Rotary Drilling System
2.3 Drilling Fluid Circulation System
2.3.1 Mud Pumps
The great majority of the pumps used in drilling operations are reciprocat-ing positive displacement pumps (PDP). Advantages of the reciprocatreciprocat-ing PDP when compared to centrifugal pumps are:
• ability to pump fluids with high abrasive solids contents and with large solid particles,
• easy to operate and maintain,
• sturdy and reliable,
• ability to operate in a wide range of pressure and flow rate.
Centrifugal pumps are very sensitive to abrasive solid contents mud, and do not offer a wide range of operation compared to PDP.
PDP are composed of two major parts, namely:
Power end: receives power from engines and transform the rotating movement into reciprocating movement.
Fluid end: converts the reciprocating power into pressure and flow rate.
The efficiency Emof the power end, that is the efficiency with which rotating me-chanical power is transformed in reciprocating meme-chanical power is of the order of 90%. The efficiency Ev of the fluid end (also called volumetric efficiency), that is, the efficiency that the reciprocating mechanical power is transformed into hydraulic power, can be as high as 100%.
Rigs normally have two or three PDPs. During drilling of shallow portions of the hole, when the diameter is large, the two PDPs are connected in parallel to provide the highest flow rate necessary to clean the borehole. As the borehole deepens, less flow rate and higher pressure are required. In this case, normally only one PDP is used while the other is in standby or in preventive maintenance.
The great flexibility in the pressure and flow rate is obtained with the possibility of changing the diameters of the pair piston–liner. The flow rate depends on the following parameters:
• stroke length LS (normally fixed),
• liner diameter dL,
• rod diameter dR(for duplex PDP only),
• pump speed N (normally given in strokes/minute),
• volumetric efficiency EV of the pump.
In addition, the pump factor Fp is defined as the total volume displaced by the pump in one stroke.
There are two types of PDP: double-action duplex pump, and single-action triplex pump. Triplex PDPs, due to several advantages, (less bulky, less pres-sure fluctuation, cheaper to buy and to maintain, etc,) has taking place of the duplex PDPs in both onshore and offshore rigs.
2.3.1.1 Duplex PDP
The duplex mud pump consists of two double–action cylinders (see Figure 2.16-a). This means that drilling mud is pumped with the forward and backward movement of the barrel.
Curtin University of Technology Department of Petroleum Engineering
Master of Petroleum Well Engineering Drilling Engineering Fundamentals
(a) Piston scheme (double action). (b) A duplex unit.
Figure 2.15: Duplex pumps.
(a) Piston scheme (single action). (b) A Triplex unit.
Figure 2.16: Triplex pumps.
For a duplex pump (2 double–action cylinders) the pump factor is given by:
Fp = π
2 2d2L− d2R LS EV . A typical duplex pump is shown in Figure 2.16-b.
2.3.1.2 Triplex PDP
The triplex mud pump consists of three single–action cylinders (see Figure ??-a). This means that drilling mud is pumped only in the forward movement of the barrel.
For a triplex pump the pump factor is given by:
Fp = 3π
4 d2L LS EV . A typical triplex pump is shown in Figure ??-b.
2.3.1.3 Pump Flow Rate
For both types of PDP, the flow rate is calculated from:
q = N Fp.
For N in strokes per minute (spm), dL, dR, and LS in inches, Fp in in3, and q in gallons per minute (gpm) we have:
q = 1
231N Fp.
Note that in this particular formulation, the volumetric efficiency of the pump is included in the pump factor.
2.3.1.4 Pump Power
Pumps convert mechanical power into hydraulic power. From the definition of power we can write:
P = F v .
In its motion, the piston exerts a force on the fluid that is equal to the pressure differential in the piston ∆p times the area A of the piston, and the velocity v is equal to the flow rate q divided by the area A, that is
PH = (∆p A) q
A = ∆p q . (2.5)
For PH in hp, ∆p in psi, and q in gpm we have:
PH = ∆p q
1714.29. (2.6)
Example 4: Compute the pump factor in gallons per stroke and in barrels per stroke for a triplex pump having 5.5 in liners and 16 in stroke length, with a volumetric efficiency of 90%. At N = 76spm, the pressure differential between the input and the output of the pump is 2400 psi. Calculate the hydraulic power transferred to the fluid, and the required mechanical power of the pump if Em is 78%.
Solution:
The pump factor (triplex pump) in in3 per stroke is:
Fp = 3π
4 × 5.52× 16 × 90% = 1026 in3 Converting to gallons per stroke and to barrels per stroke gives:
Fp = 1026 × 1
231 = 4.44gps = 4.44 × 1
42 = 0.1058bps The flow rate at N = 76spm is:
q = N Fp = 78spm × 4.44gps = 337.44gpm
Curtin University of Technology Department of Petroleum Engineering
Master of Petroleum Well Engineering Drilling Engineering Fundamentals
The hydraulic power transferred to the fluid is:
PH = 2400psi × 334.44gpm
1714.29 = 468hp
To calculate the mechanical power required by the pump we must consider the efficiencies:
P = 468hp × 1
90% × 1
78% = 667hp
2.3.1.5 Surge Dampeners
Due to the reciprocating action of the PDPs, the output flow rate of the pump presents a “pulsation” (caused by the changing speed of the pistons as they move along the liners). This pulsation is detrimental to the surface and down-hole equipment (particularly with MWD pulse telemetry system). To decrease the pulsation, surge dampeners are used at the output of each pump. A flexible diaphragm creates a chamber filled with nitrogen at high pressure. The fluctu-ation of pressure is compensated by a change in the volume of the chamber.
The schematic of a typical surge dampener is shown in Figure 2.17.
A relief valve located in the pump discharge line prevents line rupture in case the pump is started against a closed valve.
Figure 2.17: Surge dampener.