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Procedia Engineering
Procedia Engineering Engineering 14 00(2011) (2011)1690–1695 000–000 Procedia www.elsevier.com/locate/procedia
The Twelfth East Asia-Pacific Conference on Structural Engineering and Construction
Lateral Breakout Resistance of Shallowly Embedded Offshore Pipelines Y. S. LEEa*, C. C. SMITHb, C. Y. CHEUKc a
Department of Building and Construction, City University of Hong Kong, China b Department of Civil and Structural Engineering, University of Sheffield, UK c AECOM Asia Company Limited, Hong Kong, China
Abstract
Stresses induced by thermal expansion of unburied seabed pipelines can be designed to be relieved through controlled lateral buckling, which requires accurate and reliable assessment of the lateral breakout resistance. The lateral breakout resistance is strongly dependent on the pipe embedment, soil strength and loading history of the pipe. In addition, prediction of the pipe movement during the breakout process is essential as it subsequently determines the lateral resistance at large displacement. Theoretical solutions based on plasticity theory are available for combined vertical and horizontal loading on partially embedded pipes. These solutions however ignore the pre-failure pipe displacement and the associated change in soil geometry which could have significant influence on the failure load. A series of 1g model tests has been conducted to examine the lateral breakout resistance for a shallowly embedded pipe in a soft clay. Two types of tests, (1) sideswipe tests and (2) probe tests, were conducted to examine the effect of pipe embedments and loading history on the breakout resistance of a partially embedded pipeline. Results were compared with other proposed predictions in order to provide a more rigorous basis for the prediction of the breakout resistance of a partially embedded pipeline.
© 2011 Published by Elsevier Ltd. Selection Keywords: lateral breakout resistance, shallowly embedded pipe, soft clay
* Corresponding author and Presenter Email: [email protected]
1877–7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.07.212
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Author name / Procedia Engineering 00 (2011) 000–000
1. Introduction Offshore pipelines usually operate at high temperature and pressure at deepwater. The temperature changes during operating cycles induce thermal loading on the pipeline, which leads to lateral buckling and in some cases to a failure of the pipeline (Pasqualino et al. 2001). The conventional approach to prevent this buckling is to trench and bury the pipe or to relieve the stress in the pipe using inline expansion spools. However, these methods are not as cost effective as offshore development moves into deeper water with more extreme conditions. An alternative solution is to relieve the axial compressive stress in the pipe by controlling the formation of lateral buckles along the unburied pipeline (Burton 2005). This alternative design requires accurate prediction of the as-laid embedment of the pipeline and its subsequent response during lateral breakout with combined loading from thermal expansion and pipe self-weight, especially during initial expansion of the pipeline. Prediction of the horizontal load in the lateral breakout event is entirely based on empirical correlations (Verley and Lund 1995). Generally, the expressions divide the ultimate lateral resistance into two components (1) a frictional component linked to the current vertical load of the pipe and (2) a passive component linked to the pipe embedment and the soil undrained shear strength. The coefficients for the friction and passive (embedment) components vary with soil type and the ratio of soil strength to unit weights. These models only predict breakout load, but ignore the response of the pipe under various loading conditions and history. Recent investigations have focused on the construction of yield envelopes, which define the limiting combination of vertical and horizontal load and hence to provide a framework to evaluate the response of the pipe under various loading conditions. In order to provide a better understanding on the behaviour of lateral pipe-soil interaction, a series of 1g model tests was conducted to examine the lateral breakout resistance of a shallowly embedded pipeline in a soft clay. The 1g model tests were conducted in a steel tank with a perspex window equipped at the front of the tank to allow visual observation or image analysis of the failure mechanism of the breakout event. Kaolin clay was used to model the seabed. Two types of tests were conducted: (1) sideswipe tests were conducted to examine the lateral soil resistance exerted on the partially embedded pipeline at constant pipe embedments, and (2) probe tests were conducted to measure the breakout resistance of the pipe under constant vertical load. 2. Methodology 2.1. Experimental apparatus and model preparation A test tank with inner dimension of 1.5m (length) × 0.6m (width) × 0.8m (height) was used to contain the soil sample. The test tank is equipped with a perspex window at the front to allow observations of the soil deformation during the test. A two dimensional actuation system is attached on the test tank, which is controlled by two servo motors and gearheads. The actuator can provide a load capacity up to 4kN in both vertical and horizontal directions, and the speed of the actuator can be varied from 0.1mm/s to 2cm/s. The load-displacement response of the pipe is recorded by a JR3 load cell and a Linear Voltage Differential Transformer (LVDT) mounted on the guide of the actuator. Details of the experimental setup can be found in Lee et al. (2008). The test soil adopted in this study is kaolin clay, which was prepared at slurry state with an initial moisture content of about 95%. The soil sample was consolidated by a dead weight of 10kPa for two weeks until no settlement was observed. After completion of the consolidation, the dead load was removed and the soil sample was left to swell for up to two weeks. A 50mm diameter, D, smooth pipe with a length to diameter (L to D) aspect ratio of 4 was used in this study, which was penetrated to a
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particular depth, w, and moved laterally under various conditions. The tests were all conducted with a dimensionless velocity, vD/cv, larger than 30 to ensure undrained conditions in both vertical penetration and horizontal breakout tests, where v is the velocity of the pipe displacement and cv is the coefficient of consolidation. 2.2. Test programme A total of six tests were carried out, three sideswipe tests and three probe tests. Details of the tests are summarized in Table 1. In the sideswipe tests (ST1 to ST3), the pipe was initially penetrated to a particular depth, winitial, and moved laterally where the vertical position of the pipe was held constant. In the probe tests (PT1 to PT3), pipe was moved laterally under constant vertical load, V, after pipe penetration. The tests were all conducted with a minimum horizontal pipe displacement, u, of 1.5D. The loading history of the tests is described either in normally loaded or overloaded condition. The normally loaded condition represents the situation where the vertical load on the pipe during breakout is equal to the maximum experienced, Vmax, during vertical penetration, whereas over-loaded condition represents the vertical load of the pipe, V0, is reduced after the vertical penetration, but prior to lateral breakout event. An overloading ratio, R=Vmax/V0, can be used to represent the degree of unloading condition, similar to the concept of overconsolidation ratio. The overloading ratio in the probe tests R was varied from 1 to 10. Table 1: Test programme
ST1
Sideswipe test ST2 ST3
PT1
Probe test PT2
PT3
Horizontal pipe displacement, u/D
1
1
1
1.5
1.5
1.5
Pipe velocity, v (mm/s)
1
1
1
0.5
0.5
0.5
Initial pipe embedments, winitial/D Vertical load condition, V0 (N/m)
0.1 variable
0.3 variable
0.5 variable
~0.5 450
~0.5 90
~0.5 45
Overloading ratio, R
1
1
1
1
5
10
2.3. Undrained shear strength T-bar tests were conducted using a 25mm T-bar penetrometer. The undrained shear strength of the soil was interpreted using the method proposed by White et al. (2010). A small variation at shallow depths with a magnitude of about 0.2kPa is observed from the T-bar results. The average undrained shear strength at soil surface is approximately 2.33kPa, which decreases to about 1.6kPa at a depth of 150mm. The reduction in shear strength might be attributed to incomplete consolidation. 3. Results and Interpretation 3.1. General observations When pipe was penetrated to the desired depths, from 0.1D to 0.5D, a heave zone (i.e. soil berm) adjacent to the pipe was created. The size and shape of the soil heave depends on the embedment depth of the pipe, with more soil displaced in the test with greater embedment depths, such as ST3. Generally, the heave zone extended about 1D from the pipe periphery during the vertical penetration. Once the pipe
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started moving laterally, a soil berm developed actively in size with pipe movement. For the sideswipe tests with greater embedments such as ST2 and ST3, the soil berms engulfed the pipe after about 1D of horizontal displacement and partially buried half of the pipe. In the probe tests, pipe embedment could change with the lateral displacement. A significant downward movement of the pipe was observed in PT1 with a low R of 1, whereas an upward movement of the pipe was observed in PT2 and PT3 with a high R of 5 and 10 respectively. The continuous downward movement of PT1 created a larger soil berm during the lateral movement and eventually buried in the soil. For the pipes moving in an upward direction, soil was swept away from the pipe with a smaller berm size. 3.2. Load-displacement response 3.2.1. Sideswipe test Figure 1 shows the lateral response of the sideswipe and probe tests. Various predictions for the lateral breakout resistance of the pipe are also included. For the sideswipe tests, a hardening behaviour is observed, in which breakout force increases with pipe embedment. A peak horizontal force, Hmax, about 250N/m is reached for ST3 (w/D=0.5) with a small pipe displacement of about 0.15D. This is followed by a drop in resistance, which is inferred to indicate a tensile failure between the pipe and the soil behind. The lateral resistance then gradually increases with further horizontal pipe displacement. The increase in lateral resistance is caused by the active berm in front of the moving pipe, which grows in size with pipe displacement. Results of the sideswipe tests show that the lateral breakout resistance depends very much on the initial pipe embedment. After pipe breakout at large displacement, the lateral resistance is mainly contributed by the growth of the active berm. Both Verley and Lund (1995) and Merifield et al. (2008) predictions show a good agreement with the experimental results for ST1 and ST2, but underestimate the breakout resistance of ST3, greater embedment of 0.5D, by up to 30%. 3.2.2. Probe test The initial breakout resistance of the probe tests is found to reduce with overloading ratio. However, for a normally penetrated pipe in the probe test, PT1 (R=1), the horizontal force gradually increases with pipe displacement after initial breakout. This is caused by the downward movement of the pipe, more soil is being swept during the lateral movement. For the tests with high overloading ratio, PT2 and PT3, a post peak strain-softening behaviour is observed with a peak load reached at a lateral displacement of about half diameter. Results illustrate the effect of loading history on the lateral resistance of the pipe at large displacement, which should not be ignored as an overloading condition often happens when the pipe is additionally embedded due to additional hydrodynamic loads during laying.
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Figure 1 Lateral response from sideswipe and probe tests
3.3. Bounding surface A bounding surface representing the lateral breakout mechanism can be presented by normalizing the sideswipe results using Vmax, as shown in Figure 2, where yield points of the PTs at various embedments are also plotted. The failure envelopes proposed by Merifield et al. (2008) for a smooth pipe (predicting initial breakout resistance) are included for reference (the envelopes for a rough pipe are not significantly different). Results show that the peak horizontal load reaches about 0.35-0.4Vmax for various embedments and the failure envelopes are approximately parabolic in shape, similar to Merifield’s prediction. However, the peak H/V ratio significant exceeds from that Merifield’s prediction. This is attributed to the increase in berm size with the horizontal pipe displacement, which results in an increase in both vertical and horizontal forces, whereas Merifield’s envelopes predict a maximum breakout resultant force at small displacement (≤0.1D) from a wished in place configuration where the berm size can be assumed to be unchanged from its initial embedment depth (winital/D). Combing with the probe test results, alternative failure envelopes are proposed and it is expected that the bounding surface ends at a positive intersect with the vertical axis, indicating some passive resistance of a partially embedded pipeline. However, further investigation is required to calibrate the proposed failure envelopes for the prediction of lateral resistance of a partially embedded pipeline in soft soil. 4. Conclusions Two types of lateral breakout tests were conducted to investigate the breakout resistance of a partially embedded pipeline. Sideswipe tests show that the horizontal breakout resistance is dependent on the initial pipe embedment and the undrained shear strength of the soil, whereas probe tests demonstrate the significant of the loading history on the resulting pipe movement during breakout and the softening behaviour of the lateral resistance. The experimental results indicate that bounding surface is in parabolic shape, and possibly with a positive intersect with the vertical axis. However, further study is required to calibrate the prediction of the bounding surface proposed.
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Figure 2 Bounding surfaces for shallowly embedded pipelines under combined loadings
References [1]
Burton D, Carr M, Crawford M, and Pioate E (2005). The safe design of hot on-bottom pipelines with lateral buckling using the design guidance developed by the SAFEBUCK Joint Industry Project. Proceedings Deep Offshore Technology Conference, Vitoria, Espirito Santo, Brazil. Pennwell Houston, Tex.
[2]
Lee YS, Smith CC, and Cheuk CY (2008). Bearing capacity of embedded foundations. Proceedings 2nd BGA International Conference on Foundations, Dudeen, UK, 1, pp. 962-970.
[3]
Merifield RS, White DJ, and Randolph MF (2008). The ultimate undrained resistance of partially embedded pipelines. Géotechnique. 58(6), pp. 461-470.
[4]
Pasqualino LP, Alves JLD, and Battista RC (2001). Failure simulation of a buried pipeline under thermal loading. Proceedings 20th International Conference on Offshore Mechanics and Arctic Engineering (OMAE), Rio DE Janiro, Brazil, OMAE2001/PIPE-4124.
[5]
Verley R, and Lund KM (1995). A soil resistance model for pipelines placed on clay soils. Proceedings 14th International Conference on Offshore Mechanics and Arctic Engineering (OMAE), Copenhagen, Denmark, 5, pp. 225-232
[6]
White DJ, Gaudin C, Boylan N, and Zhou H (2010). Interpretation of T-bar penetrometer tests at shallow embedment and in very soft soils. Canadian Geotechnical Journal, 47, pp. 218-229.
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  • Author : Yara Sharif
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