Abstract:
A system for measuring the attitude of an object in a fixed frame of reference from a moving frame of reference, comprising a first imaging device for producing image data for the object from a first direction, a second imaging device for producing image data for the object from a second direction having a component perpendicular to the first direction and an attitude sensing arrangement for determining an attitude of the first and second imaging devices with respect to the fixed frame of reference at the time the respective images are taken and for generating attitude data. An image processing system analyses and combines the respective image data and attitude data to determine the attitude of the object. The system is particularly useful for determining attitude of offshore piles during piling operations or for monitoring the departure angles of pipes and cables during laying thereof.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to devices and systems for determining an attitude of an object in a fixed frame of reference by observations from a moving frame of reference and more particularly to a system capable of accurately measuring the verticality of a pile or similar object from a floating platform or vessel. The invention also relates to a method of performing such measurements. 
         [0003]    2. Description of the Related Art 
         [0004]    Engineering operations at sea often involve considerable difficulty due to the constant movement of the waves. This is especially the case when accuracy is required in performing measurements. For a vessel at sea, measurements may be made in a fixed frame of reference relative to the earth or in a moving frame of reference relative to the vessel. 
         [0005]    One situation where measurements in the absolute frame of reference are relevant is in the installation of fixed structures from a floating platform or vessel. In recent years large numbers of offshore structures have been installed on or in the sea bed, including wind turbines, drilling platforms, bridges, cables, pipelines and the like. The installation work is generally performed from a suitable barge or vessel using cranes and other lifting facilities. In the case of wind turbines, a single pile may be driven into the seabed using a hydraulic hammer or other pile-driving facility mounted on a heavy lift vessel. These monopiles should be installed vertically and it is essential that during driving the verticality is continuously monitored and, where necessary, corrected. At present, the procedure for performing such measurements involves a human operator approaching the monopile and performing a manual measurement of the structure using a handheld inclinometer. This measurement thus takes place directly in the fixed frame of reference of the monopile. During measurement, the pile-driving must be stopped. After evaluation of the results, the crane operator must be informed of the results and instructed with respect to the required corrective action. 
         [0006]    In the case of laying of cable from a cable lay vessel, it may be desirable to closely monitor the angle of deployment of the cable in order to determine its trajectory to the seabed. In conventional systems this may be achieved by locating an angle sensor against the cable at the departure location. The sensor may be a pitch and roll type angle sensor which requires direct contact with the cable for operation. Positioning of such sensors can be difficult and they may require frequent attention e.g. on passing joints on the cable. They may also be easily damaged, especially when laying multiple cables and at present are not applicable to all situations. Similar considerations apply during pipe-laying. 
         [0007]    It would be desirable to provide a system that simplified such measurements and could be implemented in real time without interrupting operations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    According to the invention there is provided a system for measuring an attitude of an object in a fixed frame of reference from a moving frame of reference, the system comprising: a first imaging device for producing image data for the object from a first direction; a second imaging device for producing image data for the object from a second direction having a component perpendicular to the first direction; an attitude sensing arrangement for determining an attitude of the first and second imaging devices with respect to the fixed frame of reference at the time the respective images are taken and for generating attitude data; and an image processing system for analysing and combining the respective image data and attitude data to determine the attitude of the object. By the use of imaging devices, observations may be made at a distance from the object to be measured. Measurements may thus take place without requiring anyone to be in the direct vicinity of the object. The measurements may be taken from any distance. Preferably, the distance is between 0.5 metres and 100 metres, more preferably between 5 metres and 30 metres. In most cases, the first direction and the second direction will be generally perpendicular to each other, although there is usually no requirement of exact perpendicularity. In the present context, attitude is intended to refer to the orientation of a coordinate frame attached to the object with respect to another frame, such as a geodetic reference frame, and may be expressed in terms of the degrees of deviation with respect to the reference axis. In general, attitude will be understood to cover three degrees of freedom, namely pitch, roll and yaw. 
         [0009]    Nevertheless, it will be understood that in certain circumstances, just two degrees of freedom such as pitch and roll may be monitored within a defined body frame having an arbitrary heading such as the vessel reference frame. 
         [0010]    The attitude sensing arrangement may be any suitable device capable of recording the local orientation or attitude of the respective image device with respect to the fixed reference frame. In many cases such devices are referred to as inertial monitoring units (IMU) but this is not intended to be restrictive on any particular principle of operation. In particular, the attitude sensing arrangement may be based on mechanical gyroscopic action, fibre optic gyroscopes (FOG), laser ring gyroscopes (LRG), MEMS based technologies, accelerometers, magnetometers, pendulum, electrolytic inclinometers, liquid capacitive inclinometers or any combination of the above. Most preferably, the attitude sensing arrangement also comprises a heading indicator allowing momentary determination of the attitude of the imaging devices with respect to the earth, in particular, geodetic or magnetic north. The attitude sensing arrangement should preferably have a dynamic accuracy of better than 1.0°, more preferably better than 0.5° and most preferably better than 0.2°, at least in the pitch and roll directions. 
         [0011]    In a simple embodiment, the attitude sensing arrangement may comprise a single attitude sensor for both imaging devices. This single device could be a pitch and roll sensor calibrated to the vessel or could be a three axis attitude sensor with north-seeking capability. In both these cases, the positions of the first and second imaging devices must remain fixed relative to each other within the moving frame of reference from initial set-up. Such a system may be used in combination with a ship&#39;s IMU or attitude sensor, whereby the first and second imaging devices are located at fixed locations. In a more preferred embodiment, first and second attitude sensors are provided, each integrated with a respective imaging device for movement therewith. Each local attitude sensor may be a non-north seeking attitude sensor whereby readings within a vessel frame of reference may be determined if the position of the imaging device relative to the vessel is accurately established. It will also be appreciated that each local attitude sensor could even be a single axis attitude sensor, whereby more constraints would be required on the positioning of the imaging devices and on the resulting attitude determination. The system has considerable additional advantage if each of the attitude sensors is a 3-axis attitude sensor with north-seeking capability. In this case, each imaging device can be relocated together with its attitude sensor and can instantly commence measurement, since it automatically knows its own heading. Since the heading of the observation is now defined in the geodetic reference frame, it allows pitch and roll to be defined and computed for any arbitrary absolute heading. This heading could be e.g. North or, when the vessel heading is known, the heading of the vessel. This allows considerable versatility since an imaging device can easily be repositioned if its view is obscured or if it is otherwise inconvenienced. As a further consequence of the dedicated 3-axis north-seeking attitude sensor, each imaging unit may in fact be located in its own frame of reference, moving independently of each other. It will thus be understood that they may even be located on different vessels or that theoretically one or both of the imaging devices may be portable and hand held during measurement. 
         [0012]    According to an important aspect of the invention, the system further comprises a clock, arranged to time stamp the attitude data. In order to effectively process the image data and the attitude data, there must be close correlation in time between the respective measurements. For operation on board a moving vessel, the attitude may change very rapidly and the attitude data should preferably be time stamped to an accuracy of at least 1 ms. Most preferably, the time stamp will be accurate to around 0.1 ms. The required timing accuracy, including unaccounted processing and communication latencies, may be expressed as a function of the required measurement precision and the expected movement rate of the moving frame. For example on board a vessel, it may be desirable that the timing accuracy should be better than the required pitch and roll measurement accuracy divided by the maximum expected angular rate of the vessel. The required relative timing accuracy between the first and second imaging devices may be less critical, especially if an expected angular rate of movement of the object is very low compared to variations in the moving reference frame. Even the difference between capture time and transmission time may need to be taken into account in order to ensure that the correct data items are associated to one another. The clock may be a local clock applicable to the attitude sensor and its associated imaging device or devices. Most preferably, the clock is linked to a global timing system such as GPS, which allows readings to be correlated with other events such as a global position indication. Preferably the clock also issues a clock signal for synchronising a capture time of the image data. The skilled person will be well aware of various options for ensuring that the image data and attitude data are accurately synchronised to such a clock signal. 
         [0013]    As discussed above, the invention is particularly applicable for mounting aboard a vessel. In that case, according to a preferred embodiment of the invention, the attitude of the object may be given with respect to a heading of the vessel. For a vertical object such as a pile, the attitude will preferably be the inclination of the pile and its direction. The direction of inclination of the top of the pile with respect to the base can then be displayed relative to the bow of the vessel. For a crane or pile driver mounted to and moving with the vessel, corrective action may be more easily taken in this local reference frame. It is however not excluded that the attitude of the object is also or alternatively given with respect to a geodetic co-ordinate frame. 
         [0014]    In an alternative configuration, the device may be used for monitoring a pipe or cable laying operation by monitoring the direction of departure of a pipe or cable string from a vessel. The attitude of the string at departure can be used to compute the trajectory of the string to the seabed. Of particular interest is the angle of the string with respect to the vertical (or horizontal direction). The departure heading may be determined based on position data for the vessel and the point of contact with the seabed, although it is not excluded that the heading of the string could also be determined by the claimed device. 
         [0015]    According to a yet further advantage of the invention, the system also comprises a calibration facility. This may be arranged to calibrate the total body-frame roll axis error of a respective imaging device and the attitude sensing arrangement. In this context, total body-frame roll is intended to refer to the combined error due to both the attitude sensor accuracy and any misalignment between the mounting of the camera and attitude sensor. This may be accomplished by taking an image of the horizon, which by definition must be horizontal. Frequent self-checks can be carried out, which have the advantage that a device can be verified to work correctly in the marine environment. Distortions arising from e.g. shipment of the unit, resulting in increased measurement errors, can be easily detected using such a method. 
         [0016]    According to a still further aspect of the invention, the system allows measurement and determination of the attitude in real time. There may also be an attitude display providing a real-time representation of the attitude of the object. One particularly convenient display is in the form of a sight glass or spirit level, allowing an operator to monitor deviation of the object from a desired attitude. The system may also or alternatively comprise a user interface and an image display. In one convenient form, the image processing system may be arranged such that a user can manually pick a portion of an image of the object on the display for use in determining the attitude of the object. The portion of the object may be a characteristic edge or marking on the object and picking may take place using a mouse or similar pointer to identify the portion on the image display. Additional software may be provided for locking the image based on image recognition of the characteristics of the portion. This can be of use in situations where the object becomes partially obscured and a human operator may more reliably identify the relevant portions. 
         [0017]    Although the system may be used for determining the orientation or attitude of many alternative objects, it is particularly suitable for use in determining the verticality of a pile, more specifically the sort of pile or monopile used for offshore wind turbine generators. In one embodiment of the system, the image processing system is arranged to: determine left and right borders of the pile from the first imaging device and identify a location of the plane therebetween that passes through the first imaging device; determine left and right borders of the pile from the second imaging device and identify a location of the plane therebetween that passes through the second imaging device; and combine the location of the planes from both the first and second imaging devices to determine the attitude of the axis of the pile. Assuming that the pile is rotationally symmetric, the mid-plane between the edges will identify the central axis irrespective of the direction of viewing. This makes it considerably easier to evaluate the results, as it is independent of the distance between the imaging devices and the pile. It is also applicable even when the pile tapers. 
         [0018]    The invention also relates to a device for measuring an attitude of an object in a fixed frame of reference from a moving frame of reference, the device comprising: an imaging device for producing image data for the object; an attitude sensor for determining an attitude of the imaging device with respect to the fixed frame of reference at the time the image is taken and for generating attitude data; and a communication port for transmitting data to a data processing system for determining or displaying the attitude of the object. In a simple form, the attitude sensor may be a one- or two -axis attitude sensor without north-seeking capability. It will be understood that north-seeking devices are considerably more expensive and a simple non north-seeking device may be more than adequate for many purposes, especially when used e.g. in combination with a ship&#39;s north-seeking facility or with additional GPS positioning. A more complex embodiment of the invention provides a single device in which the imaging device and a north-seeking attitude sensor are integrated. Such a device is extremely versatile and can in fact be moved to a given location and immediately provide relevant input for determining the attitude of the object. This may be the case, even when the absolute position is unknown, based merely on the attitude information. It will be understood that the data transmitted may be the image data and the attitude data. In this case, the remote data processing system may include an image processing system for analysing and combining the image data and attitude data to determine the attitude of the object as described above. Alternatively, an image processing system may be provided in the device and the transmitted data may comprise partially processed data related to the attitude of the object. The device may also include a local image display and user interface. 
         [0019]    According to an embodiment, the device may comprise a clock, arranged to time stamp the image data relative to the attitude data. The clock may be a local clock or may be a clock signal derived from an external source such as the data processing system or a satellite navigation system. Stamping the image and attitude data in an accurate manner is of importance in ensuring that they are coherent, especially when analysis takes place subsequently. The clock or time stamp facility may be part of a real-time signal multiplexer that combines the attitude data and the image data. 
         [0020]    Most preferably, the device is portable and the imaging device and the attitude sensor are fixed to each other to prevent relative movement. By portable, it is understood that the device is of a size and weight to be easily carried and set up by one person. It will be understood that such a device may even be used alone to provide measurements from two directions by moving from a first position to a second position. Such a device could be mounted on a separate vessel, movable with respect to the object being measured. 
         [0021]    It will be further understood that various imaging devices may provide the desired function. These may include laser imaging devices, infra-red imaging or conventional camera based imaging devices. Most preferably, the imaging device is a camera having automatic iris control, allowing use in varying lighting conditions. It will be understood that, throughout the day, the position of the sun will vary and it may be undesirable to shift the imaging device when the lighting condition is bad. Automatic lighting control can mitigate this difficulty, allowing the operator to continue measurements from the same location. Additional desirable characteristics of the camera include anti-smearing and anti-blooming correction to further improve image quality. 
         [0022]    In a still further embodiment, the device further comprises a calibration facility arranged to calibrate the attitude sensor for determining an attitude of the imaging device with respect to the horizon. The mounting of the attitude sensor to the imaging device is preferably robust. Nevertheless, when using the device for extended periods or in the event that it has been subject to shock, it may be desirable to have a simple self test possibility. This may be achieved by calibrating it against the horizon, which, for offshore locations, provides an accurate horizontal axis. The calibration facility may be partially local and partially remote e.g. with an image processing system being used to identify the horizon in the same way as identification of the object takes place. 
         [0023]    According to a further desirable embodiment, the device may also comprise a satellite positioning system providing position data for the imaging device. Although it is not directly required to know the exact location of the device in order to determine attitude of the object, additional benefits may be achieved by knowing location. It will be understood that position data for the device in combination with position data for the object could also be used to provide the heading requirement of the device. The satellite positioning system may be any GNSS (e.g. GPS, Glonass or both) or based on any other system capable of reliably determining position. 
         [0024]    The invention also relates to a vessel comprising the system or the device as defined above. The vessel may be any vessel, including but not limited to, a heavy lift barge performing pile driving operations, a cable or pipe laying vessel, an ROV or a support vessel to one or more such vessels. 
         [0025]    The invention further relates to a method of determining an attitude of an object in a fixed frame of reference based on observations from a moving frame of reference and a computer program product for implementing the method. The method may comprise: viewing the object from a moving frame of reference in a first direction to produce a first image data set; viewing the object from a moving frame of reference in a second direction to produce a second image data set; collecting attitude data representing a relative attitude of the moving frame of reference with respect to the fixed frame of reference for each image data set; and analysing and combining the respective image data and attitude data to determine the attitude of the object. The method may be carried out using the system or device as described above or hereinafter and is particularly directed to determining the verticality of an offshore pile during driving thereof or of a cable or pipe during an offshore laying operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The features and advantages of the invention will be appreciated upon reference to the following drawings of a number of exemplary embodiments, in which: 
           [0027]      FIG. 1  shows a schematic view of a first embodiment of the present invention; 
           [0028]      FIGS. 2A and 2B  show images taken by the first and second cameras of the embodiment of  FIG. 1 ; 
           [0029]      FIG. 2C  shows an attitude display of a monopile during use of the invention of  FIG. 1 ; 
           [0030]      FIG. 3  shows a device and system according to a second embodiment of the invention; 
           [0031]      FIG. 4  shows a device according to the invention in use on a cable-lay vessel; and 
           [0032]      FIG. 5  shows a schematic view of the orientation of the cable of  FIG. 4   
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows a schematic view of a system  1  for measuring the attitude of a monopile  2  according to a first embodiment of the invention. The system  1  is mounted upon a barge  4 , which is being used for installation of the monopile  2 . Cranes and pile-driving equipment is not shown for the sake of convenience but may be otherwise conventional. 
         [0034]    The system  1  comprises a first camera  6  and a second camera  8  mounted on the barge  4  within view of the monopile  2  and directed towards it. The cameras  6 ,  8  are mounted to view the monopile from orthogonal directions i.e. the lines of sight from the respective camera to the monopile are perpendicular to each other. As will be understood by the skilled person in the following, these directions need not be perfectly perpendicular. In the present embodiment, the first camera  6  is aimed along a left hand edge of the monopile  2  and is aligned across the barge  4  in a direction denoted X. The second camera  8  is aimed at a right hand edge of the monopile and directed in the direction denoted Y with respect to the frame of reference of the barge  4 . Also aboard the barge  4  are a 3-axis north-seeking attitude sensor  10  and an image processing system  12  including an image display  14  and a user interface  16 . The image processing system  12  is in fact a standard computer running dedicated software. 
         [0035]      FIG. 2A  is an image taken by the first camera  6  at a time T 1  as viewed on the image display  14 . It shows the monopile  2  and an indication of the Z and Y directions, together with an indication of the time T 1 . The Z direction is the vertical direction within the moving frame of reference of the barge  4 . V is the true vertical within the geodetic co-ordinate frame. The image display  14  also includes pointers  18 A, B that can be manipulated through the user interface  16 , e.g. by use of a mouse or the like. 
         [0036]      FIG. 2B  is an image taken by the second camera  8  at a time T 2  as viewed on the image display  14 . It shows the monopile  2  and an indication of the Z (local-vertical), V (geodetic vertical) and X directions, together with an indication of the time T 2 . Also shown are the pointers  18 A, B. 
         [0037]      FIG. 2C  shows an attitude display  20  in a single spirit-level representation of the attitude of the monopile  2 . A bubble  22  indicates the position of the monopile with respect to the vertical V in the fixed reference frame. The attitude display  20  shows the magnetic or geodetic North direction N, giving attitude in the fixed reference frame and can also show the X and Y directions allowing the attitude of the monopile  2  to be seen within the local frame of reference. The attitude display  20  may be provided on the crane operator or pile driver&#39;s display and may also be displayed on the image display  14 . 
         [0038]    Operation of the system  1  will now be described with reference to  FIGS. 1 and 2A ,  2 B. In use, once the monopile  2  has been placed at the correct location and driving has commenced, it is effectively fixed within the geodetic frame of reference of the earth. It still moves slowly due to the driving operation but this slow movement may be ignored for the further discussion. Images are taken and displayed on the image display  14  as shown in  FIGS. 2A and 2B  with an update frequency, e.g., of around  1  Hz. A user operating the system  1  uses the user interface  16  to identify two points on the left hand edge of the monopile  2  in  FIG. 2A  using pointers  18 A and  18 B. The image processing system  12  accurately identifies the chosen edge and locks onto it using an edge detection software module. Such a module is generally conventional software capable of pixel analysis to determine an edge of an object and is not further discussed here. Once the edge is detected, the image processing system  12  can accurately represent the position of this edge within the ZY frame of reference of the barge  4  for the time T 1 . Thereafter, the image processing system  12  can continue to follow the edge without requiring reselection, unless the image should become obscured for some reason. The image processing system  12  also receives attitude data from the attitude sensor  10  which is time-stamped as having been measured at time T 3 . The attitude data is sampled at a rate of e.g. 100 Hz and interpolation is used between these values to match the image data at times T 1  and T 2  to the attitude data at time T 3 . This allows an evaluation of the offset of the true vertical V with respect to the local vertical Z at the time T 1 . The same is repeated for the image of the second camera  8  taken at time T 2  as shown in  FIG. 2B . Once both images have been analysed, the results may be combined in the single spirit-level representation of the attitude display  20  as shown in  FIG. 2C . As the first and second cameras  6 ,  8  track the monopile, the position of the bubble  22  changes in real time and the engineer or surveyor can give directions for corrective action to the operator of the pile driver. 
         [0039]    In the first embodiment of  FIG. 1 , a single attitude sensor is used, which is the ship&#39;s own IMU. The cameras  6 ,  8  are high resolution digital cameras operating according to GigE vision protocol and having automatic iris control and anti-smearing and anti-blooming correction. This allows them to compensate for changing light conditions. Nevertheless, movement of the cameras  6 ,  8  with respect to the barge is undesirable, as this would require significant recalibration in order to define their relative positions in the local reference frame. Consequently, such an arrangement may be unsuitable where the location of the monopile  2  with respect to the barge  4  may change e.g. from one operation to the next or where significant chance of obstruction of one of the cameras is present. 
         [0040]    A second embodiment of the invention is shown in  FIG. 3 , in which like elements to the first embodiment are denoted with similar reference numerals preceded by 100. The device  101  of  FIG. 3  comprises a camera  106  and 3-axis north-seeking attitude sensor  110  integrated together in a single portable unit. This means that relative movement between the camera  106  and attitude sensor  110  is prevented. Furthermore, since the device  101  includes its own attitude sensor  110 , the momentary attitude of the camera  106  in the geodetic reference frame can be precisely monitored. The device  101  also includes a 1 GB Ethernet switch  126  and a time stamp generator  128 . The device  101  communicates through switch  126  with a data processing system  130  which has its own communications port  132 . It will be understood that although an Ethernet connection is shown, communication may also take place by wireless means. The data processing system  130  includes an image processing system  112 , image display  114  and user interface  116 . According to the second embodiment of the invention, the image processing system  112  operates slightly differently to that of the first embodiment in that it identifies left and right edges of the monopile  102 . This may be done automatically or with manual pointers as in the first embodiment. Once these edges as observed in the images are determined, the image processing system  112  calculates a plane through the centreline CL of the monopile and the camera centre by determining the middle between the planes defined between the camera and the two observed edges. Assuming a surface of revolution, this will be true, irrespective of whether the monopile  102  varies in width and leads to greater result accuracy. Together with the image data from the camera  106 , the data processing system  130  also receives attitude data from the attitude sensor. Since this includes heading data as well as data related to pitch and roll, the image processing system  112  can use this data to determine the precise direction from which the image has been made and orientate the centreline CL accordingly. The attitude data is time-stamped by the time stamp generator  128  which also issues a clock signal for the camera  106  to synchronise the image data acquisition. The image data and attitude data are together communicated via the 1 GB Ethernet switch  126  to the data processing system  130 .  FIG. 3  also shows a second device  101 B which provides data for determining the 3D orientation of the centreline 
         [0041]    CL from another direction. Based on the two readings, the data processing system  130  can determine the attitude of the centreline and display it as described above and shown in  FIG. 2C  and as an attitude display  120  on the image display  114 . 
         [0042]    As mentioned above, the device  101  is portable and can be moved to a location from which the required image is to be taken. Since the attitude sensor  110  provides real time attitude data, it may even be hand-held. In order to ensure that the device  101  is correctly calibrated to the geodetic reference frame, it further includes a self calibration button  134 . Operation of the self calibration button  134  requires the camera  106  to be directed towards an open expanse of horizon. Activation of the self calibration button  134  generates an image of the horizon H and correspondingly time-stamped attitude data. The image processing system  112  identifies the horizon H either automatically or with the assistance of an operator and compares the viewed horizontal with the value for horizontal measured by the attitude sensor and transmitted as attitude data. If there is a discrepancy, the image display indicates the difference and provides an offset to the device  101  for all further calculations. If the offset is too great, the operator is warned that the device may be faulty. 
         [0043]    The embodiment of  FIG. 3  has been described for use in a system in which two devices  101 ,  101 B provide attitude data which is combined to determine in real time the attitude of an object. It is also possible to determine the attitude in quasi-real time using a single device  101 . In that case, the device  101  is moved around the monopile, either by being moved around on the deck of the barge or by being moved around the monopile aboard a support vessel. In that case, the image data and attitude data can be supplied intermittently to the data processing system  130  which updates the attitude display  120  as new angles of view are provided. This can be sufficiently accurate if the monopile is driven relatively slowly compared with the change in position of the device  101 . The data processing system  130  may provide a suitable alarm if insufficient data is provided from a certain direction within a given time. 
         [0044]    An alternative embodiment of the invention is shown in  FIG. 4 , for use in determining the angle of departure of a cable during a cable-lay operation. Like elements to the first embodiment are provided with similar references preceded by  200 . 
         [0045]    According to  FIG. 4 , cable-lay vessel  204  is provided at its stern with a stinger or chute  211  over which cable  202  is deployed to the seabed. Attitude measuring devices  201  according to the invention are mounted overboard at the stern. The attitude measuring devices  201  are similar to the devices  101  of the second embodiment except that they incorporate attitude sensors without north-seeking capability. Cameras  206  are directed towards the cable  202  at the point where this leaves the chute  211 . Data acquisition takes place as in the previous embodiments, with image data and attitude data being time stamped and processed to determine the attitude of the cable  202  at its point of departure from the vessel  204  in the geodetic frame of reference.  FIG. 5  is a schematic view of the cable  202  at this point, indicating the geodetic vertical direction V and the horizontal plane H. The angle of the cable  202  with respect to its orthogonal projection onto the horizontal plane H is given by α. In this configuration, the actual heading of the cable  202  is not required, since this may be determined by other data, including the location of the vessel and the touchdown point of the cable at the seabed. 
         [0046]    Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. In particular, while the above techniques have been described in the context of driving monopiles for offshore wind turbine generators and cable laying, the invention may also be applied to other offshore structures, notably but not limited to offshore oil and gas installations, underwater seabed structures as foundation piles, oil well templates, underwater pipelines, pipe-laying, and underwater installation equipment such as frames and lifting facilities. Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.