[QUOTE=injunear;176577]There were several nav systems in use. In the GOM before GPS, Offshore Radist, Lorac and Decca were the most common using phase differential instead of TDs like Loran. My time in Africa in late '70s was mostly repair and salvage but what I saw of rig location was done with a small survey boat that also did sub-bottom profiles and side-scans of the sites. I’ve seen several portable master/slave nav systems set up in South America but the early sat-nav systems that were mounted in 19 inch racks were reduced to suitcase size around '80. The tech advances in my 39 years in the business are amazing. My cell phone gps is more accurate than the sat-nav survey vessels with doppler sonar.[/QUOTE]
A lot of people were employed in the positioning business, on the survey boats, on the rigs to set up and man the tracking system 24/7 and get final position by satellite. Also ashore to set up based stations in sometime very remote and difficult to reach places, which could involve camping out for days to get accurate fixes. (Not me)
Some unusual problems could occure. Off Tierra del Fuego in Argentina we had a problem with sheeps that found the base stations VERY attractive to use as rubbing poles. Fence had to be erected to keep them out.
The most accurate positioning I have been involved with was the Ekofisk Barrier installation in 1989:
I was Project Mariner for the Warranty Surveyors from the construction of the bottom slabs in Rotterdam, transport to Aalfjord in Norway, mooring during slip-forming, tow to field and final installation. I believe this to be the most complex Marine Operation ever performed anywhere.
For the tow-out and installation we had ALL the latest and best positioning and tracking equipment available at the time, with different teams of technician for each system (Three Containers full) and more monitors in the control room than you could shake a stick at.
The instrumentation included:
Syledis for the open water tow,
Mini-rangers in the fjords and for approach to the Ekofisk Field,
Laser for relative horizontal distance above water,
Acoustics for UKC and relative distance at several levels below surface.
The equipment used was:
4 x 150 m.t. hydraulic winches installed on the second half,
2 x AHTS used as “Static points” (Winch platform w/anchor point)
4 x AHT used in ranging mode
1 x Docking pin.
Weight of each half at tow-out was abt. 230,00 m.t., which increased to abt. 260,000 m.t. at the mating stage. One half was already set in position first while the second half was brought into position for mating.
Which of the positioning systems available were used for the final bit of positioning?
The acoustics for UKC and an ordinary hand held metric tape measure.
The given tolerance for mating the two halves of the barrier was +/- 75 mm. (3")
Final position obtained: +/-35 mm. (1 1/2")