The most common drilling rigs in use today are rotary drilling rigs. Their main tasks are to create rotation of the drillstring and facilities to advance and lift the drillstring as well as casings and special equiptment into and out of the hole drilled.
In general, rotary rigs can be distinguished into:
Land rigs ( onshore):
- Conventional rigs: Small, medium and large land rigs.
- Mobile rigs: Portable mast, jacknife.
- Bottom anchored rigs: artificial island, TLP, submersible, jackup, concrete-structured, etc.
- Floating rigs: drillship, semi-sumersible, barge.
For offshore rigs, factors like water depth, expected sea states, winds and currents as well as location ( duppy time) have to be considered as well.
It should be understood that rig rates are not only influenced by the rig type but they are also strongly dependent on by the current market situation ( oil price, drilling activity, rig availabilities, location, etc). Therefore for the rig selection basic rig requirements are determined first. Then drilling contractors are contacted for offers for a proposed spud date ( date at which drilling operation commences) as well as for alternative spud dates.
Rig Power System
The power system of a rotary drilling rig has to supply the following main components: Rotary system, hoisting system and drilling fluid circulation system. In addition, auxiliares like the blowout preventer, boiler-feed water pumps, rig lighting system, etc. have to be powered. Since the largest power consumers on a rotary drilling rig are the hoisting and the circulation system, these components determine mainly the total power requirements. At ordinary drilling operations, the hoisting ( lifting and lowering of the drillstring, casing, etc.) and the circulation system are not operated at the same time.For that reason the same engines can be engaged to perform both functions.
The power itself is either generated at the rig site using internal-combustion diesel engines as electric power supply from existing power lines.
The main task of the hoisting system is to lower and raise the drillstring, casings, and other subsurface equiptment into or out of the well. The hoisting equiptment itself consists of: Draworks, Fast Line, Crown Block, Traveling Block, Deadline, Deal line anchor, Storage reel, Hook, Derrick.
“Making a connection” is defined as the periodic process of adding a new joint of drillpipe to the drillstring as the hole deepends is referred.
Making a trip is the process of moving the drillstring out of the hole, change the bit or alter the bottom-hole assembly (BHA), and lower the drillstring again into the hole is referred.
Derricks are classified ( or rated) by The American Petroleum Institute (API) according to their heigh as well as their ability to withstand wind and compressive loads.
Block and Tackle:
The crown block, the travelling block and the drilling line comprise the block and tackle which permits the handling of large loads. To lift and lower the heavy loads into and out of the borehole, the drillling is strung multiple times among the crown and the travelling block.
The drilling line is a wire rope that is made of strands wounded around a steel core. Its classification is based on the type of core.
The purpose of the drawworks is to provide the hoisting and breaking power to lift and lower the heavy weights of drillstring and casings. The drawwork itself cosists of: Drum (which provides the movement of the drilling line), Brakes, Transmission and Catheads.
The principle components of the mud circulation system are as follows: Mud pumps, flowlines, drillpipe, nozzles, mud pids and tanks ( settling tank, mixing tank, suction tank), ,ud mixing equipment ( mud mixing hopper) and contaminant removal equipment ( shale shaker, desander, desilter, degasser).
The flow of circulated drilling mud can be described as from the mud pit ( storage mud) via the mud mixing hopper. where various additives like weghting material erc. can be mixed into the mud, or the suction line to the mud pumps. At the mud pumps the mud is pressure up to the required mud pressure value.
From the mud pumps the mud is pushed through the stand pipe (a pipe fixed mounted at the derrick), the rotary hose ( flexible connection that allows the fed of the mud into the vertically moving drillstring), via the swivel into the drillstring. Inside the drillstring ( kelly, drillpipe, drill collar) the mud flows down to the bit where it is forced through the nozzles to act against the bottom of the hole. From the bottom of the well the mud rises up the annuli ( drill collar, drillpipe) and the mud line (mud return line) which is located above the BOP.
From the mud line the mud is fed to the mud cleaning system consisting of shale shaker, settlement tank, de-sander and de-silte. After cleaning the mud, the circulation circle is closed when the mud returns to the mud pit.
Nowadays there are two types of mud pumps in use ( Duplex pump which consists of two cylinders and is double-acting, triplex pump which consists of three cylinders and is single-acting), both equipped with reciprocating positive-displacement pistons. The amount of mud and the pressure the mud pumps release the mud to the circulation system are controlled via changing of pump liners and pistons as well as control of the speed (stroke/minute) the pump is moving.
When the mud returns to the surface, it is lead over shale shakers that are composed of one or more vibrating screens over which the mud passes before it is feed to the mud pits.
The mud pits are required to hold an excess mud volume at the surface. Here fine cuttings can settle and gas, that was not mechanically separated can be released further. In addition, in the event of lost circulation, the lost mud can be replaced by mud from the surface pits.
The function of the rotary system is to transmit rotation to the drillingstring and consequently rotate the bit. During drilling operation, this rotation is to the right.
The main part of the rotary system are as follows: Swivel, Rotary hose, Kelly, Rotary drive ( master pushing, kelly pushing), Rotary table and Drilling string.
The swivel which established a connection among hook and kelly, has to be constructed or built extremely robust since or because it has to carry the total drillstring weight and simultaneously, provide a high pressure seal ( connection between flexible, non-rotating rotary hose and the rotation kelly).
The kelly has a square or hexagonal cross-section and provides the rotation of the drillstring. Because the kelly is made of high quality, treated steel, it is a flashy part of the drillstring. Thus to prevent the kelly from excessive wear caused by making and breaking connections, a kelly sub is mounted at the bottom end of it. To prevent backward flow of the mud in case of an kick, a kewlly cock providing a backflow restriction valve is often mounted between kelly and swivel.
The rotary drive consists of master pushing and kelly pushing. The master pushing receives its rotational momentum from the compound and drives the kelly pushing which in turn transfers the rotation to the kelly.