Method and system for measurement of fluid flow in a drilling rig return

Abstract

Improved methods and systems are provided for determining a volumetric flow rate of mud in a return line without the aid of a velocity probe. A mud level sensor is located in a return line chimney preferably in close proximity to the bell nipple. The sensor senses the height of the mud flowing in the return line. Where an ultraonsic transceiver is used as the sensor, temperature sensors in the chimney and return line determine a temperature gradient in the return line which provides a speed of sound correction as does a target at a known distance from the transceiver. A computer receives the uncorrected mud height and the speed of sound information, and determines the flow-out volume of the mud. Flow-out volume is related to the mud height, the geometry of the return line, and mud parameters. In a more complete system, the computer calibrates the determined flow-out volume with the flow-in volume, and provides a correction factor which is used to provide a corrected flow-out determination. The computer also monitors delta flow (flow-out minus flow-in), and based on the rate of change of delta flow determines when an undesirable condition such as a kick or fluid loss occurs. Where the system is to be used on rig which is subject to movement (e.g. a floating rig), an angle sensor is employed to help determine instantaneous flow-out, and the flow-out determinations are averaged over the movement cycle of the rig as determined by heave position sensors available on the rig.