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 Unsung hero of WW2 - the C2 design protected petroleum storage tank

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Tim of Aclea
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PostSubject: Unsung hero of WW2 - the C2 design protected petroleum storage tank   Sun 19 Feb 2012, 11:24

In 1936 the RAF held small amounts of fuel on its airfields and only 8,000 tons of fuel in reserve with commercial companies. To put this figure in context, 8,000 tons was the daily usage by RAF and USAAF planes operating out of GB at peak during WW2. The RN had large amounts of storage but it was all in densely packed above ground tank farms located close to the main ports. Commercial storage was also in closely packed above ground tanks. In July 1936 the Air Raid Precautions Department of the Home Office concluded that ‘it is perfectly plain that if a determined attack was made on these installations nothing could save them’. It also concluded that similar petroleum storage facilities on the Humber and along Southampton Water were equally vulnerable.

Starting in 1936 plans were put forward for the building of large volumes of protected tank farms both for the military and for civil storage. Initially 90,000 tons was planned in 1936 for the RAF and then this was raised to 290,000 tons. Following Munich this was increased to 436,000 tons and then 850,000 tons. By 1943 two million tons of storage had been constructed for the RAF.

Although there were a number of different types of storage tanks constructed, the most common and long lasting was designated the C2 design. I have a copy of a 1938 design drawing for one of them as well as photos of their construction.

The C2 design was that of a steel lining of a concrete chamber, and the experience of over 70 years of operation has confirmed the wisdom of the original selection. It was a vertical, cylindrical, reinforced concrete chamber which was semi-buried; typically half of the tank was below ground and half above ground. It contained, in the form of a lining, a mild steel all-welded open-top tank. The steel tank was built on a concrete foundation, and when completed was hydrostatically tested by filling it with water and then checking for leaks. The concrete chamber and roof were built to withstand the side and overhead loads, the roof slab being supported on 45 columns within the tank. The tank was to prove to be relatively cheap to build and excellent in service, fulfilling all the requirements of protected storage. The tank was finally mounded over with the spoil from the excavation both making it better protected against air attack and also less visible from the air.

The tanks were built with ¾ inch thick bottom plates; the sides were made up of 1inch shell plates welded together. A girder kept the tank cylindrical and helped it withstand the stresses during the period before the concrete walls were cast around it and when the tank was under hydrostatic test. Investigation showed that 6 feet 6 inch x 20 feet was the optimum size for the floor plates that made up the 118 feet diameter tank bottom. As the tank bottom had to support the tank columns that held up the concrete roof slab, it needed to be as flat as possible. It was, however, no easy matter to keep an area of 10,000 square feet flat and various welding sequences were tried until this was achieved. Most of the tank bottoms were laid directly and welded on the concrete foundation and hydrostatically tested after the shell plates had been erected on the sides of the tanks.

The concrete foundations were designed to be 125 feet 6 inch in diameter. The steel walls were utilised as shuttering on the inside for the concrete wall but wooden shuttering had to be constructed for the outer face. The shell walls were reinforced with steel rods inserted into the shuttering before the concrete was poured in. The walls were fixed to the floor by means of steel rods already set in the concrete foundation and pointing up into the wall shuttering. The concrete roof was placed on top of the walls and the tank columns and again shuttering was utilised to form the shape for the roof. The first tanks were constructed with tubular columns but these were later replaced with built-up welded channel sections which were made up and welded in situ.

In a paper written after the war M.Noone: Wartime Underground Oil Storage in Britain he noted that “possibly the most remarkable thing about the whole programme was the reliance upon welding. The writer is not aware of an installation of underground tanks which used riveting as the means of joining the shell plates together. The fact that these underground tanks have given remarkably good service over a number of years has added materially to the confidence of the engineering industry in its use and application of electric arc welding.”

Tim






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