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Rig selection depend on capabilities, limitations, and cost. These capabilities are the Drawworks horsepower, mast size, hookload and rig depth rating. To specify a rig depth rating you need to know the well profile, mud weight, drillstring design, casing design and condition of hole. It’s very important to know how much the rig can pull and that depend on mast, drilling line and drawworks. The highest hookloads of a well is required when a pipe is stucked and when running casing. And tripping operations require the highest hoisting speeds, and the hoursepower of the drawworks defines hoisting speed and hookload. Another important characteristic of the rig is how much pipe it can rack and the weight that the racking floor suports.

A brief description of the most common offshore rig types will help us to choose the best option for our well.

A jackup rig operates in water as deep as 350 feet. These rigs are very stable because they rest on the sea floor. The rig jacket is

slowly towed to the location during calm seas. Then, the legs are lowered by jacks

until they rest on the sea floor below the deck. The legs continue to lower until the

deck is lifted off the surface of the water (sometimes 60ft) and the deck is level.

Fixed-platform rigs, yet one more of many kinds of offshore vessels are

pinned to the sea floor by long steel pilings. These platforms are very stable but are

considered permanent and virtually immobile. (The jackup, on the other hand, can be

moved from location to location.) Semi submersible rigs can also operate in water

from 200-500 meters deep and they are equally stable, but they are not fixed. These

rectangular floating rigs carry a number of vertical stabilizing columns and support a

deck fitted with a derrick and related equipment. But the rig that offers the greatest

mobility and that can operate in almost any water depth is the drillship, a ship

specially constructed or converted for deepwater drilling. Dynamic positioning

equipment keeps the ship above the wellbore using a thruster with controllable pitch

propellers.

a) Jackup Rigs

The jackup rig is the most common offshore drilling rig. The jackup rig is towed to location with its legs elevated. Once on location, the legs are lowered to the bottom and the platform is "jacked up" above the wave actions by means of hydraulic jacks. The jackup rig has many advantages, including a stable work platform, good availability, relatively lower mobilization costs, versatility to work over a platform or drill in open water and generally competitive day rate. A jackup can have 3 or 4 legs. Unlike drillships and semisubrmersibles, they can easily be updated, enhanced or renovated. The modifications are extensive, but includes installation of a top drive, converting slot to a cantilever unit, leg strengthening and lengthening with more preload tanks, and improved environmental capabilities. The jackup rigs were generally considered to have a useful service time of 12 to 15 years, but with rigorous maintenance coupled with modernization, the rigs may be operable for 30 years. (Reference 2, II-612)

There are two types of jackup rigs, the independent-leg type, usually three legs with lattice construction and the mat type, in which the legs are attached to a very large mat that rests on the ocean bottom. The choice is based on ocean bottom conditions and weather general conditions and variability. The hull, barely out of the water is loaded up (water pumped into seawater tanks) driving the legs down. This preloading sequence is repeated until the legs no longer sink further in the ocean floor.

Having a mat on a muddy/ soft ocean floor might seem better option. than having to drive the legs into oceans bottom (as with the independent leg), making the operation vulnerable to weather changes. However, the mat is susceptible to damage by workboats, or to movement of objects on the ocean bottom. Mat types may be more limited in depth (>250 ft). Mat types also tow more slowly.

These reasons have lead to a preference to the independent leg jackup. Also, newer independent-leg jackups use a larger jack, so preloading can be done in one operation, with the hull jacked up with all the seawater aboard.

b) Semisubmersible rig

If drilling from a bottom supported unit is not possible, the semisubmersible is the most popular floating unit. The semisubmersible rig is a more costly option than jackup rigs. The semisubmerible is a column stabilized vessel, because of the deep draft of its columns, wave energy is dampen, minimizing roll, pitch, yaw, surge, sway and heave.

It is common to subdivide semisubmersibles rigs into generations, as in table 14.13, on II-617. Generations 4 and 5 are the deeper water rigs, with generation 4 being able to drill in 3500 to 4,000 ft and generation 5 being able to drill in 5000 ft plus depths. Rig that drill in these deep waters, must be considerably more sophisticated. The variable deck load (VDL) must be higher. Deep water semisubmersible use of large motors to position the rig over the well (dynamic positioning) can drill in deeper waters. Generation 1, 2 and 3 use mooring systems and operate in waters less than 3500 ft.

In evaluating any floating vessel for drilling, computer programs are available to calculate vessel motions by entering in the wave heights and periods. The result will be a motion history of that particular rig for a specific drilling period.

c) Drillships

The drillship were popular in the late 1950's to the late 1960's. They could be mobilized quickly. However, the semisubmersible was far more a stable drilling platform. The long narrow hull of the drillship results in motion in all six degrees of freedom (can you name them?), particularly pitch, roll and heave. Mooring systems and dynamic positioning systems have greatly improved the drillship's capability of drilling in harsh environments.

Rigs selection based on Depth Criteria

Any well to be drilled in 100 to 350 ft of water, can be drilled using any of the rig types, so depth alone is insufficient. Additional criteria will be discussed later. Deep water however rules out the most common drilling rig- the jackup