Consolidation-induced improvements in plate anchor capacity

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Cone Penetration Testing 2022

This volume contains the proceedings of the 5th International Symposium on Cone Penetration Testing (CPT’22), held in Bologna, Italy, 8-10 June 2022. More than 500 authors - academics, researchers, practitioners and manufacturers – contributed to the peer-reviewed papers included in this book, which includes three keynote lectures, four invited lectures and 169 technical papers. The contributions provide a full picture of the current knowledge and major trends in CPT research and development, with respect to innovations in instrumentation, latest advances in data interpretation, and emerging fields of CPT application. The paper topics encompass three well-established topic categories typically addressed in CPT events: - Equipment and Procedures - Data Interpretation - Applications. Emphasis is placed on the use of statistical approaches and innovative numerical strategies for CPT data interpretation, liquefaction studies, application of CPT to offshore engineering, comparative studies between CPT and other in-situ tests. Cone Penetration Testing 2022 contains a wealth of information that could be useful for researchers, practitioners and all those working in the broad and dynamic field of cone penetration testing

Physical Modeling of Suction Caissons Loaded in Two Orthogonal Directions for Efficient Mooring of Offshore Wind Platforms

Over the past decade a number of Federal and State policies and programs have promoted the development of the wind energy industry, including the establishment of offshore wind. A strategy by the Department of Energy set objectives of reducing cost and reducing time to deployment through specific deliverables such as innovative anchor and mooring design for floating offshore systems and hardware design concepts including turbine array grids. This research program proposes and investigates use of suction caissons as combined anchors to resist line loads from multiple platforms as an efficient solution for anchoring a network of wind turbine platforms. Suction caissons are a ‘mature’ anchor technology in the offshore oil and gas industry, yet there is minimal experience with application for offshore wind platforms. Established design methods, standards and recommended practices from the oil and gas industry, serve as a starting point for further adaptation. Considerations of the differences in conditions (e.g. loads, risk, failure and serviceability tolerances) between the two applications, is important for developing efficient anchor design suited to offshore wind platforms. A physical modelling program was developed to investigate the behaviour of caissons subjected to orthogonal cyclic and post-cyclic monotonic line loads, compared to the behaviour of single line loaded caissons. Modeling was performed in a geotechnical centrifuge in order to simulate in-situ stress profile at model scale, as stresses are critical to soil and foundation behavior. Load tests were performed on a model suction caisson anchor installed by jacking into normally consolidated kaolin clay (in-flight). Baseline tests were performed with single line loading for comparison to the multi-line loading tests. Line loads were applied in orthogonal directions for the multi-line load tests. The effect of varying cyclic mean load and cyclic load amplitude was also investigated. Comparison of test results was based on line displacement, applied line load, caisson rotation and internal pore pressure at the underside of the caisson cap. Centrifuge test results appear to indicate that the line load-displacement response during monotonic loading is similar for the multi-line and the single line loaded suction caisson anchors. The post-cyclic peak monotonic line load resistance provided by the caisson loaded in multiple directions was greater than the resistance provided by the caisson loaded in a single direction (accounting for the total resultant load applied). For all selected load cases, the accumulated permanent displacements during the cyclic loading did not result in a displacement (serviceability) failure of the suction caisson nor contribute significantly to the displacement correlating to the peak line load resistance of the caisson. Test results indicate that the resistance capacity of a given caisson is not reduced by applying line loads in multiple directions, when considering the resistance to the total resultant load. Test observations appear to support conceptualizations of a modified “zone of influence” (active/passive earth pressure wedges) due to the changing load orientation from resolving multiple out of phase line loads

Centrifuge and Numerical Modelling of Sand Compaction Pile Installation

Ph.DDOCTOR OF PHILOSOPH

Scaling of the reinforcement of soil slopes by living plants in a geotechnical centrifuge

The research described here in was funded by a EPSRC (EP/M020355/1) project in collaboration with the University of Dundee, the University of Southampton, the University of Aberdeen, the Durham University and The James Hutton Institute. The authors thank Professor Mike Humphreys (IBERS, Aberystwyth University) and Scotia seeds for providing seeds used in this study and Dr Gary Callon (University of Dundee) for arranging indoor growing area. The James Hutton Institute receives funding from the Scottish Government (Rural & Environmental Services & Analytical Services Division).Peer reviewedPublisher PD

Fatigue damage assessment of steel catenary risers in the touchdown zone with incorporation of time-dependant seabed interaction effects

Steel catenary riser (SCR) design is heavily affected by fatigue performance in the touchdown zone (TDZ). Within the TDZ, the riser cyclically interacts with the seabed, resulting in the progressive generation of excess pore water pressure, leading to soil softening and remoulding. However, the soil may undergo a consolidation process during the intervening pause period, i.e., the calm weather or the inactive periods of SCRs oscillation, allowing the pore water pressure to dissipate. This process results in the regaining of effective stress and consequently achieving a higher soil strength, which is detrimental to the fatigue damage accumulation in the TDZ. The existing advanced hysteretic non-linear riser-seabed interaction models do not account for the consolidation effects. Besides, a secondary mechanism, i.e., the seabed erosion due to combined vortices generated by subsea currents and seawater entrapped between the oscillating riser and the trench may contribute to the cyclic riser embedment. The existing riser-seabed interaction models do not capture this secondary mechanism as well. In this research project, first, these key knowledge gaps were addressed by developing global and local riser-seabed-seawter interaction models and incorporation of the consolidation and erosion effects. Later, the developed models were used to propose novel methodologies for incorporation of the trench effects into the riser fatigue analysis. The consolidation effects was added by coding an effective stress framework into a user-defined subroutine (UEL) in ABAQUS. This was integrated with the global riser model to determine the long-term soil stiffness associated with damage accumulation during SCR cyclic motions and soil strength recovery during the intervening pause period. The seabed soil erosion effect on trench formation was investigated by developing a three-domain model of riser-soil-fluid interaction. The model was used to study the combined effect of soil erosion, soil fluidization, and cyclic riser oscillations on the plastic soil deformation and riser embedment. A Coupled Eulerian-Lagrangian (CEL) technique was employed and the strain rate and soil softening effects in an Eulerian domain were coded into the VUSDFLD subroutine of ABAQUS. The developed models were successfully verified against the experimental studies from the literature. As proven by subsea surveys, all of these riser-seabed-seawater interaction mechanisms result in a trench formation several riser diameter deep (3D to 7D) that can significantly affect the fatigue life in the TDZ. Although most of the studies in the literature show a beneficial effect of the trench on fatigue, there is still no coherent agreement amongst researchers on the beneficial or detrimental trench effects. To further investigate the trench effect on fatigue, first, two new methodologies were proposed, i.e., i) an alternative vessel excitation algorithm called the equivalent motion method (EMM) and ii) an equivalent soil stiffness approach called the hybrid trench model (HTM). The first method was investigated to predict the fatigue damage of the riser in the linear elastic seabed using the same riser on the rigid seabed but with a virtual vessel motion algorithm. An equation was extracted from a comprehensive set of analyses for a given riser resting on an elastic seabed to obtain an equivalent vessel motion amplitude on a rigid seabed with the same cyclic damage. As an alternative solution, the proposed EMM was found to be a promising basis for further extension into the non-linear riser-seabed interaction. The second methodology provides equivalent soil stiffness to simulate the target riser embedment, which is usually obtained from non-linear hysteretic riser-seabed interaction models. The capability of HTM in developing deep trenches, e.g., 5D, was examined along with perfect compatibility with the natural catenary shape of the riser to resolve any pressure hot spots and premature stabilization problems frequently reported in the literature. This novel methodology was integrated with the effective stress analysis developed earlier in this study to deeply investigate the trench effect on the fatigue performance of SCR in the TDZ, while considering the consolidation. Besides developing several advanced tools for enhanced analysis of SCR-seabed interacation, the study extended the insight into the fatigue performance of steel catenary risers in the touchdown zone that can be used by field operators and riser life extension authorities

Deep ocean clay crusts: behaviour and biological origin

In water depths of 500m to greater than 2,000m, off the West coast of Africa, sediments comprise very soft clays with extremely high water contents and plasticity. In situ CPT and T-bar testing in these areas have identified 'crusts' with undrained shear strengths of up to 15kPa at 0.5m depth, before the strength reduces by an order of magnitude to normally consolidated strengths by 2m depth. This thesis presents an investigation into the behaviour and origin of these crusts. Mini ball-penetrometer tests on natural cores confirm the crustal strength, and indicate a sensitivity of 3 within the crust. However, pipeline interface tests using the Cam-shear device demonstrate a significant variability in the measured interface friction coefficient. Particularly low strengths are observed when shearing in an undrained manner on a rough interface. These results are attributed to the heterogeneity of natural samples, and demonstrate the need to better understand the origin of the crust material in relation to interface micro-mechanics. A microbiological investigation of crust material by extracting bacteria DNA from clay samples is described, and identifies the presence of the bacterium, Marinobacter aquaeolei. This bacterium is then used to inoculate sterile samples to determine its ability to produce crustal strength. Through this work, it is concluded that M. aquaeolei is unable to create crustal strength, although extracelluar polysaccharides produced by this bacterium will influence the permeability of sediments through the clogging of voids. It is therefore also concluded that future geotechnical investigation into marine sediments should consider the presence of bacteria and their ability to influence the soil properties. Wet sieving of crust material shows that the crust comprises a mixture of burrowing invertebrate faecal pellets and clay. Pellets are found to represent 20% to over 55% of the crust material by dry mass. Individual pellets are shown to exhibit unconfined compressive strengths of between 5kPa and 50kPa, thus demonstrating their strength and robustness. Consolidation behaviour is governed by the percentage of pellets in natural samples. Based on their location, abundance and strength, it is concluded that the origin of crustal strength lies with the presence of burrowing invertebrate faecal pellets. When sheared on rough pipeline interfaces, however, pellets are observed to crush, expelling void-filling fragments that may generate positive excess pore pressures. Smeared clay produced when shearing natural samples obstructs the dissipation of pore pressures, which may encourage hydroplaning, and explain the observation of very low interface friction coefficients. It is therefore suggested that smooth pipelines offer more sliding resistance by minimising the risk of pellet crushing. This thesis proposes that wet sieving of core samples should be undertaken during the site investigations for future deep-water, hot-oil pipeline installations to provide design information on both the consolidation and strength behaviour of natural sediments

Numerical modeling of large deformation behaviour of offshore pipelines and risers in soft clay seabeds

Deepwater oil and gas development activities have increased significantly in the last few decades to meet the global demand for energy. One of the key components of these developments is the oil and gas transportation pipeline. Deepwater pipelines are often laid on the seabed and may vertically penetrate into the seabed sediment (primarily clay) or remain suspended in the case of uneven seabed profiles. Partially embedded pipelines might displace laterally during operation due to changes in internal pressure and temperature. The displacement of the pipeline depends on soil resistance, which is also related to the initial embedment. The suspended pipelines might also be impacted by soil blocks moving out from submarine landslides. Moreover, in deepwater, steel catenary risers (SCR)—a long pipe of 150–600 mm typical diameter—are often used to transport hydrocarbon from the seabed production system to floating production facilities. The interaction between soil, water and pipes (partially embedded, suspended or SCR) involves significant large deformations, which cannot be modeled properly using traditional Lagrangian-based finite element (FE) techniques and therefore improved numerical modeling is required for safe and economic design. In the present study, simulations of the large deformation behaviour of deepwater pipelines and SCRs are performed using two numerical approaches. First, simulation is performed using the Coupled Eulerian-Lagrangian (CEL) approach available in the Abaqus FE software. In CEL, the soil is modeled as an Eulerian material that flows through the fixed mesh and therefore numerical issues related to mesh distortion at large displacements are avoided. Simulations are performed for undrained loading conditions implementing a strain-rate and strain-softening dependent undrained shear strength model for clay in Abaqus CEL through user subroutines. For partially embedded pipelines, numerical simulations are performed for vertical penetration and subsequent lateral displacements. In addition, dynamic penetration of the pipeline into a deepwater soft clay seabed is simulated. The penetration and lateral resistances are compared with the results of previous physical model tests, and numerical and analytical solutions. Recognizing the limitations of Abaqus CEL and other FE modeling techniques to simulate the role of water, ANSYS CFX—a finite volume software—is used in the second approach for numerical modeling. A technique is developed to implement strain-rate and strain-softening dependent undrained shear strength of clay in ANSYS CFX. The comparison between penetration resistances obtained from CEL and CFX shows that the latter approach can simulate the effect of water in the cavity formed behind the pipe when it penetrates to a sufficiently large depth into the clay seabed, with a transition between shallow and deep failure mechanisms. In the SCR–seabed–water interaction modeling, in addition to undrained remoulding, the reduction of undrained shear strength due to other factors such as water entrainment is considered using “shear wetting”. Cyclic degradation of penetration and uplift resistance, development of suction under the riser during uplift, and the formation of a trench are successfully simulated for a large number of cyclic motions near the seabed, where a significant shear strength reduction occurs, as reported from physical model tests. The impact force on suspended offshore pipelines by submarine landsides is also simulated using both Abaqus CEL and ANSYS CFX. The development of forces on the pipe with its penetration into the soil block shows that the trapped water behind the pipe influences the failure mechanisms and magnitude of force. The suction in the trapped water and flow of free water through the channel formed behind the pipe is simulated using ANSYS CFX. Based on a comprehensive parametric study with calibration against a series of centrifuge test results, a set of empirical equations are proposed to calculate the impact force on suspended pipelines