sentences:  Therefore the cavities hired by Shell and the most frequently used for the Stanlow refinery had the largest accumulation of sludge.A second problem with the cavities used for the storage of crude oil was that the extraction rate, at 7,200 cubic metres per day, was too slow.A system for the removal of fuel from the cavities at a much higher flow-rate of around 25,000 cubic metres per day had been installed as part of the original design.