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Hydrographic SurveyMarcus Street |
I joined the Royal Navy as a Junior Seaman in 1966, and after basic seamanship training went on to specialise as a Sonar Operator. My first ship was HMS Grafton, officially described as a Third Rate Anti-Submarine Frigate, which spent a lot of time chasing submarines around the Clyde Estuary. In general my only visits to the Bridge were for duties as a Look-Out ; navigation was something that happened inside or behind me on the chart table. In 1969 I joined HMS Hydra in Chatham and found that what appeared to be a small cruise liner was in fact an Ocean going Survey Vessel. We left Chatham in October and after travelling via Cape Town and Mauritius we arrived in Singapore in time for Christmas. Most of the next eight months were spent carrying out Hydrographic Surveys to confirm the Deep Water Route through the Malacca Straits.
As I understood it at the time, Navigation was done by the generally accepted Dead Reckoning, Star and Sun shots as well as the Radar and Gyro Compass - all of which were an improvement on the method of heading toward land until you recognised something which was my early understanding of how Navigation started. However, Hydrographic Surveying required a more precise method of establishing the ship's position. Now an Able Seaman, one of my duties was Quartermaster, which at sea meant Helmsman, so I got to see a lot more of what happened on the Bridge.
The vessel position was obtained using Decca Hi-Fix, effectively two shore based radio stations, the signals from which could be plotted on a drawn up Lattice Chart. In theory, X marked the ship's position though the more academically minded will point out that there is scope for error here. The shore stations were run by Decca Engineers contracted in by the MOD. These were set up where required by the ship using helicopters and boats. The system range was expected to be around 100 miles from the shore stations with an accuracy of 0.01 of a lane on the baseline which implied +/- 10 feet for normal fixing. This is according to my old paperwork.
Water depth was measured by a single beam Echo Sounder, and in theory the seabed was scanned for wrecks using a sonar developed toward the end of World War II. In order to cover a wider area, the ship would often deploy the Survey Motor Boats (SMB) on either side and carry out formation sounding, (see photo), with the SMB position obtained from compass bearing and vertical sextant angle. Fix timing was by a shout over the radio.
I found this work to be far more interesting than submarine chasing so I volunteered to change over to Survey Recorder. This was accepted, and I was sent to the Royal Naval Hydrographic School at HMS Drake in Plymouth where I completed the Survey Recorder 3rd class (SR3) course. A lot of time was spent out on Plymouth Sound learning how drive a Survey Motorboat and how to take a Horizontal Angle with a Sounding Sextant (an HSA). This differs from the standard Navigational Sextant in that there are no shades for sun shots, the telescope is bigger and the graduated arc is up to 140 degrees. I am not sure when position fixing with two HSAs came into use, but I was still using it on a Large Scale Survey of the approaches to Dover Harbour just before I left the Royal Navy in 1978.
On the successful completion of the course I joined HMS Enterprise which at the time was part of the Inshore Survey Squadron based at Chatham. The ISS worked mainly in the Thames Estuary and up the East Coast of the UK, carrying out surveys to check on the movement of the sandbanks in the Estuary, including the Goodwin Sands. Another regular job was checking the depth over known wrecks by wire sweeping. Positioning for this work was usually by the Decca Hi-Fix, using established Hyperbolic chains, lane checks to confirm the readings were carried out by going around the Forts in the Thames Estuary, which used to cause considerable upset to the owner of Sealand. A lot of work further north was fixed using the trusty HSA, which apparently was why the Inshore Survey Vessels had an open Bridge - very bracing!
January 1972 found me back on HMS Hydra, I joined in Mauritius in time for some boatwork in the Seychelles, where a Fleet Tanker had recently struck a rock. (HSA again). Quite a lot of the next eighteen months were spent in the Solomon Islands, with the 2 Range Hi-Fix and HSAs for boatwork. The vessel had also been fitted with the new Transit Satellite system ; six satellites were in orbit, though only five produced any data, with a planned pass about every 50 minutes. Satellite data was passed through an Elliot 903B computer, which was about the size of a small wardrobe and programmed with an inch thick holed paper roll about six inches in diameter. Not every Pass provided a position fix, due to unacceptable elevation or numerous other errors. Apparently after collecting data for some time whilst alongside in Honiara the actual position of Gaudalcanal may have been adjusted, but as an Able Seaman I just collected the data. Obviously the system accuracy was of a very high order, I discovered many years later that the Transit fix was in fact comparable to an uncorrected GPS derived position.
Following more courses in Plymouth, in early 1974 I was back on HMS Enterprise, working on the east coast again, still using the Decca Hi-Fix. The following year I joined HMS Fox, a Coastal Survey vessel working in the Irish Sea. A large survey in an area between Holyhead and the Isle of Man was completed with positioning by Del Norte's Trisponder system This was a micro wave based system with a maximum range of around 40 kilometres and a reported accuracy of 3 metres, following a good system calibration and reasonable weather conditions. A survey of the Calf Sound, (to the south of the Isle of Man) was carried out by the ship's SMB, positioning by HSA and made more interesting by an extremely fast tidal stream through the Sound.
After HMS Fox, I went back to school for my SR1 course, then returned to HMS Enterprise. As the North Sea was then opening up, the Inshores had shifted operations northwards, completing surveys at Invergordon as well as the Approaches to Peterhead and Aberdeen, as these areas were to become very busy over the next few years. As a Petty Officer I was now more involved in Bridge Watchkeeping as an Officer of the Watch as well as assisting on the Survey Operations. My last ship in the RN was HMS Bulldog, another Coastal Survey Vessel ; the main survey carried out was in the English Channel on the Sandettie Banks. As an OOW running survey lines across the shipping lanes, with a towed sonar, life was very interesting. I was more than surprised to see how many vessels could happily steam through the Dover Straits without any visible Bridge Crew, although on one occasion I did notice a large dog keeping lookout. SMB surveys were also carried out on the River Blackwater and the Approaches to Folkestone and Dover, positioning in all cases by Trisponder backed up by HSA. A further addition to the positioning was the introduction of Desktop Computers and Flatbed Plotters which were used to plot the fixes and provide heading information for the autopilot.
I left the Royal Navy in 1978, and joined Gardline Surveys, which in some ways was almost a continuation of service. The rapidly expanding exploration for Oil and Gas in the North Sea required a lot more survey personnel, , many of whom like myself had trained in the RN. As well as the exploration work, in 1982 the MOD started to farm out Hydrographic Instructions to commercial survey companies in order to carry out work that could no longer be completed by RN vessels.
Exploration surveys differed from the basic Hydrographic surveys, being a smaller area and requiring more information with regard to what was on, and under the seabed. A drilling location would be selected following geophysical interpretation of seismic data, and a Site Survey carried out, nominally a square kilometre for a Jack-Up Drilling Rig. The basic survey would be in three sections; an Analogue survey using Echo Sounder, Sidescan Sonar and a Sub-bottom Profiler. These would provide information on the water depth, any obstructions on the seabed, and an idea of the make up of the first few metres of the seabed.
A Digital or Shallow Seismic survey using a less powerful version of the equipment used for the original seismic was required to check for shallow gas in the first few thousand feet of seabed . This would not necessarily show up on the original seismic data but it's presence would cause a lot of problems if encountered during drilling operations. In addition to these, a grab or corer would be deployed at various locations on the site to confirm the seabed make-up.The larger Jack-Up Rigs can work in water depths up to 90 metres, which covers most of the Central and Southern North Sea, further North and West of the Shetlands, Semi-Submersible Rigs are used. A Semi-Submersible Rig Site Survey would require similar Digital data acquisition, but with between 8 and 12 anchors to set, the Analogue area would usually be extended to a 3 kilometre square.
If the drilling was successful, it would be followed by the installation of Platforms and Pipelines, which all required Route and Installation Surveys. Whilst the Micro-wave systems, (Trisponder, Miniranger), could be used for close to the shore, the bulk of offshore positioning in the late seventies and onward (in my experience), was by Decca's Pulse/8, a Low Frequency System that covered most of the North Sea. The accuracy depended upon who was talking but could be regarded as in the region of +/- 20 metres. The receiver had a habit of lighting up like a Christmas Tree in heavy rain storms, but if one resisted the temptation to press all the re-set buttons it would recover in it's own time. One noticeable asset was the repeatability, if you wanted to return to a previous location fixed by Pulse/8, so long as identical readings and corrections were used, you would be on the same location. Of course, this may not have been the spot one originally intended but it would be where you were.
In the Eighties, the French company Sercel produced the Syledis UHF positioning system, and several chains were set up utilising the offshore platforms. Depending on the height of the transmitting antenna, ranges up to 90 kilometres were achieveable, and once calibrated, and confirmed by Baseline Crossings, an accuracy of within +/- 3 metres was supposed to be possible. Both Pulse/8 and Syledis produced three 'ranges' or Lines of Positioning (LOP), generally giving a small 'cocked hat', the centre of which should probably be one's location.
Of course, along with the improvements in electronic positioning were the swift developments in computing. Hand written records and plotting were all replaced by computer programmes driving flatbed or drum plotters with everything recorded on cassettes, then discs, drives, memory sticks and now, I believe, a cloud.
Returning briefly to the positioning of Drilling Rigs ; they were guided on to location by a buoy pattern dropped by the survey vessel. This consisted of an 'L' shape for Jack-ups providing two transit lines and a location, and for Semi-submersibles just markers for the rig's intended final location and anchors. Once on location, the survey ship would complete a transit fix on the rig to check that it was on the required location. Confirmation of the actual location was by Transit Satellite fix, and it could take anything up to a fortnight to gather and process the data. This was sped up by putting a Positioning team with all of the required equipment onboard the Drilling Rig. Once in place and working, the Towmaster on the rig had constant access to the rig position and heading, along with the location of the assisting Tugs and Anchor Handling Vessels. Positioning of the AHVs progressed from the use of a positioning team and equipment on the vessel to remote fixing by a laser shot from the rig, then independent systems installed on the vessel which could be remotely controlled by the team on the rig.
In the early nineties came the introduction of the Global Positioning System, (GPS), bringing much improved accuracy to navigation and positioning across the world. Within a couple of decades we have progressed in positioning terms from a predicted accuracy of around 20 metres to a known figure in centimetres. Initially designed for the American Military with various security attachments, the introduction and expansion of the Satellite systems has changed how the world operates. Along with the original American system, there are now myriad systems well known to the world's navigational society, but of course not everybody may be aware of how dependent the world has become on the data from the wide variety of satellites comprising the Global Navigation Satellite System.
On the Hydrographic Survey front we have gone from a ship full of people using a single beam echo sounder to swaythe systems, laser-based aircraft surveys, (LIDAR) and Satellites to collect bathymetric data. We use Remotely Operated Vehicles fitted with cameras so that we can see what is happening underwater, and now with the development of Autonomous Vehicles both Underwater and Surface, it is possible to programme and conduct a Hydrographic Survey without actually leaving the office. I am sure it is both cost effective and extremely efficient though I somehow doubt that it is quite as much fun.
On a personal basis, I moved from Gardline Surveys to what was the Stolt Nielsen Offshore Technology, (to learn about ROVs), then on to International Survey Consultants to work as a 'Client Rep'. I have spent the last thirty odd years working as an independent Offshore Client Representative, covering just about all aspects of Offshore work with the exceptions of Dredging and Deep Seismic.
Recently, a year or so ago now, I was on the Bridge of a very new, very large Norwegian Multi-purpose vessel chatting to the Captain. Following a question from the onboard survey team, we had already confirmed that the ship did not carry a sextant and paper charts had been discarded for the electronic versions. The front of the Bridge resembled a Startrek set, no discernible steering wheel, just a couple of finger-sized joy-sticks, a few dials, a lot of computer screens and two very comfortable chairs. I asked the Captain, just as a matter of interest, given the problems that could arise from GPS spoofing and blocking, along with other assorted electronic failures, how he would get by. His response was that basically he would steam towards the land and hope that he recognised something.
Plus ça change la meme chose.
