A Heave-Compensated System To Determine Returning-Mud Flow Rate Aboard Floating Drilling Vessels
Authors: Barton C.E. 
Published: December 2007

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Introduction

The first indication of a well kick or lost-circulation problem is a change in drilling fluid flow returning from problem is a change in drilling fluid flow returning from the wellbore. Both kicks and lost-circulation problems must be detected early so that they can he controlled without damaging the well.

The return flow rate of drilling fluid is difficult to determine on a heaving, floating drilling rig The problem is caused by the pumping action of the telescopic problem is caused by the pumping action of the telescopic joint in the marine riser as the drilling rig moves in the sea waves. For example, a 6-ft vessel motion with a period of 15 seconds causes a 1,200-gal/min-amplitude oscillatory pumping action in a 2O-in.-diameter telescopic joint. pumping action in a 2O-in.-diameter telescopic joint. The pumping action of the telescopic joint alters both the quantity of fluid stored in the marine riser assembly above the telescopic joint and the flow rate of fluid leaving the marine riser through the flowline. This variable flow rate of drilling fluid in the flowline masks changes in the returning flow rate caused by down-hole phenomena. phenomena. Until now, it has been customary to use the mud-pit-level indicating system to detect kick or lost-circulation problems on floating drilling rigs. Rig motion affects problems on floating drilling rigs. Rig motion affects readings of the mud-pit-level indicating systems because the mud sloshes in the pits as the floating rig moves with the sea. Variations in the mud pit level resulting from motion are relatively less severe than the variations in the returning-mud flow rate measured at the flowline. Response of the pit-level indicating system is slow because the mud flow rate must act for some time to change the amount of mud in the pits.

This system measures the returning-mud flow rate on a floating rig by monitoring all fluid stored in and leaving the upper riser assembly. These data then can be manipulated to yield the flow rate of drilling fluid entering the upper riser assembly, which is presumed to be the flow rate of fluid leaving the wellbore.

Equipment

The system comprises four basic types of elements. Magnetic flowmeters measure the flow rates of drilling fluid going to the mud pumps and exiting from the returning flowline. Magnetically actuated tank-level transmitters measure the amount of drilling fluid in both the surge tank and the telescopic joint. A surge tank is used in the upper riser assembly to attenuate vessel-motion-induced return flow variations. The surge tank also provides a protected location for the sensors that measure the protected location for the sensors that measure the amount of fluid contained in the upper riser assembly. An electronics unit combines the signals from the tank-level transmitters and the magnetic flowmeters. The electronics unit also supplies signals to drive indicators, recorders, and alarms.

A block diagram of the system, installed aboard the Discoverer 511, is shown in Fig. 1. Magnetic flowmeters are used because they have no interior obstructions that could impede the passage of drilling fluids. While the meters remain in service, the electrodes can be cleaned ultrasonically. The magnetic flowmeters are located in the rig mud system and always are filled with drilling fluid and positioned to minimize the buildup of drill solids in the meters.

Magnetically actuated, tank-level transmitters measure the volume of drilling fluid in the surge tank and the telescopic joint.

JPT

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