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ABSTRACT This paper describes a range of techniques for estimating the capacity of offshore foundations, which has always been a main issue in foundation design and remains so today. The paper, however, takes a tack that is slightly out of the mainstream - by emphasising methods of plastic limit analysis rather than more traditional approaches. It begins with a brief history of offshore geotechnical developments, describing how design methods have evolved for shallow foundations and pile foundations, and the types of loads, site conditions and foundation geometries encountered. A number of simple solutions are provided with detailed example problems. This paper proposes that plastic limit analysis methods have the potential to supplement and enhance more traditional methods. 1. Introduction I am sincerely honoured to be invited to give the inaugural McClelland Lecture. I am humbled by the task before me as I sincerely wish to produce something that Bram McClelland would have appreciated. At first I leaned toward a subject that more characterised his expertise and interest - engineering geology, site investigation and foundation design. On further reflection however I concluded that Bram delighted in developing engineers who followed their own interests, not in his image, but in their own unique ways. That is the kind of leader he was. This epiphany led me to select a topic that has long been a passion of mine - bridging the gap, sometimes chasm, between theory and practice. I believe this is what he would have wanted from me. Estimating foundation capacity has always been a central issue in foundation analysis and design. Various different methods are employed in this practice, many of which involve ad hoc assumptions and empirical models. This paper focuses on one such advancement, plastic limit analysis (PLA), a methodology that is theoretically sound, internally consistent and surprisingly simple to apply.
- Europe (0.68)
- North America > United States > Texas (0.46)
ABSTRACT This paper reviews some key issues regarding the cyclic loading response of offshore piled foundations. Starting with axial loading it considers: the cyclic loading that can be expected; the fundamental responses of piles driven in clays and sands; frameworks for understanding axial cyclic response and specifying cyclic soil testing; and approaches for practical application in design. The review then moves to consider pile responses to moment and lateral loading, distinguishing between flexible and relatively rigid piles and anchors. A range of possible design approaches is considered and it is argued that current routine practice needs to be reconsidered. Practical methods now exist to address the potentially highly significant effects on axial capacity of piles that experience high ratios of cyclic to average loads. New research and calculation procedures are emerging that offer significant improvements in a broad spread of topics. 1. Introduction Interest in the behaviour of piles under cyclic loading grew in the 1980s to meet challenges posed by inherently fail-unsafe Tension Leg Platforms (with the first TLP being installed at Hutton in 1984) and heavily loaded deeper water fixed platforms, such as the Cognac jacket set in 320m water. Briaud and Felio (1986) assembled for API a database intended to resemble fine marine sediments covering the cyclic behaviour of clays in: laboratory tests, cyclic model experiments and axially cyclic field pile tests. They considered 16 studies on piles with diameters greater than 150mm, most of which were strain-gauged to measure axial load distributions. Local shaft friction, pore pressure and radial stress measurements were attempted in some cases, although these parameters are notoriously hard to sense reliably. The response of piles driven in sands was not addressed. The piles were submitted to significant numbers of load cycles (typically 100 to 1,000) with frequencies generally around 0.1 Hz.
- North America > United States (1.00)
- Asia (1.00)
- Oceania > Australia > Western Australia (0.28)
- Europe > United Kingdom > Scotland (0.27)
- Geology > Geological Subdiscipline > Geomechanics (0.90)
- Geology > Sedimentary Geology > Depositional Environment > Marine Environment (0.34)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
- Health, Safety, Environment & Sustainability (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Platform design (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
Abstract Premature refusal can pose a threat to driven pile installation. One contingency measure that has been applied is pre-drilling ahead of the pile tip to ease driving. While some practitioners may assume that carefully conducted drilling will not give any adverse effect, little evidence is currently available to assess the possible impact on long-term axial capacity. This paper describes a standard gravity (1g) small-scale model laboratory study in which 18 small closed-ended piles were jacked into constrained 300mm cubes of stiff high overconsolidation ratio (OCR) natural London clay, with and without pre-drilling. Four control piles were also installed without any boring. Pile loading tests performed after appropriate equalisation periods showed that pile capacity reduced systematically as the ratio of the drill hole-to-pile solid area ratio increased. The model experiments showed comparable reductions in capacity to those in earlier model and field studies on soft Mexico City clays. An effective stress analysis is suggested to help the application of the model tests to other soils types and pile geometries. Capacity reductions are concluded to be likely when pre-drilling is performed in highly overconsolidated offshore clays. The results have implications for both new piling operations in exceptionally hard clays and the re-analysis of existing installations in a broader spread of soil types, where refusal may have resulted from pile hammer capacity limitations. 1. Introduction 1.1 Use of pre-drilling to ease pile installation Driving refusal has been encountered frequently during offshore foundation pile installation. One field contingency measure applied to achieve target penetration with tubular piles has been pre-drilling ahead of the tip to ease driving. The development of more powerful hammers and improved driveability predictions has reduced the incidence of refusal events, but consideration still has to be given to this possibility when dealing with unusually hard or dense materials.
- Europe > United Kingdom (0.46)
- North America > Mexico > Mexico City > Mexico City (0.25)
- Europe > United Kingdom > Atlantic Margin > West of Shetland > Faroe-Shetland Basin > Rona Ridge > Block 206/9 > Clair Field (0.99)
- Europe > United Kingdom > Atlantic Margin > West of Shetland > Faroe-Shetland Basin > Rona Ridge > Block 206/8 > Clair Field (0.99)
- Europe > United Kingdom > Atlantic Margin > West of Shetland > Faroe-Shetland Basin > Rona Ridge > Block 206/7 > Clair Field (0.99)
- (2 more...)
Centrifuge Modelling of a Hybrid Foundation For Subsea Equipment
Gaudin, C. (Centre for Offshore Foundation Systems, University of Western Australia) | Randolph, M.F. (Centre for Offshore Foundation Systems, University of Western Australia) | Feng, X. (Centre for Offshore Foundation Systems, University of Western Australia) | Clukey, E.C. (BP Exploration) | Dimmock, P. (BP Exploration)
Abstract The performance of a hybrid foundation for subsea systems, defined as a shallow skirted mat foundation featuring short piles at the mat corners, has been investigated through a series of centrifuge tests. The foundation, with and without corner piles, was subjected to eccentric monotonic loading along the x, y and z axes, resulting in combined loading over 6 degrees of freedom. Modes of yielding were identified and the contribution of the corner piles to the bearing, sliding, overturning and torsional capacities of the hybrid subsea foundation was quantified. Results revealed that (a) centrifuge modelling could capture the strain hardening arising from the plunging nature of the mat foundation yield; and (b) the addition of the corner piles to the shallow mat resulted in a change of yielding mode from shearing at the mat invert to overturning, with a significant increase in the foundation capacities. Consequently, corner piles appear to be an efficient option to reduce the size of the subsea system foundation required to withstand a given set of combined loading. 1. Introduction Subsea mats are used in deep waters as foundations in soft normally consolidated (or lightly overconsolidated) clay to support facilities such as pipeline terminals, jumpers, riser bases and manifolds. They are typically subjected to various combinations of loading in all six degrees of freedom. In some cases, a mat may not provide sufficient resistance against bearing, sliding or overturning failure. The use of pinned piles in each corner of the mat may then be considered to increase the sliding and overturning capacity of the foundation. Such a foundation is defined here as a hybrid subsea foundation. Hybrid subsea foundations have already been deployed in situ, but neither formal guidance nor experimental data existed at the time to assist in their design.
- Oceania > Australia (0.29)
- North America > United States (0.28)
Abstract Current offshore foundation technology is being transferred successfully to the renewable energy sector. Still, there is clearly scope for developing foundations that are more tuned to the needs of the renewable power systems such as wind turbines. One such approach is the hybrid monopile-footing system with a proven record of improving the ultimate lateral resistance, particularly in cohesionless soils. This paper builds on to the previous studies by investigating the behaviour of the hybrid system, such as the effect of footing size, the magnitude of pre-loading and its significance in developing sufficient contact pressure beneath the footing, and the importance of the degree of rigidity. 1. Introduction Due to the needs of ongoing developments in the oil and energy sector, the design of offshore foundations is constantly evolving. In the hydrocarbon extraction sector, exploration and development is moving into ever deeper water, resulting in extremely challenging geotechnical conditions. The development of sites for offshore wind farms (such as Round 2 and 3 in the UK) is also extending into deeper water. The increase of wind turbine generator capacity is requiring significant development in foundation design to generate economic and practical solutions to the installation of these deepwater wind farms. Offshore foundations are generally subject to combined loading conditions consisting of self-weight of the structure (V), relatively high horizontal loads (H) and large bending moments (M). The preferred foundation system to date has been the monopile, which has been successfully employed for the majority of the offshore wind turbines installed. The advantage of the monopile is that it can be installed in a variety of different soil conditions even when loading conditions are very high. For instance, in many of the proposed offshore wind farm locations superficial seabed deposits are often underlain by weak rocks, such as mudstones and chalk.
SOLCYP: A Four-Year Joint Industry Project On the Behaviour of Piles Under Cyclic Loading
Puech, Alain (Fugro-France) | Canou, Jean (ENPC-Navier) | Bernardini, Christian (IREX) | Pecker, Alain (Geodynamique et Structure) | Jardine, Richard (Imperial College) | Holeyman, Alain (Universite Catholique de Louvain)
ABSTRACT SOLCYP is a research and development project conducted in France to:understand the physical phenomena conditioning the response of piles to vertical and horizontal cyclic loads; develop advanced design methods; and initiate pre-normative development of methodologies that may later be included in national and international codes or professional standards. The potential applications include conventional structures, such as electricity pylons or chimneys, high rise towers and high speed train bridges. However, a central emphasis is also given to more novel foundations for offshore and onshore renewable energy engineering. The paper describes the objectives and overall technical content of the project. Several companion papers focus on more specific aspects and the results obtained so far. 1. Introduction The oil and gas industry has developed various procedures for considering the effects of large wave cyclic loads on foundations for offshore structures. Design guidelines include the American Petroleum Institute (API) RP 2GEO (2011); Det Norske Veritas (DNV) Foundations (1992) and the International Organization for Standardization (ISO) 19901–4 (2003). In addition, the offshore turbines industry is progressively adapting such methodologies given in DNV-OS-J101 (2011) and Federal Maritime and Hydrographic Agency (BSH) publication entitled, Design of Offshore Wind Turbines (2007). Surprisingly, the effects of cyclic loading on foundations are largely ignored in most civil engineering and building activities. French codes and Eurocode7 (2007) reflect this poor level of consideration. A committee working under the umbrella of the French national agency, IREX, called for national engagement in an ambitious research and development project to address the present lack of guidance regarding piles under cyclic loading. This paper describes the objectives and overall technical content of the project. Several companion papers focus on specific aspects and initial outcomes. Cyclic loads may be essentially environmental (e.g. wave, wind) or operational in origin.
- Europe > France (0.90)
- North America > United States (0.69)
ABSTRACT This paper presents the results of a series of laboratory tests performed on model piles in dense sand to investigate the axial response and tensile capacity. The test schedule was designed to impose varying cyclic loads followed by static tensile tests to measure the improvement or degradation in pile capacity. The degree of degradation was calculated by using a simplified axial degradation model, a formulation that encompasses average and cyclic loads along with a prediction of the number of cycle to failure of the pile. The results indicated that few intensity cyclic loading rapidly leads to shakedown behaviour. The model predicted no capacity degradation during low-intensity cycling at low load levels with minimal degradation in the high-intensity cyclic loading tests. Measurements of tensile capacity after high intensity compressive cycles, which showed varying amounts of degradation, indicate that very low cyclic load levels need careful consideration in any degradation formulation. Load levels that did not cross a threshold, dependent on the load application history, showed little or no degradation. This threshold will be higher if low load level cycling has improved capacity and lower if degradation has occurred in previous loading cycles. 1. Introduction A large body of literature is available on the subject of loading of piles used for offshore structures of different types, such as oil and gas platforms. In the case of sand, cyclic loading is expected ultimately to reduce the axial capacity of the pile itself. As noted by Abdel-Rahman and Achmus (2011), there is a need for a method of calculating axial pile capacity degradation with regard to load magnitude and number of load cycles. This is important in relation to the improvement of economic design methods and the number of wind farms proposed to be constructed in the coming years.
ABSTRACT A precise modelling of the soil behaviour requires accurately taking into account the internal state of the material and its changes during a complex loading. The aim of this work is to design a constitutive model that can reproduce the soil response under a cyclic loading in drained and undrained conditions with a unique set of parameters. Specifically, the Cambou, Jafari and Sidoroff (CJS) model - an elastoplatic model compounded with two plastic mechanisms, isotropic for the first one and deviatoric for the second one - has been improved to include the internal state of the soil better by means of a volumetric state variable and a variable related to the anisotropy of normals at contacts between grains. 1. Introduction The complex nature of loading of pile foundations in offshore applications (oil production), as well as in in-shore applications (aeolian), requires equally complex constitutive models for soils and pile-soil interfaces. In such systems, the internal state of the material (volume element of soil or interface pilesoil) is bound to evolve greatly throughout the cyclic loading. This change can be observed through two internal variables: the void ratio in relation with non-oriented phenomena, the anisotropy for oriented phenomena. Although void ratio changes are easy to assess throughout experiments, simple access to information related to anisotropy is more difficult to obtain. Thus constitutive models for soils and interface pile soils generally take into account the changes of the material properties by considering the mere influence of density change. In this work, an existing constitutive model for soils called the Cambou, Jafari and Sidoroff (CJS) model, which was previously developed and named after that research team (Cambou et al., 1989), has been modified to take into account better the internal state changes throughout cyclic loadings.
Development of Pile Design Methodology For an Offshore Wind Farm In the North Sea
Merritt, A.S. (Geotechnical Consulting Group LLP) | Schroeder, F.C. (Geotechnical Consulting Group LLP) | Jardine, R.J. (Imperial College London) | Stuyts, B. (Arup Geotechnics) | Cathie, D. (Cathie Associates SA/NV) | Cleverly, W. (GL Noble Denton)
ABSTRACT This paper describes aspects of the foundation design methodology developed for the Borkum West II offshore wind farm in the German North Sea, comprising 40 turbines supported on piled tripods in water depths of approximately 30m. The foundation design evolved during a technical due diligence process, which offered the opportunity to review the site investigation data and cyclic loads, to reconsider the effects of cyclic loads on pile resistance and to modify pile lengths and wall thicknesses to mitigate pile tip integrity risk during driving in very dense sands. The re-evaluation of the design storm concluded that axial pile capacities could decrease by up to 25% because of cyclic loading at some turbine locations, but could be almost unaffected by cycling at others. The technical review involved a collegiate process that contributed to the development of acceptable foundation designs and mitigated risks relating to pile installation and foundation performance. 1. Introduction The Borkum West II wind farm is currently being developed by Trianel Windkraftwerk Borkum GmbH in the North Sea, approximately 45km offshore northern Germany (Figure 1). The first phase of this project includes the construction of forty 5MW turbines. The hub height is approximately 90m above sea level, and the rotor diameter is 116m. The turbines are supported in water depths of 26m to 33m by tripod structures designed by Offshore Wind Technologie GmbH. Figure 2 illustrates the general arrangement of the steel tripods, which have an outer footprint diameter of 28m. Geotechnical engineering was undertaken for the project by Cathie Associates (CA) SA/NV, Belgium. The ground investigations, interpretation and foundation designs were performed in accordance with the standards for offshore wind farms, published by the Bundesamt für Seeschifffahrt und Hydrographie (BSH, 2007, 2008; also known as the Federal Maritime and Hydrographic Agency).
Development of Degradation Laws For Describing the Cyclic Lateral Response of Piles In Clay
Khemakhem, M. (LUNAM Universite, IFSTTAR Nantes, Departement GER) | Chenaf, N. (LUNAM Universite, IFSTTAR Nantes, Departement GER) | Garnier, J (LUNAM Universite, IFSTTAR Nantes, Departement GER) | Favraud, C. (LUNAM Universite, IFSTTAR Nantes, Departement GER) | Gaudicheau, P. (LUNAM Universite, IFSTTAR Nantes, Departement GER)
ABSTRACT Centrifuge tests were performed to investigate the cyclic response of piles in soft clay under lateral loads. Typical data are presented and discussed. The accumulation of pile-head displacements and maximum bending moments with number of cycles in particular are examined. Power law models fitted on experimental data are proposed to describe the effect of large numbers of cycles. 1. Introduction Pile foundations of offshore structures used for the oil and gas or wind farm industries are subjected to large cyclic horizontal loads resulting mainly from the action of waves and wind. Pile design procedures are essentially based on the application of recognised standards or professional recommendations, among which the American Petroleum Institute (API) RP 2A (2000) is the most commonly used. It proposes a design methodology for horizontally loaded piles based on the use of local pile-soil transfer (p-y) curves. Monotonic, as well as so-called cyclic, p-y curves are provided for both sands and clays. The cyclic p-y curve concept requires some attention. It was derived in the 1970s on the basis of pile tests performed on relatively small-diameter (123/4–24-inch) piles for soft clays (Matlock, 1970), stiff clays (Reese et al., 1975; Reese and Welch, 1975) and sands (Cox et al., 1974; Reese et al, 1974). The piles were subjected to a series of cyclic loads representative of load histories imposed by Gulf of Mexico storms to jacket piles. The final result was an ‘envelope curve’, which is aimed at reproducing the response of a pile monotonically loaded at the end of the extreme event (e.g. the centennial storm). More recent laboratory tests (Craig and Kan, 1986; Kitazume and Miyajima, 1994; Jeanjean, 2009; Zhang et al., 2011) emphasise the significance of progressive accumulation of pile head displacements with applied number of load cycles.
- Europe (0.69)
- North America > United States (0.49)