We present the results from a search for gravitational-wave transients associated with core-collapse supernovae observed within a source distance of approximately 20 Mpc during the first and second observing runs of Advanced LIGO and Advanced Virgo. No significant gravitational-wave candidate was detected. We report the detection efficiencies as a function of the distance for waveforms derived from multidimensional numerical simulations and phenomenological extreme emission models. The sources with neutrino-driven explosions are detectable at the distances approaching 5 kpc, and for magnetorotationally driven explosions the distances are up to 54 kpc. However, waveforms for extreme emission models are detectable up to 28 Mpc. For the first time, the gravitational-wave data enabled us to exclude part of the parameter spaces of two extreme emission models with confidence up to 83%, limited by coincident data coverage. Besides, using ad hoc harmonic signals windowed with Gaussian envelopes, we constrained the gravitational-wave energy emitted during core collapse at the levels of $4.27\times {10}^{-4}{M}_{\odot }{c}^2$ and $1.28\times {10}^{-1}{M}_{\odot }{c}^2$ for emissions at 235 and 1304 Hz, respectively. These constraints are 2 orders of magnitude more stringent than previously derived in the corresponding analysis using initial LIGO, initial Virgo, and GEO 600 data.

Plate anchor technology is an efficient solution for mooring offshore floating facilities for oil and gas or renewable energy projects. When used with a taut mooring, the anchor is typically subjected to a maintained load component and intermittent episodes of cyclic loading throughout the design life. These loads, and the associated shearing, remoulding and consolidation processes, cause changes in the anchor capacity, particularly in soft, fine-grained soils. The changing anchor capacity affects the mooring performance by changing the safety margin and also the overall system reliability. In this paper the changing anchor capacity in reconstituted, normally consolidated natural carbonate silt was assessed through a series of beam centrifuge tests on horizontally loaded circular plate anchors. The results demonstrate that full consolidation under a typical maintained load leads to a 50% gain in the anchor capacity, and subsequent cyclic loading and reconsolidation can triple this increase. An effective stress framework based on critical state concepts is employed to explain and support the experimental observations. This study shows that, when viewed from a whole-life reliability perspective, maintained and cyclic loading provide a long-term enhancement of anchor capacity in soft, fine-grained soils. This beneficial effect is currently overlooked in design practice, but can be predicted using the framework shown here, which can form the basis for a digital twin that monitors the through-life integrity of a plate anchor.

Some offshore foundations are subjected to intermittent episodes of remoulding and reconsolidation during installation and operational processes. The maintained and cyclic loads, and subsequent reconsolidation processes, cause changes in the geotechnical capacity, particularly in soft clays. This changing capacity affects the in-service behaviour, including changes to the safety margin, the extraction resistance, the stiffness and structural fatigue rates and also the overall system reliability. This paper provides a new analysis framework to capture these effects, based on estimation of the changing soil strength. The framework is developed using critical state concepts in the effective stress domain, and by discretising the soil domain as a one-dimensional column of soil elements. This framework is designed as the simplest basis on which to capture spatially varying changes in strength due to maintained and cyclic loads, and the associated remoulding and reconsolidation processes. The framework can be used to interpret cyclic penetrometer tests, as well as foundation behaviour. This provides a basis for the approach to be used in design, by scaling directly from penetrometer tests to foundation behaviour. Centrifuge tests are used to illustrate the performance of this approach. The penetration resistance during cyclic T-bar penetrometer tests and spudcan footing installation with periods of maintained loading and consolidation is accurately captured. The framework therefore provides a basis to predict the significant changes in penetration resistance caused by changing soil strength, and can bridge between in situ penetrometer tests and design assessments of soil–structure interaction.

Steel catenary risers (SCRs) are an efficient solution to transfer hydrocarbons from deep-water seabeds to floating facilities. SCR design requires an assessment of the fatigue life in the touchdown zone, where the riser interacts with the seabed, which relies on reliable estimates of the SCR–seabed stiffness over the design life. Current models for SCR–seabed stiffness consider only undrained conditions, neglecting the development and dissipation of excess pore pressures that occur over the life of the SCR. This consolidation process alters the seabed strength and consequently the SCR–seabed stiffness. This paper summarises experimental data that show that long-term cyclic vertical motion of an SCR at the touchdown zone leads to a reduction in seabed strength due to remoulding and water entrainment, but that this degradation is eclipsed by the regain in soil strength during consolidation. The main focus of this paper is on prediction of the temporal changes in seabed strength and stiffness due to long-term cyclic shearing and consolidation, to support calculations of SCR–seabed interaction. The predictions are obtained using a framework that considers the change in effective stress and hence soil strength using critical state concepts, and that considers the soil domain as a one-dimensional column of elements. The merit of the model is assessed by way of simulations of SCR centrifuge model tests with over 3000 cycles of repeated undrained vertical cycles in normally consolidated kaolin clay. Comparisons of the simulated and measured profiles of SCR penetration resistance reveal that the model can capture accurately the observed changes in SCR–seabed stiffness. Example simulations show the merit of the model as a tool to assess the timescale in field conditions over which this order of magnitude change in seabed stiffness occurs. It is concluded that current design practice may underestimate the seabed stiffness significantly, but the new approach allows rapid checking of this for particular combinations of SCR and soil conditions.