After the first few days of the invasion, it was hoped that petroleum could be supplied in bulk.[1] Pipelines were not the sole or even the principal means by which Combined Operations was contemplating supplying bulk petroleum; it intended to rely primarily on small shallow-draught coastal tankers, of which thirty were under construction.[5][6] American 600-deadweight-ton (610-deadweight-tonne) "Y" tankers began arriving in the UK in the spring of 1944. In 1943, the British also initiated a programme to construct 400-deadweight-ton (410-deadweight-tonne) Channel tankers (Chants), but only 37 were completed by May 1944.[7] It was hoped that petroleum products might also be supplied by ocean-going T2 tankers lying offshore through ship-to-shore pipelines. The project to develop these pipelines was codenamed Operation Tombola, and the pipelines themselves became known as Tombolas.[5] The submarine pipeline had sufficient advantages to make it worthwhile to explore as a backup means of supply.