Rig Types(Part-I)
1. LAND RIGS
Land rigs vary considerably in size, lifting capacity, power generation, ability to circulate fluids... etc. See figure 01 below for an example of a land rig.
Figure:1
2. OFFSHORE RIGS
JACK-UP RIGS A.
A Jack-up rig is a movable platform that can be jacked up and down three or four supporting legs. It supports drilling in relatively shallow water depths (down to 400 feet). To move rig between close locations the platform is lowered down the legs till it floats then the legs are jacked up to the maximum height. The whole rig can then be towed by means of two boats. In long rig moves or across oceans the whole rig is normally carried on a huge carrier.
ack-up rigs are the most utilized type of rigs nowadays. Recently, legs design has been modified to support drilling in deeper waters. See figure 02 b e l o w 
Figure 02
The following lists some advantages and disadvantages of the Jack-up rigs:
Advantages
1.Provides a fixed platform at low initial cost. Can operate in soft bottom deltas.
3.Can withstand storms. Does not need marine risers or sub-sea stack.
Disadvantages
Poor safety records.
2.Difficult to tow.
Has moving parts in jacking mechanism. Hazardous for going on and off locations
SEMI-SUBMERSIBLES B.
A semi-submersible is a floating drilling rig. A typical layout is shown below in
figure 03.
Figure 03
The following are some advantages and disdvantages of Semi-submersibles:
Advantages
- Has a good safety record.
- Provides a relatively stable platform.
- Can function under more severe weather conditions.
- Can drill in deep water - up to 2000 ft.
- Can be self-propelled.
Disadvantages
- Requires marine risers and a sub sea stack.
- Has a limited cargo capacity.
- Requires support vessels.
DRILL SHIPS C.
These are special types of ships that are built for deep water drilling. They range in length between 200-450 feet. Their cargo carrying capacity and general mobility make them especially useful for drilling in remote areas. An example is shown in figure 04 below.
Figure 04
Some advantages and disadvantages of drill ships are:
Advantages
1. High carrying capacity.
2. Can drill in remote areas.
3. Can operate in deep water.
4. Self propelled.
Disadvantages
1. The lack of suitable risers to support drilling mud circulation between well head and drilling floor.
2. Not as stable as jack-ups and semi-submersibles.
3. Requires a sub-sea stack.
PLATFORM RIGS D.
A platform is a fixed installation offshore from which development drilling and petroleum production is carried out. A steel platform design is shown as an example in figure 05 below.
The deck, supported by a steel jacket, carries equipment, accommodation modules and a helicopter pad (helideck). It also supports one or more drilling rigs with associated equipment.
Figure 05
THE DRILLING RIG
The basic function of a drilling rig is to drill a bore hole. This hole must be drilled in an accurate, economic and safe way. Drilling rigs are of two main types:
rotary and cable tool.
1. Rotary drilling:
The rotation is generated either by a rotary motor to a rotary table or a top drive motor that directly rotates the drill string. The rotary table transfers power to the bit via the rotary and Kelly bushings. Weight is supplied by allowing a small amount of the drill string weight to rest on the bit.
2. Cable tool drilling:
With the wireline method, a cable is used to connect the bit to the surface and a pounding motor is used to effect the weight on the bit.
RIG COMPONENTS
Rigs are made up of various components. However the following discussion is restricted to land rig components, most of these components are also found offshore. Jack up rigs and fixed platforms are very similar to land rigs. They are not subject to wave and tidal movements as being fixed in relation to the sea bed during drilling operations. Therefore, they have no need for heave components or sub sea stack BOP’s. In the following few pages we will discuss the main components of drilling rigs taking land rigs as an example.
LAND RIG COMPONENTS
Mast or Derrick 1.
a) Masts
Masts are assembled on the ground from large welded sections fastened together with pins. They may then be raised to the vertical position by using the rig’s own power unit and hoisting line. Small masts may be truck mounted while some are telescopic. This rigging-up time for masts tends to be less than that for conventional derricks. Rigging-up time is the time spent to assemble a mast into the vertical position on-site. It also includes the time to install the power unit, all cables and piping. Masts are used for lighter work. Cantilever masts (also known as jack-knife derricks) are also common. Figure 06 shows a typical mast layout.
Figure 06
b) Derricks
There is no clear-cut distinction between a mast and a derrick. Often, the two terms are used interchangeably. To keep things simple we will regard a derrick as the framework-tower type of support usually associated with oil well drilling. Typically derricks are assembled on-site bolting individual pieces together. The rigging-up time for this method is of course, longer than for a cantilever mast. (Figure 07)
Figure 07
2. Substructure
This is the support on which the derrick rests. It also acts as a support for the heavy equipment on the rig floor. The top of the substructure varies in height from 10-30 feet above the ground. This clearance is provided so that wellhead equipment can be installed underneath the rig floor. (Figure 08)
Figure:8
3. V-Door
This is a triangular opening on the front of the derrick to allow drill pipe and equipment to be picked up from the catwalk and brought into the derrick.
4. Monkey Board
This is a platform situated at a specific height from the rig floor, typically 60 to 90 feet, on which the derrick man works during trips. This platform also supports the fingers that are used to rack the stands of drill pipe. Figure 08
The top of the derrick is known as the crown, it is usually a small platform designed to carry the crown blocks. The crown blocks are the uppermost set of sheaves on which the drilling line is strung; most crowns of recent manufacture have from four to six sheaves. (Figure 09)
Figure 09
6. Travelling Block
This is merely the travelling pulley (sheave) assembly that is slung from the crown blocks by the drilling line. It connects the drilling line to the hook and swivel. (Figure 09)
7. Hook
This is suspended directly from the block. It is composed of a main hook which carries the swivel bail and two smaller offset hooks carrying the elevator arm bails. The hook can rotate in the axis of the travelling block, limited by a lock device ; it is also sprung. (Figure 09)
8. Elevators
Two elevators are hung from the hook on the elevator bails and are used for latching around the drill pipe in order to lift it. Elevators are of many slightly differing designs and sizes for use with different pipe sizes, drill collar and casing sizes. They are not used during the drilling operation but are necessary for lifting the pipe during a trip
Pipe Elevator Figure 10
9. Slips
These devices are used to hold the weight of the drill string when it is not supported by the hook (during connections or tripping time). Slips are made of hinged sections with a single opening. They are placed around the pipe, their tapered outer sections fitting against either the inside surface (bowl) or the master bushing or against the inserts. As the pipe is lowered, the slips tapered section causes them to close tightly around the pipe. (Figure 11)
Slips Tongs
Figure 11
10. Tongs
A pair of special type of spanners is used to breakout and; or to tighten the pipe connections. One of the tongs is called “Back-up” and is attached to the drill pipe and is anchored to the derrick structure by a chain. The other one is attached by to the connected pipe box or saver sub to be unscrewed and is connected via a chain to the Cat-head of the draw works. The rotation power of the draw works is used to breakout or tighten the connection. On most modern rigs “Power Tongs” are used instead, They are hydraulically self operated able to breakout the connection without the use of chains. These are safer devices. (Figure 11)
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