Uplift Resistance Of Buried Pipelines Research Articles

Uplift resistance of buried pipelines: The contribution of seepage forces

Pipelines are commonly buried, and can buckle upwards when heated if there is insufficient soil uplift capacity. Interface tension beneath the buried pipe significantly influences the uplift capacity at shallow embedments. Conventional design approaches, which consider either zero or unlimited interface tension, do not assess and quantify the effect of interface tension on uplift capacity. The present study bridges the gap between conventional “no tension” and “full tension” capacities. Mobilisation of interface tension is governed by seepage forces which in turn directly control the formation of a gap beneath the pipe. A large deformation finite element approach, which simulates this phenomenon of gap formation using a thin layer of gap elements below the pipe, is adopted to study the soil response for various cases of uplift velocity, embedment and soil weight. The enhancement in undrained shear strength of soil at higher uplift velocities due to strain rate effects has also been considered. The interface tension mobilised at these different velocities and embedments varies systematically in a way that is expressed by modifying Hvorslev's intake factors. The proposed expressions may be used with the existing methodologies to assess pipe stability during operation, demonstrated here through a design example.

Numerical finite element modelling of soil resistance against upheaval buckling of buried submarine pipelines

Submarine high-temperature and/or high-pressure (HT/HP) pipelines used to transport oil and gas in different areas of the world are susceptible to global buckling. If the pipelines are simply laid on the seabed, buckling will likely occur in the horizontal plane, while if buried in a trench, buckling will probably arise in the vertical direction. The latter buckling mode may lead to catastrophic pipeline failure potentially associated with massive oil leakage and severe environmental damage. Concerns about upheaval buckling have motivated a number of researchers to investigate the uplift resistance of buried pipelines. The objective of the work presented in this paper is to evaluate the effects of various parameters on the resistance to upheaval buckling of buried submarine pipelines. A numerical approach was adopted to build representative and reliable 3D models of 100 m long embedded pipelines, using the specialized finite element software ABAQUS. Starting with a baseline model of a pipe buried in a medium dense sand with fines, the effects of pertinent parameters on the uplift resistance were explored for two scenarios: (1) uplift of the full pipeline length, which in essence captures the plane strain 2D response and (2) uplift of a central middle length of 20 m. Specifically, the effects of the apparent cohesion of the soil due to the presence of fines, pipeline diameter, embedment depth and diameter to wall thickness ratio were investigated. The uplift forces obtained were normalized in reference to actual pullout and/or effective pipe lengths. The results indicated that the normalized uplift forces obtained in the 3D analyses at high pipeline displacements were in close agreement with those yielded by plane strain models. The model analyses indicated that the contribution of soil apparent cohesion to the uplift resistance was significant. The work indicated that as the level of confinement increases (due to an increase in embedment depth and/or pipeline diameter) the uplift resistance increased, whereas the relative contribution of soil cohesion to the resistance decreases. The paper includes a comparison of the FE results obtained in this study with the available predictive/analytical methods, along with a discussion of which best captured the failure mode and resistance to uplift values of the numerical model.

Probabilistic analysis of the uplift resistance of buried pipelines in clay

Uplift resistance provided by soil cover is a key aspect in the design of a buried offshore pipeline. The capacity must be sufficient to avoid upheaval buckling but prediction of the uplift resistance is complicated by disturbance of the soil structure during installation. Previous numerical investigations of pipeline uplift capacity in undrained clays have employed simple elasto-plastic models and consider a homogeneous clay. These simplifications neglect critical features of soil-pipeline interaction and may not describe the real mechanical behaviour. In this paper, an advanced kinematic hardening model implemented in a finite element code is used to capture the degradation of structure as a pipeline buried in a natural clay is lifted upwards. The spatial variability of clay structure is represented by a random field and Monte Carlo simulation used to characterise the response. This novel framework shows that clay structure has a significant effect on the failure mechanism and uplift capacity of a buried pipeline. The probabilistic approach accounts for the uncertainty in the condition of the clay backfill and reveals that the spatial distribution of intact and remoulded material can change the mode of failure, emphasising the importance of considering clay disturbance in design.

The effect of radial fins on the uplift resistance of buried pipelines

This paper investigates the potential for increasing the uplift resistance of buried pipelines through the addition of radial fins on the pipe circumference. Experiments conducted in loose sand showed that fins extending by 20% of the pipe diameter increase the vertical peak uplift resistance by up to 25%, depending on embedment depth and fin configuration. A limit equilibrium solution – based on known values of peak friction and dilation angles – predicts the uplift resistance within 13% of the measurements. The trends of peak uplift resistance with embedment and fin configuration were also replicated in numerical analyses conducted using a non-associated Mohr–Coulomb soil model. The numerically predicted peak uplift resistances were within 10 and 21% of the experimental values for rough and smooth interfaces, respectively. Soil failure mechanisms from the numerical analyses were broadly consistent with that assumed in the limit equilibrium solution. However, the experimentally observed mechanisms differed subtly, with a limited extent of lifted soil above the pipe and circulatory flow occurring from above to beneath the pipe. This mechanism was approached in the numerical analyses for a smooth interface by specifying a small negative dilation angle, which had minimal effect on the predicted peak uplift resistance.

Numerical modeling of uplift resistance of buried pipelines in sand, reinforced with geogrid and innovative grid-anchor system

Reinforcing soils with the geosynthetics have been shown to be an effective method for improving the uplift capacity of granular soils. The pull-out resistance of the reinforcing elements is one of the most notable factors in increasing the uplift capacity. In this paper, a new reinforcing element including the elements (anchors) attached to the ordinary geogrid for increasing the pull-out resistance of the reinforcement, is used. Thus, the reinforcement consists of the geogrid and anchors with the cylindrical plastic elements attached to it, namely grid-anchors. A three-dimensional numerical study, employing the commercial finite difference software FLAC-3D, was performed to investigate the uplift capacity of the pipelines buried in sand reinforced with this system. The models were used to investigate the effect of the pipe diameter, burial depth, soil density, number of the reinforcement layers, width of the reinforcement layer, and the stiffness of geogrid and anchors on the uplift resistance of the sandy soils. The outcomes reveal that, due to a developed longer failure surface, inclusion of grid-anchor system in a soil deposit outstandingly increases the uplift capacity. Compared to the multilayer reinforcement, the single layer reinforcement was more effective in enhancing the uplift capacity. Moreover, the efficiency of the reinforcement layer inclusion for uplift resistance in loose sand is higher than dense sand. Besides, the efficiency of reinforcement layer inclusion for uplift resistance in lower embedment ratios is higher. In addition, by increasing the pipe diameter, the efficiency of the reinforcement layer inclusion will be lower. Results demonstrate that, for the pipes with an outer diameter of 50 mm, the grid-anchor system of reinforcing can increase the uplift capacity 2.18 times greater than that for an ordinary geogrid and 3.20 times greater than that for non-reinforced sand.

Effects of soil reinforcement on uplift resistance of buried pipeline

Pipeline failure caused by low uplift resistance in soil is a serious environmental and economic issue. Hence, having access to higher uplift resistance of pipeline through soil reinforcement has received considerable attention. The objective of this study is to examine the effect of performing geogrid to increase the uplift resistance of buried pipelines. To examine the effect of burial depth, pipe diameter, length of geogrid layers and the number of geogrid layers on the peak uplift resistance (PUR) of loose sand, 33 small-scale tests were performed in the laboratory. Results of laboratory tests reveal that depth of burial and pipe diameter have a direct effect on the PUR results. The findings show that the number of geogrid layers does not have a remarkable influence on PUR values. While the residual PUR values are of interest, for the same length of geogrid, the use of two layers of geogrid instead of one is advantageous. To verify the experimental results, 33 experiments were back analyzed using “PLAXIS 3D TUNNEL” program. It was found that experimental and numerical results are in good agreement.

Uplift Resistance of Buried Pipelines Enhanced by Geogrid

Failure of an oil or gas pipeline due to low uplift resistance of soil has serious economic and environmental consequences; hence, increasing the uplift resistance through soil reinforcement is of interest. The main purpose of this paper is to investigate the possible use of geogrid to enhance the uplift resistance of buried pipelines. For this reason, 11 small-scale laboratory tests were performed. The tests were conducted to investigate the effect of pipe diameter, burial depth, as well as length and number of geogrid layers on the uplift resistance of sandy soils. The experimental results suggest that although pipe diameter and burial depth are directly related to uplift resistance, the direct effect of geogrid incorporation is more pronounced. While Peak Uplift Resistance (PUR) is of interest, findings indicate that the number of geogrid layers does not have a pivotal influence on PUR. In addition, for verification purpose, the PUR of 11 laboratory tests were back analyzed numerically using finite element software PLAXIS 3D TUNNEL. The findings show that numerical and experimental results are in good agreement.

Enhancement of the Uplift Capacity of Buried Pipelines by the Use of Geogrids

Abstract The paper presents the results of preliminary tests that were conducted to investigate the possible use of geogrids for increasing the uplift resistance of buried pipelines. The results of the preliminary experimental investigations suggest that the uplift capacity of a pipeline section can be increased quite significantly by the incorporation of geogrids at the crown region of the pipeline in an advantageous and practicable configuration.