Among the greatest uncertainties in future energy supply is the amount of oil and gas yet to be found in the Arctic: the US Geological Survey estimates the region to hold as much as one-fifth of the world's untapped reserves of hydrocarbons. E&P in this environmentally sensitive area is just one of many challenges. MCS Kenny's Jianfeng Xu, Ayman Eltaher and Paul Jukes look at advanced analysis for Arctic pipeline design.
Arctic reserves are becoming increasingly important as reserves more easily reached elsewhere become depleted or politically inaccessible. A race has begun among Arctic nations, including the United States, Russia and Canada, for control of these resources. The exploration and development of these Arctic reserves however, will depend upon meeting the technical challenges of the harsh Arctic environment. On the other hand, the footprint of the exploration and development activities must be limited to protect the fragile Arctic environment.
For pipelines going through the Arctic territory, most of the challenges for design, construction and operation come from ice loading, permafrost interaction, coldness, remoteness and darkness. Of these challenges, ice gouging for offshore and permafrost thaw settlement for onshore, nearshore and shore-crossing are the two main challenges, and both require considerable engineering attention.
Ice gouging, or ice scour, is a common phenomenon in shallow water along the coast of the Arctic Ocean. It starts when wind and current forces pile sea ice into large masses called ice ridges. These ice ridges have a keel that extends below the water surface and moves with the ice sheets and packed ice. When pushed toward shallower water with depths less than the keel, ice masses gouge the seabed. Most commonly, the gouge depth is less than 1m, but the deepest gouges can reach to 4m. Offshore pipelines installed in ice gouging areas must be buried deep enough to avoid direct contact with the ice keel or excessive soil deformation below the gouge. Often, deep trenching or dredging in the Arctic offshore can be technically challenging and may make a project economically unfeasible.
Permafrost - soil with a temperature below 0°C for over two consecutive years - is another common feature in Arctic onshore and nearshore shallow water regions. In permafrost territories, poorly designed pipelines are likely to suffer from thaw settlement and frost heave. Likewise, large-scale permafrost degrading due to pipeline interference might lead to major environment issues and take hundreds of years to recover. To protect a pipeline project from these permafrost issues, thermal interactions between the pipeline and the environment must be minimized. Simply put, if the soil is frozen, keep it frozen; and if the soil is thawed, keep it thawed. Different methods can be used to achieve this goal, such as lifting the pipeline above the ground, massive thermal insulation and passive cooling to keep the permafrost from thawing. These methods, however, can cost up to two or three times that of the pipeline itself.
For both ice gouging and permafrost issues, careful assessment and accurate evaluations of the problem are crucial to an economically feasible solution. Many research activities and a number of projects have been carried out since the middle of 20th century in both North America and Russia, which have provided valuable experience as well as lessons learned. Experience is still very limited compared to the complex nature of the problems and the huge diversity of the Arctic environment. Due to the huge financial and time costs, full-scale field testing of individual projects is usually not feasible. As an alternative, numerical simulation is used increasingly in modern engineering design due to its accuracy, speed, low cost, flexibility and versatility; it has already become an indispensible tool for engineering practice.
The ice gouging process is highly non-linear and involves complex interaction between ice and soil and can be further complicated by the presence of a buried pipeline. An accurate prediction of stresses and strains in a buried pipeline when it is displaced by surrounding soil is the key to the safe design of the pipeline in areas subject to ice gouging. A main challenge to modeling ice-soil-pipe interactions in an ice gouging incident is the large soil deformation and the severe mesh distortion that traditional (Lagrangian) finite element (FE) analysis would undergo. Coupled Eulerian-Lagrangian (CEL) or Arbitrary Lagrangian-Eulerian (ALE) algorithms are usually applied when tackling this problem. One FE model using these algorithms is the ice-gouge model MCS Kenny developed, which uses the CEL capability available in the Explicit package of the general FE platform Abaqus. The CEL formulation can accommodate extreme soil deformations that occur during the ice gouging process by fixing the FE discretization mesh and tracking the soil material incrementally while it moves relative to the mesh. The model is constructed such that an overall domain encompasses all Eulerian material such as native soil and trench backfill and Lagrangian bodies such as the ice keel and pipeline and maintains a void region to accommodate displaced soil during the ice-gouging simulation.
The coupled formulation allows interaction between the usually stiffer Lagrangian bodies and the usually softer Eulerian material, and at the same time avoids numerical stability problems common with a pure Lagrangian FE analysis. The CEL formulation realistically models complex 3D interaction effects that occur during ice gouging events and is apt to reduce conservatism associated with other lessrealistic models. The CEL-based model developed by MCS Kenny can be used as a tool to optimize pipeline burial depth, which can result in significant trenching cost savings.
Thaw settlement is considered the most direct problem related to having warm pipelines buried in permafrost. It also occurs in shallow waters and at shore crossings where ice-bonded permafrost soil underlies the pipeline. With the heat released from the pipeline, the surrounding permafrost may gradually thaw over years of operation and create a permafrost thaw bulb, thus reducing load-carrying capacity of the soil. The pipeline may no longer be supported, while at the same time carrying the overburden weight of the soil. It causes the pipeline to deflect into the void created by settlement, which could result in overstress or even damage to the pipeline, depending on the amount of heat released from the pipe, the soil type, and the distribution of ice in the permafrost.
To simulate this thaw settlement, MCS Kenny developed a 3D FE model to predict the thermal and mechanical interactions between a buried pipeline and surrounding permafrost. The analysis using the model is applied in two steps. First, the model simulates the heat transfer process for a period of time, and then it calculates the soil settlement and pipeline deflection based on the thaw bulb predicted during the heat transfer simulation.
The model accurately predicts the settlement-induced pipeline strains using FE continuum modeling rather than discrete soil modeling, such as springs. The model also assesses the accurate size of the thaw bulb and corresponding settlement at any given time, thus alleviating over-conservatism in the design. Different soil types in multiple layers can be accommodated in such analysis, and the model can be used in the initial or final design of the pipelines, as well as in the assessment of existing pipelines embedded in permafrost soils.
Both the ice gouging and the permafrost models have been implemented using the general purpose FE software Abaqus, known for its non-linear, multi-physics capacity and extensive range of material models. The developed FE models can help optimize pipeline designs for Arctic projects, to ensure the safety of both the pipeline and the environment without unnecessary over-design.
MCS Kenny is involved in the DNV Ice Pipe Joint Industry Project, which aims to evaluate and present design methods and recommendations related to the installation, operation, and maintenance of pipelines in areas of extreme cold and ice.
So far, most of the Arctic pipeline projects have been located in Russia, Alaska and Canada. Offshore Arctic projects are generally concentrated in a few locations, namely the North Slope of Alaska in the Beaufort Sea, Sakhalin Island, the Caspian Sea and Barents Sea of Russia, and Canada's Grand Banks. The unique challenges facing Arctic pipeline designs have been under consideration for decades, and precious experiences and lessons have been accumulated. Our understanding of the Arctic environment and Arctic engineering is now more advanced than ever. With the new developments in analysis methods and in the technology, soundly designed Arctic pipelines are becoming more achievable, and they will eventually contribute to the breakthrough in the exploration of Arctic energy resources. OE
About the authors
Jianfeng Xu is a senior specialist with MCS Kenny in Houston. He has a PhD in mechanical engineering, an MS in thermal engineering and a BS in nuclear engineering. His experience includes cold region heat and mass transfer, Arctic engineering, and subsea pipeline design. He is also interested in the environmental protection of the Arctic from engineering activities.
Ayman Eltaher PhD, PE, is the technology and R&D manager of MCS Kenny in Houston. He has over 20 years of academic and industry experience, 10 years of them on offshore structural, geotechnical, pipeline and arctic engineering. He also has expertise in general numerical techniques in analysis, numerical material modeling and probabilistic, eg structural reliability, analysis.
Paul Jukes PhD, is the president of MCS Kenny in Houston. He has over 18 years experience in the structural analysis and design of structures, pipelines, risers and subsea components, and is responsible for the development of analysis tools for arctic applications. He holds a bachelor's degree in mechanical engineering & structural mechanics, and a PhD in structural engineering from the University of Sussex. He is a chartered engineer, a fellow and vice president of the Institute of Marine Engineering, Science & Technology (IMarEST) and has published over 70 journal and conference papers.