Pressure losses also increase with turbulence. If unconsolidated formations are to be penetrated, in an effort to minimise the potential for hole erosion, the flow regime shall be maintained in the laminar range, and nozzle velocities shall be limited to a maximum of 1 000 ft/sec. The longitudinal velocity attempts to make a fluid flow parallel to the conduit axis, while the transverse component attempts to move the fluid in a direction normal to the pipe axis. The disordered motion of fluid properties in turbulent flow results in two components of velocity, longitudinal and transverse components. In turbulent flow the pattern is random in both time and space. In the flat centre there is no shear of fluid layers. A special type of laminar flow with a flat centre is called plug flow. There is only one component of fluid velocity in laminar flow, a longitudinal component. In laminar flow shear resistance is caused by the sliding action and is independent of the roughness of pipe. The velocity of each layer increases towards the centre until a maximum velocity is reached. In Laminar flow the pattern is smooth, with fluid layers travelling in parallel lines parallel to the conduit axis. Note: If a mudmotor is used, flow rates are depended on the mudmotor requirements. The minimum flow rate required for PDC bit = 12.72 (D) 1.47 The recommended flow rate for roller cone bit is 30 to 50 GPM/in of bit diameter. Once hole cleaning requirements are fulfilled, bit hydraulics may be optimised. In optimisation of circulation rates, the emphasis must be placed upon supplying sufficient drilling fluid, at a high enough rate, to ensure that drilled cuttings are removed from the annulus in the shortest possible time. In situations where the rock is soft to medium hard, hole cleaning is the major objective and thus bigger nozzles are used to maximise nozzle areas. The smaller the nozzle area the greater the pressure drop and velocity. The pressure loss across the bit is influenced by the size of nozzles used and also determine the amount of hydraulic horsepower available at the bit. For a given length of drill string and mud properties pressure losses P 1, P 2, P 3, P 4 and P 5 remain constant. The objective is to optimise pressure drop across the bit such that maximum bottom hole cleaning is achieved. This is the most important element in the hydraulics equation. The two major models for calculation of these losses are the “Bingham plastic model” and the “Power law model”. Annular losses takes place across the drill collars (P 4) and drill pipe annulus (P 5). Pipe losses take place inside drill pipe (P 2) and drill collars (P 3). These losses are very difficult to calculate and the general equation isĮ = Constant depending type of surface equipment Pressure losses in surface connections are losses in standpipe, rotary hose, swivel and kelly. P 5= Annular pressure loss across Drillpipe Surface Connection Losses P 4= Annular pressure loss across Drill collars To optimize hydraulics and achieve sufficent hole cleaning the following guidelines should be adhered to: Pressure Losses Through System
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