Linking carbonate sand fabric and mechanical anisotropy from hollow cylinder tests: motivation and application

In addition to density and stress, fabric is also a key state variable strongly affecting soil behavior. While fabric influence on mechanical behavior of soils has been investigated experimentally, the available database is limited in terms of boundary conditions and soil types tested. Offshore carbonate sediments are of special interest for offshore geotechnical analyses due to their prevalence in tropical waters and unique mechanical behavior that stems from their mostly biogenic origin. A key gap in the availability of experimental data on soil fabric relates to the anisotropy of offshore carbonate sediments. In practice, anisotropy studies (whether rigorously correlated to fabric or not) are typically carried out experimentally for simple boundary conditions such as idealized plane strain and axisymmetric states. In real geotechnical applications, stress paths subjected to soil elements in the field are far more complex, often involving the combined variations of both the orientation and magnitude of all three principal stresses. This paper presents a new multi- scale approach to assess soil fabric at the micro-scale level and relate it to the macro- mechanical response observed for generalized loading conditions. A new sampling method is illustrated that enables preservation and evaluation of the fabric of offshore sediments specimens following generalized stress disturbances imparted by a hollow cylinder apparatus. The link between fabric evolution and the observed stress-strain behavior of sand is discussed along with preliminary results. The approach is part of a broad framework that will be used to systematically study the evolution of soil fabric and anisotropy and their relationship to multi-directional loading scenarios

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

Shape effects on the capacity of rectangular footings under general loading

Ultimate limit states under vertical (V), moment (M) and horizontal (H) loading of rectangular footings with varying breadth-to-length aspect ratios (BIL) are compared with predictions for plane-strain conditions. Footing/soil interfaces unable to sustain tension and with unlimited tensile resistance are considered. Finite element and analytical predictions are reported, and results are presented as failure envelopes in VH, VM and VMH load space. Vertical and moment capacity of rectangular footings, with either zero or unlimited tension interfaces, is shown to increase with reducing footing length, for foundations of a given bearing area. For footings unable to sustain tension, footing aspect ratio does not affect the shape of the failure envelope: therefore ultimate limit states of a footing of any aspect ratio can be derived from a unique envelope scaled by the appropriate ultimate limit loads defining its apex points. A closed-form expression is proposed to describe the shape of the normalised VMH envelope. The shape of failure envelopes for footings able to sustain tension is dependent on footing geometry.</p

Failure envelopes for offshore shallow foundations under general loading

The interaction of vertical, horizontal and moment (VHM) loads acting on a shallow foundation is complex, and it is becoming increasingly popular to represent ultimate limit states under general VHM loading as failure envelopes in three-dimensional load space. General loading is of particular interest offshore, where harsh environmental conditions lead to large horizontal and moment foundation loads. Shallow foundations with peripheral skirts that penetrate into the seabed are used to resist large lateral and overturning forces. During undrained loading, tensile resistance can be mobilised on the underside of the base plate by suctions developed within the skirt compartment. This paper presents failure envelopes and kinematic mechanisms for undrained ultimate limit states of circular skirted foundations in uniform and heterogeneous deposits under general VHM loading based on finite element results. An approximating method is proposed that permits accurate prediction of ultimate limit states under a full range of general loading.</p