CHRONOLOGY OF JACKUP RIGS, 1953-1969
Fig. 1-5 Offshore's rig 52.
In 1955, the first 3-legged jack-up appeared on the scene [Figure 1-6). The rig was the R. G. LeTourneau jack-up, the Scorpion, for Zapata Offshore Company. The Scorpion, an independent leg jack-up, used a rack and pinion elevating system on a truss framed leg. The rig worked very successfully for several years but was lost during a move in the Gulf of Mexico. The Scorpion was closely followed by The Offshore Company Rig No. 54. For Rig No. 54, however, a hydraulic jacking system on a trussed leg was used. These jack-ups were followed by Mr, Gus II, a mat supported unit using a hydraulic jacking system, which was built by Bethlehem Steel Corporation.
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 In most cases, in the event of a pending hurricane, the rigs were withdrawn to sheltered areas.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 10 years ago were considered to be unrealistic. For example, a rig designed for 250 feet of water will have to meet the following range of criteria:
These figures, although obtained from reliable sources, should not be considered finite. Actual criteria must be deter-
rained from weather organizations in the actual geographical location of drilling. However, the differential in criteria can easily be seen.
A new breed of jack-ups [Figure 1-7) has been developed that will operate in water depths in excess of 400 fett, although in 1974 the maximum criteria unit was only operating in 350 feet of
water in the U.S. Gulf Coast, and in less than 300 feet in the North Sea.
Jack-up designs can generally be classified into two basic categories (Figure 1-8): independent leg jack-ups and mat supported jack-ups. Each unit has its particular value.
The independent leg jack-up will operate anywhere currently available, but it is normally used in areas of firm soil, coral, or uneven sea bed. The independent leg unit depends on a platform (spud can) at the base of each leg for support. These spud cans are either circular, square, or polygonal, and are usually small. The largest spud can being used to date is about 56 feet wide. Spud cans are subjected to bearing pressures of around 5,000 to 6,000 pounds per square foot, although in the North Sea this can be as much as 10,000 psf. Allowable bearing pressures must be known before a jack-up can be put on location.
The mat supported jack-up is designed for areas of low soil shear value where bearing pressures must be kept low. The mat is connected to all of the legs. With such a large area in contact with the soil, bearing pressures of 500 to SOO psf usually exist.
An advantage of the mat type jack-up is that minimum penetration of the sea bed takes place, perhaps 5 or 6 feet. This compares with a penetration of perhaps 40 feet on an independent leg jack-up. As a result, the mat type unit requires less leg than the independent jack-up for the same water depth. One disadvantage of the mat type unit is the need for a fairly level sea bed. A maximum sea bed slope of \xh° is considered to be the limit. Another problem with the mat supported unit occurs in areas where there is coral or large rock formations. Since mats are designed for uniform bearing, the uneven bottom would probably cause a structural failure.
Jack-ups can be either self-propelled, propulsion assisted, or nonpropelled. The majority of jack-up rigs are nonpropelled. The self-propelled unit, although very flexible, requires a specially trained crew of seamen as well as a drilling team,
Jack-ups have been built with as many as 14 legs and as few as 3 legs. As the water depth increases and the environmental criteria become more severe, we find that to use more than 4 legs is not only expensive but impractical. The prime forces on a jack-up are generated from the waves and currents, hence, the less exposure to the waves and currents the fewer the forces being developed on the unit. From this standpoint the optimum jack-up is the monopod (Figure 1-9) or single leg unit.
Problems other than wave forces, however, must be overcome with the monopod type unit. But in areas such as the North Sea with very rough seas there is a need for the monopod jack-up. Thus, research is now being done in this area. A monopod production platform is already being built for use in the North Sea.
jack-ups currently constitute about 50 percent of the worldwide drilling fleet, with the semisubmersibles and the drillships composing the remaining 50 percent.
Sem¡submersible Drilling Rigs
The semisubmersible evolved from the submersible (Figure 1-10) and many of today's semisubmersibles are designed to operate either resting on the sea bed or totally afloat. One of the earlier semisubmersibles was the Blue Water I (Figure 1-11)
which was converted in 1961 from a submersible by adding vertical columns for floatation.
Today's semisubmersibles are designed foroperation in water depths up to 1,000 feet and are therefore subjected to severe sea states and high winds. The general configuration of a semisub-mersible consists of two longitudinal lower hulls which are used as ballast compartments to achieve the necessary drilling draft (Figure 1-12). These lower hulls are also the primary hulls while the rig is under tow. By virtue of its size and location, the semisubmersible offers low towage resistance while providing tremendous stability.
There are other designs of semisubmersibles such as the triangular design used on the Sedco series (Figure 1-13), four longitudinal hulls used on the Odeco series (Figure 1-14), and the French-designed Pentagone rig with 5 pontoons (Figure 1-15). The Pentagone unit is possibly the most successful of the multi-hull types, offering a unique symmetry and uniformity of
stability characteristics. This unit does not offer the towing capabilities of twin hull units, but it does provide good drilling characteristics.
Semisubmersibles 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. The conventional mooring system (Figure 1-16) 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 semisubmersible has recently been contracted for 1,500 foot water depths using the anchor and chain method. Much of the necessary chain will be carried on support, vessels.
Because of the submerged ma?s of the semisubmersible, rolling and pitching is of a low magnitude. The motion that causes problems for the semisubmersible is heave (Figure 1-17), or the vertical motion. Because of forces on the drill string when the vessel is heaving, the semisubmersible with a low heave re-
sponse is considered to be the most suitable. Heave is generated in response to exposed waterplane and is expressed as 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 and floating at the column or caisson level. With the loss of waterplane area to reduce heave response, a reduction in stability follows. Therefore, the de
Fig. 1-15 Pentagone 81
signer 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.
Many of the new breed of semisubmersibles such as the Aker H3 units (Figure 1-18) are being designed to operate in specific areas of the world such as the North Sea where the criteria are very severe. These vessels require a very large consumables capacity, a low heave response, and good stability.
Another consideration in the design and operation of the semisubmersible is propulsion. There are several opinions on this matter, each based on valid reasoning. Propulsion is a large initial expense which can be recovered in a reasonable period of
30°-70° eight line symmetric nine une symmetric eigkt line symmetric ten line
Fig. 1-16 Typical mooring pattern.
EXISTING SEMI-SUBMERSIBLES OP ËP WING IN THE NORTH SEA
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