C. M. Martin

Suction Caisson Foundations for Offshore Wind Turbines

This paper outlines a £ 1.5m, three year, research project that commenced during the middle of 2002 to determine a design framework for shallow foundations for offshore wind turbines. The shallow foundations in focus are suction-installed skirted foundations otherwise known as suction caissons (Houlsby and Byrne, 2000). There are eight distinct themes to the research covering all aspects of the geotechnical performance of these foundations. The funding for the project has been obtained from the Department of Trade and Industry (£ 917k), Industrial Partners (£ 373k) and the Engineering and Physical Sciences Research Council (£ 221k). The results will feed into the design process for offshore wind turbines almost immediately.

Critical skirt spacing for shallow foundations under general loading

AbstractFinite-element limit analysis is used to identify the critical internal skirt spacing for the undrained failure of shallow skirted foundations under conditions of plane strain based on the criterion that the confined soil plug should ideally displace as a rigid block, such that optimal bearing capacity is realized. General loading (vertical, horizontal, and moment) is considered for foundations with skirt embedments ranging from 5 to 50% of the foundation breadth in soil having either uniform strength or strength proportional to depth. The results explicitly identify the number of internal skirts required to ensure soil plug rigidity under arbitrary combinations of horizontal and moment loading expressed as a function of the normalized skirt embedment and the maximum expected level of vertical loading as a fraction of the ultimate vertical bearing capacity. It is shown that fewer internal skirts are required with increasing normalized foundation embedment, but more internal skirts are required wit...

Pipeline Unburial Behaviour in Loose Sand

Small diameter pipelines are routinely used to transport oil and gas between offshore production plants and the mainland, or between remote subsea well-heads and a centralised production facility. The pipelines may be placed on the soil surface but it is more usual that they are placed into trenches, which are subsequently backfilled. For the buried pipelines a well established problem has been that of upheaval buckling. This occurs because the fluid is usually pumped through the pipes at elevated temperatures causing the pipeline to experience thermal expansion which, if restrained, leads to an increase in the axial stress in the pipeline possibly resulting in a buckling failure. A secondary phenomenon that has also been identified, particularly in loose silty sands and silts, involves floatation of pipelines through the backfill material, usually shortly after burial. At the University of Oxford a project sponsored by EPSRC and Technip Offshore UK Ltd has commenced to investigate in detail the buckling and floatation problems. The main aim of the research programme is to investigate three-dimensional effects on the buckling behaviour. The initial experiments involve the more typical plane strain pipeline unburial tests to explore the relationship between depth of cover, uplift rate, pipeline diameter and pullout resistance under drained and undrained conditions. The second and main phase of experiments involves inducing a buckle in a model pipeline under laboratory conditions and making observations of the pipe/soil response. This paper will describe the initial findings from the research including a) plane strain pipe unburial tests in loose dry sand, and, b) initial small scale three-dimensional buckling tests. The paper will then describe the proposed large scale three-dimensional testing programme that will be taking place during 2006 and 2007.Copyright

Field testing of large diameter piles under lateral loading for offshore wind applications

The nature-inspired concept of self-healing materials in construction is relatively new and has recently attracted significant attention as this could bring about substantial savings in maintenance costs as well as enhance the durability and serviceability and improve the safety of our structures and infrastructure. Much of the research and applications to date has focused on concrete, for structural applications, and on asphalt, with significant advances being made. However, to date no attention has been given to the incorporation of self-healing concepts in geotechnical and geo-environmental applications. This includes the use of concrete and other stabilising agents in foundations and other geotechnical structures, grouts, grouted soil systems, soil-cement systems and slurry walls for ground improvement and land remediation applications. The recently established Materials for Life (M4L) project funded by EPSRC has initiated research activities in the UK focussing on those applications. The project involves the development and integration of the use of microcapsules, biological agents, shape memory polymers and vascular networks as healing systems. The authors are exploring development of self-healing systems using mineral admixtures, microencapsulation and bio-cementation applications. The paper presents an overview of those initiatives to date and potential applications and presents some relevant preliminary results.By contrast to studies in petroleum geology and, despite their world-wide occurrence, geotechnical studies of ancient fluvial sediments are rare. This paper introduces the main characteristics of these sediments by reference to a classic UK example. Attention is then drawn to a number of major overseas examples where, although the principal features can be recognised, large differences arise as a result of factors such as the tectonic setting, the volume and mineralogy of the source material and the climate at the time the sediments were deposited. The first, over-riding problem for their engineering evaluation comes during the site investigation phase with the difficulty of deducing the geological structure and distribution of the widely varying lithologies.Strain accumulation in granular soils due to dynamic loading is investigated through long term cyclic triaxial tests and cyclic triaxial tests according to ASTM D 3999-91. Soil parameters, test equipment and loading conditions have a significant influence on strain accumulation, therefore a parameterization of the silica sand and a description of the cyclic triaxial test device are explained. Cyclic triaxial tests are performed and test results are presented illustrating the evolution of Young’s modulus during long term cyclic loading. The influence of the width of the stress-strain loop and the initial void ratio on strain accumulation is investigated and validated with existing accumulation models. The usefulness of Miner’s rule on sand subjected to cyclic loading is demonstrated by two tests with different packages of loading cycles.

PISA: New Design Methods for Offshore Wind Turbine Monopiles

Improved design of laterally loaded monopiles is central to the development of current and future generation offshore wind farms. Previously established design methods have demonstrable shortcomings requiring new ideas and approaches to be developed, specific for the offshore wind turbine sector. The Pile Soil Analysis (PISA) Project, established in 2013, addresses this problem through a range of theoretical studies, numerical analysis and medium scale field testing. The project completed in 2016; this paper summarises the principal findings, illustrated through examples incorporating the Cowden stiff clay profile, which represents one of the two soil profiles targeted in the study. The implications for design are discussed.

PISA: Recent Developments in Offshore Wind Turbine Monopile Design

This paper provides a brief overview of the Pile Soil Analysis (PISA) project, recently completed in the UK. The research was aimed at developing new design methods for laterally loaded monopile foundations, such as those supporting offshore wind turbine structures. The paper first describes the background to the project and briefly outlines the key research elements completed. The paper concludes with a brief description of the anticipated impact of the work and describes initiatives that have followed since.