Offshore Drilling Rigs
Two-thirds of Gulf of Mexico production now comes from deep water. Deep water drilling is expensive and challenging, but as the industry knows, can be very rewarding.
The project economics must demonstrate that the project is viable. Yet too much success in exploration can leads to lower oil prices. Thus, as stated by Mark Childer's (PE Handbook, II-589), the drilling business has been and still is "a roller coaster ride."
The history of offshore drilling began in 1897, just 38 years after the first oil well (Drake's well in Pennsylvania). It was drilled from a pier into the Santa Barbara Channel in California. The three exploration areas that motivated the development were Lake Maricaibo, Venezuela, offshore southern California and then post World War II, the Gulf of Mexico, primarily off of Louisiana. By the end of the war (1945), the surplus US Navy landing crafts were converted to tenders, equipped with mud systems, electrical generation equipment and consumables, thus reducing greatly the payload requirement of drilling rigs.
The history of offshore drilling is chronicled in reference 1 and 2. Internet links are also provided under references.
Offshore Rig Type Descriptions
Offshore rigs are referred to as mobile offshore drilling units or MODU's. Rig selection involves consideration of capabilities, limitations, and of course cost.
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.
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 rig.
- Jackup: From approximately 20 to 30 ft minimum depth to 350 to 550 ft maximum depth. Most textbooks cite 350 ft maximum depth, however the latest Petroleum Engineering Handbook (2007), cites 550 ft in the Gulf of Mexico.
The maximum depth is a function of other environmental variables, such as wind, waves, and current conditions at the site. Severe conditions tend to lower the jackup rig's maximum water depth capacity.
- Semisubmersible: Approximately 150 to 8000 ft using dynamically mooring system. Until 1978, the maximum depth was 2,200 ft using conventional mooring system.
- Drillship: Approximately 100 to at least 8000 ft with today's technology. Maximum water depth limits occur because of riser system limitations. Other constraints can limit the maximum depth as discussed below.
Other Drilling Units
In the near future, we hope to extend the discussion to fixed platform rigs. These are discussed in reference 2, page II-602 to II-606. They include modular rigs supported by spars or TLP. They are designed to be site-specific.
A final category, would be the "Ultra Deepwater Floating Vessels" which includes both drillships and semisubmersibles. This category has one distinguishing feature, beyond the obvious from its name, is its cost, which is generally > 400 million dollars. The ultra deep floating vessels are discussed II-616 to II-617.
If the untapped resources of the future lie in water 5,000 ft plus, this is where drilling will take place. But how deep can we go?
Heave - linear motion up and down, sway- linear motion side to side, surge - linear motion back and forth
Roll - rotation along axis running aft to stern, pitch - rotation along axis running side to side, yaw - rotation along vertical axis.
Vessels generally rotational movement of pitch and roll, and linear motion of heave.
1. Silcox, et al., Chapter 18, Offshore Operations, Handbook of Petroleum Engineering (Bradley, editor, 1987), 18-5 to18-10.
2. Childers, M. Chapter 14 (Volume II), Offshore Drilling Units, Handbook of Petroleum Engineering (Lake, editor, 2007), II-589 to II-619.
3. Bourgoyne et al, Applied Drilling Engineering, page 3 to 5.