EM 1110-2-1100 (Part II)
30 Apr 02
(Raichlen 1968), indicated that the periods of free oscillation were less than 10 sec. Larger seagoing deep-
draft vessels, depending on the oscillation mode being excited, will respond to the entire range of wind-wave
periods. Field measurements by van Wyk (1982) on ships having lengths between 250 and 300 m and beams
of about 40 m found maximum roll and pitch responses at encounter periods between 10 and 12 sec. By
properly designing the mooring system, the periods and amplitudes of vessel motion can be significantly
modified.
(c) The wave-induced lateral and vertical motions of the design vessel will affect the required channel
horizontal and depth dimensions, respectively. The problem of wave-induced vessel oscillations has been
addressed by analytical/numerical means (Andersen 1979; Madsen, Svendsen, and Michaelsen 1980; Isaacson
and Mercer 1982). These efforts usually employ small-amplitude, monochromatic waves and some
limitations on vessel geometry and the incident wave directions relative to the vessel.
(d) Some field measurement programs yield valuable design information. Wang and Noble (1982)
describe an investigation of vessels entering the Columbia River channel. Pitch, roll, heave, yaw, and
horizontal position were measured for selected vessels as they traversed the channel. The data were analyzed
statistically to define extreme limits of vessel motion for various wave and other conditions (Noble 1982).
van Wyk (1982) reports on a field study of vessel response to wave action at two South African ports. The
data were analyzed statistically so extreme motion probabilities could be evaluated. Other field studies of
moving large vessels have been reported by Greenstreet (1982) and Zwamborn and Cox (1982). Raichlen
(1968) and Northwest Hydraulic Consultants (1980) discuss field measurements of small moored vessels in
marinas.
(e) Most of the major coastal engineering labs have also conducted model studies of vessel response to
wave motion. Some of these tests are discussed in Mansard and Pratte (1982), Isaacson and Mercer (1982),
Zwamborn and Cox (1982), and Briggs et al. (1994).
(2) Response to currents.
(a) There are several possible causes for currents in harbors and in the vicinity of harbor entrances.
Wind, wave-induced radiation stress, rivers, and tides can all generate currents in the vicinity of a coastal
harbor entrance. Flow from a river that enters a harbor or flows past the entrance to a harbor located in an
estuary can generate significant currents. Ebb and flood tide can generate strong reversing currents through
a harbor entrance. Tidally induced longshore currents around islands can cause navigation problems at harbor
entrances. Long-wave resonant oscillations in harbors can generate noticeable currents at nodal points, which
can seriously affect moored vessels and create hazardous navigation conditions if this location is at a place
where the flow is constricted.
(b) Currents will directly affect vessel operation, particularly when they act oblique to the sailing line
of the vessel. These currents are particularly troublesome when the vessel speed is low and vessel
maneuverability is commensurately reduced. The situation is made even more difficult when there are strong
winds acting from a different direction than the currents. Physical model studies and numerical simulations
of vessel motion have been used to predict vessel paths under various wind and current conditions,
particularly as vessels enter a harbor (Bruun 1989, Briggs et al. 1994). Currents can increase vessel sinkage
and trim in restricted channels (see next section).
(3) Wave-current interaction.
(a) At harbor entrances, currents can also exert an indirect effect on vessel navigation through their effect
on waves. Ebb currents will steepen incoming waves, making the waves more hazardous to the stability of
II-7-58
Harbor Hydrodynamics
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