The present offshore drilling industry is involved in an unprecedented construction and drilling boom. Hundreds of millions of dollars are being spent to capture the much-needed energy supplies and offshore drilling activities around the world have never slowdown. The technology for drilling and producing in deeper waters and in more hostile environments have been rapidly and continuously expanding with better drilling tools and interphase software technology to enable safer and more productive when carrying out drilling and exploration.
With the advancement in technology, these factors have created an acute shortage of trained and qualified personnel to operate the rigs now under construction by various shipyards over the world.
Offshore mobile drilling units as we know them today contained alot of sophisticated machineries for the running or drilling into the deep. However, in the old days, original units were simply land rigs taken into shallow waters and placed on a structure for drilling. The same drilling techniques that had been developed on land were used on the first offshore rigs. These techniques worked for some time, but the need to drill in deeper waters created a new type of offshore structural design. And along with the new engineering concepts came the new breed of drilling rigs which we see today.
Following offshore market trends, we find that there are basic types of offshore mobile drilling units: the submersible, the jack-up, the semisubmersible, and the drillship.
The early breed of jack-ups was primarily designed to operate in the U.S. Gulf of Mexico area in water depths up to 200 feet. Wave heights in the range of 20 to 30 feet with winds up to 75 mph were considered as design criteria for these units. In most cases, in the event of a pending hurricane, the rigs were withdrawn
to sheltered areas. Today's jack-ups, however, are being used in international waters in a range of environmental conditions that many years ago were considered to be unrealistic. For example, a rig designed for 250-350 feet of water will have to meet the following range of criteria:
a. U.S. Gulf Coast- 50foot wave, 125 mph wind, minimal current.
b. North Sea- 75 foot wave, 115 mph wind, 1 to 2 knot current.
c. Southeast Asia- 30 foot wave, 100 mph wind, minimal current.
As the water depth increases, the criteria rise accordingly and for 400 foot water depths the range becomes:
a. U.S. GulfCoast- 65 foot wave, 125 mph wind, 1 to 11/2knot current.
b. North Sea- 90 foot wave, 125 mph wind, 2 to 21/2knot current.
c. Southeast Asia- 50 foot wave, 115 mph wind, 1/2 to 1 knot current.
Semi-submersibles permit drilling to be carried out in very deep waters and they are held on location either by a conventional mooring system or by dynamic positioning thrusters installed at the bottom of the floating pontoons. The conventional mooring system usually consists of 8 anchors placed in a spread pattern and connected to the hull by chain or wire rope, sometimes even a combination of both. The dynamic positioning method is an evolution of the ship sonar system whereby a signal is sent out from the floating vessel to transducers set out on the ocean floor. Dynamic positioning becomes a greater necessity as the water depth increases and is generally considered necessary in water depths beyond 1,000 feet. However, a semi-submersible in the past been contracted for 1,500 foot water depths using the anchor and chain method. Much of the necessary chain will be carried on supply vessels. Nowadays, semi-sub have enough capacity to carry such additional variable load without having to depend on supply vessels.
Because of the submerged maps of the semisubmersible, rolling and pitching is of a low magnitude, The motion that causes problems for the semisubmersible is heave, or the vertical motion. Because of forces on the drill string when the vessel is heaving, the semisubmersible with a low heave response is considered to be the most suitable. Heave is generated in response to exposed waterplane and is expressed as T = 2π/√(gt/D) where T = time in seconds; t = tons per foot immersion; D =displacement in tons.
Therefore, the smaller the waterplane area, or 't', the lower the heave response. This is achieved in the semisubmersible by submerging the lower hulls ( pontoon ) and floating at the column or caisson level. With the loss of waterplane area to reduce heave response, a reduction in stability follows. Designer must reach a compromise between acceptable heave response and adequate stability. There are, of course, other methods of reducing heave induced forces on drill string.
In selecting a semisubmersible, it is therefore necessary to consider the following criteria:
a. Water depth.
b. Drilling depth requirement.
c. Environmental criteria.
d. Motion characteristics.
e. Consumables capacity.
The last type of mobile drilling unit is the drillship. As the name implies, it is simply a shipshape vessel used for drilling purposes. Earlier drillships were converted vessels, either barges, ore carriers, tankers, or supply vessels. However, although conversions are still being done, there are now many advanced drillships being designed purely for drilling, such as the earlier ones Glomar Challenger or the Offshore Discoverer. Drillships are the most mobile of all drilling units, but they are the least productive. The very configuration that permits mobility results in very bad drilling capabilities. Drillships are being used extensively in the U.S. Gulf Coast to bridge the gap between the jack-up and the semisubmersible.
However, it is the drillship that has drilled in the deepest water, over 1,000 feet ( now 10 times the depth capability). As discussed earlier, heave is the major problem when using a floating vessel. The drillship, because of its surface contact with the sea, develops very large heave response compared to the semisubmersible. It is possible, by means of stabilizing tanks and other methods, to reduce roll on drills hips but heave cannot be reduced. A subsequent increase in "rig downtime" or "lost" time occurs. Because of this there is a bigger demand for the use of compensation devices.
Mooring for drillships is very similar to the methods previously discussed for semisubmersibles. However, there is one additional system that has been developed on a drillship-the "Turret" system.
(Source : Youtube )