I do work with Pipeline and Flow Assurance Engineers and know a little bit. Waited till the weekend to do some calcs. Hope this helps :
Lets look at Nord Stream 1 : It would actually take quite a lot. The pipeline deign pressure was in excess of 200 bar (2901 psi).
" The pipelines have a constant internal diameter of 1,153 millimetres. However, Nord Stream designed the pipeline with three different design pressure sections (220, 200 and 177.5 bar) and pipe wall thicknesses (34.4, 30.9 and 26.8 mm respectively) corresponding to the gas pressure drop over the long journey from Russia to Germany."
You can see the laying out process here : https://www.youtube.com/watch?v=rx_5I8_i_bo&ab_channel=NordStreamAG
Summary : Approximately 4 ft diameter pipe, with metal thickness in excess of 1 inch, concrete reinforced and buried several feet under the sea bottom by digging trenches.
(The pipe is buried underground (or rather under sea bottom) to avoid stresses from currents or anchors from ships or spanning (sea bottoms do not remain flat over long time ranges, you may endup with a pipe “hanging” from one “hill top” to another “hill top” thereby creating stresses. )
Use of commercially available drones : Lots of problems with this. First of all the depth is about 80m (262ft) - its not easy to make electronics that work at that depth and stay waterproof, because the water pressure is about 130 psi. Moreover at these depths drones cannot communicate using conventional wireless signals. They need to have wired communications or use sound transducers which are typically not off the shelf items. Also a confounding fact is that the maps of these pipelines are not published and locating the pipelines is not easy because they are buried.
You may find out the general location of the pipe but the pipe may have moved etc etc - this explains the huge explosion
Quantity of methane (natural gas) released : The pipeline was at a pressure of 105bar and dropped to 5 bar overnight. (Nord Stream 2 pipeline pressure collapses mysteriously overnight | Nord Stream 2 pipeline | The Guardian).
Lets assume a 4.4 C (40F) temperature and 105 bar (1523 psi) gives a density of 90.19 kg/m3 (5.63 lb/ft3). Nord Stream 1 is about (297 + 378 + 548) 1223 km (760 miles) with a total volume of 1,276,935 m3 (45 million ft3).https://www.wermac.org/pdf/nordstream1.pdf
That gives approximately 115 Metric tonnes of Natural gas release Or 0.12 Million Metric Tons. This is the same ballpark single day production of Cheniere Sabine Pass LNG (30 Million Metric Tons per year) Cheniere | Sabine Pass
Why was the pipeline pressurized when not in use ?
There are many reasons for this, I’ll present two :
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Natural Gas hydrates : Natural gas is dehydrated before entering a pipeline to lower its dew point. Sometimes methanol is also injected to correct for hydrate formation. If a pipeline at 1523 psi is de-pressured fast to say 150 psi, the gas inside will reach a adiabatic expansion temperature of -53F (-47C). At these low temperatures, you increase the risks of hydrates blocking the pipeline, especially at low points, here is a picture of a hydrate plug :
https://i0.wp.com/www.arab-oil-naturalgas.com/wp-content/uploads/Hydrate.jpg?w=255&ssl=1
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Thermal Stresses : A temperature change from 40 F to -53 F creates huge thermal stresses especially for a pipe that is 1000+ km long.
So to avoid forming hydrates, and thermal stresses and other effects, pipeline operators do not swing the pressure too much.