Drilling mast ( drilling rig ) – Readyzone

Drilling mast 

The design and development of oil well derrick has kept pace with other fast-developing segments of the oil industry. The development of the portable derrick, or drilling mast, as it is usually called, has resulted in a material reduction in drilling costs, primarily in the amount of time consumed in rigging up and tearing down.

The development of the portable well-service unit has paralleled the development of the portable derrick, for one of the principal requirement of a portable well-service unit is a drilling mast or derrick of some kind. Before the advent of the portable well-service unit, it was necessary to leave the derrick in place over the well for use in future well-service operations. The portable well-service unit eliminated the need for a permanent derrick and thus materially reduced over-all oil well costs.

Drilling - mast

Fig. 1 – A drilling mast

A drilling mast, shown in Fig. 1, is defined as a structure which can be moved as a unit, without dismantling. The masts used in oil well drilling operations may vary from a simple pole structure for drilling shallow wells to a four-leg structure closely resembling a standard derrick. Masts have been developed with heights as great as 146 feet and load capacities exceeding 1,000,000 pounds. These masts can be used safely to depths approaching 15,000 feet in many areas. The drilling masts require a simple, reliable method of raising and lowering, which is generally accomplished by using the drilling line in conjunction with an A-frame of some type.

One of the problems associated with the development of the portable derrick is providing sufficient working room on the derrick floor while limiting the derrick base to dimensions that will permit its being transported across public highways. Most designs which are available telescope or unbolt to provide the short lengths required for highway travel.

Two basic types of drilling masts have evolved as a result of the increased demand for portable derricks: (1) the free-standing drilling mast and (2) the guyed drilling mast. The free-standing drilling mast, as the term indicates, uses no guy wires for support. If properly designed, it will withstand loads as great as any guyed drilling mast available.

Construction of derrick and drilling masts

In the early days of the oil industry, oil derricks were constructed of wood, but the advent of structural steel has eliminated wooden derricks. The few remaining are those which were left standing over old wells and are still in a safe enough condition to use for well-servicing work.

The material most commonly used for derricks today is structural steel of one form or another. Angle, channel, and I-beam in various sizes are consistently used in derrick designs. Tubular steel is commonly used to reinforce derrick legs. Single-pole drilling masts are generally made of tubular steel, many of them being fabricated of oil well casing or line pipe. As the problem of weight is paramount in portable drilling mast design, some drilling masts have been fabricated of structural aluminium, which materially reduces the weight.

Derrick substructures

In drilling operations, a working space must be provided below the derrick floor. The actual space required will depend upon the type of rig being used and the formation pressure that will be encountered in the drilling of the well. In order to control excessive formation pressures and prevent the wells from getting out of control, blowout preventers must be attached to the innermost string of casing in order that they can be closed around the drill pipe, or, if the drill pipe is out of the hole, can be used as a valve to close the casing completely. Some blowout-preventer designs will close the opening at the top of the casing regardless of whether the drill pipe is in or out of the hole. The closing element is comprised of a hydraulically expanded rubber sleeve designed to withstand any formation pressures which might be encountered. The rubber element in this type of blowout preventer is sufficiently flexible to permit the rotation or even the complete withdrawal of the drill pipe while maintaining a positive seal. Other types of blowout preventers are fitted with rams, which are made of steel faced with hard rubber. The closing elements in this type of blowout preventer are not flexible and therefore will close around only one geometrical configuration. If these blowout preventers are equipped to close around the drill pipe and it becomes necessary to close the blowout preventers while the drill pipe is out of the hole, then at least one joint of drill pipe must be placed in the hole before an effective seal can be maintained. The rams, or closing elements, of these blowout preventers can be changed to fit the size of drill pipe being used, or they can be fitted with blank rams, which give complete closure with the drill pipe removed from the hole. In many areas, at least two blowout preventers are used in series, and in this case several feet of space beneath the derrick floor are required. To provide ample work room, the derrick floor is elevated above the ground level by placing it on a substructure. The substructure is normally fabricated from structural steel, and the loads it must bear are greater than those borne by the derrick, since the substructure must support not only the derrick with its load, but other loads, such as the rotary table and draw works, as well.

The derrick and substructure must be placed on foundations that will safely support all the loads to which the derrick and substructure may be subjected. Concrete provides an excellent foundation, but it is expensive, and, further, in areas where foundations must be removed after the drilling has been completed, removal of concrete becomes a problem. In efforts to eliminate concrete foundations with their objectionable features, extensive use has been made of the bearing capacity of the soil to support the required loads. Footings for each derrick leg must, of course, be larger when concrete foundations are not used. Spreading the load of each derrick leg over a relatively large area can be accomplished by placing each corner of the substructure on a base which provides the proper bearing area. These bases may be made of structural steel members laid on a wooden timber matting. The steel bases may be welded and the timber mats bolted to reduce the set-up and tear-down time when the rig is moved. To determine the size of each mat required, it is necessary to know the load-bearing capacity of the soil and the maximum derrick loads anticipated.

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.


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


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