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.

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Full-flow T-bar and ball penetrometer tests are often used to measure intact and remoulded soil 46 strengths, with the latter determined after several large amplitude displacement cycles. In 47 offshore design the remoulded soil strength is often the governing design parameter during 48 installation of subsea infrastructure, whilst a 'cyclic strength' applies for the less severe 49 operational cyclic loading. This paper utilises T-bar penetrometer tests to measure both 50 remoulded and cyclic strengths, where the latter is determined via a new test protocol involving 51 cycles between load rather than displacement limits. The tests use kaolin clay and a 52 reconstituted carbonate silt and involve three cyclic phases with intervening consolidation 53 periods. The results demonstrate the important and beneficial role of consolidation, with the 54 loss in strength due to remoulding sometimes surpassed by the strength recovery from 55 consolidation. The most significant gains in strength, to 2.5 times the initial value, were 56 measured in the load-controlled cyclic tests. These data demonstrate a novel way to characterise 57 undrained cyclic strength, taking advantage of consolidation to reduce conservatism.

This paper describes a centrifuge study using a range of penetrometer tests (T-bar, piezocone and free-fall piezocone) to explore strength changes in a reconstituted, normally consolidated, natural calcareous silt. Various penetrometer test procedures were applied to measure the penetration resistances including monotonic, cyclic and twitch-type movements as well as pauses for pore pressure dissipation. These mobilised combinations of partial or full remoulding, strain softening, consolidation and viscous rate effects. The penetrometer resistance – representing a proxy for strength – reduced by a factor of 4·1 from drained to undrained conditions (at the lowest fully undrained penetration rate). In undrained conditions, viscous enhancement of the penetration resistance raised the tip and shaft resistance in free-fall piezocone tests by ∼2·8 and ∼3·6 times, respectively. The ‘restart’ resistance immediately after the dissipation tests was ∼2·5 times higher than the resistance prior to dissipation, giving an indication of consolidation-induced strength gain. The ‘twitch’ test (using sequential steps decreasing the velocity) captured drainage and viscous rate effects, and also gave a ‘restart’ resistance that showed even greater consolidation effects than from a dissipation test. Overall, the different penetrometer test types and procedures measured resistances in the same soil sample that varied by a factor exceeding 20 from highest to lowest, resulting from different penetration rates and history, due to strain rate, strain level (or remoulding) and consolidation. An expression for the monotonic penetration resistance combining drainage and viscous rate effects was fitted to the response of all tests, spanning >7 orders of magnitude in strain rate.

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.