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.
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.
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.
Pipelines laid on the seabed are subjected to loads that may cause unacceptable displacements. On fine-grained soils, the capacity of a pipeline to resist these loads is affected by the pipe embedment and any excess pore pressures remaining in the surrounding soil from the laying process. This paper presents results from model tests, performed at near to full scale, investigating the embedment response and the subsequent pore pressure equalisation of a pipeline on a high plasticity marine clay. Existing models for the penetration and dissipation processes are compared with the experimental data. Conventional undrained bearing capacity theory, making minor allowances for strain rate and softening effects, shows good agreement with the observed penetration response. Dissipation solutions based on elastic and elasto-plastic soil models capture the general shape of the pore pressure response. The operative coefficient of consolidation varies between tests, spanning the range between the compression and recompression values observed in oedometer tests. The observations validate the theoretical solutions for penetration resistance, and highlight the uncertainty that must be considered in estimating equalisation times.
Offshore developments may typically feature a number of subsea structures for which pipelines and risers play an integral role. Especially for deepwater projects, very soft clays will be encountered at the seabed; these can be difficult to characterize by standard in-situ and laboratory testing. This means that geotechnical model tests are increasingly used to investigate the complex interaction between the seabed soils and the risers or pipelines. The prototypes for these model tests are typically at or close to the actual dimensions used by industry and often include specific coatings for the pipeline or riser section being investigated. The models may be loaded or displaced statically or cyclically in different directions to evaluate the different mechanisms involved. Although it is a challenge to recreate the undrained shear strength found at the seabed, experience and theoretical knowledge may be combined to give good agreement between the shear strength level in the test tank and on site. This may then be verified by in situ testing in the test tank and compared with high quality data from the field itself. The model test results themselves serve as input to pipeline or riser design which can incorporate geotechnical, structural and hydrodynamic effects. This paper describes the typical procedures involved in model testing and investigates the interpretation of the data using theoretical and empirical methods. The impact of results on pipeline and riser design within a project is also considered, where the focus is on very soft clays which are often encountered in offshore projects, especially in deepwater.
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