Rig Types

In a broad sense, rigs can be categorized as immobile and mobile. These terms describe the ability of the rig to depart quickly from a drillsite if necessary. This categorization is broad and provides little definition on each rig type.

Land and marine classifications provide a more logical grouping for rig types. Fig. 16-1 illustrates rigs under these categories. Although marine rigs have more types available for selection, many more land rigs are in use today.

Cable Tool Rigs. Cable tool rigs (Fig. 16-2} have played a key part in the history' of the drilling industry. Although not widely used today, these rigs accounted for about 20% of all wells drilled in the United States in 1961. Many shallow wells {3,000 ft) in areas such as Ohio, West Virginia, and Pennsylvania are drilled currently by cable tool rigs.

Drilling with a cable tool rig is accomplished by raising a special drill bit (Fig. 16-3) and dropping it on the formation. The drillstring is composed of the bit, the drillstem, jars, and a rope socket for attaching the tools to the drilling a

Portable (wheel mounted)

Cable tool

Rotary rigs

Standard derrick

Conventional «0

Classification Drilling Fluid
Fig. 16-1 Land (a) and marine (b) rig classifications

line or cable. As the bit drills, it is sharpened at the wellsite or at the nearest shop.

Drilling fluids are not used to remove the rock cuttings from the well; instead, bailers and sand pumps clean the hole batch by batch. These devices create a suction effect at the hole bottom, similar to a sand bailer used in current wireline operations.

SheaveSi Crown block . Water table-*

Sand line

Sand line

Sandline Unit Rig

Casing

Sand reel-Bpl

Sand reel post' Band \ Sampson^ wheel P°st, post Calf wheel/ Rope clamps

Drilling line.

Tool joint-

Tool joint ;

Temper screw Bailer Bull wheel

Cable Derrick floor Casing head

Casing

Rope socket I-Jars t-Drill stem ■Bit

Fig. 16-2 Cable tool rigs (Courtesy Petroleum Engineering Drilling & Well Completion)

The principal advantages of cable tool drilling, as noted by Gatlin, are as follows:

  • lower initial equipment cost
  • lower daily operating expense
  • lower transportation cost
  • lower rig-up time and expense
Cable Tool Rig
Fig. 16-3 Drill bit for a cable tool rig (Courtesy PennWell Publishing)
  • drilling rates somewhat comparable to those of rotary rigs in hard, shallow areas
  • more precise sample (cuttings) data
  • reduced formation damage from drilling fluids

These advantages may be outweighed by disadvantages, such as poor pressure control capabilities, hole stability difficulties, and the inability to use certain completion practices.

Land Rigs. The most common land rigs use either a standard derrick, a truck-mounted derrick, or a component rig. The American Petroleum Institute provides various documents relating to these rig types, including API Specifications 4E and 8A and Standards 4A and 4D.

Standard Derricks. The oldest type of rotary rigs in the drilling industry used the standard derrick. The derrick was constructed at the rig site before drilling the well and was usually torn down before moving to the next location. Quite often, the derrick was left standing above the well after it began production in case workovers became necessary (Fig. 16—4).

During the early years of its history, the standard derrick was an advancement over the cable tool rigs because it could be designed with sufficient strength and height to run longer casing joints and pull two, three, or even four joints of pipe. Unitized derricks that are raised and lowered as a whole rather than being

Standard Derrick
Fig. 16-4 Standard derrick (Courtesy Aubrey Kaigler)

constructed piecemeal have replaced the standard derrick. The rig-up and rig-down time with the standard derrick was excessive. In addition, a unitized derrick generally has fewer failures than a derrick that is built and dismantled for each well.

Portable Rig. A portable rig is usually considered to be a truck-mounted unit (Fig. 16-5). The truck bed contains the derrick, drawworks, and mud pumps. A principal advantage of the portable rig is the low rig-up and rig-down time as well as lower truck hire requirements. Portable rigs are used frequently in workover operations and when drilling to depths of about 10,000 ft. These rigs may be used on an 8-, 12-, or 24-hr/day basis.

Portable units for drilling generally are restricted to light-duty work. Where applicable, the unit is usually the most cost-effective rig. General capabilities for some portable rigs are as follows:

1900 Oil Derricks
Fig. 16-5 Portable truck-mounted rig (Courtesy NL Industries Inc)

maximum dcrrick capacity maximum pump How rate derrick height pit volume drillpipe size

350,000 lb

6-8 bbl/min single or double joints

Since most wells drilled worldwide are shallower than 8,000-10,000 ft, the portable rig plays an important role in drilling operations.

Caution must be exercised when planning a well that approaches the upper limits of portable rig capabilities. For example, drilling a well to 10.000 ft would not strain a large portable rig if 3'/z- or 4-in. drillpipe were used. However, if 23.0-lb/ft casing must be run to 10,000 ft, the total casing load (230,000 tb) does not provide an adequate safety factor with respect to dcrrick capacity. Although short-term economics may suggest using the portable rig instead of a larger, more expensive conventional rig, the larger rig may prove more economical over the long term if drilling problems should develop.

Conventional Rig. A conventional rig is considered usually as a component rig in which the key components are so large that they cannot be transported on a single truck bed. These rigs vary in depth rating from 6,000 to 35,000 ft. The conventional rig is usually operated on a 24-hr/day basis (Fig. 16-6).

Conventional rigs are the largest [and rigs available for drilling. Maximum capabilities are in the following ranges:

Most conventional rigs pull three-joint stands.

A typical rig specification sheet is shown in Fig. 16-7. These guidelines must be used to evaluate and compare rigs of various contractors before selecting a rig for the upcoming well. Information contained in the rig specs addresses the following areas:

  • drawworks
  • derrick
  • substructure
  • engines
  • pumps
  • mud mixing systems
  • drill string
  • preventers
  • miscellaneous equipment derrick capability = 2.000,000 lb pump horsepower — 3,000 hp pit volume - 300-1,500 bbl drillpipe size = 3 '/i-5Vi in.
Ideco Drilling Rigs
Fig. 16-6 Conventional drilling rig (Courtesy Goldrus Drilling)

The rig specifications usually contain a schematic of the rig's layout so the rig site can be prepared properly (Fig. 16-8).

Marine Rigs. Drilling rigs used offshore are termed marine rigs. A common grouping system for marine rigs is based on the bottom support of the rig on the seafloor (see Fig. 16-1). A bottom-supported rig rests on the seafloor or on pads built on the seafloor. Floating rigs rely on ballast systems similar to

Drawworks

Continental-Emsco C-1 type IfJ, 1:500 hp Grooved for 1W drill line Baylor 6032 dynamatic brake Derrick

Ideco Fullview 143 ft high

750,000-lb static hook load Substructure

Modified Ideco 21 ft high

700,000-lb casing capacity simultaneous with 400,000-lb setback capacity Power Source

3 Caterpillar D398-TA (diesel) 2,592 hp Pumps

1 National 10P-130, 1,300 hp 1 Gardner-Denver KXG, 1,000 hp 1 High volume-low pressure mud mixing system Dr II 1st ring

Drillpipe—AW OD various weights and grades High tensile strength drillpipe available Standard size collars available through 9 in. Preventers {HsS Trim)

1 13%-in. GL 5000 Hydril, annular

2 13%-in. type V 5000 Hydril, single gate

1 Koomey closing unit, 120-gal capacity, air and electric, 6-station accumulator with remote control

1 Lynn International choke and kill manifold, 4 in. x 2 in. x 5,000 psi W.P. Other Equipment

Crown block—Ideco seven 52-in, sheaves, 400-ton capacity

Traveling block—Continentai-Emsoo RA526, 6-55-in. sheaves, 400-ton capacity

Hook—Byron Jackson 4300

Bunk house—12 ft x 50 ft, wheeled, air conditioned

Crown-O-Matic

Desander—Swaco, six 6-in. cones, 1,200 gpm Desilter—degasser, twelve 4-tn. cones Drillpipe spinning wrench—Varco Drilling recorder Kelly spinner—Fastway

Light plants—two Caterpillar 135 kw, 110/220 v AC

Lights—Rig-A-Lite, vaporproof

Mud tanks—3-tank system with mud agitators

Radio—Motorola, 100-watt, FM

Rotary table—Continental- Emsco, 27Vh in.

Shale shaker—Brandt dual standard

Swivel—National type R

Fig. 16-7 Typical rig specification sheet (Courtesy Delta Drilling)

Htf "

360'

215'

140'

7tt7

ROAD

2 DITCH

rwmr

Fig. 16-S Typical rig layout specification (Courtesy Delta Drilling)

shipping vessels for support and do not rest on the scafloor. Rigs that fall into each category are as follows:

Bottom Supported Floating

Barges Semisubmersible

Jackups Drillship

Platform rigs Self-contained Tendered

Many design criteria are used in selecting the proper marine rig. Major criteria are as follows;

  • water depth rating
  • derrick and substructure capacity
  • physical rig size and weight
  • deck load capacity
  • stability in rough weather
  • duration of drilling program
  • rig rating features such as horsepower, pipe handling capabilities, mud mixing capacity
  • exploratory vs development drilling
  • availability and cost

Water depth rating is perhaps the first evaluative tool. Bottom-supported units can operate in a maximum of 400-ft water depths; 250-300 ft is the typical maximum. Platforms can handle any depth to 1,000 ft (and ultimately 2,500 ft with today's emerging technology), but deep platforms are justified only for longterm development drilling. Floaters can handle shortterm projects (like exploration) in water depths from 300-7,000 ft. The upper limit is being extended steadily; the lower limit depends on weather and the size of the drilling budget.

Rig size may be a limiting factor on platforms. Deck loading and stability in environmental conditions are used in selecting drillships and scmisubmersibles.

The American Bureau of Shipping (ABS) and other rating agencies have developed extensive guidelines relating to marine rigs. The ABS rules cover items such as operating ranges, structural design criteria, and classification procedures. Although these guidelines are important with respect to marine engineering, they will not be discussed extensively in this text due to their indirect application for drilling engineers. Interested readers should consult these rating agencies in the event they are required for some difficult drilling applications.

Barges. A drilling barge is used typically in 8-20 ft of water. The barge (Fig. 16-9) is towed to the location and sunk on the bottom by Hooding various vessel compartments. After drilling has been completed, the flooded compartments are evacuated, which allows the rig to float so it can be moved to the next location. Occasionally, special efforts are required to lift the barge because it becomes imbedded in the seafloor and will not float after evacuation.

The barge is generally designed as a fully self-contained vessel. In addition to a complete drilling rig, it has sleeping quarters for the crew and ancillary personnel as well as galley facilities. Crew boats are used for transportation between the rig and the nearest docking facility and for emergency personnel evacuation.

Operating water depth and weather limit the areas for the drilling barge. A minimum water depth of 6-8 ft is required for vessel draft during transport to the location. The upper range on the water depth can be extended to 30—40

Rig Offshore Type Barge Rig Cantilever
Fig. 16-9 Drilling barge {Courtesy NL Well Industries Inc)

ft if a shell mat or pad is built as a support base for the barge. Quite often, the shell pad is required due to poor seafloor supporting capabilities.

Drilling barges are used widely in areas such as the Gulf of Mexico and Niger basin marsh areas as well as coastal water immediately adjacent to land areas. Specially dredged channels may be required in marshes where no other access is available. These channels, or canals, may add significantly to the well costs for extensive dredging, such as long, deep, or wide channels, ft is not uncommon for channels to be 2-3 miles long. Barges are not used when wave height exceeds 5 ft.

Jackups. A jackup rig is perhaps the most widely used marine vessel for exploratory drilling. The principal components are a barge-type unit and three to five legs capable of supporting the vessel when extended. The rig is self-contained and requires only a standby boat for safety purposes (Fig. 16-10).

Marshes Jack Rig

Fig. 16-10 Jackup rig (Courtesy Marathon Manufacturing Co.)

The jackup is towed to the location and spotted over or near the well site, depending on the rig type. During the towing process, the legs are up in the air and the barge floats on the water. After spotting the rig, the legs are jacked down to contact the seafloor and the rig is raised into the air. When drilling is completed, the rig is lowered to the water level, the legs are raised, and the rig is towed to the next location.

Most jackup rigs have three or four legs that may be 200^00 ft long. Sophisticated jacking systems raise and lower the legs. The legs use either a spud can on each leg or a mat base connecting the legs as the support device on the seafloor (Fig. 16-11). it is not uncommon to have some difficulty removing the support base from the loosely consolidated, sticky subseafloor soils. Therefore, most rig legs are equipped with water jetting systems to help release the

Slot Type Jackup Rig

Fig. 16-11 Mai base for a jackup rig (Courtesy Western Oceanic Inc.)

cans or mats. The mat-supported rigs are generally considered preferable for areas with soft clay soils because they allow a broader distribution of the rig weight and thus produce less soil penetration. Dangerous leg breakthroughs are avoided.

Spud can designs have included a large assortment of sizes and shapes. A trend toward larger spud cans is demonstrated by the increase in footing bearing area from 200 ft2 for the Offshore Company's Rig 52 built in 1955 to 3,300 ft' for the Marathon Gorilla (1983). Footing shapes have varied widely, with most having a pointed or wedge configuration to ensure the footing can key itself into a hard bottom if encountered.

These rigs are typically designed for use in 13-25-ft minimum water depths and as much as 250-350-ft maximum water depths in some cases. The maximum operating depth is restricted in hurricane or severe weather conditions. A jackup designed for a 300-ft maximum operating depth may have upper limits of 203-210-ft operating depth in hurricane conditions. In addition, these weather-

dependent conditions vary for different areas due to wind and wave actions in a typical storm, i.e., Gulf of Mexico vs North Sea.

Jackup rigs are segregated into slot or cantilever rigs, depending on the application requirements. The slot rig has an area within the outer limits of the rig through which all drilling is done. The derrick is located over the slot (Fig. 16-12).

The cantilever rig can be positioned over or near the well site. The mast is mounted on a cantilever beam (Fig. 16-13) such that it can be moved out from the main body of the barge and positioned over the well. This flexibility allows the operator to build a lighter, cheaper platform that supports only wellheads. it also permits the cantilever rig to be used in some well servicing operations on small production platforms. Maximum safe operating derrick loads must be determined for the rig, depending on the distance starboard or port of the centerline from the rig to the rotary. Drilling contractors often provide charts to determine the safe load limits.

The barge and rig are self-contained with respect to life-support systems and drilling equipment. The barge is usually divided into two or three decks. The top deck contains the rig, pipe racks, and occasionally the living quarters. The lower decks contain all support drilling equipment, such as pumps and mud systems, as well as the auxiliary barge equipment (Fig. 16-14).

Platform Rigs. The discovery of significant petroleum reserves in an offshore area usually requires the installation of a production platform to extract the reserves economically. A number of wells are drilled directionally from the platform to exploit the reservoir. This approach is more cost effective, in most cases, than many single-well platforms with vertical wells.

Rigs that drill from platforms are either fully self-contained or are tender supported. If the production platform is sufficiently large, all drilling and personnel support equipment is maintained on the rig. A floating tender is used to support personnel and auxiliary drilling equipment such as mud pumps and bulk chemical storage if the platform working area or load is restricted (Fig. 16-15).

Drillships. Floating rigs such as drillships or scmisubmersibles do not rest on the seafioor during drilling. As such, these rigs are not restricted by the length of the rig's legs for maximum water operating depth. These two rig types have different operating characteristics that must be reviewed prior to rig selection.

Drillships use a ship-type vessel as the primary structure to support the rig (Fig. 16-16). It may be a converted seagoing vessel or, as is common in recent years, it may be a new vessel constructed for drilling. It may be self-propelled or require oceangoing tubs for transport.

Senusubmerubles. The semisubmersible drilling unit (Fig, 16-17) is a specially designed vessel used exclusively in petroleum operations. The ballasted lower hulls are below sea level to maximize rig stability. The primary advantage

Lower Hull Oilrig

3 IQ

Schematic Jackup Rig

Fig. 16-12 Slot-type jackup rig (Courtesy Western Oceanic Inc.)

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