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Rock berms are used for many different offshore pipeline applications such as protection from anchors and trawlers, mitigation against pipeline global buckling and improvement of pipelines on-bottom stability. Understanding the lateral resistance provided by rock berms to pipelines is essential for all the above applications. This paper presents insights into lateral resistance of rock berms restraining pipelines by finite element analyses. Pipelines of various diameters (0.2 m-1.5 m) within typical rock berm geometries were modelled in PLAXIS 2D to evaluate the peak lateral resistance provided by the rock berm to the pipe. The finite element model was validated against available full-scale test data. The results of the numerical analyses demonstrate that the peak lateral resistance of a pipeline in rock berm depends mainly on the unit weight and the frictional properties of the rock, while the mobilization to reach the peak resistance is dependent on the rock berm stiffness. Based on the results from this study, a simplified design chart is presented which provides the lower bound peak lateral resistance for a given pipe diameter under typical rock berm with cover to top of pipe in the range of 0.3 m to 1 m. This design chart could be used by engineers undertaking preliminary design assessment of pipelines in rock berms.
Rock berms are used for many different offshore pipeline applications such as protection from anchors and trawlers, mitigation against pipeline global buckling and improvement of pipelines on-bottom stability. Understanding the lateral resistance provided by rock berms to pipelines is essential for all the above applications. This paper presents insights into lateral resistance of rock berms restraining pipelines by finite element analyses. Pipelines of various diameters (0.2 m-1.5 m) within typical rock berm geometries were modelled in PLAXIS 2D to evaluate the peak lateral resistance provided by the rock berm to the pipe. The finite element model was validated against available full-scale test data. The results of the numerical analyses demonstrate that the peak lateral resistance of a pipeline in rock berm depends mainly on the unit weight and the frictional properties of the rock, while the mobilization to reach the peak resistance is dependent on the rock berm stiffness. Based on the results from this study, a simplified design chart is presented which provides the lower bound peak lateral resistance for a given pipe diameter under typical rock berm with cover to top of pipe in the range of 0.3 m to 1 m. This design chart could be used by engineers undertaking preliminary design assessment of pipelines in rock berms.
Pipe clamping mattresses (PCMs) are a relatively new system for providing anchoring force to pipelines, to mitigate offshore flowline ‘walking’. They represent a cost-effective and highly efficient alternative to anchor piles, rock dump and conventional concrete mattresses. The system comprises a hinged concrete structure that clamps onto a section of laid pipeline, with concrete ballast logs securing the clamping action – with the benefit that 100% of the submerged weight of the PCM contributes to axial friction. PCMs have been applied successfully to one deepwater project, but performance data showing the influence of soil type, and allowing a general design framework to be established, has not yet been available. This paper addresses this gap by investigating the performance of PCMs through three series of centrifuge tests, supported by three Operators. Each series comprises tests on a different reconstituted deepwater soil as follows: (a) West African clay; (b) Gulf of Mexico clay; and (c) carbonate silty sand. In each test, a scaled pipeline is installed in-flight and cycled axially to represent its prior operating life. Scaled PCM models and ballast units are then installed onto the pipe in-flight, mimicking the use of PCMs to mitigate pipeline walking during operation. After installation of the PCMs, further axial cycles are applied, with the system settlement and changes in axial resistance and excess pore pressure measured. The paper shows the performance and applicability of PCMs for a range of soil types, highlighting variations in axial resistance and settlement. The suite of results will help to calibrate design tools for industry, removing unnecessary conservatism and enabling an optimised pipeline anchoring solution to be designed. Key results are equivalent friction factors for the combined pipe-PCM system and PCM settlement, which both show behaviour dependent on soil type. In the clay soils, friction increases significantly over time due to ‘consolidation hardening’. This provides validation of an important effect that has only recently been recognised in pipeline design. In contrast, hardening behavior is not evident in silty sand – although the study suggests there is potential for increasing resistance associated with settlement, which appears to mobilize additional (wedging) stress around the pipeline. Upon PCM installation, the pipelines embed further due to the added weight. Additional settlement occurs during cycling of the system, due to immediate soil deformation and consolidation-related compression. The magnitude of embedment is greater for the clay soils, but in all cases does not cause the clamping action to release. Overall, the efficiency of the PCM system in providing a high level of anchoring force per unit weight placed on the seabed is confirmed. Long term anchoring forces in the range 50-100% of the submerged weight of the PCM are demonstrated. This is several times more efficient than the commonly used alternative of a rock berm.
Rock berms are used to restrain flowlines from moving axially by adding vertical load to enhance resistance. Should the contact pressure be reduced at the flowline-berm interface, such as in response to ‘arching’ of the rock berm due to pipe settlement, this resistance may be substantially reduced. This study shows how the development of axial restraint forces is complex, with shear (‘friction’) at the pipe-berm and pipe-soil interfaces influenced by system settlement. We examine the restraint provided by rock berms through three phases of centrifuge model testing. Phase 1 isolates the pipe-berm interaction through ‘trapdoor’ tests on a false (rigid) seabed using actual berm profiles and scaled rock. Phase 2 models a slightly overconsolidated clay seabed with load from simulated berms held constant, thereby isolating the effects of pipe settlement and cyclic hardening at the pipe-soil interface. Finally, Phase 3 - which most closely represents the actual field behaviour - models the berm/pipeline/seabed system to investigate the combined effect of pipe settlement, arching and frictional response. The trapdoor tests identify that berm arching can occur in situations where simulated settlement of the flowline leads to redistribution of the rock berm weight, such that the berm weight acting on the flowline falls towards zero. Axial cycling is shown to help recover the berm's effectiveness, but is dependent on the amount of settlement - with greater settlements generating less axial restraint recovery. The constant vertical load tests confirm that axial resistance developed along the pipe-soil interface is strongly influenced by the vertical load delivered by the rock berm. Testing showed that axial resistances increase with consolidation and hardening, and also that settlements were modest and observed to be a function of vertical load. Axial resistances developed in the final phase of testing were the highest of all, as the tests include all restraining actions. These tests suggest that arching does occur under ‘realistic’ conditions - but that the effect is modest and largely eliminated by ongoing cycling. The findings reduce current design uncertainties and have already been incorporated in an offshore project where rock berms are being used to mitigate axial movements of a flowline. In particular, this novel centrifuge modelling confirms the potential for berm arching and loss of restraint, but also shows that arch collapse leads to recovery of flowline restraint - enabling the potential to reduce rock berm volume compared to the case of assuming arching leads to permanent loss of resistance.
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