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Experiments by Invited Researchers

 

Large-scale measurements of extreme wave loadings on exposed jetties

Project acronym: HyIII-FZK-17
Name of Group Leader: Alberto Lamberti, University of Bologna
User-Project Title: Large-scale measurements of extreme wave loadings on exposed jetties
Facility: Large Wave Flume, GWK
Proceedings TA Project: Large-scale measurements of extreme wave loadings on exposed jetties
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Summary:

The main objective of these experiments is to calibrate loading models on jetty decks through the evaluation of scale corrections to existing or improved methods. Within the HRW-Bologna-Rome “Exposed Jetties” project wave loads were measured in a small-scale hydraulic model. Unfortunately compressibility of air does not allow applying Froude scaling law to slamming and impact pressures/ forces.

During the experiments wave loads were measured on beam and deck elements of an idealized pier deck at close-to-full scale, which may be used for scale corrections to existing loading models based on small-scale data.

“Wave-in-deck forces” cover all wave forces applied to a platform or jetty deck. Wave-induced vertical forces on horizontal decks or platforms may be classified as impulsive forces large in magnitude and short in duration occurring at the instant of contact between the wave crest and the platform of the deck, followed by a pulsating positive and/ or negative force. Different methods were used to separate impulsive and pulsating loads, and thus to distinguish between loads that can certainly be applied using quasi-static assumptions, and those that will require consideration of the dynamic characteristics of the jetty.

Regarding the structure, the following configurations were tested: a flat deck and a deck with lower beams forming three bays with and without venting. Wave pressures on the front face, on upper deck surface and on lower deck surface were measured by 24 pressure transducers. Impulsive loads are proportional to sound celerity in water, in its turn dependent on air entrainment; therefore aeration is measured with three combined pressure and aeration transducers (developed by University of Plymouth), installed on the deck front, on the deck central bay and on the channel bed. Forces on the deck are obtained by integration of measured pressures. Some of the transducers are mounted facing upward to measure inundation loadings. Data were acquired at 4 kHz sampling frequency.

Wave loading on a suspended deck was measured under several wave attacks. Regular and irregular wave attacks were used in 4 and 5 m water depth, as well as focused wave groups to generate the highest waves in irregular attacks that could not be generated in their entirety.

Measurements included incident, reflected and transmitted waves, and velocity profiles along the vertical in front of the structure. Videos are available for all tests.

Wave loading was measured under very well documented conditions and with a very high time resolution. Data are compared with small scale tests results according to Froude scaling law and differences were identified. A scaling law for loading accounting for scale effects due to air compressibility and different air content was proposed and verified.

The focused wave group technique, which could allow much shorter tests enabling evaluation of extreme loads, was verified by comparison with long duration irregular wave tests.

The existing guidelines (K. McConnell et al., 2004) will be updated if necessary accounting for the new results, providing in any case more reliable estimates of full scale loading.

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