2. Write down the transcription of the following words and practise their pronunciation:

• vital

• tur­bulence

• cylinder

• hydraulic circuit

• circu­lating pressure

3. Read and translate the text.

The hydraulic horsepower of the circulating fluid at the bit is vital to rotary bit performance. In older conventional bits, the watercourses are positioned for cleaning only the bit cutters, leav­ing only a small portion of total fluid energy for bringing cuttings out of the hole. With the development and widespread acceptance of jet bits, fluid streams are routed directly to the hole bottom through jet nozzles, creating fluid tur­bulence around the bit and keeping the bit cut­ters clean and the hole bottom free of cuttings. Fluid velocities of up to 400 feet per second are not uncommon with jet bits. Because of the high nozzle velocities required, hydraulics planning is necessary to get maximum use of pump capabilities. Calculating hydraulics is closely interrelated with the mechanical factors that affect bit performance.

Unless bottomhole cleaning is properly main­tained, increases in bit weight and rotary speed will not increase the rate of penetration. At shallow depths, high-velocity jetting may actu­ally gouge out new hole, but its primary use is to clean the hole so that weight and speed increases can be usefully applied. Also, hydraulic power is necessary for transporting cuttings to the surface.

Hydraulic power at the bit depends on the availability of input power for the pump, high-pressure operation of the pump, a minimum of pressure losses in the hydraulic circuit, and opti­mum volume and pressure at the bit. If the hydraulic circuit is correctly set up, about ½ - ⅓ of the pressure on the fluid as it leaves the pumps will be expended through the bit nozzles, where it does the most good. Hydraulic horse­power is determined by pump output and circu­lating pressure. A change in either pump volume or circulating pressure directly affects total hydraulic horsepower.

Horsepower capability by itself is of little importance to the driller. Bit hydraulic horsepower (bhhp) - the power of the circulating fluid at the bit - is what really counts. Pressure losses throughout the circulating system lessen hydraulic horsepower. The ability to calculate and minimize such pressure losses is important to more cost effective drilling.

pressure losses in the hydraulic system

The first pressure losses occur in the surface equipment - standpipe, rotary hose, swivel, and kelly. These losses are usually small, and they are usually considered together with the drill string in pressure-loss calculations. In the drill string, friction is created between the mud and thousands of microscopic imperfections in the pipe walls. In its flow through the drill string, the mud itself is a source of further friction as little whirlpools and crosscurrents are created having a pressure-reducing effect.

Some pressure loss is a natural fact of drilling, but measures can be taken to avoid pressure losses and save money. Of course, the choice of bit and the particular drilling situation will sug­gest certain limits for a hydraulics program. Large-diameter drill pipe permits greater fluid volume and pressure at the bit than that allowed by smaller pipe. Plastic-lined drill pipe offers less resistance to fluid passage than unlined pipe, so a greater portion of the initial system pressure will reach the bit. Minimum restric­tions in the annulus will enable easier fluid return to the surface and prevent unwanted back-pressure on the bottom of the hole.

Mud properties also affect pressure losses. Increases in mud density or viscosity increase pressure losses. Also, lighter density mud is more easily pumped. The use of oil-emulsion or oil-base muds can reduce pressure losses from hole friction.

Although pressure is lost as the mud is pumped through the drill pipe, there must still be power and pressure when the mud enters the bit nozzles. As the mud leaves the nozzles, the greatest part of the system's pressure should be spent in order to clean the hole bottom, but some pressure must still remain for circulating the cuttings up the annulus and out of the hole. Proper nozzle selection is the key to a good hydraulics program. By determining the initial pressure of the mud pumps and then calculating the pressure losses throughout the system, the most efficient combination of nozzle sizes can be estimated.

Hydraulics calculation begins with the mud pumps. The size of the pump liners, replaceable sleeves that house the pistons in pump cylinders, should be coordinated with pump capacity to obtain the desired pump output. Desired output depends on the type of mud being used, the drilling depth, the desired annular pressure (for removal of cuttings), and the overall expectations of the particular drill­ing situation.