Table Heating Value Of Various Fuels

Fuel Type diesel gasoline butane methane

Density Heating Value (Ibm/gal) (Btu/lbm)

19,000 20,000 21,000 24,000

Engine

Engine

Frictionless Pulley

Frictionless Pulley

P.^.r.f components, thus minimizing shock and vibration problems.

Direct-drive rigs accomplish power transmission from the internal combustion engines using gears, chains, belts, and clutches rather than generators and motors. The initial cost of a direct-drive power system generally is considerably less than that of a comparable diesel-electric system. The development of hydraulic drives has improved greatly the performance of this type of power system. Hydraulic drives reduce the shock and vibrational problems of the direct-drive system. Torque converters, which are hydraulic drives designed so that output torque increases rapidly with output load, are now used to extend the speed-torque characteristic of the internal combustion engine over greater ranges that are better suited to drilling applications. The use of torque converters also allows the selection of engines based on running conditions rather than starting conditions.

Power-system performance characteristics generally are stated in terms of output horsepower, torque, and fuel consumption for various engine speeds. As illustrated in Fig. 1.13, the shaft power developed by an engine is obtained from the product of the angular velocity of the shaft, w, and the output torque T:

2 jrrNF

The overall power efficiency determines the rate of fuel consumption wy at a given engine speed. The heating values H of various fuels for internal combustion engines are shown in Table 1.1. The heat energy input to the engine, Q-t, can be expressed by

Since the overall power system efficiency, Et, is defined as the energy output per energy input, then

Fig. 1.13- Engine power output.

Fig. 1.14-Making a connection.12

Example 1.1. A diesel engine gives an output torque of 1,740 ft-lbf at an engine speed of 1.200 rpm. If the fuel consumption rate was 31.5 gal/hr, what is the output power and overall efficiency of the engine?

Solution. The angular velocity, a>, is given by o) = 2tt(1 ,200) = 7,539.8 rad/min. The power output can be computed using Eq. 1.1: P=wT

33,000 ft-lbf/min/hp -397-5hP"

Since the fuel type is diesel, the density p is 7.2 lbm/gal and the heating value H is 19,000 Btu/lbm (Table 1.1). Thus, the fuel consumption rate uy is

J V 60 minutes'

The total heat energy consumed by the engine is given byEq. 1.2:

Qj -WjH

_ 3.78 lbm/min( 19,000 Btu/lbm)(779 ft-lbf/Btu) 33,000 ft-lbf/min/hp

Thus, the overall efficiency of the engine at 1,200 rpm given by Eq. 1.3 is

P 397.5

1.4 Hoisting System

The function of the hoisting system is to provide a means of lowering or raising drillstrings, casing strings, and other subsurface equipment into or out of the hole. The principal components of the hoisting system are (1) the derrick and substructure, (2) the block and tackle, and (3) the drawworks. Two routine drilling operations performed with the hoisting system are called (1) making a connection and (2) making a trip. Making a connection refers to the periodic process of adding a new joint of drillpipe as the hole deepens. This process is described in Fig. 1.14. Making a trip refers to the process of removing the drillstring from the hole to change a portion of the downhole assembly and then lowering the drillstring back to the hole bottom. A trip is made usually to change a dull bit. The steps involved in coming out of the hole are shown in Fig. 1.15

1.4.1 Derrick or Portable Mast. The function of the derrick is to provide the vertical height required to raise sections of pipe from or lower them into the hole. The greater the height, the longer the section of

Fig. 1.15-Pulling out of the hole.12

Fig. 1.15-Pulling out of the hole.12

Crown Block

Fast. Line

Traveling Block

Draw Works

Crown Block

Fast. Line

Traveling Block

Draw Works

(a) Arrangement and nomenclature of block and tackle.

(b) Free body diagram of traveling block.

W

(c) Free body diagram of crown block.

Fig. 1.16-Schematic of block and tackle.

pipe that can be handled and, thus, the faster a long string of pipe can be inserted in or removed from the hole. The most commonly used drillpipe is between 27 and 30 ft long. Derricks that can handle sections called stands, which are composed of two, three, or four joints of drillpipe, are said to be capable of pulling doubles, thribbles, or fourbles, respectively.

In addition to their height, derricks are rated according to their ability to withstand compressive loads and wind loads. Allowable wind loads usually are specified both with the drillstring in the hole and with the drillstring standing in sections in the derrick. When the drillstring is standing in the derrick resting against the pipe-racking platform, an overturning moment is applied to the derrick at that point. Wind ratings must be computed assuming wind loading is in the same direction as this overturning moment. Anchored guy wires attached to each leg of the derrick are used to increase the wind rating of small portable masts. The American Petroleum Institute (API) has published standards dealing with derrick specifications and ratings.13

To provide working space below the derrick floor for pressure control valves called blowout preventers, the derrick usually is elevated above the ground level by placement on a substructure. The substructure must support not only the derrick with its load but also the weight of other large pieces of equipment. API Bull. D104 recommends rating substructure load-supporting capacity according to (1) the maximum pipe weight that can-be set back in the derrick, (2) the maximum pipe weight that can be suspended in the rotary table (irrespective of setback load), and (3) the corner loading capacity (maximum supportable load at each corner). Also, in API

Standard 4A,1 three substructure types have been adopted. In addition, many non-API designs are available. The choice of design usually is governed by blowout preventer height and local soil conditions.

1.4.2 Block and Tackle. The block and tackle is comprised of (1) the crown block, (2) the traveling block, and (3) the drilling line. The arrangement and nomenclature of the block and tackle used on rotary rigs are shown in Fig. 1.16a. The principal function of the block and tackle is to provide a mechanical advantage, which permits easier handling of large loads. The mechanical advantage M of a block and tackle is simply the load supported by the traveling block, W, divided by the load imposed on the drawworks, /y:

The load imposed on the drawworks is the tension in the fast line.

The ideal mechanical advantage, which assumes no friction in the block and tackle, can be determined from a force analysis of the traveling block. Consider the free body diagram of the traveling block as shown in Fig. 1.16b. If there is no friction in the pulleys, the tension in the drilling line is constant throughout. Thus, a force balance in the vertical direction yields nFf=W, where n is the number of lines strung through the