Abstract:
The present invention relates to an underwater apparatus, system and method for inspecting and determining the condition of a submerged structure. More particularly the present invention utilizes acoustical devices for the positioning of an inspection vehicle, with respect to the structure to be inspected.

Description:
This application claims benefit of provisional No. 60/066,212 filed Nov. 20,1997. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an underwater apparatus, system and method for inspecting and determining the condition of a submerged structure whether it be a ship, drilling rig or any other underwater structure. Whether it be a regular routine inspection, or an inspection occasioned by an accident at sea, which may raise concerns respecting the structural integrity of the structure. The apparatus is particularly suited to inspect the hull of a ship without submitting the same to a costly dry dock procedure. The present invention does not require the ship to be docked, as it is highly portable and can be deployed in any appropriate body of water. Moreover, the exact position of the hull being inspected is also determined. Accordingly, the size and location of any irregularities and/or defects in the hull of the ship or the coatings that protect it are determined to a tolerance of a few centimeters. 
     BACKGROUND 
     Cost-efficient operation of a sea-going vessel, whether it be a tanker, a container ship, a floating production platform, or a cruiser, demands rigorous monitoring of the integrity of the hull. When referring to the integrity of the hull, it is important to note that this means much more than the ability of the structure to withstand the application of normal operating stresses. More particularly, in addition to the continuous relatively mild stresses applied to a vessel at rest in port, or at sea, vessels must also be able to withstand the sometimes every violent forces experienced by ships at sea during storm and other less than ideal conditions, whether they be waves, wind, or combinations of the same. 
     Key to the maintenance of the structural integrity of a ship is protection of the surface of the steel hull from the corrosive effects of sea water. As is well known, the salinity of sea water causes it to chemically attack the surface of a ship. 
     More particularly, and by way of example, the heavy salt content of ocean water will tend to eat away at the material of which metal vessel hulls are made. The result will initially be minor pitting, which tends to be localized on account of the configuration of the ship&#39;s hull, the materials of which the hull are made, joints between various components of the hull, and other factors. Eventually, such corrosion will compromise the overall integrity of the ship under normal operating conditions and, of course, could have catastrophic effects in the event of inclement weather or a collision with debris, another vessel, or on account of being run aground. 
     Even fiberglass vessels must be coated to be protected from other deteriorating elements in the sea, such as the build-up of organic materials, marine organisms such as barnacles, and the like. 
     Thus, the first line of defense is the protection of the hull of a ship with an appropriate coating. Such coatings are selected for their properties of long life in sea water, resistance to radiation, ability to discourage the growth of marine life on the surface of the hull of the ship, adhesion and general toughness, among others. Likewise, a crucial element of the survey of a ship, is the inspection of the coating, which is often the first element in the ship to indicate the onset of a problem in the overwhelming majority of situations. 
     For these reasons, the issuing and renewal of certificates of seaworthiness for ships is dependent upon a rigorous inspection of the hull of the ship in which certain characteristics are checked throughout the entire hull. In particular, the condition of the hull is checked for corrosion and/or irregularities. The nature and amplitude of any observable dents in the surface of the hull is also examined. 
     Generally, these inspections are carried out in dry dock. The experts of the official bodies authorized to carry out these inspections meticulously inspect, square meter by square meter, the hulls of ships which are submitted for inspection. 
     Putting a ship into dry dock is a very time consuming and expensive task to perform and results in greatly increasing the cost of maintaining a ship. This cost is particularly large in the case of a large container ship where each day out of service costs the owner tens of thousands of dollars, or in the case of floating production platforms, where the cost due to lost production can be orders of magnitude higher. 
     The disadvantages of this process are compounded in the case of high-tonnage ships, whether these be, for example, oil tankers, ore carriers or container ships, because the sizes of dry docks are becoming less and less able to accommodate the growing size of such ships, because the large commercial ports with a dry docks that are capable of accepting the ships are ill-equipped, and because of the actual cost of taking the ships out of service. 
     As a result of these problems, there has been proposed a process involving the installation of an apparatus incorporating a transceiver-carrying transport device or carriage which is capable of moving underwater over the surface to be inspected. In particular, the system described in the background of U.S. Pat. No. 4,308,600 contemplates a carriage which is capable of moving on the surface of the submerged structure to be inspected. The carriage carries a transducer, for example a television camera, which is connected in a closed circuit to a receiver located on the surface. The receiver is suitable for producing an image of the surface which the carriage is scanning. 
     In principle, such a system is intended to carry out, from a location on land, the multiple checks to which the hull of a ship must be subjected. Most importantly, it is intended to do this while the ship is afloat, thus avoiding the costs associated with submitting the ship to a costly dry dock procedure. 
     However, the inspection carried out in accordance with this system suffers because the position of the carriage or operating apparatus used on the surface over which it is moving cannot be determined very exactly at any instant. This information is of value, for example, in order to make it possible to make a comparison between deterioration measurements made by transceivers carried by the apparatus, during successive statutory surveys. Thus, dry dock surveys of hull condition remain the standard. 
     However, as alluded to above, present world facilities simply cannot accommodate the number of ships which need to be inspected. Accordingly, there are numerous vessels on the sea today which are far behind their inspection schedules. As is apparent, the same poses the possibility of many problems, some of potentially devastating magnitude. 
     More particularly, in addition to the cost attendant to the mechanically inefficient operation of a vessel, failure to perform maintenance on a preventative basis will, sometimes, have a dramatic impact on the cost of a repair. This is because of the fact that if a small problem is left unattended, continued corrosion will cause it to become a serious problem. Repair costs will be far in excess of the cost of a repair made when deterioration is just beginning to occur. More seriously, however, if the problem is left unattended, it may result in weakening the structure of the vessel to the point that the application of relatively routine forces will cause the structure to break down, not only again increasing the cost of repair, but also posing the potential for loss of the vessel and, far worse, environmental disaster. 
     As a consequence of the above problems, including the expense of conventional dry dock inspection procedures, attempts are being made to overcome the disadvantages of present submerged vessel inspection systems and processes. In this regard, the assignee of this application has proposed that the high resolution acoustic positioning systems developed by the assignee be employed in an underwater inspection. 
     Patent Cooperation Treaty International Application No. P. C. T./N 094/00060 discloses a diver delivered unit which may be operated directly by a diver or otherwise in a measurement area marked by a plurality of lights. The unit is delivered to the area of the hull of a ship afloat in a harbor by a diver. In accordance with the preferred embodiment described in this application, the unit is moved over the hull of the ship by a diver, although the possibility of moving the unit on the surface of the hull without a diver is vaguely alluded to. While this disclosure is somewhat general, it appears to suggest that magnets may be used to adhere the unit to the hull of a ship while the unit is being propelled along the surface of the ship by thrusters. 
     As alluded to above, the above Treaty filing suffers from the disadvantage of being dependent upon diver delivery. Moreover, the use of a plurality of lights to guide the system is an additional task with its own attendant cost and calibration problems. The use of magnetic wheels, if that is what is meant, would appear to present problems on account of surface contours, loss of engagement, and other factors. In particular, if there is a loss of engagement, it is likely that the gap created by the same will exceed the distance over which the magnets will operate to restore contact. 
     SUMMARY OF THE INVENTION 
     The invention, as claimed, is intended to provide a remedy. It solves the problem of how to provide a reliable, economical and consistent way of inspecting the hull of a ship without resorting to the costly procedure of dry docking the ship. 
     Moreover, there are additional problems which the systems, described in the above background, do not even attempt to address. Even if the components which form the hull of the ship are in good condition, there may be other problems which, in time, may cause a hull to fail. For example, if a vessel is subjected to a large force, for example by being run aground, the shape of the overall hull may be distorted. Accordingly, the profile of the hull moving through the water may not be symmetrical and may result in the concentration of forces of unusual and unanticipated magnitude at particular point or points. This could, in time, have the effect of causing a serious failure at that point or points. Thus, examination of the overall shape of a vessel, provided in accordance with the present invention, will yield useful information about the condition of the ship. Such information comes as a cost free bonus in accordance with the method of the present invention. 
     As will be described below, in accordance with the preferred embodiment of the invention, the above disadvantages of the systems in the literature are addressed. More particularly, the system of the present invention comprises a remotely operated vehicle (ROV) which can be introduced into the waters of a port from a position on the land (or even at or close to the site of an accident possibly far at sea). The ROV is equipped with inspection cameras, position sensing equipment and non-destructive test equipment adapted to inspect and measure the characteristics of the submerged hull to be inspected. The non-destructive test equipment provides measurement of key vessel parameters, including plate thickness, plate pitting, plate delamination, plate fracture, coating thickness, and cathodic protection device potential. An integrated data set, comprised of all measurements correlated in time and hull position, is generated and recorded at a controlled spatial resolution on the hull. The ROV is adapted to propel itself toward and over the surface of the submerged hull of the ship, being guided by either a human operator who steers the ROV from the control console (which may be on land or on the ship being inspected) using an on board camera and acoustic navigation system, or guided automatically by a computer which is programmed to execute a stored inspection sequence over the submerged hull. In accordance with the preferred embodiment, the position of the ROV on the hull provided by the acoustic navigation system may be initially determined by the placement of three or more electro-acoustic position transducers at known points on the hull of the ship. It is also possible in accordance with the present invention, to steer the ROV over the entire vessel, and then to process the electro-acoustic position information to create a database which comprises the shape of the vessel. This information can then be used both to create inspection results “maps” and to generate automatic scan plans of the submerged portion of the hull for subsequent inspections. 
     It is also contemplated that in accordance with the invention, after an initial survey of the full ship hull, areas which are suspect can be subjected to a slower more detailed and/or more comprehensive inspection at finer spatial resolution, and possibly with different NDT instrument settings optimized for the local conditions. 
     ROV adhesion to the surface of the submerged hull is achieved by employing a Bernoulli effect for steady state, incompressible flow across an orifice. By flowing water across an orifice formed at the interface of one or more dedicated suction zone(s) and the hull surface under and around the suction zone, a pressure drop is created in the direction of water flow. The reduced pressure within the suction zone(s) develops a suction force proportional to the product of pressure drop and suction zone area. The suction force is largely normal to the hull at the suction zone, and in a direction which results in the suction device adhering to the hull. The interface of the suction zone and the hull is formed using a compliant skirt which provides good conformance to rough, uneven or undulating hull surfaces, while generating minimal drag and compression forces during ROV motion. 
     It is also noted that in accordance with the present invention, movement of the ROV along the surface of the hull of the ship is achieved through a combination of thruster power and wheeled propulsion. The result is the positive control of position by virtue of the contacting drive wheels and drive augmentation provided by the thrusters whose thrust assists the wheeled propulsion, both in direction and magnitude. The thrusters and the driven wheels can share a common power plant, with the drive wheels driven directly by the propulsion thruster motors via mechanical connection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One way of carrying out the invention is described in detail below with reference to drawings which illustrate a specific embodiment of the invention: 
     FIG. 1 is a perspective view of a ship docked at a facility equipped with the apparatus of present invention; 
     FIG. 2 is a plan view of a ship being inspected in accordance with the invention; 
     FIG. 3 is a cross-sectional view of a ship being inspected with the present invention along lines  3 — 3  of FIG. 2; 
     FIG. 4 is a schematic diagram of a position detecting system employed in the present invention; 
     FIG. 5 is a schematic diagram illustrating the electronic inspection system carried by a ROV inspection device constructed in accordance with the present invention; 
     FIG. 6 is an elevational view of the ROV of present invention; 
     FIG. 7 is a plan view of the inventive ROV along lines  7 — 7  of FIG. 6; 
     FIG. 8 is a side elevational view of the inventive ROV along lines  8 — 8  of FIG. 7; 
     FIG. 9 is a detailed view of the right thruster of the inventive ROV along lines  9 — 9  of FIG. 8; 
     FIG. 10 is a detailed view of the thruster of FIG. 9 inventive ROV along lines  10 — 10  of FIG. 9; 
     FIG. 11 is a detailed view of the thruster of FIG. 9 inventive ROV along lines  11 — 11  of FIG. 10; 
     FIG. 12 is a detailed view of the thruster of FIG. 9 inventive ROV along lines  12 — 12  of FIG. 9; 
     FIG. 13 is a elevational view of the NDT sled of present invention; 
     FIG. 14 is a plan view of the inventive NDT sled along lines  14 — 14  of FIG. 13; 
     FIG. 15A is a side elevational view of the inventive NDT sled along lines  15 A— 15 A of FIG. 14; 
     FIG. 15B is a side elevational view of the inventive NDT sled along lines  15 B— 15 B of FIG. 14; 
     FIG. 16 is a flow chart illustrating the inventive process for inspection of a submerged structure; 
     FIG. 17 is a flow chart illustrating the inventive process for post processing and data reporting of data gathered by the inventive process of FIG. 16; 
     FIG. 18 is a flow chart illustrating an alternative embodiment of the inventive software that controls the inventive apparatus; and 
     FIG. 19 is a detailed drawing of a portion of the software of FIG.  18 . 
     The inventive method and apparatus will be described with respect to a ship hull inspection, with the understanding that the present inventive method and apparatus may also be used to inspect oil rigs and other submerged structures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a ship  10  having a hull  11  or any other submerged structure requiring periodic inspection such as a dam, may be equipped with an inspection system  12  constructed in accordance with the present invention. Inspection system  12  comprises a topside computer controlled operator interface console  14 , a plurality of underwater transceivers  16  and a remotely operated vehicle (ROV) or robot, which functions as ROV  18 . The underwater transceivers  16  are preferably mounted directly to the hull using one or more methods, however, the transceivers may also be mounted to fixed land based features (sea bed, pier pilings, etc.). In the latter case, it is also necessary to know the position of the ship with respect to the fixed transceiver network comprising transceivers  16 . In this case, it is necessary that a number of transceivers  19  sufficient to completely define the position of the ship be used. Such transceivers  19  should be secured to the hull  11  of the ship, and, in accordance with the present invention, are secured at minimum three points. See FIGS. 2 and 3. 
     Generally, the inventive system can operate under the control of a human operator at console  14 . Console  14  includes a computer network  20  (sometimes referred to herein as the “computer”), a keyboard  22 , an appropriate monitor  24  and an analog input device such as a joystick  26 . The system also includes a number of mass data storage units (magnetic disk drives, magneto-optical drives, etc)  28  which may be used for program and data storage and exchange. 
     Power to the system is provided by the power mains, which may be derived from shipboard power, pier-side power, or stand-alone generator sources. The power mains provide power for the integrated system, including topside controls, the underwater ROV, and acoustic transceivers. The underwater ROV  18  is powered through an electrical cable (tether)  30 . Likewise, transceivers  16  and  19  are also powered by appropriate electrical cables  32  connecting the transceivers to the topside control consoles  14 . In addition to carrying power, tether  30  and cables  32  also carry information, control and associated signals, as will be set forth in detail below. 
     Underwater ROVs, which are capable of movement underwater are known. Likewise, the systems for controlling thrusters on board such ROVs to achieve desired movement of the ROVs are also known in the art. Such ROVs incorporating such control systems are sold, for example, by the assignee of the present application. Such underwater vehicles may be modified in accordance with the teachings of the present specification and be employed in the system of the present invention to carry navigation equipment and appropriate inspection devices. 
     As can be seen most clearly in FIG. 3, transceivers  16  are positioned underwater at a plurality of points which are selected to provide acoustic line-of sight communication for the emission of an acoustic signal from every point on the submerged hull  11  of the ship to transceivers  16 . In accordance with the invention, a sufficient number of transceivers  16  are placed so that this can be achieved with respect to at least three, and preferably more of the transceivers  16  from any point on the submerged hull. 
     More particularly, the hull of a ship  10 , submerged in water  34 , is surveyed by ROV  18  in accordance with the present invention. One or more acoustic pingers  38  on ROV  18  receives a signal from the console  14 , which causes it to emit an acoustic signal, which is received by transceiver  16 . The distance between the transceiver  16  and the pingers on ROV  18  can then be calculated by computer network  20  based upon a calibrated speed of sound in the water and the acoustic travel time, i.e., the time between the emitted acoustic pulse at the pinger and the received pulse at the transceiver. Once the position of the pingers on the ROV are known, it is possible to calculate the position and orientation of ROV  18 , and further, because the position of each inspection device on the ROV is also known with respect to the ROV frame of reference, it is also possible to know the exact position of each inspection device and, accordingly, the specific points on the hull  11  under inspection. 
     A high accuracy sonic (acoustic) navigation and positioning system of the type sold under the trade name SNAP by the assignee of this application and specifically designed for precise underwater tracking of underwater ROVs and providing data at high update rates in high multipath environments is employed in the system of the present invention. At 100 meters, ROV pinger to transceiver range (distance) errors are less than 2 cm, repeatability is better than 0.5 cm., and sampling rates up to 10 Hz can be achieved with such prior art systems. Such high accuracy navigation systems are adaptable for use in rivers, lakes, ponds, or the open ocean, but are exceptionally well-suited to high multipath environments such as harbors, enclosed tanks, dams or tunnels, under ice cover, beneath ships, and around submerged structures. 
     Such high accuracy navigation systems are based on a set of compact and durable acoustic transceivers  16  in the water around ship  10 , transceivers  19  on ship  10 , and pingers  38  on ROV  18 , each independently capable of transmitting and receiving high-frequency acoustic pulses. A computer network  20  controls the acoustic network of transceivers and is used for data acquisition, and for the processing, display, and output of position data. Each transceiver is connected to the system controller through a twinaxial cable  32  which provides power and a data transfer path, as shown FIG.  1 . In a typical application at least three transceivers  16  are deployed to form a stationary baseline array and one or more, but preferably three, hull mounted transceivers  19  are tracked by the stationary baseline array. 
     Each transceiver in this high accuracy navigation system is independently controlled to transmit or receive acoustic pulses with an approximately hyper-hemispherical beam pattern. The transceivers are mounted in rugged, cylindrical, stainless steel, sealed pressure cases which allow easy deployment in a wide variety of applications. 
     The high accuracy of the navigation system is due to the use of broadband high frequency acoustic pulses centered at 300 kHz. As noted above, operating at the maximum range of 100 meters the range data is accurate to within at least +/−2 cm, and repeatable to within 0.5 cm. In order to obtain the accuracy stated above, speed of sound errors must be limited to 1 cm at 100 meters or 1 part per 10,000. Repeatability is not affected by sound speed errors, but will be affected by variations in the speed of sound over a period of hours or days. 
     This high accuracy navigation system is ordinarily operated in a “long baseline” mode. Other types of typical long baseline systems operate at much lower frequencies and consist of an array of transponders covering an area of 1,000 to 10,000 square meters. The ‘baseline’ transponders are usually securely positioned on the sea floor using tripods, or allowed to float above the sea floor on moorings. System accuracy is dependent upon knowledge of these baselines, and unobstructed line-of-sight is required between the baseline transponders and the target to be positioned. There are two main differences between typical long baseline systems and the high accuracy sonic navigation system of the type used in the present invention. The use of high frequency signals limits the range (100 meters, typical at 300 kHz) and each of the baseline array elements is hard-wired to the computer. 
     In order to maintain the distinction between the cabled elements of the high accuracy navigation system arrays of the type used in the present invention and the “wireless” elements of an ordinary long baseline system, we refer herein to the high accuracy acoustic devices of the type employed in the present invention as “transceivers” rather than transponders. A wired baseline array forms a network of transceivers which can be referred to as a “net”, and the individual transceivers can be considered as “net elements”. 
     To obtain a position fix, the high accuracy navigation system first commands the target pinger  38  to transmit a pulse. This pulse is received by the three (or more) network transceivers  16 , and the one-way travel times are determined. Using the estimated sound speed, these travel times are converted to slant ranges (distances). The slant ranges are then converted to the Cartesian (x,y,z) positions by knowledge of the geometry of the net elements. The computed position and orientation of the array can be adjusted during calibration so that these x,y,z positions are reported in a global coordinate system or one which is referenced with respect to some important features of the ship&#39;s hull  11 . 
     The pingers  38  are the sonic devices which the high accuracy navigation system uses to determine relative range and position of ROV  18 . Transceivers  19  are used to determine ship position. Transceivers  16  form the stationary array to which all ranges, from both ROV pingers  38  and hull-mounted transceivers  19 , are measured. All the transceivers are interchangeable, as each can transmit and receive acoustic signals. Thus, any of the transceivers may be used in the baseline array of transceivers  16  fixed with respect to the sea floor. 
     The transceivers are connected to the twinaxial subsea cable  32  consisting of a 100 meter subsea twinaxial cable and a five meter deck cable. Longer subsea cable and deck cable lengths may also be employed. The deck unit provides the connection between the cable  32  and host computer network  20 . 
     A depth sensor may also be used to measure the depths of the transceivers in the baseline array so that the coordinate system can be adjusted to be level. It can also be used during tracking as a substitute for the high accuracy navigation system acoustic indicated depth or as an input to a filter which combines the two depth measurements to obtain an improved depth estimate. 
     A tripod  40  (FIG. 3) may be used to mount the baseline transceiver in applications requiring seabed installation. Telescoping legs allow stable installation in a rocky seabed, and a rotating and tilting mount supports aiming of the transceiver tip so as to optimize signal reception. Custom tripods or mounting fixtures can be designed for other applications. 
     A more detailed understanding of the operation of the present invention may be achieved with reference to FIG.  4 . Generally, the inventive system  12  comprises a computer network  20 . Computer network  20  includes one or more computers for ROV navigation and ROV control (including position control). In the embodiment illustrated in FIG. 4, the ROV navigation and control functions are provided by two computers, the navigation computer  42  and a ROV position control computer  44 . In a configuration consisting of a single computer, these two functions, navigation and control, would be operating as two separate processes, or threads, within that computer. 
     Navigation computer  42  interfaces with appropriate navigation electronics contained on a board  46  adapted to send ROV pinger target position information to computer  42  and receive control signals from computer  42 . Navigation electronics  46  are also coupled to the in-water navigation electronic systems associated with ROV  18 . Coupling of in-water systems associated with ROV  18  to navigation electronics  46  is achieved using tether  30 , as is illustrated in FIG. 4, which illustrates the principal elements of the electronic system, and the paths and directions used by information and drive signals during operation of the system. 
     Console  20  also includes digital data interface boards  52  which are coupled to an ROV position control computer  44 . Likewise, boards  52  are coupled to vehicle control signal lines which connect to the ROV  18  over tether  30 , via the vehicle power supply  54 . Information may be input into computer  44  through boards  52  by a joystick  26  whose output is switched by a switch  58  to either boards  52  or is connected directly to vehicle controls via the power supply  54 . This switch  58  provides a method for the operator to control the vehicle manually without computer intervention, or the control computer  44  is capable of operating the vehicle in automatic mode, wherein the ROV is driven under software control through a course which is dynamically programmed. This course is determined by a series of waypoints determined by the operator, and the software controls the ROV to drive a track line, or trajectory, from one waypoint to the next. 
     Vehicle power supply  54  outputs an electrical power signal which is coupled by tether  30  to the ROV  18 . This electrical power signal provides the power to operate the ROV  18 , including power for its vehicle movement systems, its inspection systems, and its position sensing systems. 
     Interface between the operator and the console is provided by a monitor  24  and a keyboard  22 . A single monitor and keyboard can service the navigation computer  42  and a ROV position control computer  44 . 
     The in-water systems include ROV  18  and the on-board propulsion, navigation and inspection systems. More particularly, the ROV  18  carries appropriate inspection and navigation instrumentation, including inspection devices  70 , an orientation sensor  72  and a depth sensor  74 . The output of inspection devices  70  is communicated by tether  30  to an inspection computer  76  which is coupled to dedicated monitor  25  and keyboard  23 . Output from ROV navigation devices and all other on-board devices is communicated via the tether interface  60  and tether  30  to ROV position control computer  44 . The ROV  18  also incorporates two or more pingers  38 , each of which includes a transducer  68 . A navigation camera  80  is also coupled to tether interface  60 . The navigation camera  80  function is augmented by an electrically controllable tilt unit  73 , illumination lamps  75  and with laser projectors  71 . The tilt unit can point the camera and lasers through an angular arc of  180  degrees. The lasers are mounted to either side of the navigation camera, precisely aligned to the axis of the camera, to provide a visual measurement reference within the field of view of the camera image. The in-water systems for ROV  18  are completed by a drive plant  78 , which provides the motive power for movement of ROV  18 . 
     As has been noted above, the function of the navigation system illustrated in FIG. 4 is to determine the position of the ROV  18  by determination of the positions of ROV mounted pingers  38 . Because the shape of the hull  11  in most small areas is relatively flat, and the configuration of the ROV  18  is known together with the position of pingers  38  on ROV  18 , this determination of the position of pingers  38  at a large number of locations of ROV  18  on the hull  11  of ship  10  will provide the location of a large number of points on the hull  11  and thus the three dimensional shape of the hull  11 . 
     Likewise, for each of these points, measurements of hull  11  plate thickness, coating thickness, cathodic protection device potential and other measurements taken on the surface of the hull  11  of the ship may be associated with these points in alphanumeric printouts (reports), topographical maps, and other 2-D and 3-D visualization methods illustrating and documenting the condition of the ship. Such images may be enlarged and rotated in space, as required to provide the desired degree of specificity for examination of a particular condition on a particular ship. 
     Transceivers  16  each are comprised of a transducer  68  which can both transmit and receive. Transducers  68  on transceivers  16  or transceivers  19 , when driven by an appropriate electrical signal, will emit an acoustic signal. Transducers  68 , also on transceivers  16  and transceivers  19 , when excited by an acoustic signal will output an electrical signal to board  46 . The signals received and transmitted by transceivers  16  and  19  are connected via cables  32  to navigation electronics  46 . 
     Generally, the determination of position information begins with pulses output by acoustic transducer  68  on ROV mounted pingers  38 . The signals which drive pingers  38  are sent over tether cable  30  from the navigation electronics  46  which generates those signals. 
     In accordance with the preferred embodiment of the invention, transceivers  16  and  19 , and pingers  38  all operate at the same acoustic frequency. Such operation, beginning with the determination of the ship&#39;s position relative to the fixed transceiver net, includes the stimulation of one of the fixed transceivers  19  to cause its transducer  68  to emit an acoustic pulse at the desired acoustic frequency. This pulse is received by all of the net transceivers  16 . Each transceiver  16  will then signal the decoding circuitry in the navigation electronics  46  that it has received an acoustic pulse. The time between the emission of the pulse from the particular transceiver  19  and its reception of that pulse at each of the transceivers  16  gives the distance between all transceivers  16  and the particular transceiver  19 . 
     Using the well-known principle of triangulation, the position of the particular transceiver  19  in space may then be determined. In principle, it is not necessary ranges to more than three transceivers  16 , but using four or more ranges does afford the opportunity to obtain redundant information and check for errors or improve accuracy and repeatability. 
     Once the position of the first of the transceivers  19  is known, the above process can be repeated for a second of the transceivers  19 . The process is also repeated again for a third of the transceivers  19 . Knowing the position of the three transceivers  19  attached to the hull  11 , provides a unique determination of the position and orientation of the hull with respect to the fixed transceiver net  16 . 
     In similar fashion, transceivers  16  are used to determine the position of each of the pingers  38  on ROV  18 , and thus determine the position of the ROV and the corresponding points  77  (FIG. 8) on hull  11  being inspected and measured. This is done by each of the pingers  38  sending an acoustic pulse to all of the transceivers  16 , and then repeating the process for each of the remaining pingers  38  on ROV  18 , using triangulation to determine the position of each of the pingers  38 . Computer  46  then computes the position of the measurement instrumentation on ROV  18  using the same triangulation techniques it uses to determine the position of transceivers  19  and hull  11 . Since the position of the hull  11  is known, and its shape is known, the position of the instrumentation inspection points  77  on the hull  11  of the ship may be determined. 
     At that point, the positions of each of the transceivers  19  is known. The position and orientation of the ship is also known, and data collected over the cable during or about the time of the position determination operation can be associated with a particular point  77  on the hull  11 . 
     The above operations are performed through the use of the navigation electronics board  46  which sends information to the navigation computer  42 , which in turn calculates the position of the various targets. The corresponding position information, in x,y,z Cartesian form is sent to the ROV position control computer  44 . In automatic mode, computer  44  sends drive control information to the drive plant on the ROV  18  via the tether  30 . The computer software calculates the appropriate drive control values by comparing the current position and orientation of the ROV to the desired position and orientation and computing drive values which cause the measured difference, or position state error, to be minimized. In manual mode, the operator performs this function by monitoring the ROV position and driving it by use of the joystick  26 . 
     The details of the inspection devices attached to the ROV  18  are illustrated in FIG.  5 . On board inspection devices include one or more each of ultrasonic test (UT) measurement systems, eddy current test (ET) measurement systems, potential field measurement systems, and video inspection systems. Other measurement technologies may also be employed in similar fashion. 
     The ultrasonic measurement system employs one or more non-contact, water path coupled transducers  82 . When employing multiple transducers, the devices can be configured to operate at different water path distances, ultrasonic frequencies, focus characteristics, and orientations relative to the local hull surface to optimize the transducer performance for a specific desired measurement. Moreover, the electrical characteristics of the signals which drive the transducers  82  may be varied over time to achieve a wide range of functions g from a single transducer. Some of the transducers  82  may generally comprise an ultrasonic emitter and an ultrasonic receiver for, respectively, emitting a test signal and receiving the reflected measurement signal. 
     The eddy current measurement system may include one or more eddy current transducers  90 . Potential measurement system includes one or more transducers  93  for cathodic protection device potential measurement, as well as other measurement devices  94 . 
     Finally, the system also carries an inspection camera  96  which enables visual inspection of the hull  11  of the ship  10 . Camera  96  also includes appropriate controls for varying the nature of the visual inspection. More particularly, camera  96  is of the type which has electrically controllable camera pan  97 , electrically variable zoom functions and electrically controllable focusing. Operation of camera  96  can also be enhanced by provision of inspection lighting  98 . 
     Referring to FIGS. 6 through 8, the construction of ROV  18  is illustrated. ROV  18  comprises an outer body  100  which functions as a protective housing for the ROV internal components, and as the pressure boundary between the internal suction plenum  120  and external ambient pressure  121 . The frame member  116  includes mounting provisions for the internal components and the outer body  100 . The outer body  100  is removable to facilitate instrument calibration and vehicle maintenance. When assembled on frame  116 , outer body  100  forms a tight seal at the mounting interface to minimize fluid leakage into the suction zone. 
     Internal components include one or more vehicle and non-destructive test equipment control modules  118  and  119 , internal inspection camera  96  with pan unit  97  and integrated lighting  98 , and non-destructive test instruments with associated fixtures  123 . It is also possible for the ROV  18  to carry other sensors and transducers, both internal and external to the outer body  100 . Vehicle navigation lights  75  may be mounted internal to the ROV  18  in the positions illustrated in FIG.  8 . The navigation lights will illuminate the area in front of the vehicle by projecting light through cutouts in the outer body  100 . As noted above, ROV  18  carries a navigation camera  80 , with integral tilt unit  73 , and two laser sources  71  for calibration of dimensional gauging system. The two laser sources project parallel rays of laser light, with fixed, known spacing, onto the surface to be inspected. Surface features can be dimensionally gauged by imaging the feature of interest with the navigation camera  80 , then scaling the image from the visible laser “dots” projected on the image by the two laser sources. 
     Suction plenums include one or more suction thrusters  122  which flow water from the suction plenums  120  in the directions indicated by arrows  124 , and compliant skirt  127  mounted around the perimeter of the suction plenum  120 . The compliant skirt  127  provides the interface between the suction plenum  120  and the hull, and functions to provide good conformance to rough, uneven or undulating hull surfaces, while generating minimal drag and compression forces. The result of the suction thrusters  122  in conjunction with the suction plenums  120  and compliant skirt  127  when the ROV is positioned or moving over the hull  11  of a ship, is to create a flow of water “under” ROV  18  in the directions indicated by arrows  126 , as is illustrated most clearly in FIG.  8 . The flow of water through the orifice formed by the intersection of the compliant skirt  127  and the hull surface results in a pressure drop in the direction of flow. Thus, suction plenum  120  will operate at reduced pressure relative to the external ambient pressure  121 , causing ROV  18  to be forced by the pressure differential against the hull  11  of ship  10 . 
     Free-flying and on-hull motive power is provided by drive propulsion modules  102 . ROV  18  also comprises a third wheel  114 , which is mounted for free rotation on a wheel support which rotates either freely in response to vehicle motion (passive wheel) or in controlled fashion in response to a control signal from the console to provide a steering function by movement of the wheel support subassembly in the directions of arrow  142  (actively controlled wheel). Wheel  114  is not driven, but merely serves to stabilize movement of the vehicle  18  on the surface of the hull  11  of the ship. 
     The construction of the drive propulsion module  102  is illustrated in detail in FIGS. 9 through 12. The construction of the drive propulsion module includes include thrusters  104  and drive wheels  106 . Each of the thrusters  104  comprises a drive unit  130  coupled to a turbine  108 . Each turbine  108  includes a propeller  110  and a nozzle  112 . Power to the system is provided by a drive unit  130 . Drive unit  130  can be any prime mover, including electric motor, hydraulic motor, or other. Thruster propeller  110  can be directly mounted to the rotational output shaft of drive unit  130 , or mounted as a rotational element on a fixed support shaft  111  and magnetically coupled to the prime mover  130 . Thruster nozzle  112  is mechanically supported from multiple struts eminating from the drive unit housing. 
     A drive pulley  132  and belt  133  couples power from the thruster  104  to a reduction gearbox  134 . Belt  133  passes through a hole or slot  135  in thruster nozzle  112 . Reduction gearbox  134 , in turn, couples power to a drive shaft  136  which drives gears  138  and  140 . Gear  140  is directly coupled to wheel  106 . 
     In accordance with the present invention, ROV  18  is adapted and controllable to move in various directions corresponding to clockwise and counterclockwise movement of traction wheels  106 . Likewise, steering may be provided by active rotation of follower wheel  102  in the directions indicated by arrow  142 . 
     The NDT transducer fixtures (NDT “sled” assemblies)  123  are illustrated in FIGS. 13 to  15 . The NDT sled assemblies  123  maintain fixed transducer spacing (height above hull) and orientation relative to the local hull surface. The NDT sled assemblies are rigidly attached to the vehicle frame  116  via mounting plate  141 , and are spring loaded against the hull surface via four-bar linkages  142  and springs  143 . The NDT sled assembly base  144  is attached to four-bar linkage  142  via pivot pins  145  and  146 . Pivot pins  145  permit limited (+/−30 degreees) pitch rotation (relative to horizontal) of the base assembly. Likewise, pivot pins  146  permit limited (+/−30 degreees) roll rotation (relative to horizontal) of the base assembly. The base includes  3  rolling or sliding contacts  140  which contact and move relative to the hull surface with low sliding or rolling friction. The 3 point hull contact provided by  140 , in conjunction with the base pitch and roll degrees of freedom and spring loaded four-bar linkage permits the base  144  to assume an orientation parallel to the local hull surface as it travels over the hull. 
     Gimbal slide  147  is mounted to the base unit  144  in perpendicular fashion, as illustrated in FIG.  13 . Gimbal slide  147  cross-section is T-shaped to accept mating transducer gimbal assembly  148  The transducer gimbal assembly  148  is free to slide vertically on the gimbal slide  147  and locked in position via set-screws at a desired height above the base unit  144 . The transducer gimbal assembly  148  includes transducer clamp  149  for retaining cylindrical cross-sectioned transducers. The transducer clamp  149  couples to the gimbal slide  148  via pitch gimbal  150 , and roll gimbal  151 . The roll gimbal  151  contains a T-slot for mating to gimbal T shaped slide  147 . Each gimbal axis includes knurled adjustment knobs connected to worm gearing for manual adjustment of the transducer orientation. The combined pitch and roll gimbals connecting the transducer clamp  149  to the gimbal slide  147  permits orientation of the transducer relative to the hull surface at a desired angle of operation. The gimbal slide and gimbal assembly  148  with transducer permits the distance between the clamped transducer face and the local hull surface to be adjusted by moving the gimbal assembly vertically on the slide, then locking it at the desired height. The NDT sled base  144  incorporates a through hole directly under the gimbal assembly to permit a direct water path between the clamped transducer and local hull surface. 
     Standard ultrasonic test transducers for metal thickness gauging are readily adapted to underwater service. The cylindrical shape of the ultrasonic transducers permits direct mounting in the NDT sled gimbal assembly clamp  149 . The water path for the ultrasonic transducer is adjusted via the gimbal slide using the procedure describe above. 
     Standard eddy current transducers for coating thickness gauging and metal fracture detection are typically not rated for continuous underwater service. Additionally, for accurate coating thickness measurement, the flat face of the cylindrical eddy current transducer must maintain contact with the hull surface. The eddy current probe is adapted for this invention by housing it in a cylindrical water-tight tube, with replaceable end cap which maintains contact with the hull surface during the inspection process. The replaceable end cap is mad of suitable plastic material which provides low sliding friction, yet exhibits good resistance to abrasion. Once assembled in the water tight tube and placed in contact with the hull surface, the spacing between the eddy current transducer face and the hull is controlled by the thickness of the plastic endcap material between the hull contact face and the mating, internal transducer contact face. The water tight tube assembly with eddy current probe is spring loaded against the hull surface with enough force to maintain transducer contact with the hull. The NDT sled gimbal  148  assembly is adapted for the spring loaded eddy current transducer. A fixed clamp with mating T-slot is attached to the top of the gimbal slide  147 . The gimbal assembly  148  with clamped eddy current probe in watertight container is mounted on the gimbal slide  147  in normal fashion, but the gimbal assembly  148  is not clamped in place, leaving it free to move in a vertical fashion on the slide. A compression spring is located between the fixed clamp and gimbal assembly  148 , applying constant force to the eddy current water tight tube assembly in the direction of the hull surface. The compression spring is constrained by an internal metallic pin. The metallic pin is attached to the gimbal assembly  148  immediately adjacent to the gimbal slide  147 , and running vertically parallel to the gimbal slide  147 . The metallic pin is guided by and extends through a matched hole in the fixed clamp at the top of the gimbal slide  147 . The compression spring is mechanically constrained throughout it&#39;s length by the internal pin and at it&#39;s ends by the gimbal assembly  148  and fixed clamp mounted to the top of the gimbal slide  147 . 
     When it is desired to employ the system of the present invention, a ship  10  is brought into a facility where it may be docked in a stable manner. The inspection process can be carried out with the ship docked along a pier, moored within a harbor, or moored at open sea. After the ship  10  is securely docked or moored, a number of transceivers  16  are deployed at positions fixed with respect to the shore and the sea floor. Any convenient method of secure connection may be employed to achieve this result. In particular, transceivers  16  may be secured directly at positions on the bottom or the sides of the harbor or near the shore. A tripod  40  may also be used to position a transceiver  16  where the same may be otherwise difficult. Likewise, transceivers may be attached to piles  152  as illustrated in FIG.  2 . The positions of transceivers  16  are selected to provide for line-of-sight acoustic communication between the transceivers  16  and similar transceivers on ROV  18 . Once this has been achieved, the position of the ship may be determined by the placement of transceivers  19  on the ship, for example, at the positions illustrated in FIG. 2 along the centerline of the ship and about 2 meters below the surface. 
     After the fixed transceivers  16  have been placed in the desired positions, and the position of the ship made possible to determine through the placement of transceivers  19  on the hull  11  of the ship, ROV  18  is introduced into the water and caused to swim, unmanned, to the hull  11  of the ship through the use of its thrusters  104 , which propel it through the water to the hull  11  of the ship. Complete control of the flight path of the ROV  18  during the transit to the hull is achieved by varying the propulsion thrust in a forward/aft direction and vertical direction, and by varying the thrust generated moments for yaw, pitch and roll. Forward/aft directional control is provided by the drive propulsion modules  102  acting in unison. Vertical directional control is provided by the suction thrusters  122  and  128 . Vehicle yaw control is provided by asymmetrical operation of drive propulsion modules  102 . Likewise, vehicle pitch and roll control is provided by asymmetrical operation of suction thrusters  122 . 
     During the flight from the point of deployment to the hull  11  of the ship, ROV  18  is caused to swim underwater. Navigation may be automatically controlled by the computer network  20  during the initial part of the swim, or the same may be completely manually controlled. During the portion of the swim during which ROV  18  “lands” on the hull  11  of ship  10 , the same may be performed under manual control of an operator at the console. The operator is presented with visual data during this part of the operation through the use of the navigation camera. In principle, the same may be assisted through the use of sonar or other appropriate instrumentation. 
     If the shape of a ship being surveyed is known, for example, based upon the plans used when it was being built, or based upon another survey conducted in accordance with the present invention, the ROV  18  may be caused to move automatically along a predetermined path. If, on the other hand, no such information exists, various instrumentation on the ROV may be used to cause it to move along a path defined by the intersection of the hull  11  of the ship  10  and the surface of the sea. After this shape has been defined, the ROV  18  can be caused to travel along paths at successfully lower levels until the entire surface of the ship is covered. Likewise, the inspection path can be controlled to follow lines of constant girth, with the vehicle traveling between port side water line and starbord side waterline, advancing bow to stern or stern to bow at the completion of each waterline to waterline pass. 
     The inspection of the hull  11  is carried on both electronically and visually during the course of such movement. It is contemplated in accordance with the present invention, because of the shape of many hulls  11 , operator intervention and/or guidance would be a part of the control of the movement of ROV  18 . Such operator intervention is guided by readings on instruments which show the depth, position and orientation of the ROV as well as the display of the environment surrounding the ROV  18 , which display is provided by the navigation camera  80  and inspection camera  121 . After the survey and primary inspection of the hull  11  has been carried on with reference to the information produced by the various inspection devices on board the ROV  18  and with reference to the visual inspection of the hull  11  provided by the navigation and inspection cameras, problem areas on the hull are likely to be identified and targeted for a slower, and perhaps slightly or dramatically different, inspection protocol. 
     The overall inspection process is outlined in FIG.  16 . Referring to FIG. 16, the process begins by determining at step  156  whether there is an existing 3-D model of the shape of the hull  11  of the ship. If such a model exists, the software proceeds from step  156  to step  162  where it is determined if the inspection path is to be pre-programmed “off-line” (prior to the inspection). If it is desired to plan the inspection path off-line, then step  164  is performed to generate the inspection path plan. In step  164 , there are several options for generating the inspection path plan. If the ship has been previously inspected using the process described by this invention, the planned inspection path can be based on a previously stored inspection path. If there is no stored inspection path plan for the ship, then the inspection path planning can be performed using a dedicated software program which accepts the 3-D hull shape model filename as input, and generates as output a file containing a series of straight line motion segments to be executed by the ROV control system during actual survey. Each straight line motion segment consists of a segment endpoint (desired final location and orientation of the ROV at the terminus of the straight line move), desired path velocity, and desired path acceleration. Generation of the straight line motion segments comprising the inspection path can be performed automatically or manually. If generated automatically, the path planning program will automatically create a series inspection paths according to one or more of the methods which were described above. One method generates a series of inspection paths parallel to the ship&#39;s waterline. A second method generates a series of parallel paths around the girth of the ship, up to the ship&#39;s waterline. If generated manually, the inspection path will be created by an operator manually defining (via mouse or other input device) the straight line segments on the 3-D hull image displayed on the computer console. 
     If, at step  156 , it is determined there is not an existing 3-D model of the shape of the hull  11  of the ship, then the availability of ship hull plans from which a 3-D hull shape image can be created is determined (step  158 ). Appropriate plans for 3-D hull shape model generation include ship plate expansion drawings in conjunction with CAD generated 3-D hull form description drawings or ship table of offsets used for hull manufacture. Additionally, ship data such as forward, aft and mid-section drafts may be useful for inspection path planning performed at step  164 . If it is determined at step  158  that appropriate plans and data exist for 3-D hull model generation, then a dedicated software program can be used to generate the 3-D hull shape model in step  160 . The dedicated software program will accept the filenames for the various ship plans and data as input, and generate the 3-D hull shape model file as output. Some forms of ship plans to be input, such as plate expansion drawings, may exist in paper drawing form only. In cases such as this, the plan may require digitization by manual methods (i.e., digitizing tablet) or automatic methods (i.e., scanner plus CAD program) prior to input for the 3-D hull shape model generation process. 
     If at step  158  it is determined that appropriate ship plans required for 3-D ship hull shape model do not exist, then, during the course of a normal hull inspection (starting at step  166 ), the ROV and associated tracking system can be used to generate the ship&#39;s 3-D hull shape in a form suitable for 3-D hull shape model generation. Hull profile data can be generated by driving the ROV over the submerged hull surface while tracking and recording (on non-volitile media) the vehicle&#39;s position and orientation using the acoustic tracking system and ROV instrumentation. The recorded hull profile data can then be used as input to a dedicated software program for generation of 3-D hull shape model from vehicle position and orientation tracking data. Additionally, the ROV and associated tracking system can be used to identify and record the positions of the ship&#39;s plate vertices for incorporation of plate geometry information into the 3-D hull shape model. 
     The on-hull survey starts at step  166 . The first step in the process (step  166 ) is deployment of the acoustic tracking system per methods described previously. After deployment of the acoustic network, the network is registered to the ship&#39;s hull in step  168 . This process involves driving the ROV to 3 positions on the submerged portion of the hull within the range of the acoustic network, and recording the positions (in acoustic network based coordinates) of three hull features with known locations in the hull based reference frame. The control software program operating in the navigation computer  42  is then directed to translate the acoustic network reference frame to the hull based reference frame. Following completion of step  168 , all ROV position and orientation data generated by the acoustic tracking system will be referenced to the hull based reference frame. 
     After deployment and registration of the acoustic tracking system in steps  166  and  168 , the ROV is directed to proceed to the inspection steps. At step  170 , if it is determined that automatic ROV path control is not desired, then the operator can manually pilot the vehicle to conduct the on-hull inspection in step  172 . During manual operation, the ROV is manually piloted over the surface of the hull, while the NDT instrumentation continuously tests and records the condition of the hull  11 . If automatic ROV path control is selected at step  170 , then the pre-planned path is displayed by the monitor in step  174 , and means are provided in the form of step  176  for a human operator to edit the path calculated by the computer. At completion of pre-planned path display and review, the operator then directs the computer to proceed to step  178  and controls the ROV to follow the prescribed inspection path. While this is being done the computer also directs the ROV NDT instrumentation to test and record the condition of the hull  11 . During this part of the operation, the computer is responsive to manual input control by a human operator who views the environs of the ROV through the navigation camera. 
     At completion of the planned inspection path sequence in which data has been gathered and recorded with respect to position and the corresponding hull  11  condition, the operator is given the opportunity to review the hull condition data (step  180 ), and, if available, historical data for the same ship. After review of the survey data in step  180 , the operator determines in step  182  whether more information needs to be taken in certain areas through the use of a detailed survey of that area, as discussed above. If specific areas are in need of more detailed inspection, the sequence repeats from step  170 . If there is no need to further inspect the hull within the current area of coverage provided by the acoustic tracking network, then it is determined at step  184  whether the complete submerged surface of the hull has been inspected. If there are additional areas of the submerged hull that require inspection, and additionally, require repositioning of the acoustic tracking network to provide required tracking coverage, then the sequence starting at step  166  is repeated. If there are no additional areas of the submerged hull to be inspected, then the survey portion of the process is complete, and data post-processing can be started. 
     The overall post-processing operation is outlined in FIG.  17 . Referring to FIG. 17, the process begins by determining at step  186  whether a it is desired to develop a 3-D model of the shape of hull from the acquired and recorded survey tracking data. If existing hull shape models do not exist, or the hull plans required to construct a 3-D hull shape model do not exist, then an empirical model can be developed in step  188  from the ROV tracking data taken during a survey. Additionally, if 3-D hull shape models do exist, it may still be desirable to generate an empirical model from actual survey tracking data for model refinement purposes. 
     In step  190 , the acquired data is analyzed and processed for data reporting and visualization. This step may involve identification and removal of “bad” data from the data set, data filtering (smoothing), data tagging (a hull plate identification can be associated with each data sample, based on sample location on the hull), and data statistical analysis. An industry standard alphanumeric text report is generated in step  192 , consisting of tabulated NDT data measurements (hull thickness, coating thickness, cathodic protection device potential, etc.) and the positions on the hull where the measurements were taken. Additionally, the data can be sorted and reported on a hull plate by plate basis. Graphical representations of the survey data are generated in step  194 . Graphical representations include two dimensional and three dimensional views of the hull or sections of the hull, with the data value at a location on the surface encoded using color, texture, or height above the surface. Hull material cross-sections can be generated along operator defined lines of intersection (e.g., making it possible to view hull thickness at a specified cross-section of the ship). 
     In addition to printed reports, all reports generated by the process described by this invention can be delivered in various “electronic” formats to the end user. The tabulated and graphical data reports can be converted to HTML files, making it possible for the customer to review the survey reports using a standard internet web browser. The HTML report files can be delivered on CD media for “local” browsing on a customer&#39;s PC, or the files can be embedded within a password protected web site for review over the internet. 
     In step  196 , it is determined if historical survey data exists for the specific ship. If previous survey data generated by the process covered by this invention exists, then step  198  can be used to provide documented hull condition trend analysis and predictive maintenance planning. It is also contemplated that changes in the overall shape of the inspected structure as compared to construction plans and/or a prior survey or surveys may indicate particular problems and appropriate measures taken. 
     Alternatively, computers  42 ,  44  and  76  cooperate to execute the software algorithm of the present invention. Referring to FIG. 18, generally similar components or parts performing analogous, corresponding or identical functions to those of the previous embodiment are numbered herein with numerals which differ from the previous embodiment by multiples of one thousand. The alternative process, begins by determining at step  1156  whether there is an existing model of the shape of the hull  11  of the ship. If such a model exists, the software proceeds from step  1156  to subroutine  1158  where a determination of the intersection of the surface of the sea and the hull  11  of the ship when the ship is afloat is made. 
     Once the shape and position of the intersection in space has been determined by subroutine  1158 , the ROV  18  is caused to travel along the shape surface of the hull  11  corresponding to the intersection in order to verify the accuracy of the computer generated intersection shape and position. Subroutine  1160  thus comprises a sequence of thruster commands and position sensing commands responsive to positional information feedback to cause the ROV to trace the shape of the intersection on the hull  11  of the ship at a point just below the surface of the water. Thereafter, the hull is surveyed by the ROV in the manner detailed below. 
     If there is no model of the shape of the hull  11  of the ship, at step  1162 , the question is asked whether plans for the ship exist. If such question, which is shown on the screen of the computer, is answered in the affirmative, the user is directed to put the plans on a scanner and the program proceeds to subroutine  1164 , which is a package for scanning such plans and artificially generating a three dimensional picture in space as a matrix of numbers which describe the shape of the hull  11  of the ship. Once this has been done, subroutine  1164  directs the machine to proceed to subroutine  1158 . In this regard, the information respecting the weight of the ship is also introduced into subroutine  1158  in order to determine the waterline based on displacement of a volume of water equal to the weight of the ship. 
     After such determination has been made at subroutine  1158 , the shape of the intersection and its position is verified by subroutine  1160 , by an attempt by the ROV to trace the shape in the water. 
     After the shape of the intersection has been verified, the computer proceeds to the inspection steps. 
     If no plans exist for the ship, it is necessary to generate some hull  11  shape information. This is done by the computer using subroutine  1166  to direct the ROV to trace the shape of the intersection by crawling around the hull  11  while trying to stay just below the waterline. In doing this work, a depth transducer adapted to provide very accurate depth information at depths close to the surface may be used as a feedback device when such transducer is mounted on ROV  18 . This information can be input into the computer to allow the computer to generate appropriate control signals to cause the ROV to trace the intersection, while the position determining ultrasonic system gathers intersection shape information. 
     The movement of ROV  18  is controlled by subroutine  1166 . After the intersection has been determined, ROV  18  is directed to proceed along a path parallel to the intersection and displaced downwardly from the intersection by a desired distance. This is achieved by subroutine  1168 , which provides this function in a manner substantially similar to similar functions in computer aided drafting applications and based upon an operator input or computer generated prediction of below the water hull shape. 
     After the parallel path has been generated, subroutine  1170  controls ROV  18  to follow a path on the hull  11  as close as possible to the generated parallel path and to generate actual path shape based on its movement. During the execution of subroutine  1170 , the navigation camera on board ROV  18  provides a display to the operator at the console, and the human operator may vary the path followed by the ROV through an interactive subroutine  1171 . 
     At the same time, subroutine  1172  is comparing actual path of movement information to generated path information to determine the difference between predicted hull shape and the actual contours of the area of the hull that the ROV is passing over. At the same time, a separate subroutine  1174  provides the computer network  20  in the console with measurement information from the various transducers on the ROV in order to map inspection information on the condition of the hull  11 . 
     Next, at step  1176 , the question is asked by the computer as to whether it appears that the entire surface of the ship that is underwater have been surveyed. If the answer is yes, a complete map of the shape of the hull  11  has been produced and this information can then be used to proceed to the detailed portion of the inspection, as will be described below. If more of the hull  11  needs to be surveyed, the computer is directed to return to subroutine  1168  and generate the next lower parallel path. This process is repeated until the entire hull  11  is surveyed. 
     In the case of an inspection of a hull  11  for which there is a model, after the shape of the intersection has been verified at step  1160 , the machine is directed to proceed to the inspection steps. The first of these inspection steps is subroutine  1178  which determines and displays the path for the ROV  18  to follow in order to inspect the entire surface of the hull  11  of the ship. The path is displayed by the monitor and means are provided in the form of a subroutine  1180  for a human operator to edit the path calculated by the computer operating in accordance with subroutine  1178 . The computer then proceeds to step  1182  and controls the ROV to follow the path determined by subroutine  1178  as edited by subroutine  1180 . While this is being done the computer also directs the ROV instrumentation to test the condition of the hull  11 . During this part of the operation, subroutine  1182  is responsive to human user input control provided by subroutine  1184 . Such interactive human intervention is provided by a human operator who views the environs of the ROV through the navigation camera. 
     After data has been gathered with respect to position and the corresponding hull  11  condition, whether through the completion of subroutine  1182  or the answering in the negative of the question of step  1176 , the computer is directed to proceed to assemble a model of the data as a function of position through the execution of subroutine  1186 . 
     After this is done, at step  1188 , the question is asked as to whether previous data exists. If the answer is in the affirmative, the data is secured and a comparison is made by the computer at step  1190 . If no such data exists, the computer proceeds directly to step  1192  where a subroutine determines whether or not closer inspection of some areas is necessary. 
     Likewise, it there is available data and subroutine  1190  has made a comparison, that information is output, together with the recent data to subroutine  1192  to make a determination as toward whether more information needs to be taken in certain areas through the use of a detailed survey of that area, as discussed above. If the determination is made that such additional detailed inspection needs to be done, step  1194  directs the system to repeat subroutines and steps  1186  through  1192 . Once step  1192  is performed and a determination is made that no further detailed inspection is necessary, the report subroutine  1196  is actuated, report printed and the inspection is complete. 
     In accordance with the present invention, it is also contemplated that changes in the overall shape of the inspected structure as compared to construction plans and/or a prior survey or surveys may indicate particular problems and appropriate measures taken. Likewise, historical tracking of deterioration in long and short term conditions to optimize the time at which the problem is addressed is possible and contemplated to be within the scope of the invention. 
     While an illustrative embodiment of the invention has been described above, it is, of course, understood that various modifications will be apparent to those of ordinary skill in the art. Such modifications are within the spirit and scope of the invention, which is limited and defined only by the appended claims.