Seafloor Site Characterization for a Remote Island OWC Device Near King Island, Tasmania, Australia

We present findings from a geotechnical survey for a gravity-based Wave Energy Converter (WEC) to be installed in King Island, Tasmania. The goal of this work was to assess the deployment location for a 200 kW Oscillating Water Column (OWC) and to identify possible challenges for the foundation of the structure to make it Australia’s first operational offshore OWC for a remote offshore island. The proposed location for this OWC is the southeast coast of King Island, Tasmania, approximately in a depth of ~5.5 m LAT. The survey included sub-bottom profiling, sediment cores, surficial sediment strength by penetrometer drops, seabed imagery, as well as long-term deployment (>6 months) of pressure sondes and an acoustic wave current profiler (AWAC). Our findings demonstrate that the WEC can be installed in the proposed location with significant wave height Hs ~1–1.5 m and peak period Tp of 12–14 s, and that the site exhibits sufficient sand coverage and quasisteady bearing capacity. The period between the survey and prospective deployment is only one year, demonstrating the efficiency of the survey methods (in particular, the use of the penetrometer) and OWC design but also the suitability of the candidate site for this device design.

CROSS-SHORE VARIATIONS IN SEDIMENT STRENGTH AT A SANDY BEACH

The potential for erosion and accretion of coastal sediments is governed by local hydrodynamics and sediment characteristics. These factors are highly spatially and temporally variable and strongly interact in the surf zone. It is known that for any change in the wave load, the system quickly seeks and adopts a new state of equilibrium. It follows that the understanding of the interactions between hydrodynamics and sediment properties is crucial for beach engineering. Particularly, there seems to be a lack of knowledge regarding the role of geotechnical sediment properties. The research goal of this study is to characterize the cross-shore variation of sediment strength across a sandy, erosive beach on the island of Sylt, Germany.

Influence of vertical cylindrical tetrahydrofuran hydrate veins on fine-grained soil behaviour

Over the last 10 years, marine gas hydrate drilling expeditions utilizing advances in pressure coring techniques and imaging have routinely encountered gas hydrates residing in fine-grained sediments. The hydrate typically occurs as fracture-filling, near vertical, veins that displace the sediment, potentially leading to increased sediment strength that may prevent normal consolidation of the sediment thus leading to underconsolidation. Destabilization of this hydrate, through climate change or human activity on the seafloor, may cause dramatic loss of strength of the sediment and pose a significant geohazard. To assess the impact of hydrate veins on sediment behaviour, this paper reports on a series of consolidated (CU) and unconsolidated undrained (UU) triaxial tests carried out on fine-grained soil specimens hosting simplified, vertical, cylindrical tetrahydrofuran (THF) hydrate veins of varying diameters. The results show that increasing hydrate vein diameter significantly increases strength and stiffness, including the development of post-peak strain-softening. The mode of failure of the hydrate veins influenced the specimen strength, but did not affect the specimen stiffness. Hydrate dissolution during CU tests prevented quantitative comparison with UU tests. However, CU test results on the soil specimens containing the largest hydrate vein suggest that increasing lateral confining stresses increase the sediment strength.