It has been discussed that about a quarter of the world untapped oil and gas reserves are located in the Arctic. Development of such prospects has faced challenges, both operational and design-related. Operational challenges include geographic location and associated working conditions, such as remoteness, prolonged darkness, communications and equipment reliability; climate conditions, such as low temperatures and ice coverage; unique characteristics of arctic development, such as high reliability requirements and extremely long tie-backs; and environmental conditions and regulations due to the extremely sensitive Arctic ecosystem. The design related challenges include design conditions, loadings and material issues that are unique to the area and environment and that arise from unique arctic operational and environmental conditions. Due to those challenges, the design of pipelines in the Arctic area requires the consideration of design conditions that do not apply elsewhere, such as ice gouging, frost heave and permafrost. Advanced numerical analysis is apt to play an important role in addressing these challenges and to bridge the technology gaps that still exist in order to enable optimized developments to proceed. This paper opens with a review of the main arctic challenges for pipeline design followed by the benefits of using advanced numerical techniques to address problems relevant to the design of pipeline in the arctic environment.

As the demand for energy resources grew, the oil and gas industry looked north for exploration and development of offshore hydrocarbon resources. Significant discoveries were made in Arctic and sub-Arctic regions; however, these environments can be particularly unforgiving on pipelines and offshore structures with a host of geohazards unique to the area. Burial under the seabed became the common practice of protecting offshore Arctic pipelines. Innovative design considerations including installation techniques, leak detection monitoring of the pipelines during operation, and bundling pipelines together aided in the successful completion of these projects. As design engineers on the three subsea pipelines completed in the US Arctic - Northstar (BP), Oooguruk (Pioneer), and Nikaitchuq (ENI) - all in the Beaufort Sea on the North Slope of Alaska, INTECSEA have been able to progressively improve upon design evaluations, building upon learnings from each of the previous projects. Traditional subsea pipeline design is a stress-based approach with individual pipelines either laying on the seabed or buried to a shallow depth to protect against hazards including fishing gear or dragged anchors or for protection against upheaval buckling. In Arctic pipeline design, the extreme environmental loadings tend to require deeper burials, and due to the short installation windows and high cost of installation on-ice, may lead to the use of pipeline bundles to facilitate installation. Strain-based design is generally used for Arctic subsea pipelines due to the extreme displacement-controlled loading conditions. This paper will discuss the evolution of the design considerations and methodologies for Arctic subsea pipelines subjected to the unique Arctic environmental loadings such as strudel scour, permafrost thaw settlement, and ice gouging. These evolving design methodologies are comprised in INTECSEA's in-house suite of Arctic Tools and pipeline design standards. Focus is also placed on the importance of appropriate environmental and geotechnical data collection, the use of limit states design, and the various trenching and backfilling aspects of pipeline design.

Abstract The Analytical Hierarchy Process (AHP) is employed to structure and prioritize the criteria (issues) that most strongly affect pipeline routing decisions for Arctic and sub-Arctic offshore projects. An AHP model is created incorporating these criteria and the pair-wise comparisons technique is used to establish weighting of the criteria. The collaborative pair-wise comparison approach allows all team members to explore and sound out each other's perspectives on the importance of each of the routing assessment criteria in a disciplined way that builds consensus around the model adopted for the cases under investigation. This AHP model can then be tagged to a geomatics database automatically linking expertise in the pipelining disciplines with advanced geomatics capabilities for assessing export pipeline routing schemes to directly account for key considerations like flow assurance, geohazards, on bottom stability, bottom roughness, bathymetry, seabed morphology, sediment types, existing infrastructure, economics, public use regions, and environmentally sensitive areas. The paper describes how this linkage allows an automated scoring procedure that is then tested for robustness by sensitivity checks and demonstrates the methods via case studies. When this method is applied to offshore Arctic pipelines, information can be incorporated that reflects the engineering, environmental, social, administrative, and infrastructure input that must be considered in routing/design. The model is then applied to challenging pipeline planning cases with sensitivities investigated in a way that confirms the robustness of the routing recommendations. The paper clarifies how the issues and key technical information can be efficiently captured and applied within Arctic or sub-Arctic full field development planning studies that reflect real-world information (geomatics). The ability to easily accommodate changes in the engineering/technical basis and/or corporate priorities is highlighted, as well as the consensusbuilding strengths of this advanced methodology.

Abstract Ice gouging or scour may damage structures buried in seabeds, hence research on this subject is important. Numerical modelling is one of the most flexible and least costly methods of studying ice gouging. Previously, information on existing numerical models and their results was scattered in the literature. A new database has been created that tabulates this information. The database can be used to search numerical results and analyze knowledge gaps and correlations that might exist, in order to better understand and further advance knowledge of ice gouging phenomena. The database contains information on 206 runs from 18 major numerical studies. Using the database, knowledge gaps have been assessed. A list has been made of topics which were given little attention despite their probable importance, including deformable ice keel, different-from-seabed trench backfill soil, and simultaneous interaction of pore water and soil matrix in cohesionless seabeds. The available numerical model results show that pre-set gouge depth and the maximum depth of subgouge soil deformation are nearly linearly correlated. Maximum pipeline strain as a function of test set-up parameters is assessed. Profiles of subgouge soil deformation with depth from various sources are also combined and compared in this paper.

Abstract The objective of this paper is to demonstrate how modelling can be used to account for the inherent correlations while assessing the pipe response to ice gouging – and thus narrow down the uncertainties associated with the pipeline design process. A custom-developed numerical model of ice gouging has been developed and exercised to better understand how the environmental conditions affect the gouging process. Further, numerical simulations of the keel-soil-pipe interaction have been performed, relating to the input and output of the ice gouging model. The ice gouging simulations quantitatively demonstrated the effect of the governing parameters on the gouge depth. Geotechnical conditions are very important as the main source of resistance against the driving force from ice, making a noticeable difference in the gouge depth. Force balance is also important, in particular how the vertical forces are generated or/and limited by natural phenomena such as the shape of an ice feature, seabed topography and tides. The effect of the driving force and the keel resistance limits have not been dealt with within the scope of the present study. The ice gouging simulations demonstrated that ridges with steeper rake angles result in deeper gouges. Similar gouge depths have been attained irrespective of the path – via water level changes or via seabed slope - as long as the driving force was available. The keel-soil-pipe simulations demonstrated that increasing the rake angle results in lower pipe response. Deeper gouges give larger effect at the same clearance between the top of the pipe and the gouge bottom. Considering the performed simulations jointly, it can be concluded that selecting the n -year gouge depth implicitly sets the conditions for assessing the variability in the additional governing parameters. This is mainly applicable to the keel's rake angle as the factor having the strongest correlation with the gouge depth, keeping other parameters unchanged. Appreciating the correlation above and its effect on the n -year pipeline load effect can prevent possibly overly-conservative parameter combinations. It is argued that steeper keels have the highest potential to produce deepest (extreme) gouges. For these conditions, the extreme gouge load effect seems to be acceptable as long as there is a gap between the pipe and the ice keel and the pipe wall thickness is relatively large. Thus, the pipeline design against ice gouging load effects is likely to be governed by the maximum gouge depth, as the gap itself is found to define a sharp limit between an acceptable and an unacceptable design condition. In turn, this sets high requirements to reliable determination of the gouge depth.

Abstract Subsea pipelines located in ice environments need to be protected from potential ice gouging (also known as ice scouring) created when a moving ice keel interacts with the seabed. The integrity and operability of the pipeline can be affected by direct contact between the ice keel and the pipeline, or from loading imposed on the pipeline through soil deformation caused by ice gouging. The typical method considered for protecting against the risk of damage caused by ice gouging is through pipeline burial. Conventional methods of pipeline burial use equipment such as ploughs, mechanical trenchers, and jetters, the majority of which have been designed to accomplish a maximum of 2 to 3m of pipeline burial. Dredges can be used but they have water depth limitations and limited productivity. Land based equipment has been used for shore crossings, but is limited to shallow water depths where temporary construction berms can be used as a working platform. The capabilities of existing technologies are based on current industry practice developed for pipeline burial in ice free environments. Pipeline burial requirements have generally been for improved hydrodynamic stability, mechanical protection (e.g. from anchors), controlling pipeline movements (e.g. expansion and buckling) and/or for flow assurance purposes (reducing heat loss). A 2 to 3m burial depth has been generally adequate to satisfy these conventional requirements. As developments are proposed for areas that experience relatively deep ice gouging (up to 5m), burial depth requirements will exceed the capabilities of current pipeline burial technologies. New technologies capable of working in deeper water, achieving greater burial depths, achieving reasonable trenching advance rates, operating in harsh environments, and trenching through variable and difficult seabed soils will be required. This paper highlights the issues and challenges surrounding pipeline trenching and burial in ice gouge environments. Current state-of-practice is discussed along with the technology gaps that need to be addressed to prepare for future offshore developments in ice covered waters where there is the potential for seabed ice gouging.

Abstract With the oil industry's continued quest for oil and gas in frontier offshore locations, several developments have taken place in regions characterized by seasonal ice cover including the US Beaufort, North Caspian, and Sakhalin Island. In these projects, pipeline transportation systems have been used, which are a cost effective, safe and reliable mode of hydrocarbon transport to shore. Ice gouging is one of the key design issues that affect engineering considerations with respect to strain based design, target burial depth requirements, cost, and safety. It is generally accepted that offshore pipelines in ice environments will need to be trenched and backfilled for protection. Burial depths can be greater than those that might normally be required for pipelines in temperate climates. Burial depth requirements will be a function of the design ice gouge depth (to prevent interaction between the ice and pipe) and an acceptable level of subgouge deformation beneath a gouging ice keel (which potentially strains the pipeline). There still exists uncertainty on the magnitude and extent of subgouge soil deformations due to ice gouging and the importance of sediment transport mechanisms for biasing gouge statistics. Other challenges that must be considered include strudel and hydrodrnamic scour, thaw settlement and frost heave, and upheaval buckling. These considerations may also influence burial depth requirements.

Abstract There are many challenges associated with the design and installation ofArctic subsea pipeline. A leading example is the Northstar developmentpipelines currently operating safely offshore the Alaskan North Slope. Uniqueoffshore Arctic environmental loading conditions, such as ice gouging, influence each pipeline design differently. Statistical distributions andprobabilistic assessments of ice gouge records can be used to predict designextreme gouge depths which can then be used to determine pipeline burial depthsrequired for protection against ice keels. The Northstar subsea pipeline project used the statistical ice gougeanalysis method described by Lanan et al. (1986), based on the exponentialprobability distribution, to select design pipeline burial depths forprotection against ice gouging. This method was applied to publicly availabledata and site-specific survey data collected prior to the pipeline installationin 2000, to predict the design extreme ice gouge depths expected along thepipeline route. Each year since the pipeline installation, new ice gouge datahas been collected by BP Exploration (Alaska). This paper reviews additional ice gouge data collected since installation ofthe Northstar pipelines and has assessed the use of alternate ice gougeanalysis methods to predict extreme ice gouge design depths for future pipelineinstallations in the Beaufort Sea, Data available from all Alaskan Beaufort Seaice gouge surveys in the Northstar pipeline area was also included in some ofthe statistical comparisons. Results obtained using the exponential analysis method were compared toanalyses using alternate probability distribution functions (PDFs), such as theWeibull, gamma, and log-normal. Data thresholds have also been investigated forPDF fitting. Work by Caines (2009, 2011) has shown that alternate probabilitydistribution functions might be more appropriate for modeling ice gouge depthdata, compared to the traditional exponential method. A brief comparativeanalysis was conducted using known age Northstar pipeline route data toinvestigate the effects of using all available gouge depth data versus annualmaximums only.

Abstract The Pipeline Ice Risk Assessment & Mitigation JIP (PIRAM) developed aset of engineering models and design procedures for implementation intoindustry best practices for risk mitigation and protection of pipelineinfrastructure from ice keel loading. The models established the pipelinemechanical behaviour in response to ice keel load events, and assessedengineering concepts for protection and risk mitigation strategies. Improvedmethodologies for contact frequency and ice keel loads determination formedpart of the integrated model. Pipeline protection against ice gouging is overviewed. A review of subgougeresponse and physical model tests provided a basis for refinement of threedimensional continuum finite element analyses of steady state gouging includingthe implementation of an effective stress based soil plasticity constitutiveroutine. A fully coupled ice, seabed and pipeline interaction model is used tocalibrate a simpler pipeline design approach for design purposes. Thestructural model, improved by considering 3D interaction effects, comparesstrains within the pipeline to those from continuum analyses and from physicalmodel tests. The PIRAM pipeline model provides the engine for the probabilisticassessment of pipeline cover depth using a GIS-based decision-support-systemfor route planning.

Abstract Continuously increasing energy demand combined with depleting traditional fields have pushed the industry to explore oil and gas into frontiers such as arctic regions and ultra deepwater. These areas pose new challenges to the industry in all aspects of exploration and development. In the arctic regions, subsea pipelines must be designed to survive extremely harsh conditions such as low temperatures, thermally induced fatigue, and high stresses from ice gouging. In a study supported by a recent Ice Pipe JIP, ice-soil-pipeline interactions of buried pipelines were investigated by numerical simulations using select modeling techniques. This paper presents results from this study using the Coupled Eulerian-Lagrangian (CEL) technique, and attempts to identify the important interactions between the governing parameters. Ice gouging is a major subsea pipeline safety concern in the arctic regions, in which iceberg grounding can cause large soil movements around a buried pipeline inducing excessive deformation and high stresses, severely affecting its integrity. Pipeline design in arctic regions, therefore, must account for potentially serious effects of ice gouging. Current industry knowledge of the phenomenon is limited and the subject Ice-Pipe-JIP was an effort to help enhance the understanding. It is believed that the results presented here will help pipeline engineers understand the effects and interactions of ice gouging depth, angle and width, pipeline's burial depth and soil cover, etc in clayey or sandy seabed. The results will help contribute to increased confidence in the formulation and development of arctic pipeline design best practices for the subsea pipeline engineering community, going forward. Also, the paper demonstrates CEL-based finite element analysis (FEA) as a competent and dependable numerical tool for modeling, analyzing and studying soil-structure problems with characteristic extreme large soil deformations, which are generally unamenable to more traditional Lagrangian numerical techniques. Introduction Faced with ever increasing demand for oil and gas and with the traditional fields depleting [Ref. 1], the industry is putting increasing emphasis on exploration and development of frontier areas such as the arctic regions. The Arctic Circle contains 22% of the undiscovered, technically recoverable hydrocarbon reserves in the world, 84% of which is estimated to be in the offshore [Ref. 2]. However, many challenges exist with the exploration in the arctic offshore: harsh climate, remote location, limited daylight, ice cover, sensitive ecosystem, etc [Ref. 3], including ice gouging (or scouring). Ice gouging poses an important design consideration for any on-bottom pipeline or offshore structure in the arctic regions. In the arctic and sub-Arctic areas it is common to observe ice gouging such as in the shallow Beaufort Sea and offshore Newfoundland. Environmental forces drive ice features (icebergs or ice-ridges) that extend deeper than the water column imprinting scours in the seabed. Ice scouring can result in significant seabed surficial soil movements, and has the potential to cause devastating effects on buried seabed pipelines. The pipelines are thus designed to be buried below the mudline so that contact with the gouging ice is avoided. However, surrounding sub-gouge soil displacements can get transmitted to the pipeline inducing excessive deformation; therefore, accurate determination of sub-gouge soil displacements and the impact on the pipeline is critical for appropriate design of burial depth.

Abstract The BP Northstar, Pioneer Oooguruk and Eni Nikaitchuq pipeline projects provide a significant experience base for designing, installing and operating future offshore arctic pipelines. Each pipeline is located in the nearshore zone of the Beaufort Sea, offshore the North Slope of Alaska. Maximum pipe diameters range up to 18 inches, water depths to 37 ft and lengths to 6 miles. Unique offshore arctic environmental loading conditions influenced each pipeline design differently for defining the required pipe bundle configuration, thermal insulation and trenching requirements. Similar differences affected the winter construction procedures used for each project and would be expected to impact future pipeline projects in deeper waters. A total of 13 years of pipeline operational experience has been gained and key features such as routine seabed surveys, in-line-inspection pigging and effective leak detection systems are important to the continued safe operations of these pipeline systems. Introduction Development of offshore oil and gas reserves in Arctic and sub-Arctic regions of the world have progressed relatively slowly (INTECSEA, 2011). This slow pace results largely from the remote field locations, unique arctic environmental load conditions and the lack of existing hydrocarbon export facilities. Subsea pipeline systems are often needed to connect offshore fields with onshore pipeline networks, for intrafield flowlines or for export tanker loading lines. Safe and economical design, construction and operational procedures are needed to address the unique conditions encountered for offshore arctic pipelines. Three pipeline systems have been installed in the Beaufort Sea, offshore the North Slope of Alaska for BP's Northstar, Pioneer's Oooguruk and Eni's Nikaitchuq field development projects (Figure 1). The Northstar and Oooguruk projects have been operating for a combined total of over 13 years and the Nikaitchuq project is scheduled to start operations during 2011. Each of these offshore arctic pipeline systems transports oil and gas from gravel island structures to shore. This Beaufort Sea pipeline experience can be applied on future Arctic and sub-Arctic projects having first-year sea ice, multi-year ice or iceberg load conditions. Seasonal sea ice is found at varying latitudes in both marine and inland lake locations, not just north of the Arctic Circle (or surrounding Antarctica). Other potential applications of Beaufort Sea pipeline experience include projects having complex thermal interactions with the local environment and offshore arctic projects requiring conventional marine construction procedures during the short Arctic summer season. Additionally, pipeline operations, maintenance and potential repair procedures form an integral part of an economical, safe and reliable offshore arctic pipeline system. Beaufort Sea Environmental Conditions The nearshore Alaskan Beaufort Sea ice typically breaks up or melts in place starting in June. Summer open water conditions then prevail from late July through mid-October. The summer polar pack ice edge recedes varying distances offshore based on the site location, water depth and year to year variations or long-term climate trends. Even during the open water season, however, ice floes are often present and their movement is influenced by winds and currents. Along much of the Alaskan North Slope coast, low barrier islands prevent multi-year polar pack ice from entering the nearshore lagoon areas. Ice can affect offshore marine operations outside the barrier islands at any time during the summer.

Abstract Ice loads on buried pipelines through ice-soil-pipeline interaction is a field with limited industry experience. As exploration in Arctic and cold climate regions increases it is of great importance to understand and to design for the challenges offshore pipelines must be able to withstand in these harsh areas with a suitable level of confidence. Together with thirteen industry partners, DNV has carried out a joint industry project in order to better define and therefore understand the Arctic specific challenges to pipelines. This project has developed initial industry guidelines for offshore pipelines in cold climate regions, in general and for ice scour in particular. A major design issue for Arctic offshore pipelines in shallow water is ice keels gouging the seabed and imposing loads on the pipeline through ice-soil-pipeline interaction. The industry has somewhat limited experience with deep ice gouges and ice keels interacting with buried pipelines. A common practice has been to bury the pipeline well below the deepest registered gouge mark in the relevant geographical area with the objective to limit the pipeline response. In order to render future projects in regions with severe ice gouges economically feasible whilst still being sufficiently safe there is a need to utilise modern design principles based upon statistical methods, advanced FE models calibrated to ice gouging tests, limit state design principles and strain based design. The ICE PIPE JIP provided the introduction of a more systematic approach for the identification, quantification and ranking of design uncertainties, and how this could be adopted to existing pipeline design methodologies and criteria. This paper presents some pertinent results and recommendations from the ICEPIPE JIP. The paper presents a sample case study for how the assessment of some elements of statistical uncertainty can be used in the definition of the 100 year return period ice gouge depth. This paper highlights one methodology for approaching this and it is emphasised that not all the steps that would be required in design are included in this brief paper. A discussion on the use of advanced FE tools in the ICE PIPE JIP is also presented.