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Derrick In Drilling Rigs Complete Guide

The derrick and substructure play an important role in drilling rig operations. The derrick provides the height necessary for the hoisting system to raise and lower the pipe. The greater the height, the longer the section of pipe that you can handle and, thus, the faster a long string of pipe that you can run into or remove from the hole.

Derricks can handle sections (stands), which have a composition of two, three, or four joints of drill pipe. Because common drill pipes are between 8 and 10 m long (approximately 26 to 33 ft), a derrick designed to handle three-drill pipe stands will be taller than a 10-story building. The substructure provides the height required for the blowout preventer stack on the wellhead below the rig floor. The derrick and the substructure must have enough strength to support all loads, including the hook load, pipe set in the derrick, and wind loads.

Storage of doubles inside the derrick
Storage of doubles inside the derrick

In general, Derrick In oil and gas drilling rigs is a pyramidal steel framework with square or rectangular cross-sections assembled as a fixed structure.

Derrick, large load-bearing structure from which the hoisting system and therefore the drill string is suspended.

A semipermanent structure of square or rectangular cross-section having members that are latticed or trussed on all four sides. This unit must be assembled in the vertical or operation position, as it includes no erection mechanism. It may or may not be guyed

API Definition

Substructures

Derricks and portable masts other than the telescopic type are supported by a heavy steel sub­ structure. The derrick transmits all the vertical loads through four points at the lower ends of the legs, and the purpose of the substructure is to transmit these loads to the ground via the rela­tively large area of the flanges of the basal I-beams. The required bearing capacity of the ground is thus kept within reasonable limits and less expensive foundations are required.

The height of the substructure is chosen so that in combination with the cellar depth it will accommodate below the derrick floor a BOP stack appropriate to the rig.

Telescopic masts are normally mounted on wheeled units and the loads are supported by jacks. The bearing capacity is provided by inserting plates or beams under the jacking units. Such masts are only used for relatively light work, thus the vertical loads are not as great as are experienced with derricks and the other type of mast.

Purpose & Application

Rig derrick and mast in oil and gas have two functions.

  • The secondary function, which is not fulfilled by the masts used with lighter types of well servicing rigs, is to provide support so that tubular (drill pipe, drill collars, or tubing) can be stored vertically, or “racked”, in “stands” of three lengths.

By doing this only one in every three connections has to be unscrewed when tripping the pipe out of the hole to change the drilling bit or perform other work on the lower end of the drill-pipe string, thus saving time.

Most drilling units (Types of drilling rigs) use “stands” consisting of three (Range 3) drill pipe joints. However, some small drilling rigs, especially small workover units, use stands consisting of two joints, and some heavy rigs use stands of four joints.

Types of Derricks

There are mainly 2 types of derricks in drilling rigs:

  • Stationary Type: Derrick used on offshore fixed structures
  • Dynamic Type: These derricks are primarily used on board floating drilling rigs. They are much stronger than the API standard derrick. Given that they rarely have to be dismantled they consist of welded sections that are pinned together instead of being constructed of individual bolted beams.

Construction Of Standard Derrick In Drilling Rigs

The so-called API standard derrick is shown in Figure 2. It consists of four main legs of beams that are bolted together plus a series of horizontal girts and diagonal cross bracing, also bolted. The material is normally structural steel with a minimum yield of 224.4 N/mm2 (33,000 psi) and a hot-dipped galvanized surface finish. One side of the derrick has a higher opening at the bottom than the other sides; this is known as the V­ door and is required in order to give suf­ficient clearance to life forty foot (12 m) lengths of pipe from the catwalk into the derrick.

A standard derrick
Figure 2: A standard derrick

Some eighty feet above the derrick floor is the derrickman‘s platform, called the “monkey board”, where he has the equipment to handle the upper ends of the stands of pipe and rack them neatly. This consists of “fingers” hinged to a bar which is fastened across two legs of the derrick. Figure 3 shows the details.

Racking equipment in drilling derrick
Figure 3: Racking equipment in derrick

At the top of the derrick is the crown block with the axle supported in a structure called the water table. Above the crown is one single beam, called the gin-pole, which is used to support a pulley block used solely to life the crown into position.

This construction allows the standard derrick to be transported on normal trucks and to be erected without heavy lifting equipment. A special­ ist rig-building crew (Rig personnel) is used, who construct it from the bottom up, with each successive layer lifted into place by a gin-pole supported by cables attached to the previous layers. This is illustrated in Figure 4. Since every component of a derrick is designed to carry its share of the load any parts omitted, improperly placed or needing to be forced into place may contribute to the failure of the derrick. For this reason, the nuts and bolts are tightened only slightly more than finger-tight (where safety permits) during the initial construction. Once the entire structure has been erected all nuts and bolts are then tightened to the required tension. This method ensures a more even distribution of stresses.

Unfortunately, the name “gin-pole” has been given to two different pieces of equipment related to the derrick – the connection is that they are both tempo­rary lifting devices.

Gin-pole for rig building
Figure 4: Gin-pole for rig building

Derrick Sizes and General Dimensions In Drilling Rigs

Derrick Size No.Height (A) – ftHeight (A) – inNominal Base Square (B) – ftNominal Base Square (B) – inDrawworks Window Opening (C) – ftDrawworks Window Opening (C) – inV Window Opening © – ftV Window Opening © – inOpening (D) – ftOpening (D) – inGin-Pole Clearance E – ftGin-Pole Clearance E – in
ftinftinftinftinftinftin
10800200762385680
11870200762385680
12940240762385680
161220240762385680
1813602607623856120
18A13603007623856120
1914003007626676170
2014703007626666170
2518903707626676170
Derrick Sizes and General Dimensions
Tolerances For Standard Derricks in drilling rigs:

Tolerances For Standard Derricks in drilling rigs:

  • A, The vertical distance from the top of the base plate to the bottom of the Crown Block support Beam, +/- 6 in
  • B, The distance between heel to heel of adjacent legs,+/- 5 in
  • C, The window opening measured in the clear and parallel to the center line of the derrick side from top of the base plate, + 3 ft, 6 in
  • D, The smallest clear dimension at the top of the derrick that would restrict the passage of crown block, +/- 2 in
  • E, The clearance between the horizontal header of the gin pole and the top of the crown support beam.,+/- 6 in

Derricks Vs Masts

Conventional or standard derricks are used:

  • where there are many wells in the same location: without dismantling, the derrick is rolled to the next cellar.
  • on small, tender supported platforms where there is insufficient space to assemble a mast horizontal! y.
  • in situations where mast sections cannot be transported.
  • on very heavy-duty rigs for deep wells where the inherently stable layout of a standard drilling derrick can be made stronger than a portable mast and has more racking capacity.

Disadvantages of such derricks

  • Specialist rig builders are required.
  • Erection and dismantling are time-consuming. (But time delays can be avoided by “leap-frogging” two derricks.)

Manufacturer & IADC Specifications

Derricks are classified (or rated) by the American Petroleum Institute (API) according to their height as well as their ability to withstand wind and compressive loads. API has published standards for the particular specifications. The higher the derrick is, the longer stands it can handle which in turn reduces the tripping time. Derricks that are capable to handle stands of two, three, or four joints are called to be able to pull “doubles”, “thribbles”, or “fourbles” respectively.

They are manufactured in accordance with API 4F or related ISO – This specification covers the design, manufacture, and use of derricks, portable masts, crown block assemblies, and substructures.

  • Spec 4E, Specification for Drilling and Well Servicing Structures, covers steel rig derricks, portable masts, and substructures suitable for drilling or well servicing. It includes stipulations of materials, loadings, conditions of loading, and allowable unit stresses intended to meet the requirements of present and future operating conditions, such as deeper drilling, offshore drilling from floating devices, and the effect of earthquakes, storms, and other adverse conditions. An appendix contains recommendations for maintenance and use of drilling and well-servicing structures.
  • Spec 4F, Specification for Drilling and Well Servicing Structures, this specification covers the design, manufacture, and use of steel derricks, portable masts, crown block assemblies, and substructures suitable for drilling and servicing of wells. It includes stipulations for marking, inspection, standard ratings, design loading, and design specifications of the equipment.

Drilling Rig Mast and Derrick Nameplate Information

  1. Manufacturer’s name
  2. Manufacturer’s address
  3. Specification 4F
  4. Serial number
  5. Height in feet
  6. Maximum rated static hook load in pounds, with guy lines if applicable, for the stated number of lines to a traveling block
  7. Maximum rated wind velocity in knots, with guy lines if applicable, with a rated capacity of pipe racked
  8. The API specification and edition of the API specification under which the structure was designed and manufactured
  9. Manufacturer’s guying diagram-for structures as applicable
  10. Caution: Acceleration or impact, also a setback, and wind loads will reduce the maximum rated static hook load capacity
  11. Manufacturer’s load distribution diagram (which may be placed in mast instructions)
  12. Graph of maximum allowable static hook load versus wind velocity
  13. Mast setup distance for a mast with guy lines.

Periodic Inspection

The API applicable references are API RP 4G, API RP 54, and the Manufacturer’s recommendations. Some companies are more strict and require the API Category IV inspection (as per API RP 4G) every 5 years instead of 10. Mast/derricks and substructures on mobile offshore drilling rigs or fixed platforms are exempted from the requirements of a Category IV inspection.

If any section or part of the structure is damaged or if concealed damage is detected, report it immediately and paint the damaged area with a highly contrasting color of paint. Even slight damage in certain areas may be sufficient cause to condemn the structure until it can be repaired. Contact the manufacturer, give him the size, type, and serial number of your structure; detail the damage for him and follow his advice concerning structure loading and repairs.

Fixtures and accessories are preferably attached to a structure by suitable clamps. Do not drill or burn holes in any members or perform any welding without obtaining approval of the manufacturer.

Rig Derrick Capacity In Drilling

The ratings for all types of derricks and masts are specified by the manufacturers in accordance with the standards These include a design is given in API Specification 4F “Drilling and well-servicing structures”. These include a design factor of 50% for steel derricks when new. The ratings of used equip­ment, however, should be reviewed regularly after inspection by the manufacturer or insurance company surveyor.

Ratings given include:

  • Maximum hook load i.e. the maximum string weight including the weight of the trav­elling block.
  • Maximum setback capacity i.e. the maximum weight of drill pipe and drill collars that can be racked in the derrick or mast.
  • Maximum wind velocity with the full setback.
  • Pitch and roll tolerances (for offshore applications).
  • General depth rating for a given size and weight of drill pipe.

It is therefore essential to be familiar with the capacity of the derrick or drilling rig in use. The neces­sary information can be obtained from:

  • the manufacturer’s nameplate which gives “Mast and Derrick Name Plate Information”.
  • substructure Name Plate Information.
  • manufacturer’s operating instructions.

The following strength and capacity factors should be taken into account to prevent the der­rick or mast from being overloaded:

  • Maximum anticipated casing load design. (Although a rig may be rated to drill to a certain depth, it may not be designed strong enough to run casing/conductor heavier than drill pipe to that same depth.)
  • Anticipated wind load or other exceptional conditions e.g. dynamic derrick foundations as on a drill ship.
  • Substructure (base) setback capacity.

Field welding is not normally allowed on masts or derricks. Where this is unavoidable high grade welding rods have to be used under special procedures. During welding a repre­sentative of the manufacturers will have to be present. It will then be necessary to have the mast or derrick inspected by the certifying company.

Rig Derrick & Mast Drilling Loads

During operation derricks and masts are subjected to vertical forces arising from the load car­ried by the traveling block and hook and horizontal forces arising from the pressure exerted by the wind.

On a floating unit, the derrick will oscillate as the vessel rolls and pitches which will result in the hook load having a lateral (relative to the derrick) component. The movement of a floating vessel will also generate inertial loads in the derrick, but the period is so long that these will be insignificant relative to the other loads.

Vertical forces

Figure 4 shows one possible arrangement of the drilling line, the fast-line, and the dead­ line (the fast line and the deadline are the names given respectively to the sections of the drilling line between the crown and the draw-works and between the crown and the point where the line is made fast on the substructure. The latter is known as the deadline anchor). The arrange­ment shown would be described as having six lines strung because there are six segments of the line supporting the traveling block.

arrangement of the drilling line in derrick mast
Figure 1

Figure 4: Ignoring friction, the tension in the line is = Hookload / N

  • N = the number of lines strung.
  • Hook load = load on hook + weight of traveling block

It is important to note that because the ends of the drilling line are attached at rig-floor level there are two more lines pulling down on the crown block than there are pulling up on the traveling block. It follows that:

Derrick load = Hook load + Pase line load + Dead line load

Static Derrick Load

The static derrick load (ignoring the weight of the derrick plus crown block) occurs when the block is not moving but is carrying the full hook load. Because there are no friction effects the fast line load and the deadline load are both equal to the line tension and:

static derriclk load equation

Derricks and masts in drilling rigs are designed to have a static load capacity for a specified number of lines and with an established position for the deadline anchor. Changing the number of lines strung or moving the deadline anchor position will alter the static load capacity considerably.

Dynamic Derrick Load (crown load)

The dynamic rig derrick load is the load that occurs while running in or pulling out a drilling string or casing string. The derrick load is still the sum of the hook load, the fast line load, and the dead-line load, but in this case, friction plays a part – which is a combination of the bearing friction of the sheaves and the internal friction within the line as it is bent round the sheaves.

To balance frictional forces the tension in a line increases by a factor “k” as it passes over each sheave. This assumes that the frictional forces are directly proportional to the load on the sheave and that the line wraps around 180° of the sheave. The latter is true for the lines which pass round the sheaves in the traveling block and is approximately true for the fast line and the deadline.

In drilling line calculations, allowance is made for friction by means of factors derived from the above-mentioned factor “k” and the number of lines strung.

The \hspace{0.1cm} fast \hspace{0.1cm} line \hspace{0.1cm} factor =  \frac{Fast \hspace{0.1cm} line \hspace{0.1cm} tension}{Hook \hspace{0.1cm} load} =    \frac{k^{N}  \big(k-1\big)}{ k^{N} - 1 }

The \hspace{0.1cm} dead \hspace{0.1cm} line \hspace{0.1cm} factor =  \frac{Dead \hspace{0.1cm} line \hspace{0.1cm} tension}{Hook \hspace{0.1cm} load} =     \frac{k-1}{ k^{N} -1}

Thus \hspace{0.1cm} the \hspace{0.1cm} load \hspace{0.1cm} on \hspace{0.1cm} the \hspace{0.1cm} crown \hspace{0.1cm} block \hspace{0.1cm} = \frac{ k^{N+1} +k-2}{ k^{N} -1} x hook \hspace{0.1cm} load

API RP 9B quotes a value of 1.04 for “k” for roller bearing sheaves, which are the most com­mon type. Substituting this figure in the above equation and comparing the result with the static derrick load indicates that the dynamic load on the derrick is insignificantly higher than the static load. The increase due to friction varies from 1 % with only two lines strung (i.e. a single sheave traveling block) to 0.5% with ten or twelve lines strung.

Nore however, that the changes in fast line and deadline tensions are far from insignificant. With ten lines strung the dynamic fast line tension will be some 23% higher than the static line tension, and the dynamic deadline tension will be 17% less. What is even more signifi­cant is that if the weight indicator sensor is on the deadline, which it usually is, the dynamic fast line tension with ten lines strung will be almost 50% higher than the figure shown by the weight indicator.

Shock Loads

As shock loads are difficult to calculate rigs are designed to twice their rated strength (the design factor of 50% previously mentioned). Shock loads occur when a string is picked up out of the slips and there is the acceleration of the string and when the brake is applied decelerate it. The tripping of a hydraulic jar or mechanical jar will induce shock loads (shock absorber) although most of the energy is transmitted to the stuck pipe.

The harsh acceleration or deceleration of a heavy string can result in large inertial forces devel­oping which are capable of overloading the derrick or mast. It is also possible that the block line or the string could break in such circumstances. For this reason, the inertial forces should be reduced as far as possible when the rig is handling heavy loads by braking and picking up the string with care.

Wind load

The wind load on a derrick with a rack full of drill pipe results in considerable transverse forces.

Derricks and masts are constructed in accordance with API Spec 4F to comply with a maxi­ mum wind load specified for that particular drilling rig. Most derricks and masts can withstand a wind load of 45 – 58 m/s (100 – 130 mph) with the racks full of pipe.