Buried pipeline response to ice gouging

Arctic region is rich in abundant discovered and undiscovered hydrocarbon resources and is an important area for energy development. In the arctic and other cold regions, subsea pipelines, which are considered to be an economical and convenient way of oil and gas transportation, are exposed to various geohazards such as pressure ridges or icebergs gouging the seabed. These floating ice masses could impose distress to the pipe through interaction with the seabed and ultimately jeopardize the integrity of the pipeline structure. To protect the pipelines the most common and efficient practice is to bury them into the seabed. Efficiency refers to both cost and performance of this method. Comparing to alternative methods, such as ice management or construction of protective structures along the length of the pipeline, trenching would be a more manageable option. In addition to protecting against ice features, trenching can help maintain the structural integrity of the pipelines against other hazards such as lateral buckling or hydrodynamic loads. Trenching is very useful to cope with uneven seabed and mitigate free spans. As the result, finding a safe but economic burial depth to install the pipelines in the subsea is important in offshore pipeline projects. The key to determination of a safe and economical burial depth is the proper understanding of the seabed soil response to the ice gouging and accurate prediction of the sub-gouge deformation under gouging loads. Numerical analysis could be an efficient tool to capture the seabed behaviour during the ice gouging event and simulate the sub-gouge deformations provided an appropriate soil model is applied. The soil constitutive model should be able to account for different stress paths. It should also be simple in terms of estimating input parameters with small number of common tests. Most of the constitutive models available in commercial finite element packages do not appropriately simulate the dilative behaviour of sand. An improvement in hardening law could also enhance their accuracy in predicting soil stress-strain behaviour. In this research, ABAQUS Explicit Finite Element (FE) software is used for numerical analyses. Some of the limitations of built-in soil constitutive models in the ABAQUS Explicit for capturing the ice gouging mechanism are shown. A variant of the Drucker- Prager Cap model is therefore proposed to capture the behaviour of sand under large deformation more realistically. NorSand plasticity model, developed on critical state framework, has shown good performance in modeling various laboratory test results of sand and has been used for a variety of geotechnical applications. In this research, the proposed Drucker-Prager Cap model as well as the NorSand critical state model have been implemented in ABAQUS Explicit using the user subroutine VUMAT and are used to simulate the seabed response to the ice gouging event within the Arbitrary Lagrangian Eulerian framework (ALE). Through the application of volume constraint method, these constitutive soil models are extended to predict the undrained behaviour of soils which is lacking in ABAQUS Explicit. The developed constitutive models are verified and validated against triaxial drained and undrained tests. The finite element simulations using these constitutive models are also validated against the centrifuge ice gouging test results. In addition to ice gouging mechanism the effect of some of the more influential factors of this process is investigated. Through the numerical analyses it is demonstrated that: (i) the critical stress ratio of soil directly correlates with the keel reaction forces; (ii) in a specific soil at a denser state (i.e. more dilative), larger keel reaction forces are required in order to reach the steady state condition; (iii) the higher attack angle of the keel results in the lower keel reaction forces; (iv) deeper gouges yield larger keel reaction forces; (v) it is possible to normalize the keel reaction forces based on the keel geometry and soil material properties; (vi) the developed frontal berm height consistently increases with the increasing shear strength of the soil; (vii) larger frontal berm is developed in denser soil in steady state; (viii) the increase of keel attack angle and gouging depth result in larger frontal berm height. The variation of the attack angle modifies the mechanism of the frontal berm development; (ix) smaller sub-gouge deformations are observed in denser soil; (x) the sub-gouge deformation increases in the soils with higher shear strength. The critical stress ratio is more influential for denser soils compared with the loose soils; (xi) the increase of the attack angle reduces the vertical extension of the sub-gouge deformation and (xii) as expected, the increase of the gouging depth extends the subgouge deformations deeper into the seabed. These results are confirmed through physical tests published in literature. The developed numerical models are used to simulate the results of some of the physical tests of research programs that are carried out in C-CORE such as the PIRAM project. The results of numerical analyses show good agreement with results obtained from centrifuge tests. Although the proposed Drucker-Prager Cap model is able to simulate different behavior of sands but because some issues such as the effect of intermediate principal effective stress and the definition of the yield surface are better addressed in critical state NorSand model, the latter model is found to be more preferred in the analyses of this research.