GB2125965A - Underwater surveying - Google Patents

Abstract

An underwater surveying arrangement includes a "fish" 8 in the form of a self propelled torpedo- shaped body, and an array of markers 4 located on the sea bed. The "fish" 8 once released from a "mother" ship 2 is controlled to drop to a predetermined level above the sea bed and then to be maintained at that level. Upon reaching the required level the "fish" is controlled to follow a predetermined scanning pattern relative to the markers and to record survey information while performing the scanning pattern. A sea bed survey can thus be carried out regardless of depth and the "mother" ship can be released for other duties while the surveying operation is taking place. <IMAGE>

Description

SPECIFICATION Underwater surveying methods and arrangements The present invention relates to underwater surveying methods and arrangements.

To make a seabed survey for an oil rig or platform sites it has been proposed to locate an array of markers on the sea bed around the site to be surveyed. These markers are in the form of transponders. A "mother" ship on the surface of the sea is arranged to communicate acoustically with the transponder network to determine the exact location of the markers on the sea bed in relation to theoretical and/or physically known positions on the earth's surface.

In this way the location and orientation of the selected seabed site relative to the surface can be precisely determined.

In order to carry out a comprehensive survey of the seabed site the "mother" ship is provided with a tethered "fish" which is towed in a regular scanning pattern across the site using the markers to ensure that the regular scanning pattern is maintained. The "fish" is in the form of a torpedoshaped submersible body which can be controlled to maintain a predetermined depth and carries side scan sonar for surveying the sea bed. In addition to a towline which links the "mother" ship to the "fish," a communication line transmits all data gathered by the "fish" to the "mother" ship for processing.

A disadvantage of this method and arrangement is that in practice the depth at which the fish can be towed is limited.

As a result if a deep water survey is required the "fish" will be located too far from the seabed for an accurate survey to be made.

A further disadvantage of this method and arrangement is that the "mother" ship is actively employed on the survey for the duration of the survey.

According to the invention there is provided an underwater surveying arrangement comprising an array of markers arranged to be located about a site on the seabed and a self propelled underwater survey body, the body being provided with means for maintaining the body at a predetermined height above the sea bed, means for causing the body to follow a predetermined scanning pattern over the sea bed relative to the markers, means for sensing a predetermined characteristic and means for storing the sensed data.

According to the invention there is further provided a method of surveying an underwater bed site comprising the steps of placing an array of markers in the vicinity of a site to be surveyed, determining the location of the markers in relation to above water positions, releasing a self propelled untethered body into the water, bringing the body to a predetermined height above the bed and maintaining it there, causing the body to follow a predetermined scanning pattern relative to the markers sensing a predetermined characteristic on the bed from the body as the body follows its scanning pattern, storing the sensed data in the body and recovering the body.

An underwater survey method and arrangement embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is perspective view of the arrangement; and Figure 2 is block diagram of the control system of the "fish" of the arrangement of Figure 1.

As shown in Figure 1 a mother ship 2 has dropped an array of markers, in the form of recoverable transponders 4, around a selected site on the sea bed. The transponders 4 are spaced 1-2 km from one another.

The ship 2 carries a transducer 6 which is arranged to interrogate each transponder 4 in turn. A ranging receiver (not shown) on-board the ship 2 processes the received signals to determine the distance of each transponder from the ship 2. These distances are then used to calculate the distance of separation between adjacent transponders and from all the available data the "mother" ship can determine the precise location of the markers in relation to fixed land surface positions.

The "mother" ship carries an untethered "fish" 8. The "fish" 8 is in the form of a torpedo shaped body carrying its own propulsion system and has an on-board navigation computer package for controlling the movement of the "fish" below water.

The "fish" 8 carries a transducer 6 which is arranged to communicate with the transponders.

In operation the "fish" 8 is lowered into the water from the "mother" ship and is automatically controlled to sink to a desired level above the sea bed and to move into a predetermined start or datum position in relation to the markers. A computer control system in the "fish" then automatically causes the "fish" to follow a predetermined and regular scanning pattern over the sea bed relative to the markers.

Advantageously the scanning lines of the pattern are spaced at 300 m. intervals.

During this scanning process a side scan sonar system carried by the "fish" surveys the sea bed and the resulting survey signals are stored by a recording system such as a tape recorder.

When the "fish" has completed the scanning pattern it then adopts a recovery mode in which it is caused to rise to the surface. A radio beacon carried by the "fish" is then activated to enable the "mother" ship to home in onto the "fish" and effect the recovery thereof.

On board the "mother" ship 2 the stored survey signals are read out and processed in the usual manner to enable a complete survey report to be compiled. It will be appreciated that the provision of an untethered "fish" to automatically scan the sea bed enables the "fish" to be used at almost any depth. Furthermore since the "fish" can perform its scanning operation and a sea bed survey independently of the "mother" ship, the "mother" ship is freed for other duties.

For example where a number of sites are to be surveyed the ship can drop a fish at each site in turn and then arrange for the recovery of each fish at the completion of the surveys.

Figure 2 is a block diagram of the various circuits carried by the "fish".

As shown a transducer control circuit 10 is arranged to cause a transducer 11 to communicate with each marker 4 in turn and a computer 12 responds to data received back by the transducer 11 to determine the precise location of the "fish" relative to the markers 4. A start-up and recovery circuit 14 is activated during the initial period of operation of the "fish" to actuate a depth control assembly.

The depth control assembly includes an echo sounder 22 for giving an indication of the actual level of the "fish" above the sea bed, a reference level generator 24 for generating a signal indicative of a desired level above the sea bed, and a comparator 26 for comparing the desired level with the actual level. A depth control circuit 28 responds to the output of the comparator 26 in a sense to bring the "fish" to the desired level and thereafter to maintain it at the desired level.

Also during the initial period, the start and recovery circuit 14 switches the computer 12 into a mode in which it will cause a drive control circuit 1 6 and a rudder control circuit 1 8 to drive the "fish" into a predetermined datum position relative to the markers. As soon as the datum position is reached the computer deactuates the start up and recovery circuit 14, and switches to a mode in which it will control the drive control circuit 1 6 and rudder control circuit 18 to follow a predetermined scanning pattern which has been preprogrammed into the store 20.

At the same time a tape recorder 32 and side scan sonar system 30 are actuated.

During the scanning period the data obtained by the side scan sonar 30 is stored by the tape recorder 32 together with discrete positional data supplied by the computer 12.

When the scanning pattern of the store has been completed, the computer 12 reactivates the start up and recovery circuit 14, which then operates in a recovery mode. In the recovery mode the depth control assembly is deactuated to allow the "fish" to rise to the surface and a beacon 33 is actuated to enable the "mother" ship to home in on the "fish" and effect recovery.

While the arrangement described is particularly suitable for use in sea bed site surveys it will be appreciated that it can equally be used to locate sunken aircraft or ships.

Furthermore the arrangement can be adapted to carry out pipe line surveys by locating the markers along the length of an underwater pipeline. The side scan sonar is then arranged to detect any damage which the pipe may have sustained.

In addition to sea bed surveys and searches for lost ships or aircraft the "fish" could be adapted to perform deep water surveys for manganese nodules or provided with other types of sensors such as profilers or hydrocarbon detectors.

The transponders used may take the form of the Models 722A or 722B supplied by EGG Sea Link Systems of the USA and the data processed by the ATNAV II system supplied by the same company. Transponders type 7064 manufactured by Sonardyne Ltd. of the UK can be used instead.

In place of recoverable transponders, nonrecoverable transponders may be used for example Sonardyne types 7062-3.

Claims (10)

1. An underwater surveying arrangement comprising an array of markers arranged to be located about a site on the sea bed and a self propelled underwater survey body, the body being provided with means for maintaining the body at a predetermined height above the sea bed, means for causing the body to follow a predetermined scanning pattern over the sea bed relative to the markers means for sensing a predetermined characteristic and means for storing the sensed data.

2. An arrangement according to claim 1 wherein the body is further provided with means for bringing the body into an initial datum position relative to the markers prior to the body following the predetermined scanning pattern.

3. An arrangement according to claim 1 or to claim 2 wherein the body is provided with release means for allowing the body to return to the surface of the water upon completion of the scanning pattern.

4. An arrangement according to claim 3 wherein the body includes a beacon for enabling location of the body when at the surface.

5. An arrangement according to any preceding claim wherein each marker comprises a transponder and wherein the body is provided with a transducer for interrogating each marker in turn to determine the position of the body relative to the markers.

6. An arrangement according to any preceding claim wherein the means for storing the sensed data comprises a tape recorder and including means for feeding positional data to the tape recorder simultaneously with the sensed data.

7. An arrangement according to any preceding claim wherein the body includes a store for storing a predetermined scanning pattern which the body is required to follow.

8. A method of surveying an underwater bed site comprising the steps of placing an array of markers in the vicinity of a site to be surveyed determining the location of the markers in relation to above water positions, releasing a self propelled untethered body into the water, bringing the body to a predetermined height above the bed and maintaining it there, causing the body to follow a predetermined scanning pattern relative to the markers sensing a predetermined characteristic on the bed from the body as the body follows its scanning pattern storing the sensed date in the body and recovering the body.

9. An underwater survey arrangement substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of surveying an underwater bed substantially as hereinbefore described with reference to the accompanying drawings.