Offshore Drilling Rigs

The sequence of operations is as follows when a land well is drilled:

–  Prepare location before rig arrives.
– Dig cellar.
– Install conductor pipe.
– Prepare support pad for rig, camp, etc
– Build roads, fencing, dig pits.
– Sometimes drill water well.
– Move rig on to location, rig up and prepare to start drilling.

Offshore Drilling Rigs:
     Two main types: floating and bottom-supported unit.

   Floating unit include: semi submersible (bottle-type, column stabilized), barge rig and drill ship.
   Bottom-supported unit include: submersible (posted barges, bottle-type submersibles, arctic submersibles),
jackups and platforms.

(1). Semi Submersible

     This floating drilling unit has columns when flooded with seawater, cause the structure submerge to a
predetermined depth.
Although it is moved by wave action, it sits low with a large part of its structure under water combined with
eight huge mooring anchors, make it a very stable installation.
This type of rig drills a hole in the seabed then it moves to the next location. With advancing technology
some semi submersibles can drill in water depths over five thousand feet.

(2). Platform
This immobile structure can be built from concrete or steel and rests on the seabed. When oil or gas is
located a platform may be constructed to drill further wells at that site and also to produce the hydrocarbon.

Steel Jacket Platform
Most common type of platform consist of the jacket, a tall vertical section made of tubular steel members.  Supported by piles driven into the seabed.
Additional sections on top of the jacket provide space for drilling rig, crew quarters, and other equipment.

Concrete Gravity Platform
Build from steel reinforced concrete Tall caissons, or column are the dominant feature of this platform. Sometime, special concrete cylinder are fixed at the base of the
caissons on the sea floor to store crude oil.

see our Offshore Movies section

Steel-Caisson Platform
Specifically for use in cold area – where fast-moving tidal currents carry pack of ice that can destroy steel-jacket.  The caissons are made of two layers of thick steel to
prevent ice damage.

    Compliant Platform
  Using rigid platform in water much over 1000 feet depth is not practical – very much expensive to build. In deep water, most companies use compliant platform, which
contain fewer steel parts and are lighter than rigid steel-jacket.  Guyed-tower platform and tension-leg platform.

(3). Jack up
This is a mobile drilling rig, different from the semi submersible. Instead of floating over its drilling location the Jackup has long leg structures, which it lowers to and into the seabed raising the rig out of the water.  The obvious limitation with this type of installation is the depth of water it can operate in. The maximum being five hundred feet.

    (4). Drill Ship

   As the name suggests this is a ship shaped drilling vessel. Unlike the semi submersible and the Jackup, it does not require tugboats to tow it to location.  Although they are not as stable as semi submersibles they also drill in very deep waters.

read more about Drilling Bits

Rotary Drilling
Rotary drilling uses a sharp, rotating drill bit to dig down through the Earth’s crust. The spinning of the drill bit allows for penetration of even the hardest rock.
The actual mechanics of modern rigs are quite complicated. In addition, technology advances so rapidly that new innovations are being introduced constantly.
A rotary drilling rig with some of its major components identified is illustrated in the next figure.

The basic rotary drilling system consists of four groups of components:
Prime movers – Hoisting equipment  –  Rotating equipment –   Circulating equipment

Prime Movers
The prime movers in a rotary drilling rig are those pieces of equipment that provide the power to the entire rig. Recently, while diesel engines still compose the majority
of power sources on rotary rigs, other types of engines are also in use. Some rotary rigs may use electricity directly from power lines. Most rotary rigs these days
require 1,000 to 3,000 horsepower, while shallow drilling rigs may require as little as 500 horsepower.
The energy from these prime movers is used to power the rotary equipment, the hoisting equipment, and the circulating equipment.

Hoisting Equipment
The hoisting equipment on a rotary rig consists of the tools used to raise and lower whatever other equipment may go into or come out of the well.
The most visible part of the hoisting equipment is the derrick, the tall tower-like structure that extends vertically from the well hole.
The hoisting system is made up of the drawworks, derrick, crown block, traveling block, hook and wire rope.
If a drill bit needs to be changed, either due to tear or a change in the subsurface rock, the whole string of pipe must be raised to the surface. The hoisting equipment is
used to raise all of this equipment to the surface so that the drill bit may be replaced.

   Whenever the drillstem is suspended by the traveling block and drill line, the entire load rests on the derrick. The standard pyramid derrick is a structure with four
supporting legs resting on a square base.
In comparison, a mast is much more slender and may be thought of as sitting on one side of the rig floor or work space. The derrick is erected on a substructure which
supports the rig floor and rotary table and provides work space for the equipment on the rig floor.

  The derrick and its substructure support the weight of the drillstem at all times, whenever it is suspended from the crown block or resting in the rotary table. The height of the derrick does not affect its load-bearing capacity, but it is a factor in the length of the sections of drill pipe that can be removed.

Hoisting Equipment
Traveling Block, Crown Block, Drill Line & Hook
Use to connect the supporting derrick with the load of drill pipe to be lowered into or withdrawn from the borehole. During drilling operations, this load usually consists of
the weight of the drill pipe, drill collars and drill bit. The drill line passes from the drawworks to the top of the derrick. From here is sheaved between the crown block and traveling block to give an eight, ten or twelve-line suspension. It is then clamped to the rig floor by the deadline anchor. Suspended from the traveling block, on standard drilling systems, is the hook which when drilling carries the swivel and kelly and when tripping it lifts the drill string.

The Drawworks
The drawworks is a mechanism commonly known as a hoist. The main purpose of the drawworks is to lift the drill string out of and to lower it back into the borehole.
The drill line is reeled (spooled) on a drum in the drawworks.  When engaged, the drum turns and either reels in the drill line to raise the traveling block, or lets out the drill line to lower it. Because the drillstem is attached to the block, it is raised or lowered. One outstanding feature of the drawworks is the brake system, which enables the driller to easily control a load of thousands of pounds of drill pipe or casing.
An integral part of the drawworks is the gear (transmission) system. This gives the driller a wide choice of speeds for hoisting the drill string.

The drawworks also has a drive sprocket that drives the rotary table by means of a heavy-duty chain. In some cases, however, the rotary table is driven by an independent engine or electric motor.  Another feature of the drawworks are the two catheads. The make-up cathead, on the drillers side, is used to spin up and tighten the drill pipe joints. The other, located opposite the driller’s position on the drawworks is the breakout cathead. It is used to loosen the drill pipe when the drill pipe is withdrawn from the borehole The rotating equipment consists of components that actually serve to rotate the drill bit.  Rotating equipment from top to bottom consists of swivel, a short piece of pipe called the kelly, rotary table/topdrive, drill string and bit. A component called the swivel, which is attached to the hoisting equipment, carries the entire weight of the drill string, but allows it to rotate freely. The drill bit is located at the bottom end of the drill string, and is responsible for actually making contact with the subsurface layers, and drilling through them. There are four main types of drill bits, each suited for particular conditions:
–  Steel Tooth Rotary Bits (most basic type).
–  Insert Bits (tungsten carbide inserts).
– Polycrystalline Diamond Compact Bits (diamond inserts).
– Diamond Bits (diamonds implanted in them).

   Diamond bits are forty to fifty times harder than traditional steel bits.                                           

 References:
1. Drilling Equipment and Operation.
2. drilling Operation
3. Offshore Drilling Engineering.