Ayman Eltaher

Extra High-Pressure High-Temperature (XHPHT) Flowlines: Design Considerations and Challenges

Development of deep water oil reservoirs in the Gulf of Mexico may encounter conditions where the flowline product temperatures approach 177°C (350°F), water depths range to 3000 m (10,000 ft), and tie-back distances up to 40 miles are presently being considered. These high flowline temperatures, water depths and distances, present real challenges to the design of flowlines. The objective of this paper is to present the design considerations and challenges of designing for extra high pressure high temperature (XHPHT) conditions. For such conditions, a pipe-in-pipe (PIP) flowline system with thermal expansion management, and a limit state-based design are viable solutions. This paper is split into three main parts and covers (i) design challenges and how they are overcome, (ii) finite element analysis design methods, and (iii) qualification testing of PIP components. The first section presents the main design issues, and challenges, of designing flowlines for deepwater and high-temperature conditions. The paper discusses aspects of controlling the large axial loads, such as thermal expansion management using buckle initiators and end constraints for flowlines, and presents current methods. The second section describes the use of advanced finite element analysis (FEA) tools for the design and simulation of PIP systems, and presents local and global FEA models, using ABAQUS, to investigate the limit state design of XHPHT flowlines. A 3-D helical response of the inner pipe subjected to high temperature, and the sequential reeling and lateral buckling of flowlines is also discussed. The final section of the paper describes the qualification testing to be undertaken on PIP components to ensure structural integrity and long-term thermal and structural performance. Qualification testing for PIP components for 177°C (350°F) service is discussed, and includes the testing of centralizers, waterstop seals, thermal insulation and loadshares. This paper is based on both theoretical and practical research work.Copyright

Development and Validation of a Coupled Eulerian Lagrangian Finite Element Ice Scour Model

Pipelines in the arctic offshore must be installed and buried below the seabed to avoid direct contact, and to mitigate the effects of strains induced by soil displacement below the ice keel scour depth. A three-dimensional (3D) finite element (FE) model that utilizes the Coupled Eulerian Lagrangian (CEL) formulation has been developed to provide direct and explicit estimation of pipe stresses and strains. The CEL formulation is novel, and no published work has attempted to explore its capabilities and potential for ice scour modeling to date. The developed model will be helpful in solving some of the uncertainty regarding pipeline burial depth, potentially resulting in major trenching cost savings. In order to gain confidence in this numerical modeling technique, a systematic validation effort was carried out, whereby numerical predictions of subgouge displacements were compared with measured data from centrifuge tests and other published empirical and numerical data. Sensitivity analyses were then performed to investigate the effect of the scouring keel geometry, depth, and attack angle on the induced subgouge soil displacements. Preliminary conclusions were drawn and presented in this paper.Copyright

Permafrost Thawing-Pipeline Interaction Advanced Finite Element Model

The increasing energy demand has promoted the interest in exploration and field development in the Arctic waters, which holds one quarter of the world’s petroleum reserves. The harsh conditions and fragile environment in the arctic region introduce many challenges to a sustainable development of these resources. One of the key challenges is the engineering consideration of warm pipelines installed in permafrost areas; found mainly in shallow waters and shore crossings. Evaluations have to be made during the pipeline design to avoid significant thaw settlement and large-scale permafrost degrading. In this paper, a three-dimensional (3D) finite element (FE) model was developed to study the interaction between buried pipelines transporting warm hydrocarbons and the surrounding permafrost. This interaction is a combination of several mechanisms: heat transfer from the pipeline, results in permafrost thawing and formation of thaw bulb around the pipeline. Consequently, the thaw settlement of soil beneath the pipeline base results in bending strains in the pipe wall. For safe operations, the pipe should be designed so that the induced strains do not exceed the ultimate limit state conditions. The developed model helps in accurate prediction of pipe strains by using finite element continuum modeling method as opposed to the more commonly used discrete (springs) modeling and hand calculations. It also assesses the real size of the thaw bulb and the corresponding settlement at any time, thus preventing an over-conservative design.© 2009 ASME

Offshore Pipeline Embedment in Cohesive Soil: A Comparison Between Existing and CEL Solutions

A common issue confronted by engineers in analyzing high pressure high temperature (HPHT) pipelines for installation and operating conditions is pipe-soil interaction. For installation, a key concern is whether the soil can generate sufficient resistance to allow the pipeline to be laid on the curve. For operation, a concern is whether the pipeline structural stress can be controlled and mitigated, for the given soil condition, under conditions of thermal expansion and potential global buckling. In both scenarios, pipeline embedment is a critical parameter as it is directly related to soil resistances to the pipeline stability. Previous studies have used experimental, analytical and numerical methods to provide estimates to the pipe embedment during the laying operation. The recently developed Coupled Eulerian-Lagrange (CEL) finite element analysis (FEA) method provides a promising numerical technique in analyzing large-deformation geotechnical problems, such as pipeline embedment analysis. This paper uses this approach, together with currently available embedment solutions, to cross-validate these methods for cohesive soils.Copyright

The technical challenges of designing oil and gas pipelines in the arctic

The world demand of oil and gas is growing at an ever increasing rate, and as a result, there is a demand to explore new areas for more petroleum production. The arctic region is one of the remaining unexplored areas where such exploration still can be done. According to the US Geological Survey estimates, the arctic region, mostly offshore, holds as much as 25% of the worlds untapped reserve of hydrocarbons where much of the reserve is lying under seasonal or year-round sea ice. The exploitation of these remaining reserves, however, will depend upon meeting the technical challenges of design, construction, and operation of offshore installations. Despite some experience with Arctic oil and gas exploration and production during the last three decades, technology gaps still exist and will have to be bridged in order to enable optimized developments to proceed. In this paper, technical design difficulties particular to arctic pipelines are presented; these include ice gouging, permafrost thaw settlement, strudel scour, and upheaval buckling. An emphasis is then placed on advanced finite element techniques that can be used to address these issues, with an example of such techniques illustrating their ability to model highly complex and nonlinear phenomena.

Flow Induced Forces on Multi-Planar Rigid Jumper Systems

Rigid subsea jumper systems are typically used as interface between subsea structures and are required to accommodate significant static and dynamic loads. Due to constraints imposed by in-line planar jumpers (e.g. U shaped and M shaped jumpers), the industry is shifting towards the use of multi-planar jumper systems (e.g., Z-shaped jumpers). These multi-planar jumper systems have increased tolerance to end displacements and can be tailored to accommodate cyclic end motions of subsea structures. Multi-planar systems, however, come with unique challenges of their own including the coupling of flexural and torsional responses under vortex induced vibrations (VIV), fluid induced vibration (FIV) and slugging. In particular, the development of hydrodynamic slug flow is a common occurrence in oil and gas pipelines. It is understood to be initiated by instabilities of wave on the gas-liquid interface. It is also understood that slug flows are the source of vibration within pipework when a change of direction occurs e.g. 90° bend at a subsea riser base or top side piping. In standard slug flow vibration analysis, averaged slug frequency and length are used to calculate the force generated. In the case of a multi-planar rigid jumper, several changes of direction occur within a short length of pipe. After each bend the characteristics of the slug flow are modified. It is necessary to accurately capture these changes in order to reproduce the forces generated at critical points along the jumper length. This paper presents a methodology for analyzing slugging induced fatigue that has been developed in an on-going study undertaken by MCS Kenny for design of multi-planar rigid jumper systems. In this methodology, Computational Fluid Dynamics (CFD) is used to accurately simulate the flow within the jumper and provide pressure fluctuations on the internal pipe wall for the vibration analysis. The pressure fluctuations are then incorporated in a Finite Element (FE) model of the jumper system and further used to determine the slugging fatigue damage. CFD (Star-ccm+) and FE (Flexcom, ABAQUS) software programs are used to accurately capture the response of the jumper system. Key conclusions and challenges overcome during the course of this study are presented herein.Copyright

Extended Porosity Rate Function for Frost Heave

Frost heave is a common phenomenon in the Arctic, where soil expands in the direction of heat loss due to ice lens growth upon freezing. It also occurs if a refrigerated structure is buried in unfrozen frost heave-susceptible soil, and thus special considerations are required when designing chilled or LNG pipelines in the Arctic. In the past decades, many theoretical and numerical methods have been developed to predict the frost heave of freezing soil. Among them, the rigid ice model, segregation potential model, and porosity rate function model are the most popular. These frost heave models work well in predicting the soil response during a pure freezing process, but none of these methods consider a thawing and consolidation of soil, which is the opposite but integrated process when the system undergoes the annual temperature cycle.In this study, efforts are made to extend the porosity rate function to the thawing branch based on reasonable assumptions. With the extended model, a fluctuating surface temperature can be applied on top of the soil surface to simulate a continuous changing ambient temperature. The extended model is realized in ABAQUS with user defined subroutines. It is also validated with test data available in the public domain. As an application example, the extended model is utilized to simulate a chilled gas line buried in frost-susceptible soil to estimate its frost heave over a multi-year operation.Copyright

Vibration Assessment Methodology for Subsea Pipework

Vibration is an important cause of fatigue damage in subsea piping systems (jumpers, rigid spools, tree pipework etc.). Presently, guidelines are available for a qualitative and quantitative vibration assessment for process pipework, but these are more applicable to topside and refinery piping. Currently, there are limited guidelines specifically developed for vibration assessment for subsea piping. For subsea pipework, additional guidelines may be necessary to check for possible interaction between current induced VIV, wave induced VIV (for shallow waters) and slugging, and other mechanisms that result in flow induced vibration. This paper provides augmented guidelines for subsea piping, which takes into account the interaction between piping and its subsea environment. At the same time, suggestions are provided to trim the guidelines of mechanisms that does not occur subsea. The Likelihood of Failure (LOF) assessment based on the proposed guidelines provides an indication whether further detailed vibration analysis (such as CFD and/or FEA) is necessary. Based on the LOF from this assessment, suitable detailed analysis can be performed to determine the fatigue life. This enhanced methodology enables the user to perform vibration assessment and analysis in the least computational time by identifying 1) the most critical flow conditions over the production life of the field and 2) the most critical vibration mechanisms that should be analyzed. This results in a reduction in the number of conditions or load cases which require detailed analysis, without decreasing the accuracy of the assessment and analysis.Copyright

The Span Mitigation Analysis With Use of Advanced FEA Modeling Techniques

Spans occur when a pipeline is laid on a rough undulating seabed or when upheaval buckling occurs due to constrained thermal expansion. This not only results in static and dynamic loads on the flowline at the span section, but also generates vortex induce vibration (VIV) which can lead a fatigue issue. The phenomenon, if not predicted and control properly, will result in significant damage to the pipeline integrity, leading to expensive remediation and intervention works. There are various span mitigation methods in use for both over stressing and fatigue concerns. The mitigation methods, if not analyzed properly, may result in much unnecessary work or generate more problems or concerns in the future. The mitigation analysis can become very challenging due to many restrictions in the field such as the minimum and maximum heights or lift of mechanical supports or grout bags, and bearing capacity vs. cost of supports. The cost of different mitigation methods and their interactions are the other considerations along with the installation tolerances, challenges associated with the water depth and uncertainties in seabed properties. This paper describes the latest developments in use of finite element analysis to investigate associate mitigation solutions given the governing practical limitations and cost factors. The ULS and fatigue lift criteria are used as the guidelines. The methods presented within this paper are applicable for various span conditions. Conclusions are then drawn to the impact of these various scenarios so that the pipeline integrity can be assured with confidence.Copyright

Warm Pipeline in Permafrost: A Sensitivity Study of the Major Thermal Properties

Pipelines carrying warm contents through permafrost areas present a key challenge to the development of petroleum reserves in the Arctic region. Evaluations have to be made during the pipeline design to avoid signification thaw settlement and large-scale permafrost damage. Current practice in estimating pipeline thaw settlement is primarily based on the assumption of a complete consolidation of the thaw-unstable layer. This often results in an over-conservative estimate and costly over-design. An integrated three-dimensional (3D) finite element (FE) model, developed recently as part of J P Kenny’s in-house simulator tools, has been used successfully to simulate the thermal and mechanical interaction between the pipeline and surrounding permafrost and gives a better estimate of the heat transfer, thaw settlement and pipeline deformation over the pipeline’s service life. Given the lack of experimental data and the complex nature of this problem, the sensitivity study on the major variables, presented in this paper and based on the 3D FE model, offers an in-depth insight into the problem and provides better-based guidelines for proper design of pipelines in permafrost. A number of thermal parameters are studied in this paper and compared based on their impact on the final deformation of the pipeline, so as to identify key parameters in the pipeline-permafrost thaw settlement processes. These parameters include the hydrocarbon content temperature and the convection coefficient, inside the pipe; the thickness and thermal conductivity of the insulation layer, on the pipe exterior; and the initial temperature, soil void ratio and the surface condition of the permafrost/soil, as boundary conditions. Results of current study improve the understanding of the pipeline-permafrost interaction from the heat transfer perspective and provide better guidance to the pipeline design in the permafrost environment.© 2010 ASME