86 



In many aspects search and identification tasks are similar to surveying, 



in that the tools and techniques are identical. The primary difference is 



that search and identification tasks involve investigation of man-made artifacts,j 



surveying encompasses both man-made and natural objects and features. In 



many instances the task involves no more than straightforward visual inspection 



using TV and compass, in others it is more sophisticated. J.D. Westwood 



of Sub Sea Survey Ltd. described a typical search/identification mission 



conducted in the Norwegian sector of the North Sea. The following account 



is largely extracted from his paper (Westwood, 1978) . 



The aim of search/identification surveys is to determine the extent of debris 

 remaining from the drilling operations and abandoned well sites which may 

 prove destructive to fishing equipment. Such surveys are required by the 

 Norwegian Petroleum Directorate (NPD) . They require each oil company to: 

 1) conduct a side scan survey within a 2km square around the site center, 

 in the case of wells drilled by semi-submersible rigs, and a 600m square for 

 jack-up platforms, and 2) visually inspect the seabed over a 200m square centered 

 on the well position. This survey being a 'grid search' with 20m spaced lines. 

 It is also necessary to visually inspect items seen on side scan outside this 

 200m square. 



Initially, the abandoned well sites are located by towing a magnetometer. 

 Strong magnetic anomalies can be observed even when the wellheads have been 

 removed and the casings cut off below the seabed. This is followed by a side 

 scan survey of the 2km square area. A local navigation net is established 

 by laying three transponders around the edges of the site. These are calibrated 

 by one of the established methods. 



Using the acoustic navigation system the ship is brought to the dive position 

 and the submersible launched. It dives to the seabed and is piloted into 

 position to begin the survey. The vehicle is driven over the predetermined 

 200m line space grid until an item of debris is encountered. The item is 

 examined and sized using the vehicle's TV cameras and manipulator, the results 

 being recorded on video tape. If necessary, still photographs are taken, and 

 an item of interest is numbered and plotted onto the site chart. This process 

 is continued until the site has been completed. If items are discovered near 

 the edge of the site the survey grid is extended to provide coverage beyond 

 that point. In practice some items plotted are quite insignificant, however, 

 even a tin can can be a good position marker and when re-encountered can provide 

 a check on the accuracy of the navigation system. 



Using the vehicle's manipulator and specially developed tools, lifting lines 

 can be attached to items of debris which are then winched up onto the support 

 ship. All of these operations are carried out without anchoring the ship 

 which continually moves along with the submersible. Video monitors on the 

 bridge display TV from the seabed and a communications net connects pilot, 

 observer, navigator, deck and the ship's helmsman. 



Onboard the support ship preliminary charts are plotted out showing the seabed 



coverage of the side scan survey and submersible TV survey. Upon returning 



to shore, results from the preceding work period are reprocessed, video tapes 



copied, titled and annotated and final reports produced. These results are 



used in discussions with the NPD to determine which items are likely to constitute 



a hazard to fishing and therefore warrant removal. 



