5.6. FLOATING STRUCTURES
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made to generate bound long waves in the wave tank, additional “parasitic” long waves, not present in nature, will also be generated, and these
unwanted waves will influence the motion of the moored vessel. For this
reason, second-order generation of irregular waves and active absorption of
reflected waves are preferred for laboratory studies. Mansard and Pratte
(1982) discussed the importance of properly simulating wave groups in models of large moored vessels.
In addition, tests in two-dimensional wave flumes compromise the threedimensionally of the prototype, and this must be factored into the decision
to conduct tests in a flume instead of a basin. Many times economic considerations dictate whether a test is conducted in a 2-d or a 3-d facility. The
section entitled Short-Wave Model Lab and Scale Effects in Chapter 4 contains additional discussion about laboratory effects that may be important
in physical models of floating structures.
Scale effects in physical models of floating structures are negligible if
care is taken to construct the model geometrically and dynamically similar
to the prototype. For the case of linear elastic behavior of mooring lines,
minor scale effects will exist in the mooring line similitude if the model
mooring line material does not exactly fulfill the linear elasticity requirements of Eqn. 5.58. Simulating nonlinear elongation of mooring lines is
more difficult, and scale effects could occur unless calibration of the model
mooring configuration is performed. The scale effect associated with using fresh water in a model to represent a salt-water prototype is usually
considered to be insignificant.
5.6.3 Floating Structure Model Scale Selection
As is the case with most model testing of coastal structures, wave tank
capability (water depth, wave height, etc.) and facility availability play a
major role in the selection of model scales for floating structures. Most
floating structures used in coastal engineering are either floating breakwaters or floating docks. The relatively small size of these structures in
the prototype (compared to conventional types of breakwaters) has allowed
somewhat larger model scales to be employed. Hudson, et al. (1979) stated
that usual linear scales for floating breakwater studies have been between
1:6 and 1:27.
The other use of floating structures in coastal engineering models is to
evaluate the impact of harbor waves on moored vessels. Scales for these geometrically undistorted models are based on harbor dimensions, and length
scales typically range between 1:75 and 1:150.
227
made to generate bound long waves in the wave tank, additional “parasitic” long waves, not present in nature, will also be generated, and these
unwanted waves will influence the motion of the moored vessel. For this
reason, second-order generation of irregular waves and active absorption of
reflected waves are preferred for laboratory studies. Mansard and Pratte
(1982) discussed the importance of properly simulating wave groups in models of large moored vessels.
In addition, tests in two-dimensional wave flumes compromise the threedimensionally of the prototype, and this must be factored into the decision
to conduct tests in a flume instead of a basin. Many times economic considerations dictate whether a test is conducted in a 2-d or a 3-d facility. The
section entitled Short-Wave Model Lab and Scale Effects in Chapter 4 contains additional discussion about laboratory effects that may be important
in physical models of floating structures.
Scale effects in physical models of floating structures are negligible if
care is taken to construct the model geometrically and dynamically similar
to the prototype. For the case of linear elastic behavior of mooring lines,
minor scale effects will exist in the mooring line similitude if the model
mooring line material does not exactly fulfill the linear elasticity requirements of Eqn. 5.58. Simulating nonlinear elongation of mooring lines is
more difficult, and scale effects could occur unless calibration of the model
mooring configuration is performed. The scale effect associated with using fresh water in a model to represent a salt-water prototype is usually
considered to be insignificant.
5.6.3 Floating Structure Model Scale Selection
As is the case with most model testing of coastal structures, wave tank
capability (water depth, wave height, etc.) and facility availability play a
major role in the selection of model scales for floating structures. Most
floating structures used in coastal engineering are either floating breakwaters or floating docks. The relatively small size of these structures in
the prototype (compared to conventional types of breakwaters) has allowed
somewhat larger model scales to be employed. Hudson, et al. (1979) stated
that usual linear scales for floating breakwater studies have been between
1:6 and 1:27.
The other use of floating structures in coastal engineering models is to
evaluate the impact of harbor waves on moored vessels. Scales for these geometrically undistorted models are based on harbor dimensions, and length
scales typically range between 1:75 and 1:150.