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Collaborating Authors
Results
Monotonic and Cyclic p-y Curves for Clay Based on Soil Performance Observed in Laboratory Element Tests
Zhang, Youhu (Norwegian Geotechnical Institute) | Andersen, Knut H (Norwegian Geotechnical Institute) | Klinkvort, Rasmus T. (Norwegian Geotechnical Institute) | Jostad, Hans Petter (Norwegian Geotechnical Institute) | Sivasithamparam, Nallathamby (Norwegian Geotechnical Institute) | Boylan, Noel P. (Norwegian Geotechnical Institute) | Langford, Thomas (Norwegian Geotechnical Institute)
This paper presents a numerical framework for monotonic and cyclic p-y curves for piles in clay, based on soil performance observed in laboratory element tests. A framework of constructing monotonic p-y curves from the stress-strain response measured in direct simple shear (DSS) tests is firstly introduced. The framework is developed from a parametric finite element study. An extension of the framework for analysing cyclic loading is then described, with the use of the cyclic accumulation procedure established at NGI in the last few decades. Finite element analyses are performed for a pile element subjected to various cyclic load parcels in order to validate the extension, using a soil model that follows the cyclic contour diagrams. The procedure to apply the proposed framework for calculating the overall pile response under monotonic and cyclic loading within a conventional beam-column type analysis is then described. The proposed framework offers a rational approach for the p-y response of a pile element under monotonic and cyclic loading, with the possibility to consider the site specific pile-soil interface roughness, soil properties as well as the cyclic loading conditions. Finally, an example case using a program (NGI-PILE) that implements the proposed procedure is presented and validated against finite element analysis. The currently proposed framework has potential applications for design of a range of offshore structures, including offshore well conductors, anchor piles and pile foundations supporting jacket structures.
- Europe (0.47)
- North America > United States > Texas (0.29)
Model Uncertainty in Axial Pile Capacity Design Methods
Lacasse, Suzanne (Norwegian Geotechnical Inst.) | Nadim, Farrokh (Norwegian Geotechnical Inst.) | Langford, Thomas (Norwegian Geotechnical Inst.) | Knudsen, Siren (Norwegian Geotechnical Inst.) | Yetginer, Gülin Luis (Statoil) | Guttormsen, Tom Reidar (Statoil) | Eide, Asle (Statoil)
Abstract To estimate the safety level associated with the axial capacity of a pile, one needs to know the bias and uncertainty in the calculations made the design method. This model uncertainty is usually obtained by comparing the predicted axial pile capacity with the measured axial pile capacity in reliable pile load tests. Model uncertainty can have a strong influence on the calculated annual probability of failure of a piled foundation, and thus on the estimation of the safety margin. This paper studies the model uncertainty for the API-method and the NGI, ICP, Fugro and UWA methods for predicting the axial pile capacity in clays and in sands. The study also shows that the selection of the parameter to quantify the uncertainty influences the values of the mean and standard deviation. A significant factor in the evaluation of model uncertainty is the reference database of pile load tests used to quantify the uncertainty. The paper suggests an approach for quantifying model uncertainty for pile calculation methods. There is a need to quantify specific uncertainties, such as the reduced capacity in low plasticity clays and pile diameters and lengths that become much larger than the dimensions used for the pile load tests in the reference database(s). The paper recommends that an international joint industry project be initiated to look into the databases of pile model tests and to establish a consensus on the reliable pile load tests, the soil characterization and the interpretation to be used for the evaluation of the model uncertainty. Introduction The model uncertainty in a calculation method is usually quantified in terms of a mean (or bias), a standard deviation (and/or coefficient of variation) and the probabilistic density distribution that best fits the data. For methods predicting the ultimate axial pile capacity, the model uncertainty is obtained by comparing the predicted axial pile capacity with the measured axial pile capacity in reliable pile load tests. A companion paper in the same session at OTC 2013 (1) demonstrated the importance of model uncertainty in the probabilistic calculation of axial pile capacity and probability of failure. The model uncertainty was especially significant in the probabilistic analyses of the axial pile capacity of piles in sand (1). The study of model uncertainty was undertaken as part of the design of pile foundations on jackets in the North Sea. The aim was to document compliance with governing regulations in terms of annual failure probability. This paper studies the model uncertainty for the API-method and four newer design methods. Table 1 lists the methods considered and the references for each method: the current API method, the pre 1987 API method, the NGI-05 method, the ICP-05 method, the Fugro 96/05 method and the UWA-05 method. These methods became of greater interest when API RP2A RP2GEO (2) and ISO 19902 (3) introduced them as alternative methods to the API method for the design of piles in sand.
- Europe (1.00)
- North America > United States > Texas > Harris County > Houston (0.28)
SAFEBUCK JIP - Observations of Axial Pipe-soil Interaction from Testing on Soft Natural Clays
White, David | Bruton, David A.S. (Atkins) | Bolton, Malcolm (U. of Cambridge) | Hill, Andrew John (BP) | Ballard, Jean-Christophe (Fugro Engineers B.V.) | Langford, Thomas (Norwegian Geotechnical Institute)
Abstract This paper outlines recent research into axial pipe-soil interaction from the geotechnical elements of the SAFEBUCK Joint Industry Project. The operational axial pipe-soil friction strongly influences the initiation and cyclic development of lateral buckles, and also controls the magnitude of pipeline end expansions as well as rates of axial walking. Results from model tests performed at the University of Cambridge are presented in this paper, and provide new insights into the axial pipe-soil response on fine-grained clayey soils. A simple test arrangement was used to pull an 8 m long plastic pipe axially over a bed of soft natural clay collected from a deepwater location offshore West Africa. Many axial sweeps were performed, spanning a wide range of velocities (0.001 mm/s - 5 mm/s) and a wide range of intervening pause periods (up to several days). Both of these variables had a strong influence on the axial pipe-soil resistance - or ‘friction’. The peak values of equivalent friction factor were as high as 1.5 and the residual values were generally in the range 0.2 - 0.5, but fell to below 0.1 in some cases. Higher peak values are associated with longer waiting periods between axial sweeps. The lowest residual values are associated with the fastest rates of shearing. This wide range of axial resistance was observed in a single test using the same pipe resting on the same soil, which is disconcerting from a design perspective. To identify the origin of this variability, an interpretation based on the generation and dissipation of excess pore pressure is explored. This provides a reasonable explanation for the results, but some unexpected aspects of the behavior remain. The results show the important influence of pore pressure effects, consolidation, and the level of drainage during sliding. They also highlight the complexity of axial pipe-soil interaction. For these experimental results, conventional design calculations do not provide adequate predictions of the observed behavior except for during very slow drained movements. The undrained behavior is not captured by conventional design calculations, which provides a cautionary warning to designers. In particular, in the slow-draining natural clay used in this experiment, very low equivalent axial friction factors - as low as F/W' is ~ 0.1 - can be sustained for a long period of movement. The SMARTPIPE® is a recently-developed tool for performing pipe-soil interaction tests in situ offshore, using an instrumented model pipe mounted on a seabed frame. Selected results from a SMARTPIPE® cyclic axial pipe test performed at a deep water location are also presented and discussed. The results support the proposed interpretation based on the generation and dissipation of excess pore pressure. Some differences exist between the in situ and model test data but they can be explained by the smaller magnitude of axial velocity tested, the higher coefficient of consolidation of the in-situ soil and the absence of pause periods between sweeps. Minimal data from experiments on axial pipe-soil interaction is in the public domain, so the results provided here represent a significant contribution to the available knowledge. This research is continuing within the SAFEBUCK JIP, via additional model testing using a new facility that is described in this paper. The aim is to establish new and more robust design guidance for pipe-soil interaction, to support the reliable and efficient design of seabed pipelines.
- Europe > United Kingdom > England > Cambridgeshire > Cambridge (0.34)
- North America > United States > Texas (0.28)
- Research Report > New Finding (0.66)
- Research Report > Experimental Study (0.66)
- Geology > Mineral > Silicate > Phyllosilicate (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract A large subsea development will comprise a floating production system connected to subsea wells by a large network of infield flowlines. These flowlines are all susceptible to lateral buckling and pipe-walking under operating conditions, with the challenge of meeting design limits states associated with local buckling, fracture and low-frequency fatigue damage during the operational cycles. The pipe-soil interaction response is the largest uncertainty in the design of such systems, and has a significant influence on the pipe response and structural limit states. The Project therefore commissioned a series of project-specific test programmes, at large-scale and small-scale, to improve understanding of axial and lateral pipe-soil interaction under monotonic and cyclic loading. The project has also carried out novel in-situ pipe-soil interaction tests in the field. The test programme was focused on relatively heavy pipe behaviour associated with pipe-in-pipe systems in very soft deepwater clay. The findings from these tests have led to a radical reappraisal of the interaction mechanisms and provided much greater confidence in optimised design solutions for the project. The test methods are described and the results and interpretation are summarised. These illustrate the significant advance in geotechnical knowledge and understanding achieved during this project, which is expected to benefit many future projects. Background Field Description This typical deepwater subsea development will comprise a floating production system connected to subsea wells by a large network of infield flowline systems, including:Production flowlines with high performance pipe-in-pipe insulation, and uninsulated service flowlines; Water injection networks comprising plastic-lined rigid steel pipe; Gas injection flowlines comprising uninsulated pipe for a future gas export system. Susceptibility and Control Any pipeline which is subjected to above ambient temperatures and pressures has a tendency to relieve the resulting high axial stress in the pipe wall by expanding longitudinally. This expansion is resisted by the axial soil resistance between the pipe and the seabed. This restraint causes an axial compressive force to develop in the pipeline, which can cause buckling. Because these pipelines are laid in deep water there is no requirement for pipeline trenching and therefore no lateral or uplift restraint acting on the pipeline to prevent buckling, apart from the lateral resistance between the pipe and the soil. A key challenge for this subsea flowline design is the control of lateral buckling, pipe walking, or route curve instability of the flowlines between the riser base and the drill centres. Conceptual engineering and FEED (front end engineering design) confirmed that all flowlines are susceptible to lateral buckling and several are susceptible to pipe-walking. If left uncontrolled, this behaviour can have serious consequences for the integrity of a pipeline.
- North America > United States > Texas (0.46)
- Europe (0.46)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Piping design and simulation (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Offshore pipelines (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Floating production systems (1.00)
Abstract The issue of fatigue damage caused by cyclic interaction of the steel catenary risers with the seabed has gained prominence with the widespread use and lengthening of the spans for this type of system. This paper presents the findings from a series of large-scale model tests of soil-riser interaction in re-constituted high plasticity marine clay from the Gulf of Guinea. Data are presented on soil stiffness during virgin penetration, unload-reload stiffness as a function of displacement amplitude, the effects of soil-riser separation during robust load cycles, force-controlled versus displacement controlled load conditions, stiffness degradation under cyclic loading, and stiffness regain due to consolidation and thixotropy. Introduction Steel Catenary Risers (SCRs) are utilized to connect floating platforms with seabed systems and feature prominently in deepwater projects. Figure 1 gives the general arrangement of such a riser, where the ‘Touchdown Zone’ (TDZ) refers to the area where the riser is in ‘dynamic’ contact with the seabed. The interaction between SCR and seabed in the TDZ is of great importance when evaluating the structural fatigue life of the riser (Hatton, 2006); a stiffer seabed will result in greater localized stresses in the riser and vice versa. Recent work has investigated seabed-SCR interaction to improve interaction models that better capture the geotechnical behaviour within the structural analysis (e.g. Aubeny et al., 2006 and Clukey et al., 2005). Physical model testing has been an important tool to investigate seabed-SCR interaction in the TDZ, as reported by several authors including Dunlap et al. (1990), Bridge et al. (2004) and Giertsen et al. (2004). However, this work was based on test data for kaolin or low plasticity soils. The majority of deepwater projects are located in areas with clays of much higher plasticity, such as the Gulf of Guinea, Gulf of Mexico and South China Sea. Andersen (2004) has demonstrated that the cyclic behaviour of clays is dependent on both plasticity index and overconsolidation ratio. The authors therefore decided to perform seabed-riser interaction tests on a high plasticity soil taken from the Gulf of Guinea. Figure 2 illustrates several facets of soil-riser interaction behavior that were investigated. The first involves the mobilization of soil resistance during initial monotonic penetration of the riser into the seafloor. Upon completion of the initial penetration phase, two limiting conditions of cyclic loading of the riser were investigated. The first involved force-controlled loading conditions in which the riser was unloaded and reloaded to a uniform level of compression resistance in each load cycle. The second involved displacement controlled loading in which the riser is reloaded to a uniform penetration depth. It is readily apparent from Figure 2 that fundamentally different soil-riser interaction will occur under displacement-controlled conditions, as soil resistance will decline with each successive load cycle. With regard to the relevance of loading mode to actual field conditions, either can be relevant depending on the condition being considered. During the trench formation phase (which may occur repeatedly during the life of a riser), the riser successively embeds itself deeper with each load cycle, so a force-controlled mode would likely be a closer approximation to field conditions. In contrast, as the trench approaches a steady-state configuration, a soil-riser interaction behavior in a displacement-controlled mode is likely to be most relevant. Accordingly, soil-riser interaction behavior in both loading modes merit investigation.
- Geology > Mineral > Silicate > Phyllosilicate (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.99)