communication device of the first and/or second observation unit may then be configured to transmit at least part of the panoramic image data to the underwater vehicle upon request.

The underwater observation system uses multiple static cameras with omnidirectional views, which can be made to record images of the environment at command. The <NUM> DOF positions of the cameras may be accurately determined in advance by calibration techniques. The cameras may then be used to generate 3D image maps of the environment. The underwater vehicle (e.g. ROV or AUV) could project light onto the otherwise dark scene, and the observation system may be commanded to acquire panoramic images. This projected light may be diffuse light or laser stripes, and allows creation of 3D point cloud data of the environment without the use of additional equipment (e.g. a laser scanner).

In case full images are acquired (diffuse light source), the communication device may transmit the entire panoramic image. Alternatively or in addition, the processor device of the first and/or second observation unit may be configured to perform image processing tasks. The processing device may for instance reduce image content to those portions of the scene that contain light (and discard the dark content) to limit the data quantity to be transmitted. The processor device may also be configured to extract landmark features from the panoramic image data, and the communication device may then be configured to transmit the extracted features with positional reference data to the underwater vehicle upon request. Landmark features from multiple units may be combined and used to generate 3D point cloud data from the surrounding objects. Such post-processing may for instance be executed by a processor on the requesting underwater vehicle, or on a remote processing station (e.g. on a surface vessel that is coupled to the ROV).

In embodiments, the first and second observation units are configured to operate, upon request by a nearby underwater vehicle, in at least one selected from three operational modes. In a first operational mode, the communication device of the first and/or second observation unit receives positional data of both the first and second observation units, and the processor device of the first and/or second observation unit merges received positional data with timestamps, to form a dataset of time-dependent positional profiles for both observation units. In a second operational mode, the underwater imaging device acquires further image data of vehicle light sources on the underwater vehicle, the processor device receives the further image data from the imaging device, and determines further positional data of the underwater vehicle relative to the imaging device, and the communication device of the first and/or second observation unit transmits the further positional data to the underwater vehicle. In a third operational mode, the underwater imaging device acquires panoramic image data of surrounding objects in the wide FOV of the underwater imaging device, and the communication device transmits the panoramic image data to the underwater vehicle.

The system may be configured to operate in a single mode, and switch to another single mode upon request. The system may also be configured to operate in multiple modes at the same time, and de-activate one of the current modes and/or activate one or more other modes upon request. By providing an underwater observation system with different operational modes, the system may be efficiently used for different purposes and perform alternative measurements upon request, while saving energy when particular modes of operation are not desired.

According to a third aspect, there is provided a method for monitoring a spatial profile of a submerged surface or structure in time, using the underwater observation system according to the second aspect. The method comprises:.

The underwater observation system, when installed on subsea assets, can monitor relative motions between the deployed units and therefore of parts of assets on which the units are deployed. This allows accurate (e.g. sub-centimeter) determination of positional changes of submerged objects and/or surfaces in time. Depending on the acquired image data, up to six degrees of freedom (DOF) motion of the submerged objects or surfaces may be monitored. The method may for instance be used to estimate mechanical stresses between two locations of a submerged object, or of structures (e.g. conduits) interconnecting two submerged objects.

Initial deployment comprises positioning of the first and second observation units at a mutual distance on or along the submerged surface or structure. In a displacement monitoring mode, inter-unit distances of up to <NUM> meters or more may be achievable. In a dynamic observation mode wherein ROV motion is tracked, inter-unit distances of up to <NUM> meters may be achievable.

According to a fourth aspect, which is not claimed, there is provided a method for using an observation unit including an underwater imaging device with a digital imaging sensor according to claim <NUM>. The method comprises:.

Such a method is believed to be inventive in and of its own right in the context of imaging of and positional determination for a localized light source in the vicinity of an imaging device, and may be subject of a divisional application.

According to a fifth aspect, which is not claimed, there is provided a method for using an observation unit including an underwater imaging device with a digital imaging sensor according to claim <NUM>. The method comprises:.

Such a method is believed to be inventive in and of its own right in the context of imaging and discriminating multiple localized light sources in the vicinity of an imaging device, and may be subject of a divisional application.

According to a further aspect, there is provided a computer program product configured to provide instructions to carry out a method according to at least one the abovementioned aspects, when loaded on a computer arrangement.

In yet a further aspect, which is not claimed, there is provided a computer readable medium, comprising such a computer program product.

Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts. In the drawings, like numerals designate like elements. Multiple instances of an element may each include separate letters appended to the reference number. For example, two instances of a particular element "<NUM>" may be labeled as "20a" and "20b". The reference number may be used without an appended letter (e.g. "<NUM>") to generally refer to an unspecified instance or to all instances of that element, while the reference number will include an appended letter (e.g. "20a") to refer to a specific instance of the element.

The figures are meant for illustrative purposes only, and do not serve as restriction of the scope or the protection as laid down by the claims.

The following is a description of certain embodiments of the invention, given by way of example only and with reference to the figures.

In the next figures, various coordinate systems will be used to describe spatial characteristics and relations for exemplary embodiments of the observation unit and system. The "unit axis" A refers herein to a nominal axis through an observation unit, and on which an imaging device of the unit is centered. The "axial direction" Z is used herein to refer to the direction along this unit axis A. The term "radial direction" R refers herein to the directions that point radially away from the unit axis A (i.e. perpendicular to the axial direction Z), and which lie in a transversal plane for which a surface normal vector points along the axial direction Z. The "angular direction" (or "azimuthal direction") Φ corresponds to a unit-vector that initiates at a local radial position, and which points anti-clock-wise along an (infinitesimal) angle of rotation about the unit axis A, and perpendicular to both the (local) radial and axial directions R, Z. Any radial-angular plane transverse to the axial direction Z is referred to herein as an "azimuthal plane" Pϕ.

The term "surface" is used herein to generally refer to a two-dimensional parametric surface region, which may have either an entirely or piece-wise flat shape (e.g. a plane or polygonal surface), a curved shape (e.g. cylindrical, spherical, parabolic surface, etc.), a recessed shape (e.g. stepped or undulated surface), or a more complex shape. The term "plane" is used herein to refer to a flat surface defined by three non-coinciding points.

It should be understood that the directional definitions and preferred orientations presented herein merely serve to elucidate geometrical relations for specific embodiments. The concepts of the invention discussed herein are not limited to these directional definitions and preferred orientations. Similarly, directional terms in the specification and claims, such as "top," "bottom," "left," "right," "up," "down," "upper," "lower," "proximal," "distal" and the like, are used herein solely to indicate relative directions and are not otherwise intended to limit the scope of the invention or claims.

<FIG> schematically shows a perspective view of an exemplary observation system <NUM> deployed underwater. The system <NUM> includes a plurality of observation units 30a, 30b, 30c, 30d, which are all immersed in a body of water <NUM>, and are positioned at respective positions Qa, Qb, Qc on submerged structures <NUM>, <NUM>. The submerged structures <NUM>, <NUM> are arranged across a submerged surface <NUM>, which forms a water-soil interface between the above-situated body of water <NUM> and the earth layer <NUM> below. In this example, the submerged surface <NUM> forms the surface of a portion of a seabed <NUM>, and the submerged structures <NUM>, <NUM> form part of a subsea oil extraction system, which includes several wellheads <NUM> and at least one production manifold <NUM> that is connected to the wellheads <NUM> via jumpers <NUM>. The surface <NUM> of the seabed <NUM> is typically not perfectly planar, but has local height variations with respect to a vertical direction Z (corresponding with gravity).

The observation units <NUM> include watertight enclosures, and can be deployed in submerged environments for a long term (e.g. years). Once deployed, the units <NUM> are configured to establish communication channels between pairs of units <NUM> to form a meshed network. The units <NUM> are also configured to acquire image data of each other and of the surroundings at desired periods and update rates, and to exchange acquired data via the communication channels.

Although four observation units <NUM> are illustrated in <FIG>, it should be understood that a greater or smaller number of units can be employed. An increased number of deployed units in the meshed network arrangement allows a larger and/or denser spatial coverage, and may provide increased measurement redundancy which may be exploited to improve measurement accuracy and reliability of the system <NUM>.

<FIG> presents a perspective view of an exemplary observation unit <NUM>, which is part of the system <NUM> shown in <FIG>, and which is deployed underwater on the submerged surface <NUM> (i.e. seafloor) of the seabed <NUM>.

The observation unit <NUM> comprises a housing <NUM>, which accommodates various sensors <NUM>, <NUM>, <NUM> and other electronic components <NUM>, <NUM>, <NUM>, <NUM> in a watertight and pressure resistant manner. The housing <NUM> is at a lower distal portion <NUM> coupled to a base <NUM>. The base <NUM> defines a support structure for the housing <NUM>, and accommodates a power supply <NUM>, which is electrically coupled to the sensors <NUM>, <NUM>, <NUM> and the other electronic components <NUM>, <NUM>, <NUM> to provide required electrical power. The base <NUM> further includes a support arrangement, which in this example is a tripod leg structure on a lower side, and which is adapted to support the base <NUM> and underwater observation unit <NUM> relative to the seabed <NUM> or structure <NUM>, <NUM>. In this example, the power supply <NUM> is formed by a replaceable seawater battery, which is known per se. The base <NUM> is selectively detachable from the housing <NUM>, to allow the battery <NUM> to be replaced.

The housing <NUM> of the unit <NUM> includes an optically transparent medial portion <NUM>, <NUM> with an optical communication device <NUM> inside, a component casing <NUM>, and a transparent dome <NUM> with an optical imaging device <NUM> on an upper side of the housing <NUM>. The medial portion <NUM>, <NUM>, the component casing <NUM>, and the dome <NUM> jointly form an elongated body that extends along a central unit axis A. In this example, the unit <NUM> is essentially rotationally symmetric about the unit axis A. The medial portion <NUM>, <NUM>, the component casing <NUM>, and the transparent dome <NUM> are essentially continuously rotationally symmetric about unit axis A, whereas other unit components have discrete rotational symmetries about axis A (e.g. the base <NUM> has three-fold symmetry, and the communication device <NUM> has two-fold symmetry).

In a deployed state of the unit <NUM>, the unit axis A is preferably directed with a substantial component normal to the (macroscopic) orientation of the supporting submerged surface <NUM> or structure <NUM>, <NUM>, to allow the optical communication device <NUM> and the imaging device <NUM> a largest possible unobstructed FOV. Furthermore, the deployed observation units <NUM> project with at least the medial portion <NUM>, <NUM>, the component casing <NUM>, and the transparent dome <NUM> above the surface <NUM> of the seabed <NUM>. This allows the imaging device <NUM> of one unit <NUM> to observe the unit's surroundings and to provide the optical communication device <NUM> a line of sight to communication devices <NUM> of one or more other units <NUM> in the vicinity. The achievable visual and/or optical communication range between units <NUM> deployed underwater may be in the order of several hundreds of meters. In this example, the units <NUM> are relatively small; A height ΔZu of the housing <NUM> (from <NUM> to the top of dome <NUM>) along the axis A is several tens of centimeters e.g. about <NUM> centimeters, and diameter Øu of the housing <NUM> transverse to the axis A is about <NUM> centimeters.

The component casing <NUM> forms a pressure resistant shell, which consists essentially of Titanium and defines an internal chamber for accommodating an orientation sensor <NUM>, a processor <NUM>, and a memory unit <NUM>. Titanium is a strong, light, and corrosion-resistant metal. In addition, the thermal expansion coefficient of Titanium can advantageously be selected to approach or even match the thermal expansion coefficient of particular types of glass that may be used for forming the dome <NUM> and/or the medial portions <NUM>, <NUM>, to reduce differential thermal stress between these parts (and potential negative optical effects) under varying temperature conditions.

The medial portion <NUM>, <NUM> is formed by a first medial portion <NUM> and a second medial portion <NUM>, which are stacked along and centered on the unit axis A, and which accommodate distinct functional parts of the optical communication device <NUM>. The communication device <NUM> includes an optical signal transmitter <NUM>, and an optical signal receiver <NUM> of the anidolic (non-imaging) type.

The optical signal transmitter <NUM> includes a plurality of light sources (<NUM>, see <FIG>), and is configured to transmit an optical data signal via light that is emitted by the light sources <NUM>, through the second medial portion <NUM>, and into the body of water <NUM> surrounding the unit <NUM>.

The imaging device <NUM> is formed by a photogrammetric camera <NUM> with an ultra-wide field of view (UW-FOV), which is configured to acquire image data of objects located in the vicinity of the unit <NUM>. The camera <NUM> is configured to detect and acquire image data of other light sources in the vicinity of the unit <NUM>.

In this example, the camera FOV faces away from the housing <NUM> and upwards along the unit axis A, to ensure that portions of the housing <NUM> are not within the camera FOV when the observation unit <NUM> is deployed. In particular, the camera FOV faces away from the optical signal transmitter <NUM> of the optical communication device <NUM>.

The orientation sensor <NUM> is configured to acquire attitude data for the unit <NUM>, by determining at least a pitch and a roll of the underwater imaging device <NUM> relative to the surface <NUM> or structure <NUM>, <NUM> on/in which the unit <NUM> is deployed.

The processor <NUM> and memory unit <NUM> are communicatively coupled with the orientation sensor <NUM>, to receive and store the attitude data acquired by the orientation sensor <NUM>. The processor and memory units <NUM>, <NUM> are also coupled with the camera <NUM>, to receive and store image data acquired by the camera <NUM>. Furthermore, the processor and memory units <NUM>, <NUM> are coupled to the communication device <NUM>.

A cable router tube <NUM> is provided through the medial part <NUM>, <NUM> of the housing <NUM> and along the unit axis A. The processor and memory units <NUM>, <NUM> are electrically connected with the communication device <NUM>, via signal conduits. In addition, the communication device <NUM> and other sensors <NUM>, <NUM> and electronic components <NUM>, <NUM> are electrically coupled to and powered by the power supply <NUM> via power conduits. In addition, a data port (not shown) may be provided in the base <NUM> or at an underside of the housing <NUM>, which is electrically connected to the processor and memory units <NUM>, <NUM> via further signal conduits. This data port may be configured for downloading measurement data and/or uploading configuration data (e.g. for upgrading firmware) once the unit is recovered from the seafloor <NUM>. The tube <NUM> accommodates the various conduits and prevents optical masking of the signal transmitter and receiver <NUM>, <NUM>.

The processor <NUM> is configured to receive the image data from the camera <NUM>, and to determine positional data of the second light source relative to the camera <NUM>. The memory unit <NUM> is configured for storing the positional data with timestamps, to form a dataset of time-dependent positional data. The communication device <NUM> is configured to transmit the positional data to other underwater observation units 30b, 30c, 30d, a nearby underwater vehicle <NUM>, and/or an underwater processing station.

<FIG> shows a schematic side view of the upper portion of the observation unit <NUM> from <FIG>, including the transparent dome <NUM> with camera <NUM>. The camera <NUM> is accommodated inside the dome <NUM>, and includes a digital imaging sensor <NUM> and a fish-eye lens <NUM> for receiving and refracting light from the surroundings and projecting the light onto the sensor <NUM>. The imaging sensor <NUM> includes a two-dimensional (2D) array of pixels.

The transparent dome <NUM> is formed as a hyper-hemispherical shell of optically transparent material, which is sufficiently rigid to resist considerable external pressures associated with underwater deployment without significant deformation. Exemplary materials for the dome <NUM> are borosilicate glasses or non-crystalline silica glasses, preferably with low coefficients of thermal expansion (e.g. in the order of <NUM>. <NUM> K-<NUM> or lower).

The camera <NUM> is positioned with its fish-eye lens <NUM> substantially coinciding with a nominal center of curvature Cd of the dome <NUM>. The spherical portion of the dome <NUM> extends over an azimuthal range of <NUM>° and an elevational range that at least equals the elevational coverage ΔΘv of the camera's FOV.

The fish-eye lens <NUM> confers an omnidirectional UW-FOV to the camera <NUM>. In this example, the camera FOV covers <NUM>° in the azimuthal plane Pϕd. The camera FOV has an elevational coverage ΔΘv of -<NUM>° to <NUM>° relative to the azimuthal plane Pϕd. The resulting UW-FOV allows instantaneous imaging of a large portion of the unit's surroundings. The UW-FOV covers a relatively narrow elevational range around the azimuthal plane Pϕd, in which other units <NUM> are expected to be located, but also larger elevational angles corresponding with an upwards region in which an underwater vehicle <NUM> (e.g. an ROV or UAV) may move around.

The fish-eye lens <NUM> has a focal length that is slightly larger than the distance to the sensor <NUM>, so that a focal point F of an imaged light signal <NUM> from a point-like light source <NUM> will be located slightly behind the imaging sensor <NUM>. Such a point-like light source <NUM> may for example correspond with the optical signal <NUM> from an optical signal transmitter <NUM> of a visible but remote other unit <NUM>. Light received from a point-like light source <NUM> will thus be projected slightly out of focus onto the imaging sensor <NUM>, to generate an image blob that extends over multiple adjacent pixels of the array, in order to allow accurate subpixel detection of center coordinates.

<FIG> shows a schematic side view of the transparent medial portion <NUM>, <NUM> of the UWOC unit <NUM> from <FIG>. The medial portion <NUM>, <NUM> is formed by a first medial housing portion <NUM> and a second medial housing portion <NUM>. Exemplary materials for the medial portions <NUM>, <NUM> are glasses or acrylic glasses. The medial housing portions <NUM>, <NUM> are formed as truncated spheres that consist essentially of pressure-resistant and optically transparent material, and which are mutually stacked and coaxially arranged around the unit axis A. Each medial housing portion <NUM>, <NUM> has an outer surface with a convex curvature along radial and axial directions R, Z, and an associated center of curvature Ct, Cr located on the unit axis A.

The optical signal receiver <NUM> is of an anidolic type. The optical signal receiver <NUM> is accommodated in the first medial housing portion <NUM>, and includes a detector <NUM> with SiPM sensors 64a, 64b and first reflector optics 60a, 60b.

The first reflector optics 60a, 60b are positioned with volumetric center substantially coinciding with a nominal center of curvature Ct of the first medial housing portion <NUM>. The first reflector optics <NUM> includes an inverse parabolic conical reflector 60a, 60b for each associated SiPM 64a, 64b. Each reflector <NUM> is adapted to reflect incoming light signals <NUM> that radially approach with a substantial component along a first azimuthal plane Pϕr, and to project the reflected light onto the associated SiPM 64a, 64b. The reflectors <NUM> are arranged inside the first medial portion <NUM> with two-fold (<NUM>°) rotational symmetry around the unit axis A. The axis of revolution Ba, Bb of each reflector <NUM> is parallel with the unit axis A, and the (possibly truncated) top of each mirror is directed toward the associated SiPM 64a, 64b.

The optical signal transmitter <NUM> is accommodated in the second medial housing portion <NUM>, and includes light sources <NUM> and second reflector optics <NUM>. The light sources <NUM> are formed by LED units that are arranged in a regular azimuthal distribution around the unit axis A, and which are adapted to emit light with wavelengths substantially in an optical range of <NUM> nanometers to <NUM> nanometers.

The second reflector optics <NUM> are formed by another parabolic conical reflector <NUM>, which is adapted to reflect optical signals <NUM> emitted by the LEDs <NUM> outwards, with a substantial component along a second azimuthal plane Pϕt that is parallel with the first azimuthal plane Pϕr. The second reflector optics <NUM> are positioned with its volumetric center substantially coinciding with a nominal center of curvature Cr of the second medial housing portion <NUM>.

The processor <NUM> and memory unit <NUM> are communicatively coupled with the optical signal receiver <NUM> via the cable router tube <NUM>, to receive and store data acquired by the SiPMs 64a, 64b. The processor unit <NUM> is also communicatively coupled with the optical signal transmitter <NUM> via the cable router tube <NUM>, to control the emission of optical signals <NUM> by the LEDs <NUM>.

<FIG> shows a schematic perspective view of one of the conical reflectors 60a, 60b in the optical signal receiver <NUM>. The parabolic conical reflector <NUM> is formed as an inverse parabolic cone with an outer surface of optically reflective material, and with an annular focal region that coincides with the planar sensor surface Ps of the associated SiPM 64a, 64b. The conical mirror <NUM> is centered on an axis of revolution B, which extends through the sensor region Ps of the underlying SiPM <NUM>, and is substantially parallel with the axis A of the UWOC unit <NUM>. The truncated top of the mirror <NUM> faces the SiPM <NUM>.

A parabolic cone allows all light rays from a particular radial direction (i.e. particular value for the azimuth angle) to be projected onto a single focal point of the parabola. <FIG> shows radial parallel light rays of an incoming optical signal <NUM>, and reflection thereof by the reflector <NUM> towards this focal point and onto the sensor surface Ps of the SiPM <NUM>.

The parabolic conical reflector <NUM> of the optical signal transmitter <NUM> has a similar shape and will reflect optical signals <NUM> emitted by the LEDs <NUM> radially outwards. This reflector <NUM> is centered on an axis of revolution which essentially coincides with the axis A of the UWOC unit <NUM>.

<FIG> illustrate an image processing method in an observation unit with an underwater imaging device, for instance unit 30a with an omnidirectional UW-FOV camera <NUM> as described with reference to <FIG>. <FIG> schematically illustrates two representations of images 84a, 84b acquired by the camera <NUM>. It should be understood that the omnidirectional UW-FOV camera <NUM> will generate curved images. Rectangular 2D Cartesian coordinate frame are nevertheless used in <FIG>, merely for illustration purposes.

Image frame 84a in <FIG> shows an optical signal 80b from the optical transmitter 36b of unit 30b, as perceived by the camera <NUM> of the observing unit 30a. Image frame 84b in <FIG> shows an optical signal 80c from the optical transmitter 36c of unit 30c, as perceived by the camera <NUM> of the observing unit 30a. Optical transmitters 36b and 36c are activated in a directly successive and temporally non-overlapping manner.

The associated optical signals 80b and 80c are imaged by the camera <NUM> in the successive image frames 84a-b. The processor device <NUM> of the observing unit 30a then subtracts the second image 84b from the first image 84a (indicated by the Θ-symbol), to generate a difference image <NUM>. This difference image <NUM> includes an intensity maximum <NUM> associated with the (angular) location of light source 36b, an intensity minimum <NUM> associated with the (angular) location of light source 36c, and a background region <NUM> with average intensity value associated with removed background information due to the subtraction.

Pixel regions where a received light signal hits the imaging sensor <NUM> of the camera <NUM> can be associated with a set of two angular coordinates (e.g. an azimuth angle Φ and an elevation angle O, or direction cosine angles) relative to a local reference frame defined with respect to the camera <NUM>. By subtracting images of successive light signals <NUM> from different units <NUM> in the vicinity, identification and discrimination of corresponding pixel regions can proceed with increased accuracy, and based on a relatively small number of image frames. The removal of irrelevant background image data from the subtraction operation increases the accuracy of detecting relevant pixels associated with the light signals <NUM>.

A plurality of the proposed units <NUM> from <FIG> can be deployed underwater to form an observation and monitoring system <NUM>. <FIG> shows part of the exemplary system <NUM> in a deployed state, and illustrates a method for structure/asset displacement monitoring. The units 30a, 30b, 30c, 30d, 30e are configured to operate without external control, and to establish optical communication channels between pairs of units <NUM>. The resulting communication channels may form a meshed network, wherein the units <NUM> form network nodes that cooperate to perform one or several observation and monitoring functions. The displacement monitoring method is explained with reference to the exemplary units <NUM> from <FIG>, but it should be understood that equivalent units may be used as an alternative or in addition to such units <NUM>.

In an initial deployment stage for the system <NUM>, the observation units 30i (i = a, b, c,. ) are placed at non-coinciding locations Qi on the submerged surface <NUM> or structures <NUM>, <NUM>, such that each unit 30i is within visual and/or optical communication range with at least one other unit 30j (j = a, b, c,. The relatively small units <NUM> may initially be placed by an underwater vehicle <NUM>, for instance a ROV <NUM>. The units <NUM> are deployed with inter-unit distances ΔRij between each pair of units 30i, 30j (e.g. ΔRab between units 30a and 30b). In a (quasi-static) displacement monitoring mode, inter-unit distances ΔRij of up to <NUM> meters or more may be achievable.

In order to conserve electrical power, the units <NUM> are configured to remain in a dormant mode for extended times, and to activate at predetermined time intervals and/or upon external request to perform measurements, to store measurement data, and/or to exchange measurement data. The processor <NUM> of each unit <NUM> is programmed with timing and/or external instruction protocols for activating the sensors <NUM>, <NUM>, <NUM> at predetermined periods and/or external request, and for storing the acquired data in the memory unit <NUM> and/or optically transmitting the acquired data to other units <NUM> in the network.

The signal transmitter 36a of a first observation unit 30a may emit light signals 80a, 81a (or 82a; not indicated in <FIG>). The emission of light may occur continuously, intermittently after predetermined time intervals, or upon request by the ROV <NUM> or another nearby underwater vehicle (e.g. an UAV). A portion of this light signal 80a may be received by other cameras (e.g. 40b) of nearby observation units (e.g. unit 30b), yielding image data for each unit <NUM> within visual range. Via initial calibration procedures, the pixel region where a received light signal hits the imaging sensor <NUM> of the camera <NUM> can be associated with a set of two angular coordinates (e.g. an azimuth angle Φ and an elevation angle Θ, or direction cosine angles) relative to a local reference frame defined with respect to the camera <NUM>.

During imaging with the camera <NUM>, the orientation sensor <NUM> of each unit <NUM> acquires attitude data for this unit <NUM>, by detecting changes in at least pitch, and roll angles for the camera <NUM> relative to the surface <NUM> or structure <NUM>, <NUM> on/in which the unit <NUM> resides. The processor <NUM> of each unit generates positional information with angular coordinates for the detected external light sources, on the basis of the acquired image data and attitude data. The positional information is referenced with respect to a common coordinate frame and provided with a timestamp corresponding to the time of measurement. The resulting data with timestamp is locally stored in the memory unit <NUM>. The acquired image data and attitude data may also be separately stored in the memory unit <NUM>, for downloading and post-processing purposes.

In addition, each of the units <NUM> may be configured to send optical interrogation signals <NUM> to another unit <NUM> via its signal transmitter <NUM>, and to respond to an interrogation signal <NUM> received via the signal receiver <NUM> by emitting an optical response signal <NUM> via the signal transmitter <NUM>. The processor <NUM> of each unit <NUM> may then be configured to execute a ranging function between this unit <NUM> and a specific other of the surrounding units that is within optical range, by determining time of flight (TOF) between the emitted interrogation signal <NUM> and a received response signal <NUM>. The resulting TOF data may be stored in the memory unit <NUM> and/or transmitted via the communication device <NUM> to another unit <NUM>. As illustrated in <FIG>, the first unit 30a emits an optical interrogation signal 81a. The second unit 30b sends and optical response signal 82b to the first unit 30a, upon receiving the optical interrogation signal 81a from the first unit 30a. The first unit 30a measures a time difference between transmission of the interrogation signal 81a and receipt of the response signal 82b from the second UWOC unit 30b, to derive a TOF. The processor <NUM> of the first unit 30a then determines an inter-unit distance ΔRab on the basis of the TOF.

Apart from the above, each unit <NUM> is configured to receive positional data of the other units <NUM> at predetermined times or upon request. The positional data is to be transmitted by each unit <NUM> via its communication device <NUM> to the other units <NUM>. The processor device <NUM> of one unit <NUM> is configured to merge positional data (including timestamps) received from the other units <NUM>, to form a merged dataset of time-dependent positional profiles for all observation units <NUM>, which is stored in the memory unit <NUM>.

The node positions can be computed from the recorded positional data (e.g. angular data and attitude data) and at least one known distance to determine the scale of the deployed system <NUM> (e.g. from one or more TOF-based inter-unit distances). The deployed system <NUM> can thus be used to accurately detect (e.g. sub-centimeter) relative motions (e.g. subsidence) between the deployed units <NUM>, and parts of the surface <NUM> and assets <NUM>, <NUM> on which the units <NUM> are deployed, by retrieving the merged dataset and analyzing the time-variations in the positional data. The method may for instance be used to estimate mechanical stresses between two locations of a submerged object (e.g. wellheads <NUM> and manifolds <NUM>), or of structures (e.g. jumpers <NUM>) interconnecting such objects, to provide a timely indication of potential structure failure.

The underwater vehicle <NUM> may include a wireless optical communication device (not shown), which is configured to address any unit <NUM> and request for a transmission of positional data. Such a vehicle <NUM> may move within communication range of a selected unit <NUM>, and request the unit <NUM> for a transmission of network measurements. The addressed unit <NUM> may then upload its current merged dataset of time-dependent positional profiles to the vehicle <NUM>.

The system <NUM> may additionally include an acoustic modem (not shown), configured to upload positioning data to a vehicle at the surface of the sea <NUM>. Alternatively or in addition, one or more of the units <NUM> may be in signal communication via a wired connection with a nearby underwater data access point (also not shown).

<FIG> shows part of the exemplary observation system <NUM> from <FIG> in a deployed state, and illustrates a method for enhancing position information for an underwater vehicle <NUM> e.g. an ROV. In an initial calibration stage for the deployed system <NUM>, the observation units <NUM> may exchange interrogation and response signals <NUM>, <NUM>, in order to determine TOF and inter-unit distances ΔRij via methods described herein above.

The ROV <NUM> may be fitted with a plurality of wireless optical communication devices <NUM>, each including an optical signal transceiver that is configured to emit light <NUM>, and to receive optical signals <NUM> from the signal transmitters <NUM> of nearby observation units <NUM>.

The ROV <NUM> may be configured to serve as a master node for the system <NUM>. This master node is configured to establish the common network time of the system <NUM>, and to synchronize time for all the units <NUM> in the network by broadcasting timing information via its optical communication device to the units 30j. The master node is further configured to define tasks that individual units 30j need to execute per measurement cycle, and to transmit instructions to a specific unit 30j via communication device. Alternatively, one or several of the observation units <NUM> in the system <NUM> may be configured to function as the master node during different periods.

The underwater imaging devices 40a, 40b observation units 30a, 30b in visual range of the ROV lights <NUM> acquire image data of these lights <NUM>. The processor device 44a, 44b of each unit <NUM> may then determine positional data associated with of the ROV relative to the imaging device <NUM>, via methods described herein above. The communication device 35a, 35b of each unit <NUM> may then transmit this positional data to the ROV <NUM> upon request, via optical signals <NUM> that may be received by any or all of the ROV's communication devices <NUM> that have a line of sight to that unit <NUM>. Only optical signal 80a from unit 30a to ROV communication device 19d is shown in <FIG> for clarity, but it should be understood that other units <NUM> may communicate positional information to any or all ROV devices <NUM>.

The system <NUM> may be kept deployed in dormant mode on the seafloor <NUM> and structures <NUM>, <NUM> for a long time, but may be woken up by the ROV <NUM> (or another underwater vehicle) entering the site, and ordered to start tracking and broadcasting the 6DOF position of the ROV <NUM>.

Any or all units <NUM> in the system <NUM> can also be ordered by the ROV <NUM> to record images of the environment with the static cameras <NUM> with omnidirectional views. During such recording, the ROV <NUM> may project light (e.g. diffuse light or laser stripes) onto the otherwise dark scene. Full panoramic image data, or landmark features extracted from those images by the units <NUM>, may be transmitted together with positional reference data to the ROV <NUM> upon request.

The system <NUM> may be configured to operate in a single mode, and switch to another single mode upon request. The system <NUM> may also be configured to operate in multiple modes at the same time, and to de-activate one of the current modes and/or activate one or more other modes upon request by the ROV <NUM>. The system <NUM> may thus be efficiently used for different purposes and perform alternative measurements upon request, while saving energy when particular modes of operation are not desired.

The present invention may be embodied in other specific forms without departing from its scope of protection, defined by the claims. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice. All changes which come within the meaning of the claims are to be embraced within their scope.

In the above examples, the camera FOV had an azimuthal coverage ΔΦv of <NUM>° and an elevational coverage ΔΘv of -<NUM>° to +<NUM>°. Depending on the application and desired vertical observational range, the elevational coverage ΔΘv may be reduced to the range -<NUM>° to +<NUM>°, or may be increased to the range -<NUM>° to +<NUM>°.

The skilled person will appreciate that the component casing <NUM> may consist essentially of materials different than titanium. Other suitable materials are e.g. stainless steel, electrogalvanized steel, aluminum, or other sufficiently rigid materials that are corrosion-resistant or otherwise provided with an external coating of anti-corrosion material.

Also, the shape of the medial portions <NUM>, <NUM> of the unit's housing <NUM> should not be considered limited to stacked truncated spheroids. Instead, a medial housing portion with a cylindrical shape, or another shape with rotational symmetry about the unit axis A, would be possible.

Furthermore, the power supply <NUM> was formed in the above examples as a replaceable seawater battery, but may alternatively be formed by other suitable water-compatible and pressure-resistant power supply arrangements. For instance, a pressure-tolerant non-rechargeable alkaline battery pack may be used in monitoring units for long-term deployment (e.g. long-term displacement monitoring mode), or rechargeable nickel-metal hydride (NiMh) batteries in a pressure housing may be used in monitoring units that are only deployed for a short period (e.g. in positioning mode).

Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both.

For instance, the image processing functionalities on the one hand, and the optical communication and network management tasks on the other hand, may be controlled by separate processor devices provided in the same underwater observation unit.

Claim 1:
An observation unit (<NUM>) for underwater deployment on or in a submerged earth layer (<NUM>) or a submerged structure (<NUM>, <NUM>), comprising:
- a housing (<NUM>) adapted for supporting the underwater observation unit relative to the submerged layer or structure;
- an underwater imaging device (<NUM>) attached to the housing, and configured to acquire image data of a second light source (36b, 70b) located within a wide field of view, FOV, covering the surroundings of the observation unit;
- a processor device (<NUM>) configured to receive the image data from the imaging device, and to determine positional data of the second light source relative to the imaging device, and
- a communication device (<NUM>), comprising an optical signal transmitter (<NUM>) with a light source (<NUM>) that is fixed to the housing, the optical signal transmitter being configured to transmit the positional data by emitting a light signal (<NUM>) with the light source into the surroundings of the observation unit and towards at least one of: another underwater observation unit (30b, 30c, 30d), an underwater vehicle (<NUM>), and an underwater processing station.