Arrangement for controlling volume in a gas or oil well system

An arrangement to control volume of fluids in a gas or oil well system having a riser extending from a well to a rig, the riser having an increased diameter section relative to a major part of the riser, the increased diameter section being situated below the upper end of the riser and above sea level or ground level, and above any slip joint in the riser;

• the arrangement further comprising a sensor to continuously measure the position of the slip joint;
• the increased diameter section being coupled to an outlet that is in fluid communication with a mud return system, wherein the arrangement further comprises a return pump coupled between the outlet and the mud return system, the outlet being arranged at a lower level than the mud return system, the pump being positioned to pump mud from the outlet to the mud return system, and level sensors measuring the level of liquid within the increased diameter section, wherein the liquid level in the riser is adjusted, by using the return pump, to a level that is within the increased diameter section.