This Géotechnique themed issue covers a wide range of the latest work in offshore geotechnics, representing state-of-the-art output from some of the leading international research groups and companies. The call for papers went out in early 2011 and was met with a strong response, indicating the depth and quality of work being carried out around the world. Places in this issue were therefore hotly contested. The sub-panel responsible for the issue believes that the resulting selection of papers will have considerable impact on offshore practice, and is pleased to draw this work to the attention of the wider geotechnical community.
Much of the research that is presented consists of high-quality experimental work, coupled with detailed theoretical considerations, reflecting that much of our understanding of soil behaviour is still founded on detailed observations in the laboratory. The papers by Sultan et al. (2012) and Torisu et al. (2012) tackle the intrinsic response of soil in two different scenarios. Sultan et al. (2012) are concerned with the effect of gas, which frequently occurs at offshore development sites, on the behaviour of soil and particularly when the gas comes into or out of solution. Torisu et al. (2012) are more concerned with rate effects on the response of deep marine clays, and present experimental results with loading rates spanning two orders of magnitude. Both pieces of work reflect the very detailed and specialised nature of element testing for offshore problems. Of course, while having a detailed understanding of soil behaviour is essential, it is not sufficient to solve many of our geotechnical problems. The paper by Kovacevic et al. (2012) demonstrates the application of complex soil models in an advanced finite-element simulation of an underwater slope. The modelling follows the entire evolution of an escarpment over many thousands of years, necessary to reproduce the complex geological profile of the slope, and to capture the instabilities that develop over time.
Three papers focus explicitly on pipeline design, an area that has rapidly expanded over the past 10 years, with many current and future offshore developments involving large networks of pipelines stretching over potentially difficult terrain. With considerable investments at stake, the geotechnical understanding can be critical to the success or otherwise of these projects. The paper by Westgate et al. (2012) considers the embedment of pipelines during the laying process and compares field measurements to a simple prediction method, showing good agreement. This paper highlights the role that field measurements can play in helping to develop design methodologies further, a lesson that is applicable to a range of other geotechnical problems both offshore and onshore. Randolph et al. (2012) consider axial pipe friction, developing a new model for understanding the development of sliding resistance on soft materials, important for considering what happens to pipes as they heat up and cool down, and thus seek to expand or contract. Finally Martin & White (2012) present numerical work that demonstrates the powerful nature of the finite-element limit analysis method. In their paper the method is applied to the capacity of a pipe segment on, or embedded in, undrained clay under vertical and horizontal loading. Some 10 000 individual results are presented, bracketing the exact solution for each case to within 1%, and arguably providing the definitive study of this particular problem.
A themed issue on offshore geotechnics would not be complete without papers describing the development of macro-element plasticity models. This type of approach is now well established for a wide range of problems, particularly for the modelling of shallow foundations, and more recently in the modelling of subsea pipelines. Cassidy et al. (2012) apply the approach to a suction embedded plate anchor (SEPLA), a recent anchoring concept where a plate anchor is embedded using a suction pile, and then deployed once the suction pile is removed. The key issue addressed by the paper is the second part of the process, and the plasticity model provides a good description of the deployment process. The paper by Knappett et al. (2012) considers grillage foundations, as used for subsea structures, and their response under horizontal loading (which for many applications is the dominant loading condition). Again, the plasticity model provides a good description of the response of the grillage. Finally, the paper by Xie et al. (2012) considers the complex problem of a spudcan penetrating next to a pre-existing pile foundation and reports the results of a parametric study of the problem using high-quality centrifuge tests.
Although this issue is not a symposium in print, there will be an opportunity to listen to a presentation of the papers at the Society for Underwater Technology's 7th Conference on Offshore Site Investigation and Geotechnics. This conference will be held in London at the Royal Geographical Society from Wednesday 12 September to Friday 14 September 2012. The Géotechnique papers will be presented in two special sessions on one of the afternoons, and special rates are available for attending the conference just for these sessions. The sessions will reflect the order of papers in the issue, with the first four presented in the early afternoon, and the final five in the later afternoon. The sessions will include an opportunity for discussion of the papers. The final timetable can be found on the conference website at http://events.sut.org.uk/osig/.
The editorial sub-panel consisted of Byron Byrne, Bill Craig, Richard Jardine, Chris Martin, Dave White and Lidija Zdravkovic, and we are all grateful for the very quick responses by reviewers and authors which made possible the publication of this themed issue, from first submission of manuscripts to printed copy, in less than 12 months.
