API REPORT 82-31 Hydrodynamic Coefficients for Cylinders Roughened by Marine Growths from the Gulf of Mexico
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API REPORT 82-31 Document Information:
Title
Hydrodynamic Coefficients for Cylinders Roughened by Marine Growths from the Gulf of Mexico
American Petroleum Institute
Publication Date:
Jan 1, 1983
Scope:
Abstract
Research results reported in (1,2,5,6) showed that rigid
marine growths in the Gulf of Mexico can be very lumpy and covered
with hydroids, tunicates and other fuzzy organisms, attaining
average thicknesses of up to two to six inches depending upon location.
In the Santa Barbara channel various types of growths of up to 12 to 24
inches in thickness were observed (6). Laboratory tests were made
on horizontal cylinders by towing and in waves for smooth, sand
roughened and artificially marine roughened cylinders. The results from
the smooth and sand roughened cylinders corresponded well with
expectations [i.e. the tow results correspond to those from other
investigators and the wave results were reasonable with respect to
results from plan aroscillatory flow (3,5)] and the results from the
artificially marine roughened cylinder showed drag and added mass
coefficients considerably greater than those for the send roughened
cylinder (with sand sizes such that k/D = .02).
Although the results from the 1980 and 1981 tests show
general trends, the coefficients for actual marine growths remain to
be determined. Therefore, Chevron USA positioned test cylinders in the
So. Pass area of the Gulf of Mexico to obtain actual marine growths for
1, 2 and 3 years, at various depths. This report covers the results
from the 1982 tests of one year of growth at depths of −55
and−140 feet, which were funded by the American Petroleum
Institute. The results for steady towing showed that the −55 ft.
growth was about equal to that for the sand roughened cylinder, and the
−140ft. growth was in-between the conditions for a sand roughened
cylinder and a smooth cylinder. However, the wave drag force
coefficients for the −55 ft. cylinder were about 29% higher than
those for the sand roughened cylinder and the inertia coefficient was
about 57% higher. These are significant differences that may indicate
some particular influences from biological growths on the forces
on horizontal cylindrical members from waves. They also show that
testing in waves may be more sensitive to variations in roughnesses
than testing in steady towing. The values of the coefficients
presented herein are derived from a specific method of analysis (1) and
from a specific range of laboratory tests. It is not the purpose of this
work to make a judgment on which values should be used for
design conditions.
A second feature of the 1982 work was to compare the influence
on the inertia coefficient from a force transducer that is
completely sealed from the ambient pressure fluctuation and one that is
unsealed, so that the ambient pressure fluctuations are free to be
transmitted to the inside of the cylinder. The results showed that
between the two tests performed the inertia coefficients were equal
for Keulegan-Carpenter number equal to or less than 10 when the gap
between the test section and the dummy section was 3/8 inch. However,
some question may still remain for higher K and for other values of the
gaps between the dummy cylinder sections and the force transducer
unit. Additional analyses have been proposed for 1983.
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