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Suction bucket installation in pure clay or pure sand is well established. However, in layered soils, particularly in sand over clay soils, the soil plug uplift is considered a major installation risk and feasibility of installation is often questioned. In this paper, the experience from installation of two suction bucket jackets at two offshore wind farms in layered soils is reported. The installation at both sites was successful and no excess plug uplift was observed. The transfer of suction to the clay-sand interface is believed to have contributed to the success of the installations. The two cases presented in this paper provides confidence in suction bucket foundation installation in similar soil conditions. [ABSTRACT FROM AUTHOR]

Suction-bucket-supported jacket foundations are being used increasingly for offshore wind turbines, the advantages being large bearing capacity and efficient installation. For a large-capacity wind turbine, the pullout capacity and displacement of the bucket in the tripod or tetrapod jacket can be critical to the safety of the structure. The loading rate, which is commonly dominated by the offshore environmental conditions, has a significant effect on the ultimate capacity and stiffness of the bucket foundation. In this paper, a numerical model is developed to investigate the response of a bucket foundation under vertical pullout loading at different rates. A bounding-surface model based on critical state soil mechanics is introduced to account for the soil behaviour under different stress states and drainage conditions. The model is modified and calibrated against a comprehensive set of laboratory tests, including both triaxial compression, triaxial extension and direct simple shear tests in drained and undrained conditions. How partial drainage affects the load–displacement response and pore pressure field are thus presented. A parametric study on the selection of soil parameters is also conducted to shed light on practical foundation design. [ABSTRACT FROM AUTHOR]

As an alternative to monopiles, suction bucket jacket foundations are gaining increasing popularity in China for supporting offshore wind turbines. One of the major design challenges and governing factors for foundation sizing is the long-term tilt caused by the differential settlement between the buckets due to a combination of prevailing wind directions and soft seabed conditions. The assessment of the long-term consolidation settlement is complicated and subjected to uncertainties, such as the load sharing between the skirt and the bucket lid, and load re-distribution with time as the soil response transits from short-term undrained behaviour to long-term drained behaviour. This paper presents an attempt to understand the short-term and long-term load sharing mechanisms for the suction bucket foundation by means of finite element analysis. Without modelling the large deformation installation process explicitly, the initial (undrained) load sharing mechanism and induced additional stress (or excess pore water pressure) in the soil body is first examined. The re-distribution of the load between the skirts and the lid as the excess pore pressures dissipate is subsequently investigated. The study examines a range of soil conditions and foundation aspect ratios. It is found that the undrained skirt wall friction capacity relative to the load level has an important impact on the initial load sharing mechanism. As consolidation takes place, significant load redistribution occurs, with the loads carried initially by the lid partially or fully transferred to the internal and external skirt wall frictions. The load sharing at completion of consolidation heavily depends on the drained skirt friction capacity relative to the load level. Guided by the numerical findings, a tentative analytical model for practical design purpose is proposed. [ABSTRACT FROM AUTHOR]

China National Offshore Oil Corporation (CNOOC) currently is developing a mat-supported wellhead production platform concept as an alternative solution to conventional jacket platforms in shallow-water oil fields. The platform is towed to the site with the mat foundation elevated. The mat then is lowered and embedded in the seabed as the hull is jacked out of the water. Steel pipe piles subsequently are driven through the tubular legs of the platform. After grouting the connection, the pile foundations provide permanent support for the platform. The mat foundation therefore provides only temporary support to the platform. This paper presents a case study focusing on the installation behavior of the mat foundation in a soft soil site as part of the platform concept development and suitability evaluation. The study determined (1) the expected penetration depth under the platform's self-weight considering its irregular geometry and soft over stiff clay stratigraphy; (2) the target center of gravity to achieve even penetration and the potential for tilt development in the case of offset from the target position; (3) the effectiveness of using ballast tanks to correct the tilt; and (4) settlement after the installation due to dissipation of pore pressure and creep. Advanced large-deformation finite-element simulations featuring a strain softening soil model and analytical analyses were performed. A dedicated lab testing program geared was designed and executed to provide the necessary soil parameters for the analyses. This paper presents the techniques and considerations that were applied in this study and insights gained into the installation and subsequent postinstallation behavior of large-area mat foundations. [ABSTRACT FROM AUTHOR]

For offshore wind development in Southern China, typhoon represents the extreme environmental loading condition to be considered in the design of the turbine structures. For the geotechnical design of the foundations, it is specified by the design code that the effect of cyclic loading on the soil strength and stiffness must be considered. This necessitates the complete time history of loads on the foundation experienced during a design typhoon. Idealised storm profiles with staged mean wind speeds and significant wave heights are commonly used as input to time-domain simulations to generate the foundation load history. However, no such design typhoon profile is provided in the Chinese design standard which creates considerable difficulty and uncertainty in the design process. In this paper, the methodology and the rationale that were applied to derive the design typhoon profiles for an offshore wind project in Guangdong province are presented. The wind and wave characteristics generated by typhoon are described in detail and the implications for the foundation design are discussed. To demonstrate application of the proposed typhoon profile, a case study for the ULS design of a suction bucket jacket is presented. • Design typhoon profiles are developed for an offshore wind farm in Southern China. • The typhoon wind and wave characteristics are described and implications for geotechnical foundation design are discussed. • The methodology and the rationale for developing the design typhoon profiles are presented. [ABSTRACT FROM AUTHOR]

Pile foundations supporting offshore structures, such as jacket platforms and wind turbines, are subjected to cyclic lateral loading. The capacity and deformation of these pile foundations under cyclic lateral loading are important and challenging design aspects. This paper presents the results of a series of four lateral load pile tests in an overconsolidated clay at the Haga site and their back-analysis using a p-y framework based on fundamental soil behavior measured in the laboratory at the element level. This framework can account for site-specific cyclic soil properties, summarized in classic contour diagrams, and site-specific cyclic loading characteristics in design. The paper first provides a summary of the framework and outlines the important assumptions and calculation procedures. It then presents a detailed description of the test setup and soil conditions. The pile load test results and the back-analysis are subsequently presented. Generally, a good match between the test results and the model predictions was observed, demonstrating the model's capability to capture the essential behavior of pile foundations under monotonic and cyclic lateral loading. [ABSTRACT FROM AUTHOR]

Abstract Large diameter steel pipe pile foundations, typically known as monopiles, are the predominant foundation solution for offshore wind turbines. This paper deals with soil reaction curves for monopile analysis under lateral loading in clay. Developed from extensive parametric finite element analyses, models are proposed that allow for construction of site-specific soil reaction curves based on the stress-strain responses measured in laboratory tests. Assisted by failure mechanisms revealed by finite element analyses, a conceptual framework is proposed to analyse the monopile response under lateral loading using the beam-column approach. The conceptual framework makes use of the p-y model for wedge failure developed in this study along the monopile above the rotation point, existing p-y model for the flow-around failure suggested in the literature below the rotation point and base shear s-u model developed in this study at the pile tip. Validation exercises against finite element simulations and field pile testing results demonstrate excellent capability of the framework to predict monopile responses. The soil reaction models proposed in this paper provide practising engineers with a simple yet powerful approach to use site-specific soil reaction curves in monopile design based on element soil behaviour measured in laboratory, without the need for advanced numerical analyses. Highlights • Soil reaction models that use stress-strain responses measured in laboratory tests as input. • Site-specific soil reaction curves can therefore be used in design, without the need for advanced site specific finite element analysis. • A conceptual framework for monopile analysis using a conventional beam-column approach. [ABSTRACT FROM AUTHOR]

Abstract Large diameter monopiles are typical foundation solutions for offshore wind turbines. In design of the monopile foundations in sand, it is necessary to understand the drainage conditions of the foundation soil under the design loading conditions as the soil performance (strength and stiffness) is highly dependent on the drainage conditions. This paper presents a numerical investigation into this issue, with a purpose to develop a simple design criterion for assessing the soil drainage conditions around a monopile in sand. It is found that for typical monopile foundations in sand, the drainage condition during a single load cycle is generally expected to be undrained. However, the current state-of-practice uses p-y springs derived for drained soil responses for monopile design. The impact of this discrepancy on monopile foundation design was evaluated and found to be insignificant due to the relatively low level of loading as compared to the capacity of the soil. [ABSTRACT FROM AUTHOR]

The fatigue assessment of a well conductor is a critical design task during offshore exploration and development of hydrocarbons. In recent years, major progress has been made towards the modelling of soil-conductor interaction. Upgraded soil-conductor interaction springs (i.e., p-y curves) and damping models are developed and are gaining acceptance by the industry. However, there is a general lack of simplified procedures to incorporate the soil damping into the time-domain structural analysis of a wellhead system. Thus, the motivation of this paper is to close this gap by proposing a simplified procedure for incorporating the soil damping in the well conductor time domain analysis. The procedure is generic and allows for convenient adoption of alterative p-y spring and damping models. Design charts for equivalent soil system damping ratio versus the lateral deflection of the conductor are developed through parametric analyses based on representative wellhead configurations with typical soil strength profiles in the Gulf of Mexico to facilitate practical design. • A simplified procedure for incorporating the soil damping in time domain analysis of well conductors. • Design charts are developed for representative wellhead configurations and soil strength profiles in the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

This paper deals with the static lateral load-displacement response ( p-y spring) of a pile slice in soil. The response is governed by a localised flow-around soil failure mechanism. A model is proposed that allows for construction of site-specific p-y springs by directly scaling the soil stress-strain response measured in laboratory tests. This model is based on an extensive parametric finite element study and can explicitly account for the effect of pile-soil interface roughness factor on both the strength and shape of the p-y spring. The model demonstrates excellent agreement with the p-y responses calculated numerically. An example application illustrates the capability of the model to predict the overall pile response by comparison with full three dimensional finite element analysis. The proposed model is compared with existing models in the literature, where similarities and differences are discussed and highlighted. The model provides practising engineers with a simple yet powerful approach to use site-specific p-y curves in design based on element soil behaviour measured in laboratory, without the need for advanced numerical analyses. [ABSTRACT FROM AUTHOR]

The paper re-visits the topic of ultimate bearing capacity of laterally loaded piles in clay. The paper first presents a review of various recommendations made by design guidelines and industry practitioners which illustrates inconsistency and need for further work. A literature study is therefore performed and a generalised recommendation of the ultimate lateral capacity of piles in clay that is self-consistent and flexible for a wide range of conditions is made. The paper further investigates two practical considerations often encountered in design, namely the effect of axial loading and the effect of soil strength anisotropy by means of finite element analyses. Practical methods to account for these effects are proposed. [ABSTRACT FROM AUTHOR]

The discusser is correct that the construction of the cyclic contour diagrams for a new soil without any prior knowledge or experience would have required more cyclic soil element tests than those indicated in Fig. These diagrams can also be used to inform the shape of diagrams for other clays, when test results are limited. This procedure is based on the use of cyclic contour diagrams, which are fundamentally based on observed soil responses at the element level. [Extracted from the article]

Pile group foundations are widely used in the offshore industry to support large jacket structures, such as oil platforms and substations for offshore wind farms. The pile group foundations are commonly designed using methods developed for single piles but modified to account for the group interaction effect. The modification for pile group effect has been conventionally based on elasticity solutions or empirical observations from field/model pile load tests. In the past decade, considerable progress has been made with respect to modelling single pile response under lateral loading in clay. However, there appears to be limited progress for modelling pile groups in the offshore industry. The design practice has stayed unchanged for several decades. This paper presents a numerical study in an attempt to come up with design recommendations for typical pile group configurations used in the offshore industry. A comprehensive numerical investigation of the group effect for several typical pile group configurations are performed, examining the effect of pile spacing and the loading direction. From the parametric analyses, the so-called p and y modifiers which are to be applied to correct the single pile p-y curves for consideration of the group effect are proposed. An example is presented to demonstrate the application of the modifiers. • Group interaction effect modifiers are developed for representative pile group geometries used in the offshore industry. • Closed-form equations are presented, allowing for consideration of pile spacing, loading direction and pile group layout. • The modifiers are general and are not tied to a specific strength profile or stress-strain response of the soil. [ABSTRACT FROM AUTHOR]

Abstract: With the development of the offshore oil and gas industry, mobile jack-up drilling platforms are increasingly utilized in deeper waters and harsher environments. Excessive conservatism in jack-up site specific assessment may result in undue rejection of a unit. In soft clayey seabeds, the spudcan foundations of the jack-up platform penetrate deeply into the soil with partial or complete backflow. Although the deep penetration and associated backflow are widely perceived to increase horizontal and moment foundation bearing capacities in particular, the problem is not well understood. In this paper therefore, a plasticity foundation model is proposed that accounts for the effects of backflow. It has been developed through a combined numerical and experimental study. The model is suitable for performing integrated soil–structure analyses. Results of such analyses of a jack-up under quasi-static push-over load are discussed to highlight the impact of the model in the context of site specific assessment of jack-up rigs in soft clay. [Copyright &y& Elsevier]

Abstract: The spudcan footings of mobile jack-up platforms have been known to embed several diameters during their installation in soft clay soils. During penetration soil can flow around the spudcan back filling the cavity above. This paper provides the elastic stiffness coefficients for surface to deeply embedded spudcan, as required for predicting the in situ load–displacement behaviour of a jack-up from the wind, wave and currents loading of a large storm. Results from 264 finite element analyses provide 352 dimensionless coefficients for the combined vertical, horizontal and moment elastic stiffness matrix. Coefficients are provided for (i) a representative spudcan shape, (ii) seabed with constant or linearly increasing shear modulus with depth, (iii) spudcan embedment from the surface to up to five diameters, and (iv) the limiting conditions of either an open and completely filled cavity above the spudcan. Recommendations for an interpolation procedure for partial backflow conditions are also provided. These improve the coefficients provided in existing jack-up site assessment guidelines, which do not account for spudcan shape or an increasing strength soil profile. [Copyright &y& Elsevier]

Estimation of a footing's bearing capacity under general loading is a topic of wide interest, especially for the offshore energy industry. Although there have been several studies in the past few decades on shallow footings under general loading, limited work has been done for deeply buried footings. This paper presents results of a numerical investigation into the undrained bearing capacity of deeply buried flat circular footings in clay of both uniform and linearly increasing shear strength profiles. The footings' uniaxial bearing capacities and failure surfaces under combined vertical, horizontal, and moment loading are reported. The results are analyzed in the context of application by the mobile jack-up industry, for which understanding of the circular spudcan footing's bearing capacity under general loading is required for site-specific assessments. The paper also presents a parametric study on the effect of a footing's aspect ratio (thickness/diameter) on its bearing capacity. To conclude, a closed-form expression is proposed to formulate the failure surfaces of the footings under general loading. [ABSTRACT FROM AUTHOR]

Abstract: Mobile jack-up drilling rigs are typically supported by individual, large diameter spudcan foundations. Before deployment, the suitability of a jack-up to a location must be shown in a site-specific assessment under loads associated with a 50-year return period storm, which ultimately need to be resisted by the foundations. The capacity of the spudcans under combined vertical, horizontal and moment loading is therefore integral to the overall site-specific assessment of the jack-up. In soft clays, spudcans can penetrate deeply into the seabed, sometimes up to several footing diameters, with soil flowing around the downward penetrating footing, sealing the cavity. Although this is generally believed to provide some additional bearing capacity to the footing, no detailed study or formal guidance is available to date. This study, therefore, investigates the influence of soil back-flow on the failure mechanisms and quantifies the effect on the capacity of a spudcan under general loading through finite element analyses. A closed-form analytical expression is developed that describes the capacity envelope under combined loading, applicable to embedment depths ranging from shallow to deep. [Copyright &y& Elsevier]

Fatigue assessment is a critical design aspect for many offshore structures. Soil-foundation interaction has a direct impact on the system dynamic response of these structures. While the stiffness of the soil-foundation interaction influences the system's natural frequency, the damping influences the amplification of the structural response to environmental excitations. This paper presents a simplified model for estimating the soil damping due to nonlinear soil response for pile foundations, which have wide applications in the offshore industry, such as for supporting jacket platforms, wind turbines and wellhead facilities. The proposed model is fundamentally linked to the damping response of the soil measured at element level therefore it offers design engineers an efficient and accurate way to estimate soil-pile interaction damping based on site-specific soil data. Approaches to include the suggested model for structural analysis are also proposed. • A simplified model for estimating hysteretic soil damping for laterally loaded pile foundations. • Scaling from damping behaviour measured in soil element tests. • Approaches to incorporate soil damping into time domain structural analysis are proposed. [ABSTRACT FROM AUTHOR]

Suction caisson foundations are increasingly considered as a foundation solution for offshore wind farm development in China. This paper outlines the design considerations for developing a suction caisson supported jacket solution for an offshore wind farm in Southern China. Geotechnical analyses for four major aspects, ultimate limit state (ULS), serviceability limit state (SLS), fatigue limit state (FLS) and installation are discussed. The design challenges encountered in the project due to soft seabed and metocean characteristics (severe typhoon loading and dominant wind directions) and technical approaches adopted by the project are presented. Areas for further studies are identified and discussed. The purpose of this paper is to disseminate this knowledge and raise awareness of several important aspects. [ABSTRACT FROM AUTHOR]

A novel side-rounded suction caisson foundation is proposed in this study. Compared to a conventional circular shape in plan, it has a rectangular middle section inserted in between the two circular halves for increased moment capacity. This paper presents an investigation into the bearing performance of this novel foundation in clay under uniaxial and combined loading by means of an extensive finite element parametric analysis. It is found that by adopting a dimensionless equivalent embedment ratio and a dimensionless equivalent strength heterogeneity ratio, which account for the side-rounded shape of the proposed foundation, its design approach can be unified with an existing framework established for conventional skirted circular footings. The advantage of the proposed foundation and the application of the proposed design method is demonstrated through an example application. [ABSTRACT FROM AUTHOR]

The offshore wind industry in China has seen a rapid development in recent years and is projected to account for half of global yearly installed capacity in the years to come. Monopiles are a popular foundation solution for supporting offshore wind turbines. However, due to challenging seabed soil conditions, which often feature thick normally consolidated soft clays and harsh environmental loading (i.e. frequent occurrence of typhoons), extremely long monopiles are often designed. The large monopiles are costly to fabricate and install and sometimes kills the viability of the concept in cases where the bedrock is relatively shallow and expensive piling in rock is otherwise required. However, the state-of-practice for designing these monopiles in clays is typically by using the API p-y springs, which are widely known to underestimate soil-pile interaction stiffness and capacity for large diameter monopiles. Improvement to the design method can therefore have significant economic implications to the industry. The paper presents an effort toward this direction. A multi-spring beam-column model suitable for monopile design in soil conditions in China is proposed. The model features three soil spring components, namely the lateral p - y spring, the pile tip base shear s - y spring and rotational m - θ springs along the pile shaft. The validity of the model is verified by a comprehensive suite finite element parametric analyses. Guidance for incorporating the cyclic loading effect into design is also provided. The model proposed in this paper has large potentials for application in design practice. • Multi-spring beam-column model for monopile analysis in soft clays. • Captures lateral p-y, base shear s-y and side shear M-θ resistances. • All springs are fundamentally linked to stress-strain response of the soil. • Model demonstrates a clear improvement over the current design method. [ABSTRACT FROM AUTHOR]

The hemiball penetrometer is a geotechnical device developed to measure the property of surficial seabed sediments (<0.5 m), which is of most significance for pipeline design and for situations in which traditional site investigation methods face challenges in accurately characterizing the soil properties. This study developed a novel method to derive the axial frictional resistance along the pipeline through a simple scaling relationship from the torsional resistance measured by the hemiball penetrometer in situ. To achieve this, the undrained and drained torsional resistances of the hemiball for idealized conditions were theoretically explored using the plasticity theory and principles of critical state soil mechanics. The link between undrained and drained torsional resistances for the idealized conditions then was established. Calibrated against those theoretical solutions, coupled finite-element analyses were conducted to explore solutions for more-realistic real-life conditions and to study the effects of hemiball embedment:diameter ratio, initial strength profile, and consolidation after hemiball or pipe installation. Based on these analyses, a simple model using two scaling factors, the effective radius of the hemiball and the equivalent length of the pipe, was proposed to derive the pipe–soil interface friction from the measured torsional resistance of the hemiball. The method presented in this paper provides a simple yet powerful approach for interpretation of hemiball testing results, and is an important step toward the eventual application of the hemiball in deep-water pipeline engineering. [ABSTRACT FROM AUTHOR]

Pile foundations supporting offshore structures, such as jacket platforms, are subjected to cyclic lateral loading. The capacity and deformation of these pile foundations under cyclic lateral loading are important and challenging design aspects. The purpose of this paper is to present verification of a framework for analysing the lateral pile response under cyclic loading in clay, based on fundamental soil behaviour measured in the laboratory at the element level. This framework can account for site-specific cyclic soil properties, summarized in classic contour diagrams, and the site-specific cyclic loading characteristics in design. This is a step-change improvement from current codes and standards recommendations which are based on a series of tests on a single pile at a single site (i.e. the Sabine River tests, Matlock 1962, 1970). The paper first provides a brief summary of the framework and outlines the important assumptions and calculation procedures. The paper then presents a comprehensive validation exercise of the framework through back-analyses of three sets of 1-g and centrifuge tests, covering different soil conditions (strength profile, OCR and plasticity) and loading sequences. This hindcast demonstrates the model's capabilities to capture the essential behaviours of pile foundations under cyclic lateral loading and its added value as a design tool, when compared with current practice. [ABSTRACT FROM AUTHOR]

Suction anchors are popular anchoring solutions for station-keeping of offshore floating facilities. For the in-place holding capacity design of these anchors, there is a debate in practice regarding the possibility of developing a tension gap on the rear side of the anchor and how to account for it in design. This study is carried out to shed further light on this debate. Analytical and finite element analyses are reported in this study which investigate: i) the optimal padeye depths and the influencing factors, for both free gapping and no gapping interface conditions; ii) the effect of tension gap on the anchor's holding capacity when the padeye is placed at optimal depths; iii) the effect of tension gap on the anchor's holding capacity when the padeye is placed at non-optimal depths. Strategy to account for tension gap in practical design is provided. • Analytical equations to predict the optimal padeye depth. • Analytical equations to predict the maximum reduction in holding capacity due to tension gap. • Strategy to account for tension gap in practical design. [ABSTRACT FROM AUTHOR]

The plate-fin heat exchangers with pressure-resistant offset strip fins are widely used in the C3MR and other liquid natural gas (LNG) systems, and the two-phase pressure drop characteristics of flow boiling in pressure-resistant offset strip fin channels is a crucial factor for designing and optimizing the compact heat exchangers. A cryogenic experimental rig was established to obtain two-phase pressure drop data in the present study and the fin thickness of 0.4 mm is selected based on an actual compact heat exchanger in the C3MR LNG liquefaction system. The tested fluid is R22, and the experimental conditions include mass fluxes of 64–128 kg/(m2·s), vapor qualities of 0.1–1.0, saturation pressures of 0.22–0.26 MPa, and heat fluxes of 0.7–5.5 kW/m2. The comparison between the obtained data for fin thickness of 0.4 mm and those for fin thickness of 0.1–0.4 mm in literatures show that, the friction factor f of gas flow in channels increases with the increasing fin thickness, and the increment is 306%–451% as the fin thickness increases from 0.1 mm to 0.4 mm. The research results of two-phase flow show that, as the vapor quality increases, the frictional pressure drops initially increase and then decrease, representing a maximum at vapor quality of 0.90. With the increment of heat fluxes, the average vapor quality increases, resulting in the increasing frictional pressure drops. The two-phase multiplier of two-phase pressure drop varies from 3 to 35 and increases with the decreasing fin thickness and hydraulic diameter. A new correlation of two-phase pressure drops in offset strip fins with different fin thicknesses was developed within a deviation of ±25%. • New experimental data for cryogenic compact heat exchanger are obtained. • Friction factors of pressure-resistant offset strip fins are correlated. • Effects of mass fluxes, heat fluxes on two-phase pressure drop are analyzed. • Structural parameters were introduced into new developed correlations. [ABSTRACT FROM AUTHOR]

Large-diameter monopiles are the predominant foundation solution for supporting offshore wind turbines. They are conventionally designed using soil reaction curves developed for long slender piles used for supporting offshore oil and gas platforms (e.g., the API p-y model). However, due to the difference in the length/diameter ratio and the resulting soil mechanisms, the use of p-y curves alone can lead to significant under-prediction of the lateral stiffness and capacity of monopiles. To overcome the shortcoming, the authors have previously proposed a conceptual two-spring framework, i.e., the so-called ' p-y + M R - θ R ' model, to capture the monopile response in soft clay under lateral loading. The framework uses distributed p-y springs to consider the lateral soil resistance along the pile above the rotation point (RP) and a single moment-rotation (M R - θ R) spring attached at the RP to capture the entire soil resistance below the RP, i.e., the distributed resistance along the pile, base shear and base moment at the pile tip. The proposed p-y and M R - θ R springs were curve-fitted to the results of 3D numerical analyses. However, as the stress-strain response and the shear strength profile inevitably influence the p-y and M R - θ R springs, the applicability of the empirical formulations to soil conditions other than those examined is uncertain. This study proposes an enhancement to the ' p-y + M R - θ R ' framework, in which the p-y and M R - θ R springs are not tied to a specific soil and strength profile but fundamentally linked to the properties that can be measured directly in the site investigation and laboratory. This extension is achieved through analytical analyses and an extensive parametric numerical study. The predictive capabilities of the model are demonstrated by back-analyses of finite element analyses and centrifuge model tests. The proposed model provides practising engineers with a simple yet powerful approach to use site-specific soil reaction curves in the design of monopiles embedded in soft clay. • A two-spring framework for monopile analysis in soft clays. • Soil resistance below the rotation point is lumped into a single moment-rotation spring. • Both p-y and M R - θ R springs are fundamentally linked to soil stress-strain response. • Validation against 3D finite element analyses and centrifuge model tests. • Model demonstrates a clear improvement over the current design method. [ABSTRACT FROM AUTHOR]

Installation of a mobile jack-up rigs with spudcan foundations are sometimes challenge due to uncertain offshore environments. Prediction of penetration resistance during installation is often necessary to avoid hazards such as punch-through failure. When the field monitored data deviates with these predictions, engineers have to decide how severe this deviation is based on their experience. This paper provides a method that can quantify the installation risk by a set of percentile curves of penetration resistance against depth. The percentile curves can be quantified through combining a random large deformation finite element method with Monte-Carlo simulation. The spatial variability in seabed soils can be considered and the penetration resistance curves through the random soils can be obtained. This model was verified by a spudcan installation penetration into deeper seabed. Finally, four field cases in North-East Brazil and Gulf of Mexico are investigated. Results show that the installation risk during spudcan installation can be estimated by comparing the measured preloading and penetration depth values with the percentile curves, which indicates the proposed method is capable to help decision making during spudcan installation. • This paper provides a set of percentile curves to quantify the installation risk of spudcan foundation in random soils. • The percentile curves are quantified through combining RLDFE method with Monte-Carlo simulation. • Four field cases are investigated and the results indicate the proposed method can help decision making of the engineer. [ABSTRACT FROM AUTHOR]

The offshore wind farm industry in China has been growing exponentially in recent years, and suction bucket foundations are increasingly applied as an effective solution. This paper presents a comprehensive site investigation and outlines the key soil parameters for offshore suction bucket foundation design, by taking the Houhu wind farm in southern China as an example. The in-situ cone penetrometer (CPTU) tests and sampling borehole were carried out to investigate the geotechnical features of soils. The soil conditions are characterized by multilayers and complicated sediment components. The classification and mechanical properties, such as stress history, static and cyclic shear strengths, and shear modulus of soils were obtained and compared by CPTU interpretation and advanced laboratory tests. Then the probable foundation optimization design solution from the perspective of soil parameters in the light of set up effect and cyclic behavior was discussed and highlighted. Based on the experience of this project and other projects in offshore China, some common challenges were summarized and the relative recommendations were proposed finally. The main objective of this study is to provide more reliable guidance of geotechnical investigation for the rapid development of wind farms in offshore China. • A comprehensive site investigation was carried out for the offshore suction bucket foundation design in southern China. • The classification and mechanical properties of soils were obtained and compared by CPTU interpretation and advanced laboratory tests. • The probable foundation optimization design solution was discussed and highlighted. [ABSTRACT FROM AUTHOR]

Foundations for Offshore Wind Turbines (OWTs) are designed following the limit state philosophy. One of the considered states is the Serviceability Limit State (SLS), which verifies that the permanent rotation of the foundation generated from accumulated strains in the soil is below a project specific criterion. Despite design codes requiring an estimation of the permanent rotation, there is not clear guidance on how to implement this. This paper describes a methodology to estimate the monopile permanent rotation for SLS and discusses its advantages and limitations. The methodology combines an accumulation method with results from 3D Finite Element Analyses (FEA) and a soil model that accounts for strain accumulation as a function of the number of cycles, relative density and load characteristics. The performance of the proposed methodology is compared against experimental centrifuge tests and results from advanced 3D FEA, indicating that it can predict the permanent rotation with satisfactory accuracy, and with a considerable reduction in computational effort. This is important for the design of OWTs, where different load histories might be required to be checked – often under tight time constraints – to find which load history leads to the largest permanent rotation, and therefore is more critical to SLS design. • The Serviceability Limit State in OWT design requires estimating monopile permanent rotations, but there is no guidance. • We present a methodology to predict permanent rotation that combines Finite Element Analyses with an accumulation method. • Input is based on laboratory soil element tests, monopile geometry and load histories at seabed. • We compare predictions against experimental centrifuge tests and results from advanced 3D FEA, obtaining good agreement. [ABSTRACT FROM AUTHOR]