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Experiments by Invited Researchers

 

Experimental study on wave propagation in ice and the combined action of waves and ice on structures (LS-WICE)

Project acronym: HY+_HSVA-01_Kvaerner
Name of Group Leader: Hilde Benedikte Østlund
User-Project Title: Experimental study on wave propagation in ice and the combined action of waves and ice on structures (LS-WICE)
Facility: Large Ice Model Basin (LIMB)
Data Storage Report: HY+_HSVA-01_Kvaerner_LS-WICE_data_storage_plan_and_report_s.pdf
Publications: POAC17_046_Cheng-et-al.pdf
POAC17_051_Herman-et-al.pdf
POAC17_068_Tsarau-et-al.pdf
Herman_Evers_Reimers_Floe-size-distributions-in-laboratory-ice-broken-by-waves_tc-12-685-2018.pdf
Herman_Evers_Reimers_Floe-size-distributions-in-laboratory-ice-broken-by-waves_tc-12-685-2018-supplement.pdf
Herman_The_Cryosphere_Final_tc-12-685-2018.pdf

Loads on Structures and Waves in Ice

Project acronym: Hy+_HSVA-01_Kvaerner

Name of Group Leader: Hilde B. Østlund / Andrey Tsarau

User-Project Title: Loads on Structures and Waves in Ice (LS-WICE)

Facility: large Ice Model Basin (LIMB) at Hamburg Ship Model Basin (HSVA)

 

Publications:

Floe-size distributions in laboratory ice broken by waves

Agnieszka Herman, Karl-Ulrich Evers, and Nils Reimer

The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-186

Accepted article in journal The Cryosphere

 

Loads on Structure and Waves in Ice (LS-WICE) project, Part 1: Wave attenuation and dispersion in broken ice fields

Sukun Cheng, Andrei Tsarau, Hongtao Li, Agnieszka Herman, Karl-Ulrich Evers, and Hayley Shen

Proceedings of the 24th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC’17), June 11-16, 2017, Busan Korea

 

Loads on Structure and Waves in Ice (LS-WICE) project, Part 2: Sea ice breaking by waves

Agnieszka Herman, Andrei Tsarau, Karl-Ulrich Evers, Hongtao Li, and Hayley Shen

Proceedings of the 24th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC’17), June 11-16, 2017, Busan Korea

 

Loads on Structure and Waves in Ice (LS-WICE) project, Part 3: Ice-structure interaction under wave conditions

Andrei Tsarau, Sergiy Sukhorukov, Agnieszka Herman, Karl-Ulrich Evers, and Sveinung Løset

Proceedings of the 24th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC’17), June 11-16, 2017, Busan Korea

 

The decline of ice extent in Arctic regions observed during the recent decades leads to a significant change of weather conditions in these areas. As a result of the weather change stronger winds and gales can be expected. Recognizing that destructive storms with associated high amplitude waves will become more commonplace as global climate warming intensifies and that waves will be able to penetrate further into the pack ice because more open water is present, it is timely to adapt the current understanding of operational conditions and environmental loads on structures in Arctic waters by focusing on the Marginal Ice Zone (MIZ).

A multi-group investigation was conducted at Hamburgische Schiffbau-Versuchsanstalt GmbH (HSVA) from Oct. 24 to Nov. 11, 2016 under the Hydralab+ Transnational Access project: Loads on Structure and Waves in Ice (LS-WICE). There are three parts to this investigation: ice fracture under wave actions, wave attenuation/dispersion in broken ice covers, and ice-structure interaction under wave conditions.

The objective of the LS-WICE project was to shed light on both the manner in which waves adjust during their passage through broken ice and how a marine structure is affected by the combined action of waves and ice in MIZ. To achieve these objectives, three groups of experiments were performed: ice fracture under wave actions, wave attenuation/dispersion in broken ice covers, and ice-structure interaction under wave and ice conditions. 

Ice fracture under wave actions

There are hardly any comprehensive data available for ice breaking by waves, this experiment therefore provides unique data related to an important aspect of ice-wave interactions in MIZ. Wave characteristics, ice properties, ice motion and the evolution of breaking pattern were simultaneously measured.

This investigation focuses on the part of the experiment related to wave-induced ice breaking. During the preparatory stage of the tests, a continuous ice sheet was produced, and the mechanical and physical ice properties were determined at a number of selected locations in the Large Ice Model Basin (LIMB). Pressure transducers and ultrasound sensors located at different positions along the ice tank were used to monitor the wave propagation through the ice. Additionally, 3D motion of a few markers placed on the ice was tracked with an infrared camera system (Qualisys Motion Capture System). The progress of breaking was recorded with cameras placed above as well as sideways from the tank. The measurements consisted of two groups of tests, with a constant wave period in each group. The wave amplitude was increased stepwise, starting from a value too low to break the ice, until a major fragmentation of ice occurred. After each group of tests, the locations of cracks were determined and the floe-size distribution was estimated.

Contrary to the expectations, progressive breaking starting from the ice edge was not observed. Instead, the ice sheet first broke approximately in the middle of its length, and this first major crack presumably had a profound influence on the subsequent development of fractures. Thus, the breaking pattern in this experiment was very different, e.g., from that observed previously in preliminary tests at the same facility (Nils Reimer, HSVA – personal communication) or in the field (Squire et al., 1995). Obviously, with just one case, it is impossible to determine whether this kind of breaking behavior is a repeatable feature or just a singular event caused, e.g., by unrecognized defects in the ice sheet that may have been created during ice preparation. The data from the pressure and Qualisys sensors suggest the role of reflected waves in the process. Moreover, it should be remembered that the ice sheet was not frozen to the beach at its down-wave end, i.e., it was a freely floating floe roughly ten wavelengths long rather than shore-fast ice, for which most models are developed. 

A more in-depth analysis of the experiment described here will be presented in a subsequent paper, together with results of numerical modeling with a new version of the Discrete-Element bonded-particle Sea Ice model (DESIgn; Herman, 2016), extended to include a wave model coupled with an ice model.

 

Wave attenuation

The investigation focuses on the wave attenuation/dispersion. A level ice sheet was produced first. The physical and mechanical properties were measured in the Large Ice Model Basin (LIMB) before passing through a range of waves to obtain the attenuation/dispersion relation. Wave measurements were monitored with pressure transducers and ultrasound sensors. The test sequence began with a continuous ice sheet followed by broken ice sheet. The broken ice sheet was produced by cutting the continuous ice sheet into uniform size floes. A range of floe sizes and wave periods were tested. Results of the data obtained and their implications to wave propagation in the marginal ice zone populated with different broken ice floes will be presented.

Obtained data for wave dispersion and attenuation for a substantial range of period and floe sizes. Physical and mechanical properties of the ice floes are directly measured. Scaling laws of the effective modulus of an ice field may hopefully be obtained from this experiment.

Data were obtained data for T=0.9-2.3s, and floe size from 0.5-6m, unprecedented range of parameters from laboratory studies. These data include dynamic pressure measurements under the ice cover, ultrasonic sensor measurements of surface elevation, ice motion for a range of floe sizes and wave lengths.

Difficulties encountered: Larger floes (6m) broke easily. Studies are constrained to smaller floes. At lower periods attenuation is high. Sensors far from the ice edge cannot register strong enough signal. Increasing wave amplitude is not an option due to the risk of damaging floes. Floes can freeze together if room temperature is low. When this happens the effective floe size is larger than the manually cut pattern.

In addition to laboratory studies mentioned earlier, a recent field experiment (Rogers et al., 2016) also showed that wave attenuation depended on the type of ice covers. Theoretical studies of how an ice cover might attenuate waves and change its dispersion relation are many. Squire (2007) provided a review of the theories up to that publication time. These theories have only been partially confirmed by a number of field experiments.  Meylan et al. (2014) reported attenuation in the order of 10-5 (m-1) in the Southern Ocean marginal ice zone with floes that were 2-3 m near the ice edge to 10-20 m about 200 km from the ice edge. The floes were 0.5-1 m thick first year ice. Doble et al. (2015) reported attenuation in the order of 10-3 (m-1) in the Weddell Sea. The floes were newly formed pancake ice with size ~0.7 m and thickness 0.05-0.1 m. The space between floes was filled with frazil ice. The ice conditions of these two studies were very different from each other, and also very different from the much earlier study of Wadhams et al. (1988), in which they reported several field studies in the Arctic marginal ice zone including Bering and Greenland Seas. The range of attenuation was from 10-5 – 10-4 (m-1). The ice covers consisted floes from 5-50 m with a range of thickness from 0.4 to 1.2 m in the Bering Sea and 3.1 m in the Greenland Sea. These studies strongly suggested that wave attenuation is a result of the combination of ice type and floe sizes.

To isolate ice effect on wave propagation from other mechanisms such as wind wave generation, nonlinear wave-wave energy transfer, and wave breaking, laboratory studies are necessary. The present study demonstrated a systematic approach to determine floe size effect on wave propagation. To scale up the results from a laboratory size wave and floes to the field, we need to determine the dominant length scales that can link the laboratory to the field conditions.

Detailed results of this study will be reported later in a journal paper. Wave-ice interaction may include many physical mechanisms as discussed in Shen and Squire (1998). In this experiment, the total attenuation from all processes present in the experiment is measured. Likewise, wave dispersion data are also the result of coexisting mechanisms. The dataset can potentially be used in many future studies.

The preparation of ice floes in the ice tank is done manually by cutting the parental level ice sheet into pieces of required size and shape. Rectangular floes were produced by cutting the ice sheet first into 1.6-m-wide strips with the help of 5 people standing on the motor-driven carriage and holding ‘ice knives’ and then cutting these strips into L-m-long rectangles.

 

Ice-structure interaction under wave conditions

Novel tests with ice/wave/structure interaction were performed. The structure was a cylinder with a diameter of 620 mm. The visual overview of the data indicates that the peak impact loads on the structure due to ice in waves exceed more than twice the open-water wave loads under otherwise the same conditions. A complete analysis of the obtained data set, including the ice loads and the ice motion data, would hopefully help establishing an analytical relation between the floe-motion response in waves and the impact load on the structure.                  

The analysis focused on both the response of a wave-driven ice floe near the structure and the forces on the structure due to both waves and ice impacts. Regarding the surge RAO (Response Amplitude Operator) of the ice floe, its mean values were not significantly different from 1 for all considered wavelengths. However, it was found that this RAO does not solely account for the variation of the impact forces on the structure and impact occurrence in the experiment. Floe-floe collisions seemed to affect impact occurrence as well.

Among the parameters influencing the impact force, following was identified:

  •    wave height and period (also wave length for shallow water);
  •    ice-floe kinematics in waves, including momentum exchange due to floe-floe interaction;
  •    interaction between the structure and the ice floe.

The effect of ice properties was not assessed in this study. A major difference of this experiment compared to previous experiments on impacts of a wave-driven ice mass on a structure is the utilisation of a wave tank fully covered with ice floes instead of considering only one isolated ice mass and a structure. Full ice coverage ensures a better representation of MIZ conditions and allows taking into account floe-floe interactions and the wave dispersion effects in ice-covered water. 

 

References

Doble, M. De Carolis, J., G. Meylan, M. H. Bidlot, J.-R., and Wadhams, P. (2015). Relating wave attenuation to pancake ice thickness, using field measurements and model results, Geophys. Res. Lett., 42, 4473–4481, doi:10.1002/2015GL063628.

Herman, A., (2016). Discrete-Element bonded-particle Sea Ice model DESIgn, version 1.3a – model description and implementation, Geosci. Model Dev., 9, 1219–1241.

Meylan, M. ., Bennetts, L. G. and Kohout A. L. (2014). In situ measurements and analysis of ocean waves in the Antarctic marginal ice zone, Geophys. Res. Lett., 41, 5046–5051, doi:10.1002/2014GL060809.

Rogers, W. E., Thomson, J., Shen, H.H., Doble, M.J., Wadhams, P., and Cheng, S. (2016). Dissipation of wind waves by pancake and frazil ice in the autumn Beaufort Sea, J. Geophys. Res. – Oceans, DOI 10.1002/2016JC012251.

Shen, H.H. and Squire, V.A. (1998). Wave damping in compact pancake ice fields due to interactions between pancakes, AGU Antarctic Research Series 74, Antarctic Sea Ice: Physical Processes, Interactions and Variability, (Ed. Dr. Martin O. Jeffries): 325-342.

Squire, V.A. (2007). Of ocean waves and sea-ice revisited, Cold Reg. Sci. Technol., 49(2), 110–133.

Wadhams, P., Squire, V.A., Goodman, D.J., Cowan, A.M. and Moore, S.C. (1988). The Attenuation Rates of Ocean Waves in the Marginal Ice Zone, J. Geophys. Res., 93(C6), 6799-6818.

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