HOISTING SYSTEM
The hoisting system is a large pulley system which is used to lower and raise equipment into and out of the well. In particular, the hoisting system is used to raise and lower the drillstring and casing into and out of the well. The components parts of the hoisting system are shown in Figure 3. The drawworks consists of a large revolving drum, around which a wire rope (drilling line) is spooled. The drum of the drawworks is connected to an electric motor and gearing system. The driller controls the drawworks with a clutch and gearing system when lifting equipment out of the well and a brake (friction and electric) when running equipment into the
well. The drilling line is threaded (reeved) over a set of sheaves in the top of the derrick, known as the crown block and down to another set of sheaves known as the travelling block. A large hook with a snap-shut locking device is suspended from the travelling block. This hook is used to suspend the drillstring. A set of clamps, known as the elevators, used when running, or pulling, the drillstring or casing into or out of the hole, are also connected to the travelling block.
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Figure 3 Hoisting system |
Having reeved the drilling line around the crown block and travelling block, one end of the drilling line is secured to an anchor point somewhere below the rig floor. Since this line does not move it is called the deadline. The other end of the drilling line is wound onto the drawworks and is called the fastline. The drilling line is usually reeved around the blocks several times. The tensile strength of the drilling line and the number of times it is reeved through the blocks will depend on the load which must be supported by the hoisting system. It can be seen from Figure 3 that the tensile load (lbs.) on the drilling line, and therefore on the fast line, Ff and dead line Fd in a frictionless system can be determined from the total load supported by the drilling lines, W (lbs.) and the number of lines, N reeved around the crown and travelling block:
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W |
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W |
(a) Free body diagram of traveling block (b) Free body diagram of crown block
Figure 4 Drilling line tension
Ff = F, = W/N
There is however inefficiency in any pulley system. The level of inefficiency is a function of the number of lines. An example of the efficiency factors for a particular system is shown in Table 1. These efficiency factors are quoted in API RP 9B — Recommended Practice on Application, Care and Use of Wire Rope for Oilfield Services. The tensile load on the drilling line and therefore on the fast line will then be :
Ff = W/EN
where E is the Efficiency of the from Table 1. The load on the deadline will not be a function of the inefficiency because it is static.
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Number of Lines (N) |
Efficiency (E) |
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6 |
0.874 |
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8 |
0.842 |
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10 |
0.811 |
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12 |
0.782 |
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14 |
0.755 |
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Table 1 Efficiency Factors for Wire Rope Reeving, for Multiple Sheave Blocks (API RP 9B) 6 |
Note: Table 1 applies to Four Sheave Roller Bearing System with One idler Sheave.
The power output by the drawworks, HPd will be proportional to the drawworks load, which is equal to the load on the fast line Ff, times the velocity of the fast line vf (ft/min.)
HPd= Ff_vf
33,000
Eight lines are shown in Figure 3 but 6, 8, 10, or 12 lines can be reeved through the system, depending on the magnitude of the load to be supported and the tensile rating of the drilling line used. The tensile capacity of some common drilling line sizes are given in Table 2. If the load to be supported by the hoisting system is to be increased then either the number of lines reeved, or a drilling line with a greater tensile strength can be used. The number of lines will however be limited by the capacity of the crown and travelling block sheaves being used.
The drilling line does not wear uniformly over its entire length whilst drilling. The most severe wear occurs when picking up the drillstring, at the point at which the rope passes over the top of the crown block sheaves. The line is maintained in good condition by regularly conducting a slip or a slip and cut operation. In the case of the slipping operation the travelling block is lowered to the drillfloor, the dead line anchor is unclamped and some of the reserve line is threaded through the sheaves on the travelling block and crown block onto the drawworks drum. This can only be performed two or three times before the drawworks drum is full and a slip and cut operation must be performed. In this case the travelling block is lowered to the drillfloor, the dead line anchor is unclamped and the line on the drawworks is unwound and discarded before the reserve line is threaded through the system onto the drawworks drum.
The decision to slip or slip and cut the drilling line is based on an assessment of the work done by the line. The amount of work done by the drilling line when tripping, drilling and running casing is assessed and compared to the allowable work done, as shown in Table 2. The work done is expressed in Ton-miles and is calculated as follows:
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Nominal Breaking strength of 6 x 19 I. W.R. C (Independant Wire Rope Core) Blockline (lbs) |
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Nominal Diameter |
Ton-miles between cuts |
Improved Plowed Steel |
Extra Improved Plowed Steel |
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1” |
8 |
89,800 |
103,400 |
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1 1/8” |
12 |
113,000 |
130,000 |
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1 1/4” |
16 |
138,800 |
159,800 |
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1 3/8” |
20 |
167,000 |
192,000 |
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1 1/2” |
24 |
197,800 |
228,000 |
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Table 2 Allowable work and Nominal Breaking Strength of Drilling Line Institute of Petroleum Engineering, Heriot-Watt University 7 |
Round Trip Operations:
The greatest amount of work is done by the drilling line when running and pulling the drillstring from the well. The amount of work done per round trip (running the string in hole and pulling it out again) can be calculated from the following:
Tr = D_(Ls + D) Wm + D (M + 0.5C)
10,560,0 2,640,000
All of the terms used in these equations are defined below.
Drilling Ahead:
The amount of work done whilst drilling ahead is expressed in terms of the work performed in making trips. Analysis of the cycle of operations performed during drilling shows that the work done during drilling operations can be expressed as follows:
Td= 3(T2-T1)
If reaming operations and pulling back the kelly to add a single or double are ignored then the work becomes:
Td= 2(J2-Tl)
Running Casing:
The amount of work done whilst running casing is similar to that for round tripping pipe but since the casing is only run in hole it is one half of the work. The amount of work done can be expressed as:
Tc = D (Lc + D) Wc + 4DM "21,120,000
Short Trips:
The amount of work done in pulling the drillstring back to the previous casing shoe and running back to bottom, for example to ream the hole can be expressed as in terms of the round trips calculated above:
TST — 2(T4-T3)
where:
Tr = Ton-miles for Round Trips
TrST = Ton-miles for Short Trips
Td = Ton-miles whilst drilling
Tc = Ton-miles for Casing Operations
Dc = Depth of hole (ft)
Ls = Length of drillpipe stand (ft)
Lc = Length of casing joint (ft)
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W m |
= wt/ft of drillpipe in mud (lb/ft) |
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W c |
= wt/ft of casing in mud (lb/ft) |
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M |
= wt. of blocks and elevators (lb) |
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C |
= wt. of collars — wt. of drillpipe |
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(for same length in mud) |
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T1 |
= Ton miles for 1 round trip at start depth (D1) |
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T2 |
= Ton miles for 1 round trip at final depth (D2) |
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T3 |
= Ton miles for 1 round trip at depth D3 |
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T4 |
= Ton miles for 1 round trip at depth D4 |
The selection of a suitable rig generally involves matching the derrick strength and the capacity of the hoisting gear. Consideration must also be given to mobility and climatic conditions. The standard derrick measures 140′ high, 30′ square base, and is capable of supporting 1,000,000 lbs weight. (Figure 5).
The maximum load which the derrick must be able to support can be calculated from the loads shown in Figure 4. The total load will be equal to:
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Figure 5 Drilling derrick |
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Exercise 1 The Hoisting System
A drillstring with a buoyant weight of 200,000 lbs must be pulled from the well. A total of 8 lines are strung between the crown block and the travelling block. Assuming that a four sheave, roller bearing system is being used.
a. Compute the tension in the fast line
b. Compute the tension in the deadline
c. Compute the vertical load on the rig when pulling the string