Coastal Geohazard and Offshore Geotechnics

With rapid developments being made in the exploration of marine resources, coastal geohazard and offshore geotechnics have attracted a great deal of attention from coastal geotechnical engineers, with significant progress being made in recent years. Due to the complicated nature of marine environmnets, there are numerous natural marine geohazard preset throughout the world’s marine areas, e.g., the South China Sea. In addition, damage to offshore infrastructure (e.g., monopiles, bridge piers, etc.) and their supporting installations (pipelines, power transmission cables, etc.) has occurred in the last decades. A better understanding of the fundamental mechanisms and soil behavior of the seabed in marine environments will help engineers in the design and planning processes of coastal geotechnical engineering projects. The purpose of this book is to present the recent advances made in the field of coastal geohazards and offshore geotechnics. The book will provide researchers with information reagrding the recent developments in the field, and possible future developments. The book is composed of eighteen papers, covering three main themes: (1) the mechanisms of fluid–seabed interactions and the instability associated with seabeds when they are under dynamic loading (papers 1–5); (2) evaluation of the stability of marine infrastructure, including pipelines (papers 6–8), piled foundation and bridge piers (papers 9–12), submarine tunnels (paper 13), and other supported foundations (paper 14); and (3) coastal geohazards, including submarine landslides and slope stability (papers 15–16) and other geohazard issues (papers 17–18). The editors hope that this book will functoin as a guide for researchers, scientists, and scholars, as well as practitioners of coastal and offshore engineering

Hydrogeological challenges in a low carbon economy

Hydrogeology has traditionally been regarded as the province of the water industry, but it is increasingly finding novel applications in the energy sector. Hydrogeology has a longstanding role in geothermal energy exploration and management. Although aquifer management methods can be directly applied to most high-enthalpy geothermal reservoirs, hydrogeochemical inference techniques differ somewhat owing to peculiarities of high-temperature processes. Hydrogeological involvement in the development of ground-coupled heating and cooling systems using heat pumps has led to the emergence of the sub-discipline now known as thermogeology. The patterns of groundwater flow and heat transport are closely analogous and can thus be analysed using very similar techniques. Without resort to heat pumps, groundwater is increasingly being pumped to provide cooling for large buildings; the renewability of such systems relies on accurate prediction and management of thermal breakthrough from reinjection to production boreholes. Hydrogeological analysis can contribute to quantification of accidental carbon emissions arising from disturbance of groundwater-fed peatland ecosystems during wind farm construction. Beyond renewables, key applications of hydrogeology are to be found in the nuclear sector, and in the sunrise industries of unconventional gas and carbon capture and storage, with high temperatures attained during underground coal gasification requiring geothermal technology transfer

Tracing back the source of contamination

From the time a contaminant is detected in an observation well, the question of where and when the contaminant was introduced in the aquifer needs an answer. Many techniques have been proposed to answer this question, but virtually all of them assume that the aquifer and its dynamics are perfectly known. This work discusses a new approach for the simultaneous identification of the contaminant source location and the spatial variability of hydraulic conductivity in an aquifer which has been validated on synthetic and laboratory experiments and which is in the process of being validated on a real aquifer

Seismic behaviour of structures with basements in liquefiable soil

Earthquake induced liquefaction can cause significant damage in the built environment. Structures on shallow foundations can suffer large settlement and rotation, and light under- ground structures can uplift. The performance of structures with basements, which intuitively combines these two problems, is not understood. In this thesis, the seismic behaviour of structures with basements in liquefiable soil has been investigated. A series of highly instrumented dynamic centrifuge tests showed that the presence of a basement can reduce the liquefaction induced settlement of a structure whilst maintaining the natural isolation provided by liquefied soil. The generation of positive excess pore pressures during shaking increased the buoyancy force provided by the basement. When the ratio of uplift to total weight during liquefaction was controlled, this buoyancy reduced settlement compared to structures with shallow foundations without basements. The centrifuge test data showed that structures with basements were susceptible to suffer a large accumulation of rotation during earthquake induced liquefaction. Compared to structures without basements, resistance to rotation provided by the soil decreased because the buoyancy provided by the basement reduced the vertical effective stresses below the structure. Rotation was exacerbated by an increase in the moment loading imposed by the structure due to the presence of the basement. An increase in the plan area of the basement was found to reduce residual rotation. A mechanical model for displacement and rotation in liquefiable soil was developed using data from the centrifuge test series. It was based on a traditional mass-spring-damper model, with the addition of slider elements to incorporate accumulation of displacement and rotation. The model was able to replicate the centrifuge test results when liquefaction occurred. Parametric studies using the model found that residual rotation was highly sensitive to the basement width and height of the centre of gravity of the structure. In summary, liquefaction induced settlement and rotation, and seismic demand of a structure can be minimised by including a basement and controlling the basement geometry, mass distribution, and total weight of the structure. The increase in usable space that a basement provides in a building is anticipated to make this mitigation method an attractive option compared to conventional alternatives such as soil improvement

Modelling the two-phase plume dynamics of CO2 leakage into open shallow waters

A numerical model of two-phase plume developments in a small scale turbulent ocean is proposed and designed as a fundamental study to predict the near field physicochemical impacts and biological risk to the marine ecosystem from CO2 leakage from potential carbon storage locations around the North Sea. New sub-models are developed for bubble formation and drag coefficients using in-situ measurements from videos of the Quantifying and monitoring potential ecosystem Impacts of geological Carbon Storage (QICS) experiment. Existing sub-models such as Sherwood numbers and plume interactions are also compared, verified and implemented into the new model. Observational data collected from the North Sea provides the ability to develop and verify a large eddy simulation turbulence model, limited to situations where the non-slip boundary wall may be neglected. The model is then tested to assimilate the QICS experiment, before being applied to potential leakage scenarios around the North Sea with key marine impacts from pCO2 and pH changes. The most serious leak is from a well blowout, with maximum pH changes of up to -2.7 and changes greater than -0.1 affecting areas up to 0.23 km2. Other scenarios through geological structures would be challenging to detect with pH changes below -0.27

Thermo-hydro-mechanical analysis of CO2 injection into deep aquifers

One of the biggest challenges for humanity is global warming and consequently, climate changes. Even though there has been increasing public awareness and investments from numerous countries concerning renewable energies, fossil fuels are and will continue to be in the near future, the main source of energy. Carbon capture and storage (CCS) is believed to be a serious measure to mitigate CO2 concentration. CCS briefly consists of capturing CO2 from the atmosphere or stationary emission sources and transporting and storing it via mineral carbonation, in oceans or geological media. The latter is referred to as carbon capture and geological storage (CCGS) and is considered to be the most promising of all solutions. Generally it consists of a storage (e.g. depleted oil reservoirs and deep saline aquifers) and sealing (commonly termed caprock in the oil industry) formations. The present study concerns the injection of CO2 into deep aquifers and regardless injection conditions, temperature gradients between carbon dioxide and the storage formation are likely to occur. Should the CO2 temperature be lower than the storage formation, a contractive behaviour of the reservoir and caprock is expected. The latter can result in the opening of new paths or re-opening of fractures, favouring leakage and compromising the CCGS project. During CO2 injection, coupled thermo-hydro-mechanical phenomena occur, which due to their complexity, hamper the assessment of each relative influence. For this purpose, several analyses were carried out in order to evaluate their influences but focusing on the thermal contractive behaviour. It was finally concluded that depending on mechanical and thermal properties of the pair aquifer-seal, the sealing caprock can undergo significant decreases in effective stress

An integrated seismic and well data study of shallow fluid accumulations in Snøhvit, SW Barents Sea

In the vicinity of the Snøhvit hydrocarbon reservoir in the Hammerfest Basin a number of fluid flow phenomena occur, e.g. free gas accumulations, pockmarks and potential indicators of gas hydrates.The presence of shallow gas may cause major blowouts during drilling. To reduce the risk, it is important to locate the shallow gas and gas hydrates. CO2 has been injected into the Snøhvit reservoir since 2008, even though there has been proven leakage from the reservoir. Leakage of CO2 from the reservoir, through the overburden reaching the seabed is a significant environmental risk. Therefore, it is important obtain a detailed understanding to processes controlling fluid flow, how deep-seated faults can act as conducts for fluid migration, as well as the origin of the shallow gas and gas hydrates located above the reservoir. Newly released 3D seismic data shows the upper few 100s meters of the overburden at Snøhvit in much more detail than previous known, due to reprocessing. In this thesis the migration mechanisms of fluids are described in detail together with seismic indications of shallow gas and gas hydrates. Further, pockmarks, gas chimneys and high amplitude anomalies were mapped out and discussed in relation to deep seated faults penetrating the reservoir. A conceptual model was carried out explaining the origin of the shallow gas above the Snøhvit field in relation to the geological history in the area and how faults play an important part. It was found that the cycles of glacial loading and unloading during the Cenozoic have caused extensive fracture developments, leading to migration of gas from the reservoir to form shallow accumulations of gas and gas hydrates with suitable pressure and temperature conditions. A simpe analytical model was used to determine the leak off factor of two faults located close to the F-2 CO2 injector, and to study which parameters might affect the migration of CO2 from the reservoir through a faoult and into overlying sand, thereby explore more about the risk of storing CO2 in the Snøhvit reservoir today. The leak off factor calculated in this study was found to be very low. The main parameters driving leakage through faults seem to be the reservoir permeability, fault permeability and reservoir thickness. The risk of injection into a low permeable reservoir with an overlaying high permeable sand was highlighted. It is also shown that the fault permeability becomes less important if the overlaying sand has low permeability and hence low flow potential

Buoyancy Force Acting on Underground Structures considering Seepage of Confined Water

The antifloating property of underground structures in areas with high underground water levels is a key design aspect. Evaluating the buoyancy forces acting on underground structures is complicated, particularly in the presence of confined water beneath the structures. Herein, the effects of the permeability coefficient of layered soil, hydraulic gradient, and embedment depth of the aquiclude on the buoyancy force acting on underground structures are investigated through three model tests: (1) calibration of the test system, (2) buoyancy force acting on a structure located in homogeneous soil considering vertical direction seepage, and (3) buoyancy force acting on a structure located in layered soil considering vertical seepage of confined water. The results show that the pore pressure along the structure and the buoyancy force acting on the underground structure considering seepage are greater than those obtained under hydrostatic conditions. The raising ratios of the pore pressure and buoyancy force are equal to the vertical hydraulic gradient when seepage occurs in homogeneous soil. In the presence of confined water, the raising ratio is significantly greater than the hydraulic gradient. In the cases studied herein, the raising ratio is approximately twice the hydraulic gradient. Simplified equations are proposed to calculate the buoyancy force acting on underground structures considering the vertical seepage of confined water. Finally, a finite element analysis is carried out to verify the conclusions obtained from the model test and the rationality of the proposed equations