Current Loads on VLCC's by OCIMF6 (Figure I-8). The sum of these forces together with the forces acting on the tug gives a reasonable estimate of the minimum bollard pull required to control a tanker in storm conditions.

Estimation of Required Tug Power

After calculating the wind and wave loads acting on the tankers, the tug/towwire resistance was added and an estimated loss of efficiency applied. This factor, which ranged from 20 to 28 percent, is an estimate of the loss of bollard pull resulting from the motions of the tug in waves. This is merely an estimate, but is in line with similar figures used by others.7

The conversion of bollard pull to horsepower must also be an estimate as it varies for each vessel. It depends greatly on the RPM and diameter of the propeller(s) and on whether the tug has Kort nozzle(s) around the propeller(s). For this paper, a factor of 75 horsepower per ton of bollard pull was used. This is typical of a twin screw tug in the 5,000- to 10,000-horsepower class with Kort nozzles.8 For a 7,000-horsepower tug, this equates to 93 tons of bollard pull.

RESULT AND CONCLUSION

The results of the analysis are displayed in Figures I-9 through I-11. In reviewing them, it is worthwhile to note the major trading routes for various size tankers:

  • VLCCs trade primarily to the U.S. Gulf with a smaller number trading ANS Crude from Alaska to the West Coast.

  • 140-MDWT tankers trade primarily to the East Coast of the U.S. with a much smaller number trading in the U.S. Gulf and to the West Coast.

  • 80-MDWT vessels trade primarily in the U.S. Gulf and on the East Coast with a smaller number trading on the West Coast.

This analysis shows that a tug of 7,000 horsepower is adequate for handling both a VLCC and a 140-MDWT tanker up to just short of a 20-foot (6-meter) sea state and for handling an 80-MDWT tanker up to a sea state of about 21.5 feet (6.5 meters). Beyond that point, a larger tug, or two tugs would be required. By referring to the joint probability curves (Figures I-1 through I-6) one can see that the probability of 6 to 6. 5 meter seas in combination with onshore winds is quite small. For each of the regions, the joint probability of having 16- to 20-foot (5- to 6-meter) waves in conjunction with onshore winds is:

East Coast

0.30%

Gulf Coast

0.05%

California

0.20%

Pacific NW

0.80%

Alaska

1.40%

These numbers represent the percentage of time in certain coastal waters when one would anticipate that a rescue tow or holding operation of a loaded VLCC would be beyond the capability of a single 7,000-BHP tug.

In 1978, DNV published Towing Operations Guidelines and Recommendations for Barge Transportation. This document was intended to provide guidance to the offshore industry on how large a tug would be required to transport major equipment offshore. These guidelines recommended using a tug capable of towing in 16.5-foot (5-meter) seas with 39-knot winds and up to a 2-knot current. This correlates very well



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