written by: Willie Scott
• edited by: Willie Scott
• updated: 10/9/2012
One method of drilling for offshore oil and gas is to use a semisubmersible drilling rig, a floating structure having vertical hollow legs connected to horizontal pontoons used for ballasting with seawater for stability. It also has numerous large heavy anchors which keep it secured to the seabed.
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The Semisubmersible Drilling Rig
In this article we shall consider a semisubmersible drilling rig being employed for the oil and gas drilling phase. This vessel floats on horizontal pontoons and vertical columns known as legs. The pontoons and columns are interconnected by circular cross members giving the hull its structural strength. The bottom pontoons and legs are hollow to allow filling with seawater to provide ballast to submerge the structure and to hold bunkers of diesel oil, drilling mud and fresh potable water.
Welded to the hull is the deck which holds the accommodation, control room, power generation unit, helideck and pad, mud system and pumps, the drill platform, rotating table, one or two cranes, lifeboats, flare tower and the all important derrick.
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The Accommodation & Control Room
The control room is the hub of the rig. Here the boss man sits – the Operation Manager (OM), he surveys all the different instrumentation and printouts which tell him how far he has drilled, the drilling rate, geologists reports on core sample analysis, and a multitude of other relevant technical information including sea and wind conditions.
Usually the accommodation is very good with satellite TV, cinema, gyms and even saunas. The cabins sleep two men in bunks, who are normally on the same shift pattern.
The food is also excellent - and it has to be as the crew work a physically hard twelve-hour shift with twelve hours well-earned rest. They get relieved from the rig every two weeks for shore-side leave; departing and returning to the rig by helicopter.
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These will either be diesel driven or on the more modern rigs, self-contained power generation units driven by gas turbines. These are normally of the RB211 gas turbines which will run on gas or gas-oil. There is also an emergency diesel generator housed in its own fire-proof room, with a supply of diesel from the emergency generator day tank. This is capable of supplying power to the emergency and essential services for 24 hours. The main switchboard will be nearby in the control room
The Beatrice field in the Moray Firth, 12 miles of the Aberdeen coast has two 5MW wind turbines installed close to the production platforms, to supplement its power usage; an innovative renewable energy project led by its owners Talisman Energy and currently the largest capacity offshore wind turbines.
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Following the Piper Alpha disaster when the crew couldn’t lower the lifeboats into the sea as it was covered in burning oil, the modern type of lifeboats are hung over the side of the deck on near vertical steep angled ramps and are free-fall launch. When launched the boats hit the water at an angle travelling under the surface for a distance before surfacing, hence avoiding the presence of fire on the surface of the sea around the rig.
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Normally there are two or three cranes with long jibs which allow them to cover most of the deck area between them These have their own operating hydraulic system driven by onboard diesel engines. The cranes are used to unload the supply boats and assist in bunkering, hoisting the hoses to and from the bunker barges and to lift items of equipment to a workspace for maintenance.
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The Drill Derrick
The derrick essentially holds the lifting gear for the drilling pipes and strings of drill pipe. At the very top is the crown block; a system of pulleys, sheaves and wire ropes, this must be capable of holding a full length of drill pipe of up to 100 tonnes. The crown block has several runs of wire wound round the pulleys then one end drops down to the travelling block and drill clamp. The other end of the wire rope comes off the crown block pulleys down to a winch bolted to the deck. The winch provides the power to drive the whole draw-wire system.
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The Rotating Table & Drill Platform
The drill platform is used when changing or adding another drill pipe to the string. It houses the rotating table which has bushings in the centre, and is normally chain driven using several heavy duty cogs and chains. The table bushings allow the drill string to move down under its own weight, the rotation of the table being transferred to the drill string turning it and drill bit clockwise as it bites into the subsea rock.
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The Flare Tower
This extends well clear of the rig at an angle and holds the pipe and flare tips containing the sour gas to be flared off. In the old days the flare gas used to be ignited using a flare gun fired into the escaping gas but now we are a bit more sophisticated using electronic igniters mounted on the flare tip. There is a heat shield on the tower to protect the workers from the radiant heat of the flare.
A few sketches of semi-sub drilling rigs are shown below.