Derrick ( drilling rig ) – Petroleum Industry – Readyzone

Derrick

The conventional derrick is a four-sided truncated pyramid ordinarily constructed of structural steel, although tubular steel is used infrequently for certain parts of the derrick. Derricks may be either portable or nonportable, the portable derrick being commonly referred to as a mast. The nonportable derrick is usually erected by bolting the structural members together. After the well has been drilled, it can be disassembled, by unbolting, and erected again at the next location. As erection of the derrick may require several days, it is the practice of some companies to use two nonportable derricks with each rig. After a well has been completed, no attempt is made to dissemble derrick No. 1 immediately, because derrick No. 2 has already been erected at the next location. Considerable rig time can be saved by making use of the two derricks, because no time is lost by having to wait for the rig-building crew to disassemble the derrick, move it to the new location, and assemble it again. After the rig is drilling on well No. 2 and when the next location is known, then derrick No. 1 can be disassembled and erected at the next location. In flat terrain it is sometimes skidded to the next location without dismantling.

The principal considerations involved in the design of a derrick are these:

  1. The derrick must be designed to carry safety all loads which will ever be used in wells over which it is placed. This is the collapse resistance caused by vertical loading, or the dead-load capacity of the derrick. The largest dead load which will be imposed on a derrick will normally be the heaviest string of casing run in a well. However, this heaviest string of casing will not be the greatest strain placed on the derrick. The maximum vertical load which will ever be imposed on it will probably be the result of pulling on equipment, such as drill pipe or casing, that has become stuck in the hole. The designer must consider that, sometime during the useful life of the derrick, severe vertical strain will be placed on it because equipment has become stuck in the hole. Several methods can be employed to provide for these maximum strains. One is to allow a considerable additional load above the maximum casing load. Another is to design the derrick to with stand loads which exceed the capacity of the wire line which will be used on the rig.
  2. The derrick must also be designed to with stand the maximum wind loads to which it will be subjected. Not only must it be designed to with stand wind forces that will act on two sides at the same time (the outer surface of one side of the derrick, and the inner surface of the opposite side), but cognizance must also be taken of the fact that the drill pipe may be out of the hole and stacked in the derrick during periods of high winds. The horizontal force of the wind acting on it and drill pipe is counteracted by the pyramidal design of the derrick, by bolting the derrick legs to their foundations, and by the use of from one to three guy wires on each leg of the derrick. These guy wires are attached to dead men location some distance from the derrick. A dead man is made from a short length of large pipe, a concrete block, or a short section of timber, which is buried in the ground to provide an anchor for the guy wire. Guy wires are small diameter wire lines, usually less than one-half inch in diameter.
Derrick

Fig. 1 – Deep – drilling rig in operation

A typical derrick is shown in Fig. 1. The component parts of it are gin pole, crow’s nest water table, derrick man’s working platform, legs, girts, braces and ladder.

The gin pole, located at the extreme top of the derrick, is used principally for hoisting the crown block into place. The crow’s nest provides a safe working surface around the crown block. The water table is the opening in the top of the derrick into which the crown block fits. The derrick man’s working platform is the working area from which the upper end of the drill pipe is handled as the drill pipe is removed from, or inserted into, the hole. The derrick legs are the principal structural members of it. Each leg, of which there are four, is a continuous member extending from the base of the derrick to the water table. The girts are the horizontal structural members connecting and supporting the four derrick legs. The braces are the structural members used to strengthen the derrick by proper bracing between girts. The derrick ladder is used to provide a convenient access to the upper parts of it, principally the working platform for the derrick man and the crown block.

Derrick

Fig. 2 – Block – and – tackle combinations.

In view B of Fig. 2 the vertical loading of a derrick can be determined. The pull on each line is shown to be W/4, where W is equal to the weigh to be lifted. The pull on each line will be called T for further development of the problem. In other words, for the specific example of view B, Fig. 2, T = W/4. The load on the derrick caused by the hanging weight, W, will be the sum of the downward line pulls. As there are six lines pulling downward, the total vertical load is 6T. Figure 3 is a plan view of derrick floor, showing the hole, the hoist, the derrick legs, and the position of the dead line. The leads on the individual derrick legs are shown in Table 1.

Table 1 assumes that there are two sheaves in the traveling block. The maximum load on any leg would be 2.5T in this particular instance, and this maximum would occur only if the dead line were attached to one of the legs which also supported the hoist-line load. In practice, this is not a recommended procedure, as the maximum total load on any derrick leg could be reduced to 2.0 T by attaching the dead line to the leg A or B. However, using the case illustrated in Fig. 3, the maximum equivalent load on it would be the maximum leg load multiplied by four, as shown below

                                      Maximum equivalent load = 4 × 2.5 T = 10 T

However,

                                     Actual load on derrick = 6 × T = 6 T

Where

                                    6 = number of lines pulling down

The ratio of the equivalent load to the actual load is called the derrick efficiency factor. This is evident because of the various derrick leg loading combinations and also because of the fact that the derrick leg is the principal load-supporting structure, and if one derrick leg fails, obviously the entire derrick will fail.

DerrickIf the dead-line load is shifted to leg A or leg B, then the maximum load on the individual derrick leg becomes 2.0 T. The equivalent maximum load will now be : 4 × 2 T = 8 T. The derrick efficiency factor will be altered from the previous calculation :

DerrickSince the derrick as a whole is no stronger than its weakest member, if one of it legs is overloaded, the derrick may fail even though the total load is well below the calculated failure load with an evenly distributed load. Therefore, the derrick load should not exceed the safe equivalent load. The derrick efficiency factor, then, is a measure of the use made of the derrick strength, and proper placing of the dead line is a major factore in the most efficient utilization of the derrick design capacity. The required derrick capacity, based on vertical loading only, can be computed as follows:

Derrick

Derrick

Fig. 3 – Plan view of derrick floor.

An example problem will illustrate the practical utilization of the equivalent load theory.

Example: What percentage of rated derrick capacity may be utilized when six lines to the traveling block and eight lines on the crown block are used, with the dead line being attached to a derrick leg opposite the hoisting drum ( equivalent to attaching to leg A or leg B in Fig. 3 ).

         Solution:

            Tension in each line =  \frac{W}{6} = T

            Total load on derrick = 8T

                   (disregarding weight of crown loa d)

            Centered load = 6T

           Portion of centered load absorbed by each derrick leg = \frac{6 T}{4}  = 1.5 T

           Hoist-line load on leg C or leg D = 0.5 T

           Total load on leg C or leg D = 2.0 T

          Dead-line load on leg A = T

          Total load on leg A = centered load + dead-line load

                                             = 1.5 T + 1.0 T = 2.5 T

           Total derrick load if all legs were carrying a load equivalent to leg A    

                 = 4 × 2.5 T = 10 T

Derrick          Therefore, 80 percent of the rated derrick capacity can be used. The weight of the crown block, derrick, and derrick accessories were not considered in the previous development since they would have been an additive term in both the numerator and denominator of the derrick efficiency factor, and would not materially affect the result, especially where the load, T, is large compared to these weights.

The effective load carrying capacity of a derrick can be increased by reinforcing its legs. This reinforcement is usually in the form of tubular steel column placed adjacent to the derrick legs.

The API has developed specifications for derricks. The size number of the derrick refers to the number of panels or bays between the uppermost and lowest girts. Each panel has a height of seven feet. The first, or lowest, girt is approximately ten feet above the derrick floor for all derricks except some very tall ones, in which the lowest girt is located approximately fourteen feet above the derrick floor. In developing standard specifications for derricks, the API has defined the most important components of it. Derrick height is measured along the derrick leg from the top of the derrick floor to the bottom of the water table beams. The base square is the distance between derrick legs and is measured at the top of the derrick floor and inside the derrick legs a distance of approximately one-fourth the width of the leg angle. The water table opening is the open distance between flanges of the water table beams.

The first girt extends around three sides of most derricks. On the fourth side greater clearance must be provided in order to allow the handling of long joints of drill pipe, casing, and other pieces of equipment. The larger opening on the fourth side is called a V-window, because of its similarity to an inverted V.

The API safe-load capacity of a derrick is determined by computing the strength of its leg at its weakest point and multiplying this result by four, since there are four legs. The API rating does not consider either the weight of the derrick itself or the wind or other loads which may be imposed on the derrick. This is an important fact to understand, because in the API specifications ( Standard 4A ) there are requirement to withstand certain wind velocities. These wind forces are used in the design of the braces and girts in the derrick.

Derrick

Table 1 – Assumes that there two sheaves in the traveling block. The maximum load on any leg would be 2.5 T in this particular

Derrick substructures

Click here to read about Derrick substructures.

Example : Calculate the size of the corner mats required for a sandy loam soil condition and a maximum derrick load of 500,000 pounds. Use a safety factor of 3.

Derrick

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Reference : Mc Cray & Cole, Oil Well drilling Technology, New India Publication.

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