|BRITAIN'S OFFSHORE OIL & GAS
There are two basic types of drilling rigs - fixed platform rigs and mobile rigs. Fixed platform rigs are installed on large offshore platforms and remain in place for many years. Most of the large fields in the North Sea such as Forties and Brent were developed using fixed platform rigs.
Mobile rigs comprise two types: jack-up rigs (F66) used in shallow water less than 100 metres deep and semi-submersible rigs (F65) used in deeper waters down to 1000 metres or more. In very deep waters, drilling ships are used. Jack-up rigs have lattice legs which are lowered to the seabed before the floating section carrying the derrick is raised above the sea surface. Semi-submersible rigs float at all times, but when in position for drilling are anchored and ballasted to float lower in the water with their pontoons below wave-level. Some have 'dynamic-positioning' propellers and can drill in very deep water.
The drilling derrick towers above the drill floor (F71) and is where most of the activity is concentrated. The derrick supports the weight of the drillstring (F72) which is screwed together from 9-metre lengths of drill pipe. Hoisting equipment in the derrick can raise or lower the drillstring. At the bottom of the drillstring is a drill bit (F67 & F70), which can vary in size and type. It is attached to the drill collars, heavy pipe-sections that put weight on the bit. On semi-submersible rigs, a compensator keeps the drillstring stationary while the rig and derrick move as a result of wave motion. The drill bit is rotated either by turning the whole drillstring ("rotary drilling") or by using a downhole turbine which rotates as drilling fluid is pumped through it. In rotary drilling, the rotary motion is imparted to the drillstring by the "top drive". This is an electro-hydraulic motor suspended in the top of the derrick. It is attached to the top of the drillstring and imparts torque to it, causing it to rotate. To add a new section of drill pipe the drillstring is clamped in the drill floor with wedges (slips) and the top drive disconnected. The new joint is screwed into the drillstring suspended in the drill floor, the top drive connected to the top of the new joint, and drilling restarted. The raising and lowering of the top drive and the maintenance of correct tension on the drillstring is controlled by the driller operating the drawworks lever in a control cabin (F69) (called the "doghouse") on the drill floor.
Drilling fluid (also called "mud"), which is mainly water-based, is pumped continuously down the drillstring while drilling. It lubricates the drilling tools, washes up rock cuttings and most importantly, balances the pressure of fluids in the rock formations below to prevent blowouts.
In offshore drilling, the first step is to put down a wide-diameter conductor pipe into the seabed to guide the drilling and contain the drilling fluid. It is drilled into the seabed from semi-submersible rigs, but on production platforms a pile-driver may be used. As drilling continues, completed sections of the well are cased with steel pipe cemented into place. A blowout preventer is attached to the top of the casing. This is a stack of hydraulic rams which can close off the well instantly if back pressure (a kick) develops from invading oil, gas or water.
A typical problem faced while drilling is the drillstring sticking in difficult rock formations such as the thick Tertiary clays in the North Sea. A hydraulic device known as a jar, mounted between the drill collars, can give the drillstring a series of jolts. If that does not work, other techniques may be used, including spotting with oil and water. Special fishing tools can also retrieve stuck pipe and broken equipment (junk).
Drilling grinds up the rock into tea-leaf-sized cuttings which are brought to the surface by the drilling mud. The drilling mud is passed over a shale shaker which sieves out the cuttings (see F115, page 64). In exploration drilling, the cuttings are taken for examination by a geologist known as a mudlogger who is constantly on the lookout for oil and gas. Oil entrapped in the mud is detected by its fluorescence in UV light. Gas is extracted from the mud in a gas trap and sent under vacuum to a gas detector and analyser. An increase in the amount triggers an alarm to alert the mudlogger and the drilling superintendent. If laboratory tests are needed on potential reservoir rock, a solid core of rock can be drilled by a special hollow drilling bit. Each short length of core retrieved calls for the entire drillstring to be pulled out of the well and then reinserted, so coring is an expensive operation not undertaken lightly.