Submersible S Under Construction Today

10 12 14 10 18 20 2 2 24 26 28 PERIOD OF SEAS (SECONDS)

Fig, 1-17 Heave response curve.

Oil And Gas Drilling Rig Models
Fig. 1-18 Aker H3.

time if mobility is required. Jn 1974 a large semisubmersible crossed the Atlantic in a record-breaking 21 days. The published data quote an average speed of 9.72 knots for the unit. This, of course, meant a considerable reduction in moving cost and a fair increase in productive time. On the other band, considering that once a unit has reached location it is generally in that area for a long time, propulsion units are not only unnecessary but they also use up valuable consumables (weight) capacity.

In selecting a semisubmersible, it is therefore necessary to consider the following criteria:

  1. Water depth.
  2. Drilling depth requirement.
  3. Environmental criteria.
  4. Motion characteristics.
  5. Consumables capacity.
  6. Mobility


The last type of mobile drilling unit to be discussed is the drillship. As the name implies, it is simply a shipshape vessel used for drilling purposes. Earlier drillships were converted vessels (Figure 1-19), either barges, ore carriers, tankers, or supply vessels. However, although conversions are still being done, there are several new drillships being designed purely for drilling, such as the Glomar Challenger (Figure 1-20) or the Offshore Discoverer (Figure 1-21). 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. 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 drillships but heave cannot be reduced, A subsequent increase in "rig downtime" or "lost" time occuts. 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. This system has been used successfully on the Offshore Company's Discoverer II and III and will be installed in Offshore's Super Discoverer (Figure 1-22).

Briefly then, drillships are versatile tools but should only be considered for use io areas of small wave heights and low wind velocities.

Offshore Drilling Storm
Fig. 1-19 Storm's Typhoon.
Glomar Challenger

Fig, 1-20 Glomar Challenger.

Rig Casualties

As a point of interest, the latest information available to date on mobile offshore drilling unit accidents still rates the jack-up as having the most casualties, with the semi submersible second and the drillship third. However, when comparing the total number of rigs built to the number of casualties by rig type (Figures 1-23 and 1-24), the casualty rates of the jack-up and semisubmersible are not as far apart.

For example, there have been 47 jack-up casualties from 1955 to 1974 amounting to approximately $122 million in damage and the total number of jack-ups was 143 (Table 1-1). For semi-submersibles, there were 12 casualties amounting to about $50 million in damage out of a total of 72 semisubmersible rigs. These figures indicate that one out of every 3 jack-up rigs ex-

Drilling Rig Photos Semisubmersibles
Fig. 1-21 Offshore Discoverer.

perienced a casualty, whereas one out of every 5 semisubmer-sibles were involved.

When dividing the number of casualties into the total costs of damages (Figure 1-25) the average loss per rig for jack-ups was $2.76 million and the average cost for semisubmersibles was $5.66 million. In conclusion, the jack-up drilling rig may have a higher casualty rate but there are almost twice as many jack-ups as semisubmersibles and the cost of damages per rig are less for the jack-up than for the semisubmersible. Jack-up rigs also rep-

Jackup Offshore Rigs

Fig. 1-22 Offshore's Super Discoverer.

Submersible Offshore


Fig. 1-23 Cumulative number of rigs built by year and by type.


Fig. 1-23 Cumulative number of rigs built by year and by type.

Fig. 1-24 Cumulative number of rig casualties (by rig type).

Fig. 1-24 Cumulative number of rig casualties (by rig type).

resent about 50% of the total rig fleet, 58% of the total value of rig casualty damages, and only 60% of the "total rig casualties. All three figures show the close relationship of the number and cost of casualties to the total number of rigs.

Overall drilling rig casualties, however, have decreased. In 1957, for example, casualties occurred to about 7% of the total rig fleet, but in 1973 only 1,47% of the fleet was involved in a mishap, and it dropped to about 1.2% in 1974 (Figure 1-26).

Rig Construction

The future of the offshore mobile drilling unit industry is extremely bright. The forecast for construction shows a very

TABLE 1-1 Rig Casualties

Total Number Estimated Total Total Number Built Value of Damages

(1955-1974) (Through 1974) (1955-1974)

Jack-up 47 143 $122 million

Semisubmersible 12 72 S 50 million

Jack Drilling Cost


Fig. 1-25 Estimated value of rig casualty damage (by rig type).


Fig. 1-25 Estimated value of rig casualty damage (by rig type).

high demand through 1985 for all classes of drilling units. The jack-up still leads the fleet in the total number of rigs in operation, with the semi submersible running a distant second and the drillship far off in the rear.

In 1974 the total worldwide offshore drilling rig fleet in operation numbered 317. This includes 143 jack-ups, 72 semisubmer-sibles, 25 submersibles, and 77 floaters (Figure 1-27). In addition to these figures in 1974 there were 42 jack-ups under contract for


















1953 I960 1965 1970 1974


Fig. 1-26 Casualty ratio of accidents compared to number of rigs operating.

Submersible Offshore


Fig. 1-27 Cumulative number of offshore rigs operating (by type).


Fig. 1-27 Cumulative number of offshore rigs operating (by type).

construction, 75 semisubmersihles, and 18 floaters. Forty-five of the semisubmersihles are destined for use in the North Sea.

Rig count projections estimate that there will be 408 offshore mobile drilling units by 1985. Of these, 183 will be jack-ups, 121 will be semisubmersihles, and 104 will be floaters (Figure 1-28).

Construction Increase Projection
Fig. 1-28 Mobile offshore drilling equipment.
Drilling Rigs

Fig. 1-29 Annual construction of rigs by water depth capability.


Fig. 1-29 Annual construction of rigs by water depth capability.

Offshore Drilling Comparison

Fig. 1-30 Chronological rig cost comparison.


Fig. 1-30 Chronological rig cost comparison.

Rig construction costs continue to increase as rig designs are "scaled up" for operation in deeper water depths (Figure 1-29) under more severe criteria. Increasing numbers of rigs are being built for over 350 foot water depths. The larger and heavier rigs require increased amounts of more expensive steel and costs for labor and materials continue to spiral. For example, construction costs in 1974 were approximately six times the costs in 1961. The average cost of construction for a jack-up rig increased from $3.3 million in 1961 to $14.5 million in 1974 and the cost is expected to be approximately $19.65 million in 1976 (Table 1-2 and Figure 1-30).

For the semisubmersible, the average construction cost in 1961 was $6.67 million. The cost in 1974 was about $20.5 million and it is expected to be around $30.76 million in 1976. These construction cost escalations are not unreasonable. In fact they may prove to be conservative. Much of this cost is due to Ihe size and sophistication of the newer drilling rigs.


Average Construction Costs (U.S. $ Million)


Average Construction Costs (U.S. $ Million)


















Because of increased construction costs, sophistication of equipment, and specialized operating personnel the day rates for the new rigs are also skyrocketing. Day rates in the past were based on investment and depreciation of equipment. Now the market is vulnerable and day rates are being decided more by demand than by dollar investment. The market has changed and the industry must change with it.

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