Pumping systems

A method comprising:

• pumping fluid with a pumping arrangement, which comprises:
• a first system for pumping the fluid, the first system comprising:
• a sequence of two or more sub-systems each being a respective positive-displacement sub-system and having a respective one-way inlet;
• one or more one-way flow paths comprising a respective one-way flow path linking each adjacent two of the sub-systems;
• and
• a one-way outlet from a last of the sub-systems;
• wherein the respective one-way inlet of each of the two or more sub-systems receives the fluid in addition to any fluid received from the one or more one-way flow paths;
• wherein the pumping comprises:
• operating the pumping arrangement in a first mode of operation in which at least some of the sub-systems are substantially in phase with respect to each other to cause the first system to draw the fluid from more than one of the one-way inlets;
• and
• operating the pumping arrangement in a second mode of operation in which at least some of the sub-systems are substantially in antiphase with respect to each other to increment a pressure of the fluid as the fluid moves along the sequence to cause the pumping arrangement to deliver the fluid at higher pressure and lower flow than the pumping arrangement delivers in the first mode of operation;
• and
• wherein at least one of the sub-systems has a variable stroke length;
• and
• wherein a ratio of a stroke length of at least one of the sub-systems to a stroke length of another of the sub-systems downstream of the at least one of the sub-systems is lower, for the first mode of operation than the ratio is for the second mode of operation, to compensate for compression of the fluid as the fluid moves along the sequence.