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This paper describes a programme of centrifuge tests investigating the behaviour of clayey seabed under wave loading using an in-flight wave loading system. Three model seabeds of kaolin clay capturing typical unconsolidated, normally consolidated and overconsolidated soil responses were considered, with each seabed experiencing several episodes of wave loading and resting. Data acquisition measures included pore pressure transducers, accelerometer, bender elements and T-bar penetrometers. The depth-wise distribution of excess pore pressure, soil strength and modulus, as well as the motion of the liquefied layer of the seabed, was monitored throughout to enable a thorough investigation into the liquefaction and reconsolidation features of the soil. For the unconsolidated and normally consolidated soils, remarkable development of residual pore pressure was observed, and there was evidence that the strength/modulus recovery cannot be achieved by the surficial soil within a prototype time of 15 days. Within a certain depth below this surficial layer, there was a drastic increase in undrained strength, and this phenomenon was carefully examined by a modified moving-boundary model. For the overconsolidated soil, the build-up of residual pore pressure was rather limited, but discernible amplification of oscillatory pore pressure amplitude was observed. Implications for practice in offshore engineering are discussed based on the experimental findings. [ABSTRACT FROM AUTHOR]

Accurate assessment of pipe-soil interaction under cyclic wave actions is of pronounced importance for the stability analysis of submarine pipelines in sandy seabed. This paper presents a plane-strain numerical study on such a problem using a finite element program DBLEAVES, which incorporates an elasto-plastic soil model that is capable of capturing the cyclic mobility behavior of sandy soils under cyclic loadings. A detailed validation against analytical solution and model test results was provided to demonstrate the robustness of the present numerical model to mimic both pre- and post-liquefaction behavior of sands, before an extensive parametric study was introduced. It was found that the accumulation of excess pore pressure in the vicinity and far field of a pipeline was strongly affected by the existence of it, with an influential range of about two pipe diameters. The influences of wave and seabed properties (e.g. relative densities) on the uplift response of pipelines were then investigated, based on which an explicit model was developed to quantify the degradation effect of waves on the uplift bearing capacity of pipelines against thermally-induced buckling. [ABSTRACT FROM AUTHOR]

Seabed soil may experience a reduction in strength or even liquefaction when subjected to cyclic loadings exerted by offshore structures and environmental loadings such as ocean waves and earthquakes. A reasonable and robust constitutive soil model is indispensable for accurate assessment of such structure–seabed interactions in marine environments. In this paper, a new constitutive model is proposed by enriching subloading surface theory with a fractional-order plastic flow rule and multiple hardening rules. A detailed validation of both stress- and strain-controlled undrained cyclic test results of medium-dense Karlsruhe fine sand is provided to demonstrate the robustness of the present constitutive model to capture the non-associativity and cyclic mobility of sandy soils. The new fractional cyclic model is then implemented into a finite element code based on a two-phase field theory via a user subroutine, and a numerical case study on the response of seabed soils around a submarine pipeline under cyclic wave loadings is presented to highlight the practical applications of this model in structure–seabed interactions. [ABSTRACT FROM AUTHOR]

• The bearing capacity of offshore pipelines buried in flat and inclined clayey seabed is investigated. • The plane along which a buckling most likely occurs could deflect largely from the outer normal of seabed surface. • The anti-buckling bearing capacity of a pipeline is lower than its uplift bearing capacity, mostly by 15% to 35%. • Un-conservation of industry-standard design approach is highlighted. • Design charts are provided for the modification of existing database. The application of marine pipelines prevails in offshore developments for the transportation of oil and gas products from offshore to onshore, and some of them are inevitably buried in continental slopes with moderate inclinations (mostly below 6°). Regrettably, very limited attention has been paid to the bearing capacity of these pipelines, which generates the very motivation of this study. A total number of 169 200 separate finite element limit analyses (FELA) were conducted on rigid plane-strain pipe sections in inclined clayey seabed, using a sound FELA code OxLim. The breakout resistances of the pipe along all possible directions are explored to determine the most vulnerable plane where a transverse displacement could most likely take place, and accordingly the minimum breakout resistance. Before reaching a full-flow state, this resistance was found to be considerably lower, mostly by 15% to 35%, than the uplift resistance. Design charts that can be immediately used for the modification of existing database are then provided, which promotes the safe design of pipelines regarding transverse displacement or buckling. The potential non-conservation of the most recent design approaches is highlighted through comparisons. [ABSTRACT FROM AUTHOR]