Surface Mud Volume Measurements on Floating Rigs
Authors: Schuh Frank J. 
Published: December 2007

Tests on rigs in the Gulf of Alaska show that variations in surface mud volume measurements can range from 20 to 60 bbl (3.2 to 9.5 M), depending on vessel motions. These variations were reduced to acceptable levels with conventional equipment by placing single or pairs of sensors at or straddling the centroid of the mud surfaces in each pit.

Introduction
Pit-volume totalizers were developed initially for Pit-volume totalizers were developed initially for conventional land rigs with steel pits. On land operations, one float or sensor is placed in each pit downstream of the constant-volume shale shaker and treating pits. The signals from each sensor are combined to provide a measurement of the total mud volume in the portion of the active system downstream of the constant-level treating pits. On most pit-volume totalizer systems, the "total volume" also pit-volume totalizer systems, the "total volume" also is displayed on a gain/loss indicator that can be set manually at the zero position. The gain/loss indicator shows the number of barrels of mud that has been gained or lost since the signal was set at the zero position. The gain/loss indicator is equipped with position. The gain/loss indicator is equipped with limit switches that sound an alarm whenever a preset gain or loss limit has been exceeded. For well control operations these limits ideally would be set at the smallest practical gain or loss. Where reasonable care is exercised in providing a constant surface-volume mud system, the gain/loss limits can be set at acceptable values. In floating drilling operations, these limits must be increased by the magnitude of any variations in the pit-volume totalizer signal due to vessel motions.This paper reports measurements of the magnitude variation in pit-volume totalizer readings that result from vessel motions if one sensor is placed in each pit and describes an arrangement of sensors that virtually eliminates the volume variations caused by vessel motions.

Theory
Although the mud systems on floating rigs are subject to complex combinations of heave, pitch, roll, surge, and sway, the principal effect on pitvolume totalizer measurements appears to be caused pitvolume totalizer measurements appears to be caused by the pitch or roll motions. Fig. 1 is a schematic of a pit that shows how pitch or roll motion affects the pit that shows how pitch or roll motion affects the pit-volume totalizer measurements. The variation in pit-volume totalizer measurements. The variation in pit volume measurements caused by angular motions pit volume measurements caused by angular motions between the sensor and centroid of the pit can be calculated from the following relation.
(1)
For the pits studied, this relationship predicts a peak-to-peak range of volume measurements of more than peak-to-peak range of volume measurements of more than 50 bbl (7.9 M) for 3 degrees of pitch and roll motions.If the single sensor is placed at the centroid of the mud-pit surface, this relationship would predict no change in volume measurements caused by pitch or roll motions. It also is apparent that the average height of two sensors placed on a line through and equidistant from the centroid of the surface will eliminate the pitch and roll motion effects.

Measurements
Pit-volume totalizer (PVT) measurements were Pit-volume totalizer (PVT) measurements were obtained on a floating rig during a period when pitch and roll motions ranged from 1 to 3.5 degrees. Fig. 2 is a schematic of the pits and PVT sensor arrangements studied. Tables 1 and 2 list the measurements of the PVT signal range and the pitch and roll motion PVT signal range and the pitch and roll motion measurements. Test periods generally were limited to periods of 1 hour. periods of 1 hour. JPT
P. 1497