Piling Pius Hull Support


Fig. 5.17. Classification of offshore mobile drilling units. After Howe,12 courtesy API.

which includes the mud pits, supplies (dry mud, chemicals, etc.), pipe racks, cementing equipment, fuel and water, and crew quarters. The principal advantages of these installations are greater mobility and lower installation costs. A disadvantage is the higher percentage of lost time due to high wind and waves; during periods of rough weather, the tenders must pull away to prevent collision with the platform. Large self-contained platform operations may proceed in any weather short of a hurricane.

Completely Mobile Offshore Units

Thq,next development in offshore equipment was the completely mobile offshore unit. This is based on an extension of the inland barge principle, in that the entire rig is floated to the location as a unit. Figure 5.17 shows the classifications of these units as given by Howe.12 Just as in land drilling, the economic advantages of complete mobility are considerable; units of this type are rapidly replacing former installation types.

The offshore drilling topic could be expanded over the entire length of this book if we were to attempt anything like a complete coverage of the subject. No mention has been made of such problems as corrosion, wave forces, and meteorological statistics, all of which are extremely important to offshore equipment design. The offshore oil potential is great; undoubtedly, an increasingly greater proportion of U. S. production will come from these fields in future years. The drilling and production problems of these operations offer a challenge to the entire industry.


  1. According to the API formula, what wind velocity is required to exert a wind loading of 0.2 psi? Ans. 85 mph
  2. A drilling rig has eight lines strung through the travelling block. A hook load of 240,000 lb is being hoisted at a velocity of 50 ft/min. Calculate:
  • a) the velocity of the line being spooled at the drawworks (this is called the fast line)
  • b) the line pull at the drawworks assuming frictional losses of 2% per working line
  • c) the output horsepower of the drawworks. Ans. (a) 400 ft/min (b) 36,000 lb (c) 430 HP

3. For the data of Problem 2 calculate the derrick load, assuming

  • a) a frictionless system
  • b) an actual system (no friction loss occurs at the dead line, since it is static) considering the fast line load from above. Ans. (a) 300,000 lb (b) 306,000 lb
  1. A 6 X 19 Seale, improved plow steel, FC cable has a breaking strength of 41.8 tons. Develop the following figures as taken from the Tool Pusher's Manual. (2% per line for friction)
  2. of Safety Hook Fast line Dead line Derrick lines factor load load load load

6 4 109,600 20,900 18,300 148,800

8 4 140,600 20,900 17,550 179,050

10 4 169,300 20,900 16,950 207,150

  1. Assuming the drawworks operating curves of Figure 5.6, what is the highest gear which would handle the hoisting conditions of Problem 2? Which gear could handle the same load at 100 ft/min hoisting speed?
  2. At what depth can an 800 HP rig hoist 4§ in., 16.6 lb/ft drill pipe at 100 ft/min? Assume 8 lines and 55% engines to hook efficiency. Ans. 8,800 ft
  3. Plot the allowable volume vs discharge pressure for a 600 HP duplex mud pump at a volumetric efficiency of 90%.
  4. A 6 X 14" power pump is operating at 40 strokes per minute. (In common nomenclature a stroke is synonymous with a crank revolution.) The piston rod diameter is 2 in. Calculate:
  • a) output volume at 90% volumetric efficiency. Ans. 233 gpm
  • b) input horsepower to pump at 85% mechanical efficiency and a discharge pressure of 500 psig. Ans. 89 HP

9. An 8 in. hole is being drilled with 4 J inch O. D. drill pipe. If an upward velocity of 150 ft/min is required in the annulus, what is the required pump output in gal/min? If circulating pressure is 1000 psi, what is the output HP requirement at the pump?


  1. McCaslin, L. S., Jr., "Southwest U. S. A. — Birthplace of Rotary Drilling," Oil and Gas Journal, Anniversary Number, May 1951, p. 226.
  2. Specifications for Steel Derricks, API Std. 4A, 14th ed. New York: American Petroleum Institute, Jan. 1952.
  3. Specifications for Portable Masts, API Std. 4D, 3rd ed. New York: American Petroleum Institute, Mar. 1955.
  4. Crake, W. S., "Application of Internal Combustion Engine Power to Rotary Drilling Rigs," The Petroleum Engineer, Dec. 1947, p. 70.
  5. Specifications for Rotary Drilling Equipment, API Std. 7, 11th ed. New York: American Petroleum Institute, Apr. 1953.
  6. Specifications for Casing, Tubing, and Drill Pipe, API Std. 5A, 22nd ed. New York: American Petroleum Institute, Mar. 1958.
  7. Bentson, H. G., "Rock-Bit Design, Selection, and Evaluation," API Drilling and Production Practices, 1956, p. 288.
  8. Tool Pushers Manual. Dallas, Texas: American Association of Oil Well Drilling Contractors, 1955.
  9. Recommended Practice on Application, Care, and Use of Wire Rope for Oil Field Service, API RP 9B, 2nd ed. New York: American Petroleum Institute, Jan. 1954.
  10. Rowan, C. L., "Submersible Barges," in Fundamentals of Rotary Drilling. Dallas, Texas: The Petroleum Engineer Publishing Co., 1955, p. 55.
  11. McGee, D. A., "A Report on Exploration Progress in the Gulf of Mexico," API Drilling and Production Practices, 1949, p. 38.
  12. Howe, R. J., "Some Factors in the Engineering Design of Offshore Mobile Drilling Units," API Drilling and Production Practices, 1955, p. 209.

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