1,721,038 research outputs found
An analytical solution for the undrained horizontal–torsional resistance of mudmats
Full text linkRectangular mudmat foundations are frequently used for supporting subsea structures for offshore oil and gas developments. The self-weight of the subsea structure and mudmat often mobilise a relatively small proportion of the vertical bearing capacity and horizontal–torsional sliding generally represents the governing load case. However, the effect of torsion on mudmat capacity is not explicitly considered in current guidelines for geotechnical design of offshore foundations. In this paper, upper-bound plastic limit analysis is used to develop explicit expressions for the combined horizontal and torsional capacity of skirted mudmats. The results of the limit analyses are compared with results from finite-element analysis, and with other published solutions. A method is proposed for estimating the translational sliding resistance from the interaction diagram for biaxial horizontal loading and a unique expression is proposed to define the normalised H–T failure envelope. The effects of foundation aspect ratio, foundation embedment ratio, skirt–soil interface roughness, direction of horizontal loading and degree of soil strength heterogeneity are investigated systematically.</p
The ultimate undrained resistance of partially embedded pipelines
No full textOn-bottom pipelines for transporting oil and gas in deep water undergo significant changes in temperature and pressure during operating cycles, which cause a tendency for lateral buckling. Prediction and control of this phenomenon are required for the safe design and operation of these pipelines. However, the soil response under combined vertical and lateral loading is a significant area of uncertainty, and current practice relies on empirical expressions for the estimation of lateral pipe-soil resistance. This paper reports the results of finite element (FE) analyses of shallowly embedded pipelines under vertical and horizontal load. These analyses have been compared with collapse loads calculated using the upperbound theorem of plasticity, and are used to construct yield envelopes defining the limiting combinations of vertical and horizontal load. The FE limiting loads were found to compare well with upper-bound plasticity solutions, and the internal soil displacements calculated in the FE analyses match the upper-bound and experimentally observed deformation patterns. The yield envelopes generated by the FE and upper-bound analyses have been fitted by simple solutions, which aid assessment of the ultimate resistance of shallowly embedded pipelines.</p
The effect of pipe-soil interface conditions on the undrained breakout resistance of partially-embedded pipelines
No full textPipelines in deep water are usually laid on the seabed, penetrating by a fraction of the pipe diameter, rather than being buried. Thermal expansion and contraction of the pipeline during operation can lead to lateral buckling. For a buckle to be initiated, the pipe must break out from the as-laid position. The seabed sediment found in deep water is typically soft clay, which remains undrained during pipe embedment and breakout. In this paper, finite element (FE) analyses of a shallowly-embedded pipeline under vertical and horizontal load are used to generate yield envelopes in V-H load space, indicating the load conditions that will lead to breakout. Two FE techniques are used: (i) conventional displacement FE analysis, using ABAQUS, and (ii) an FE-based limit analysis technique to produce upper bound plasticity solutions. In particular, these analyses examine the influence of separation between the pipe and the soil when tension is applied. Separate yield envelopes are derived for the cases involving separation (no tension) and full bonding (full adhesion / unlimited tension) at the pipe-soil interface. Simple curves are fitted to these envelopes in order for the results to be applied in the routine assessment of pipe breakout behaviour.</p
Modelling the axial soil resistance on deep-water pipelines
No full textAxial pipe-soil resistance is an important aspect of deepwater pipeline design, since it influences the longitudinal and lateral buckling responses under thermally induced expansion and contraction of the pipeline. Experimental evidence has shown that the axial resistance, expressed as a proportion of the submerged pipeline weight, can vary by an order of magnitude, depending on the rate of axial movement and cumulative time. This paper provides a theoretical framework for assessing the magnitude of axial friction. The framework is developed within a critical-state context using effective stresses, applicable to any degree of drainage in the soil, quantifying the magnitude and duration of excess pore pressures generated near the pipe/soil interface. Two other aspects of behaviour are added to match the observed velocity dependence of axial resistance: (a) a damage term, leading to contractive volumetric strain at the interface; and (b) strain-rate dependence of the mobilised soil strength. Analytical expressions are derived that capture the above features of the response. The resulting variations of normalised frictional resistance with time and velocity are then shown to match experimental data from interface shear-box tests, representing a planar idealisation of the same behaviour, and from model pipe tests.</p
Field observations of as-laid pipeline embedment in carbonate sediments
No full textReliable prediction of the embedment of untrenched subsea pipelines is of increasing importance as hydrocarbon developments progress into deeper waters, located further from shore. Pipeline design issues such as hydrodynamic stability, lateral buckling and axial walking require accurate assessment of the pipe embedment, in order to assess correctly the pipe-soil resistance forces and the thermal insulation provided by the soil. This study presents a detailed back-analysis of the laying process and the as-laid condition of a pipeline on carbonate sediments. The pipe embedment is linked to the relevant soil properties, metocean conditions, vessel motions, and lay geometry along the route. A cycle-bycycle framework is proposed for the development of embedment as the pipe is subjected to oscillations during laying. The calculations use parameters obtained from standard in situ tests, and are applied across a range of soil and lay conditions along this particular pipeline route. The proposed calculation framework incorporates the effect of the lay rate and the pipeline catenary on the embedment process. It offers a significant improvement on the current practice of applying empirical multiplicative factors to the calculated static embedment in order to account for dynamic lay effects.</p
The effects of penetration rate and strain softening on the vertical penetration resistance of seabed pipelines
No full textOffshore pipelines in deep water are generally laid directly on the seabed, without any additional stabilisation measures. Design parameters that determine the soil resistance to lateral and axial motion of the pipeline are a function of the amount of vertical embedment. However, this latter quantity is difficult to estimate, partly because of the effects of soil heave around the pipeline as it penetrates, and partly because the soil shear strength depends on the strain rate and the degree of softening as the soil is sheared and remoulded. In this paper, a large deformation finite-element approach was adopted to study pipe-soil interaction during vertical embedment of pipelines on the seabed. The simple Tresca soil model was modified to incorporate the combined effects of strain rate and softening. The present large deformation finite-element method was validated by comparing the results with data from centrifuge model tests. A parametric study was then performed, varying the strain rate and softening parameters to explore their effects on penetration resistance. Simple expressions for penetration resistance, incorporating the effects of strain rate and softening, have been developed. The effects of soil strength vertical heterogeneity and buoyancy have also been explored.</p
Numerical simulations of dynamic embedment during pipe laying on soft clay
No full textPrediction of the as-laid embedment of a pipeline, which affects many aspects of pipeline design, is complicated by the dynamic motions that occur during the lay process. These motions cause pipelines to embed deeper than predicted based on static penetration models, as the seabed soils are both softened and physically displaced by the pipeline motion. This paper describes the results of 2D numerical analyses using a large displacement finite element approach aimed at quantifying pipeline embedment due to cyclic lateral motion at various fixed vertical load levels. The validity of the numerical results is first assessed by comparison with published data from centrifuge model tests in two different types of clay. A parametric study varying the normalized vertical load is then presented, which suggests a simple approach for estimating an upper limit to the dynamic embedment.</p
Modelling the embedment process during offshore pipe-laying on fine-grained soils
No full textSubsea pipelines are becoming an increasingly significant element of offshore hydrocarbon developments as exploration moves into deep-water environments further from shore. During the lay process, pipelines are subject to small amplitude vertical and horizontal oscillations, driven by the sea state and lay vessel motions. Centrifuge model tests have been used to simulate these small-amplitude lay effects, with varying degrees of idealization relative to the real lay process. In the soft soils found in deep water, pipe embedment can exceed a diameter or more, thus significantly affecting the lateral pipe-soil interaction, axial resistance, and thermal insulation. In this paper, results from centrifuge model tests are used to calibrate a model for calculating the dynamic embedment of a subsea pipeline. The model uses elements of plasticity theory to capture the effects of combined vertical and horizontal loading, and incorporates the softening of the surrounding soil as it is remoulded due to the pipeline motions. Influences from the lay rate, lay geometry, and sea state are included in the calculation process. The model is compared with observed as-laid pipeline embedment data from field surveys at three different offshore sites. Using site-specific soil parameters obtained from in situ testing and idealized pipe loads and motions to represent the load and displacement patterns during offshore pipe-laying, respectively, the model is shown to capture well the final as-laid embedment measured in the field surveys.</p
- …
