The mud pits are usually a series of large steel tanks, all interconnected and fitted with agitators to maintain the solids, used to maintain the density of the drilling fluid, in suspension. Some pits are used for circulating (e.g. suction pit) and others for mixing and storing fresh mud. Most modern rigs have equipment for storing and mixing bulk additives (e.g. barite) as well as chemicals (both granular and liquid). The mixing pumps are generally high volume, low pressure centrifugal pumps.
At least 2 slush pumps are installed on the rig. At shallow depths they are usually connected in parallel to deliver high flow rates. As the well goes deeper the pumps may act in series to provide high pressure and lower flowrates.
Positive displacement type pumps are used (reciprocating pistons) to deliver the high volumes and high pressures required to circulate mud through the drillstring and up the annulus. There are two types of positive displacement pumps in common use:
(i) Duplex (2 cylinders) – double acting
(ii) Triplex (3 cylinders) – single acting
Triplex pumps are generally used in offshore rigs and duplex pumps on land rigs. Duplex pumps (Figure 7) have two cylinders and are double-acting (i.e. pump on the up-stroke and the down-stroke). Triplex pumps (Figure 8) have three cylinders and are single-acting (i.e. pump on the up-stroke only). Triplex pumps have the advantages of being lighter, give smoother discharge and have lower maintenance costs.
Institute of Petroleum Engineering, Heriot-Watt University