Circulating System
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Bit |
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Figure 6 Circulating system |
The circulating system is used to circulate drilling fluid down through the drillstring and up the annulus, carrying the drilled cuttings from the face of the bit to surface. The main components of the circulating system are shown in Figure 6. The main functions of the drilling fluid will be discussed in a subsequent chapter — Drilling Fluids. However, the two main functions of the drilling fluid are:
• To clean the hole of cuttings made by the bit
• To exert a hydrostatic pressure sufficient to prevent formation fluids entering the borehole
Drilling fluid (mud) is usually a mixture of water, clay, weighting material (Barite) and chemicals. The mud is mixed and conditioned in the mud pits and then circulated downhole by large pumps (slush pumps). The mud is pumped through the standpipe, kelly hose, swivel, kelly and down the drillstring. At the bottom of the hole the mud passes through the bit and then up the annulus, carrying cuttings up to surface. On surface the mud is directed from the annulus, through the flowline (or mud return line) and before it re-enters the mudpits the drilled cuttings are removed from the drilling mud by the solids removal equipment. Once the drilled cuttings have been removed from the mud it is re-circulated down the hole. The mud is therefore in a continuous circulating system. The properties of the mud are checked continuously to ensure that the desired properties of the mud are maintained. If the properties of the mud change then chemicals will be added to the mud to bring the properties back to those that are required to fulfil the functions of the fluid. These chemicals will be added whilst circulating through the mud pits or mud with the required properties will be mixed in separate mud pits and slowly mixed in with the circulating mud.
When the mud pumps are switched off, the mud will stop flowing through the system and the level of the mud inside the drillstring will equal the level in the annulus. The level in the annulus will be equal to the height of the mud return flowline. If the mud continues to flow from the annulus when the mud pumps are switched off then an influx from the formation is occurring and the well should be closed in with the Blowout preventer stack (See below). If the level of fluid in the well falls below the flowline when the mud pumps are shut down losses are occurring (the mud is flowing into the formations downhole). Losses will be discussed at length in a subsequent chapter.
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.
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Discharge Valves Discharge Valves
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Figure 7 Duplex pump
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Discharge Valve.
Piston Rod
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Intake Valve.
Figure 8 Triplex pump
The discharge line from the mud pumps is connected to the standpipe — a steel pipe mounted vertically on one leg of the derrick. A flexible rubber hose (kelly hose) connects the top of the standpipe to the swivel via the gooseneck. The swivel will be discussed in the section on rotary system below.
Once the mud has been circulated round the system it will contain suspended drilled cuttings, perhaps some gas and other contaminants. These must be removed before the mud is recycled. The mud passes over a shale shaker, which is basically a vibrating screen. This will remove the larger particles, while allowing the residue (underflow) to pass into settling tanks. The finer material can be removed using other solids removal equipment. If the mud contains gas from the formation it will be passed through a degasser which separates the gas from the liquid mud. Having passed through all the mud processing equipment the mud is returned to the mud tanks for recycling.
There will be at least two pumps on the rig and these will be connected by a mud manifold. When drilling large diameter hole near surface both pumps are connected in parallel to produce high flow rates. When drilling smaller size hole only one pump is usually necessary and the other is used as a back-up. The advantages of using reciprocating positive displacement pumps are that they can be used to:
• Pump fluids containing high solids content
• Operate over a wide range of pressures and flow rates
and that they are:
• Reliable
• Simple to operate, and easy to maintain
The flowrate and pressure delivered by the pump depends on the size of sleeve (liner) that is placed in the cylinders of the pumps. A liner is basically a replaceable tube which is placed inside the cylinder to decrease the bore.
The Power output of a mud pump is measured in Hydraulic Horsepower. The horsepower delivered by a pump can be calculated from the following:
HHP= PxQ 1714
where,
HHP = Horsepower Q = Flow rate (gpm)
P = Pressure (psi)
Since the power rating of a pump is limited (generally to about 1600 hp) and that the power consumption is a product of the output pressure and flowrate, the use of a smaller liner will increase the discharge pressure but reduce the flow rate and vice versa. It can be seen from the above equation that when operating at the maximum pump rating, an increase in the pump pressure will require a decrease in the flowrate and vice versa. The pump pressure will generally be limited by the pressure rating of the flowlines on the rig and the flowrate will be limited by the size of the liners in the pump and the rate at which the pump operates.
a
The mechanical efficiency (Em) of a pump is related to the operation of the prime movers and transmission system. For most cases Em is taken as 0.9. Volumetric efficiency (Ev) depends on the type of pump being used, and is usually between 0.9 and 1.0. The overall efficiency is the product of Em and Ev.
A schematic diagram of a duplex pump is shown in Figure 7. As the piston moves forward discharging fluid ahead of it, the inlet port allows fluid to enter the chamber behind it. On the return the fluid behind the piston is discharged (i. e. on the rod side) while fluid on the other side is allowed in. The theoretical displacement on the forward stroke is:
1 4
where,
d = liner diameter L = stroke length
on the return stroke
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V |
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n(d2 — d2r )L
where,
d = rod diameter
r
Taking account of the 2 cylinders, and the volumetric efficiency Ev the total displacement (in gallons) of one pump revolution is:
2(V, + V2 )E, = ^
The pump output can be obtained by multiplying this by the pump speed in revolutions per minute. (In oilfield terms 1 complete pump revolution = 1 stroke, therefore pump speed is usually given in strokes per minute) e. g. a duplex pump operating at a speed of 20 spm means 80 cylinder volumes per minute. Pump output is given by:
Q = (2d2 — d;)LE„R 147
where,
Q = flow rate (gpm) d = liner diameter (in.)
dr = rod diameter (in.) L = stroke length (in.) R = pump speed (spm)
These flow rates are readily available in manufacturers’ pump tables.
A schematic diagram for a triplex pump is given in Figure 8. The piston discharges in only one direction, and so the rod diameter does not affect the pump output. The discharge volume for one pump revolution is:
= 3VE = 3nd2LEv
Again the pump output is found by multiplying by the pump speed:
Q = d2LEvR Q 98.03
where,
Q = flow rate (gpm)
L = stroke length (in.) d = liner diameter (in.)
R = pump speed (spm)
More power can be delivered using a triplex pump since higher pump speeds can be used. They will also produce a smoother discharge since they pump an equal volume at every 120 degree rotation of the crankshaft. (A pulsation dampener, or desurger, can be installed on both duplex and triplex pumps to reduce the variation in discharge pressure). The efficiency of a triplex pump can be increased by using a small centrifugal pump to provide fluid to the suction line. Triplex pumps are generally lighter and more compact than duplex pumps of similar capacity, and so are most suitable for use on offshore rigs and platforms.
Exercise 2 The Mud Pumps
Calculate the following, for a triplex pump having 6in. liners and 11in. stroke operating at 120 spm and a discharge pressure of 3000 psi.
a. The volumetric output at 100% efficiency
b. The Horsepower output of the pump when operating under the
conditions above.