Concurrent sexual partnerships do not explain the HIV epidemics in Africa: a systematic review of the evidence

The notion that concurrent sexual partnerships are especially common in sub-Saharan Africa and explain the region's high HIV prevalence is accepted by many as conventional wisdom. In this paper, we evaluate the quantitative and qualitative evidence offered by the principal proponents of the concurrency hypothesis and analyze the mathematical model they use to establish the plausibility of the hypothesis

Offshore decommissioning horizon scan: Research priorities to support decision-making activities for oil and gas infrastructure

Thousands of oil and gas structures have been installed in the world's oceans over the past 70 years to meet the population's reliance on hydrocarbons. Over the last decade, there has been increased concern over how to handle decommissioning of this infrastructure when it reaches the end of its operational life. Complete or partial removal may or may not present the best option when considering potential impacts on the environment, society, technical feasibility, economy, and future asset liability. Re-purposing of offshore structures may also be a valid legal option under international maritime law where robust evidence exists to support this option. Given the complex nature of decommissioning offshore infrastructure, a global horizon scan was undertaken, eliciting input from an interdisciplinary cohort of 35 global experts to develop the top ten priority research needs to further inform decommissioning decisions and advance our understanding of their potential impacts. The highest research priorities included: (1) an assessment of impacts of contaminants and their acceptable environmental limits to reduce potential for ecological harm; (2) defining risk and acceptability thresholds in policy/governance; (3) characterising liability issues of ongoing costs and responsibility; and (4) quantification of impacts to ecosystem services. The remaining top ten priorities included: (5) quantifying ecological connectivity; (6) assessing marine life productivity; (7) determining feasibility of infrastructure re-use; (8) identification of stakeholder views and values; (9) quantification of greenhouse gas emissions; and (10) developing a transdisciplinary decommissioning decision-making process. Addressing these priorities will help inform policy development and governance frameworks to provide industry and stakeholders with a clearer path forward for offshore decommissioning. The principles and framework developed in this paper are equally applicable for informing responsible decommissioning of offshore renewable energy infrastructure, in particular wind turbines, a field that is accelerating rapidly

Drum centrifuge tests of shallow skirted foundations on soft clay

Vertical bearing capacity of a shallow foundation with short skirts in normally consolidated clay has been investigated in the UWA drum centrifuge for embedment depths up to two footing diameters. Tests were carried out in a special fabricated strong box set into the channel of the drum. A half-model test was conducted adjacent to the strongbox Perspex window, facilitating optical measurement of soil deformation during foundation loading. A full-model test was conducted in the interior of the strongbox to ascertain the true bearing capacity as the half-model results are anticipated to be distorted by friction developed along the Perspex window. This paper describes the experimental techniques adopted and the results in terms of bearing capacity factor and the accompanying soil failure mechanisms.</p

Elastic solutions for consolidation around seabed pipelines

The build-up of pipe-soil resistance that can be mobilized on a seabed pipeline, particularly in the axial direction, is affected by consolidation of the seabed beneath the pipeline after it is laid. On fine-grained soils the consolidation period may extend well beyond the laying period, and involve a significant gain in pipe-soil resistance: a process analogous to the set-up of piles driven in clay. The available level of pipe-soil resistance affects many aspects of pipeline design, such as stability on route curves and down slopes, geohazard vulnerability, lateral buckling and axial walking behaviour. This paper considers consolidation around on-bottom pipelines through a numerical study, supported by field data. The soil is modeled as an elastic material with coupled Biot consolidation. Pipe embedments of up to 0.5 diameters have been considered and the effect of soil berms adjacent to the pipe, formed of the displaced soil, has been explored. The solutions and guidance provided in this paper allow pipeline designers to estimate the 'set-up' periods over which the effective stress at the pipe-soil interface will rise in the period after laying, indicating the build-up of available axial pipe-soil resistance. The results account for the enhancement of this resistance through a 'wedging' action. The dissipation process is retarded by a factor of up to 2 if the displaced soil forms a berm that is collapsed against the pipe wall. Similar solutions for the analogous situation of consolidation ('set-up') around driven piles have existed for many years. This paper provides the equivalent design tool for on-bottom pipelines.</p

Undrained vertical bearing capacity factors for shallow foundations

The undrained vertical bearing capacity of a shallow foundation is a fundamental problem in geotechnics. Bearing capacity factors based on analytical and numerical methods have been published for strip and circular, surface and shallowly embedded foundations, with rough and smooth foundation-soil interfaces in soils with uniform and linearly increasing shear strength profiles - although not comprehensively for all combinations. Furthermore, in the case of limit analysis solutions, considerable disparity can exist between upper and lower bound solutions for some foundation and soil conditions. This letter presents a coherent set of bearing capacity factors based on finite element and finite-element limit analysis, for strip and circular shallow foundations across a practical range of foundation embedment ratio, foundation-soil interface roughness and soil shear strength heterogeneity.</p

Response of normally consolidated kaolin clay under irregular cyclic loading and comparison with predictions from the accumulation procedure

The accumulation procedure on contour diagrams of shear strain is used in geotechnical design to account for the effect of cyclic loading on offshore foundations. In this paper, contour diagrams of maximum shear strain are presented based on a series of stress-controlled, symmetrical and non-symmetrical, cyclic direct simple shear tests on normally consolidated kaolin clay that can be used to re-examine the extensive database of model testing in kaolin clay. It is shown that the failure under non-symmetrical cyclic loading can be defined at a higher maximum shear strain than under symmetrical loading, offering potential opportunities for reducing foundation sizes for structures that are less sensitive to displacement. Staged tests with parcels of uniform amplitudes have been also performed to compare the measured shear strains with the values predicted by the accumulation procedure. It is shown that the accumulation procedure predicts the strain measured in the symmetrical staged tests well but overpredicts shear strains measured in the non-symmetrical staged tests

Assessment of the consolidated breakout response of partially embedded subsea pipelines

The changing soil strength due to consolidation around a subsea pipeline can alter the lateral breakout resistance. Results of elasto-plastic coupled consolidation finite-element analyses are presented that quantify the effect of consolidation on the undrained breakout resistance and trajectory of partially embedded seabed pipelines. Breakout resistance is presented in terms of failure envelopes in vertical-horizontal load space, which also allow the trajectory of the pipe during breakout to be determined by normality. Observed responses for consolidated undrained breakout are described by simple algebraic expressions as a function of initial pipe embedment, pipe self-weight (and consequent consolidation pressure) and breakout load path. Consolidation and the associated enhancement of the shear strength of the surrounding soil can have a significant effect on the breakout response. The assumption in current practice of unconsolidated undrained soil behaviour may lead to underestimation of pipeline breakout resistance and inaccurate prediction of the trajectory of the pipe during breakout.</p

Vertical cyclic loading response of a shallow skirted foundation in soft normally consolidated clay

Skirted foundations are a potential foundation solution for a range of offshore structures including hydrocarbon and renewable energy platforms and subsea structures. Offshore foundations can be subject to cyclic loading from environmental, installation and operational events affecting the geotechnical response. A series of centrifuge tests have been performed on a shallow skirted foundation on normally consolidated kaolin clay under a range of vertical cyclic load sequences, to investigate the effect of tensile or compressive average stress, the magnitude of the applied stress and the effect of cyclic loading of low magnitude followed by consolidation on the foundation response. Results are presented as vertical foundation displacements normalised by the foundation geometry and interpreted within the traditional shear strain contour approach. The findings indicate that the average, rather than maximum, vertical stress defines the foundation vertical displacement response and failure mode; that a threshold stress exists below which a steady state is maintained even at high number of cycles; and that geotechnical resistance increases as a result of low level cyclic loading followed by consolidation