22. Casing head

Often called a casing hanger and is of multi purpose :-

• To provide a support point to suspend casing strings prior to being cemented into position.

• Used to provide a coupling between the various casing strings and the BOP stack.

• Having locked the casing with cement, the hanger then provides a seal between other casing strings and the annulus. A standard casing head is fixed to the first casing string and thereafter provides for up to three subsequent casing strings.

23. Mud pumps (Slush Pumps)

Two or three pumps are usually found on rig site, their function is to circulate the drilling fluid at the required pressure and volume. The type of pump used is a reciprocating piston pump. Pump design varies but the basic features are common Pumps may be classified on four features :-

A. Number of Cylinders:

Normally pumps have either two or three cylinders and are known as Duplex and Triplex respectively.

B. Pumping Action:

1.Double-acting Pumps - This means that both sides of the piston are used for pumping i.e. the piston is filling one side of the cylinder while fluid is being discharged on the other side. Usually Duplex pumps are double-acting. This type of pump can be easily recognised by a valve systems at both ends of the cylinder.

2. Single-acting Pumps - On these only one side of the piston is used for pumping, that is, the cylinder is then filling or discharging. Triplex pumps are usually single-acting recognizable by having a valve system at one end only.

C. Piston Stroke:-

This obviously is related to the output of the pump. It is fixed and can not be changed. The longer the stroke of the piston, the greater the pump output.

D. Cylinder diameter (Liner size): -

Each cylinder is equipped with a removable sleeve or liner. A pump, although of fixed stroke length, has a whole range interchangeable liners of different diameter available, allowing for different pressure/volume ranges with changing hole conditions. aurally, the smaller the liner diameter, the smaller the volume pumped on each stroke but as the liner walls are thicker, more pressure is available.

There are usually two to three pumps on a drilling rig. They are always of the positive displacement type. In other words plunger pumps rather like a bicycle pump.

PUMP TYPES:

*duplex, double-acting, or *triplex, single-acting

A duplex pump has two cylinders. Each cylinder has two suction and two discharge valves. As the piston moves through the cylinder it is discharging mud in front at the same time as mud is filling the cylinder behind.

A triplex pump has three cylinders with each cylinder having only one suction and one discharge valve. The cylinder is filled as the piston moves back and is discharged as the piston moves forward.

For one complete cycle of each piston a triplex pump discharges one cylinder full of mud. In a duplex pump however, because it is double acting, two cylinder volumes are discharged for every cycle of each piston.

(Figure 23) shows the pump action for each type.

Pu mps are commonly rated on the horse power they transit. Pistons are sometimes known as swabs. The discharge of the pump is fitted with a pulsation dampener and connected to the stand pipe and rotary hose (kelly hose), through the mud line via the mud line manifold or standpipe manifold.