Oil & Gas News
Wednesday, 11 March 2009
The use of Advanced Finite Element Analysis Tools for the Design & Simulation of Subsea Oil & Gas Pipelines & Components. Part 2
Pipeline Analysis for Arctic Applications
Analysis of pipelines in arctic conditions requires specialized considerations that do not apply to non-arctic fields. These considerations include loading conditions such as those associated with strudel scour, permafrost thawing and ice gouging. Analysis of upheaval buckling and free spanning associated with strudel scour is rather similar to that for non-arctic applications; the main difference is, however, in deriving the problem parameters (such as the free span length).
Analysis of the pipeline for settlement due to permafrost thawing involves a coupled or an uncoupled form of the following analyses: conduction and convection of the heat from the pipeline to the soil and permafrost; volume change due to the phase change of the permafrost ice; and soil consolidation and resulting settlement. A wide range of sophistication and degree of coupling of these analyses has been proposed, while development of a standard approach to such analysis is still underway. Finite element is usually used for the thermal analysis, with the volume change and consolidation commonly performed with simplified methods. Finite element, however, has been used to perform all the three types of analysis.
Analysis of ice gouging of arctic seabed and resulting sub-gouge soil (and buried pipeline) deformation is performed usually using either of two approaches.
The first models the seabed soil as nonlinear springs that deforms under the iceberg/ice ridge pressure and that transfers the load to the buried pipeline. The more sophisticated finite element approach involves modelling the seabed soil with an Eulerian mesh, and the less deformable objects, such as the pipeline, are modelled with a Lagrangian mesh. This "Coupled Eulerian-Lagrangian" (CEL) formulation has the advantage of being able to model the extreme soil deformations involved, while keeping good track of the stresses in the pipeline. An example of such analysis output is shown in Figure 9.
Figure 9: Sub-gouge Deformation derived using Coupled Eulerian-Lagrangian Finite Element Formulation
Local Finite Element Analysis of Subsea Components
ABAQUS is also used to model subsea pipelines and components such as Bulkheads, Flanges and Riser clamps. A typical pipe-in-pipe bulkhead is shown in Figure 10 and this is constructed using ABAQUS CAE.
Using ABAQUS the component can be loaded with pressure, temperature, and the structural response can be obtained. Macros are used to extract stresses, and to then split them into bending, membrane, and membrane and bending stresses. Appropriate code checks are then undertaken. The analysis can also be used to assess the stress loading in the girth welds as shown in Figure 11.
Figure 10: Complex Solid Modelling of Pipeline Bulkheads
Figure 11: Detailed Modelling of Girth Welds
Solid FEA can also be used to design subsea components such as clamp-on buckle arrestors which are proposed to be used for reel lay as shown in Figure 12. FEA allows to study the phenomena of collapsing/propagating of the pipe-in-pipe flowline, and to investigate the effectiveness of clamp-on buckle arrestor for deep water flowlines. Sensitivities of key design parameters can then be explored with the purpose of guiding detail mechanical design of the clamp-on buckle arrestor.
Figure 12: Clamp-on Buckle Arrestors
Micro Modelling (Sub-Modelling)
The adoption of sub-modelling can be used, to analysis the stress loading in the girth welds as shown in Figure 13 and Figure 14. The effects of radial misalignment, flaw defects, and linking this to Engineering Criticality Assessment (ECA) can also be undertaken. The use of a FEA as a design tool is very powerful, and allows one to address very complex issues at a micro level.
Figure 13: Detailed Modelling of Weld Detail with Radial Misalignment
Figure 14: Detailed Weld Geometry using the Sub-Modelling Technique
An Integrated Approach to Pipeline Route Selection
The routing of subsea oil and gas pipelines and flowlines pose particular challenges. Routing is undertaken by integrating third party software with 'Simulator' stress analysis tools, as shown in Figure 15.
Figure 15: Flowline Routing using 3-D Software
The compiled 3-D model is used to plot existing and possible flowline routes. The seabed slope, elevation, and profile can be analyzed while plotting, leading to route optimization. The flowline route XYZ coordinates are directly extracted from Fledermaus and used to create finite element models in ABAQUS for detailed span analysis. The coordinates may also be used to create flowline alignment drawings. A typical flowline profile is shown in Figure 16.
Figure 16: Route Profile
Current flowline routes from the Subsea Field Layout Drawing can be plotted in the 3-D model and analyzed, alternate routes can be identified and studied. The model can be complied from two different sets of survey information if required, for example, data sampled at 3-meter and 15-meter intervals. When selecting optimized paths the routes are preferred to stay in the detailed survey data set (3-meter) but other options which fall outside detailed survey data can be used. Figure 17 shows an example of a 3-D model, and indicating initial route slopes.
Figure 17: Initial Route Slopes
The 3-D model is assembled using the survey data and color maps are applied to visually describe the information. The model can be analyzed using color maps describing elevation and slope. The slope color map gives an enhanced perspective of the seabed floor easily highlighting avoidable and problem areas. The model shaded with elevation is show in Figure 18. The elevation color shades the model based on water depth with the minimum in pink and the maximum in dark purple. This color map can be used to analyze the model and plot the possible alternate routes. The slope legend and example profile are also shown in Figure 18. Slopes are in decimal degrees.
Figure 18: Slope Color Map Model
The models are viewed with a vertical exaggeration factor of 6 to clearly show avoidable areas, without amplifying the rate of change it would be difficult to pinpoint problem areas. Once the route has been optimized, it is exported into ABAQUS, and pipeline expansion and lateral buckling analysis can be undertaken to ensure a safe and robust design.
An optimized route using 3-D software, integrated with stress analysis, will allow significant financial savings. 3-D visualization provides significant benefits in understanding the seabed morphology and is the preferred choice for engineering applications. This methodology has been recently adopted on a project in Indonesia.
J P Kenny would like to thank all who participated in providing information for this paper. A special thank you is given to Dr. Kuka Kukathasan, J P Kenny Ltd (London Office).15:40 2 Comments
Wednesday, 3 December 2008
The use of Advanced Finite Element Analysis Tools for the Design and Simulation of Subsea Oil and Gas Pipelines and Components
Paul Jukes PhD CEng FIMarEST Ayman Eltaher PhD PE James Wang MSc Billy Duron BSc J P Kenny, Inc. Houston, TX USA
USE ADVANCED FINITE ELEMENT ANALYSIS?
The world is consuming oil and gas at an ever increasing rate and, as a result, there is demand to exploit new opportunities and make projects that were once not technically or commercially feasible, now viable in a cost-effective manner.
Technology gaps exist and the inability to bridge that gap, due to technology either being unavailable or just too expensive to implement, has put some projects 'on-hold' for many years. A number of engineering challenges, or technology gaps, have been identified that has a significant impact on the design of oil and gas pipelines, and subsea equipment [Ref. 1]. The main technical challenges that exist are identified as; deep water, high pressure/high temperature, flow assurance, and thermal buckle management. It is now common place for subsea systems and pipelines to be installed in water depths in excess of 1,000 meters (3,300ft). Flowline designs are presently being considered, for a major operator, in the Gulf of Mexico (GoM) for water depths down to 3,000 meters (10,000ft). High Pressure and High Temperature (HP/HT) with pressures in the order 700bar (10,000PSI) or more, and temperatures being considered up to 160°C (320°F) are not uncommon. For a major operator in the GoM, temperatures up to 177°C (350°F) are presently being considered. This can present real design challenges in the choice of materials and in the design methodology. Stress based design codes are no longer applicable at these high temperatures, and the solution is to design such pipelines using a limit state methodology.
Routing and Survey
The routing of subsea oil and gas pipelines and flowlines pose particular challenges. The importing of 3-D survey data efficiently is a numerical challenge and can be computational expensive to undertake. Routing can have significant financial benefits if the length of the pipeline is reduced, and it can also minimize undue bending and stress on the pipeline if the pipeline is re-routed around onerous undulating seabed, rocks, or imperfections. Importing survey data into a 3-D visualization tool quickly, and efficiently, is key to success. Generally, routing is undertaken by integrating third party software with FE analysis tools, as will be demonstrated within this paper.
In recent times, there has been a greater requirement for pipeline and subsea design companies to tackle these engineering challenges in a cost-effective manner. One such way is to use more advanced analysis tools, such as finite element analysis to model, simulate, and design both pipelines and subsea components. This will allow designs to be optimized with a greater understanding of the Engineering complexities, without having to undertake expensive large scale tests, and hence a viable, workable, solution can then be obtained.
An optimized design will provide 'added-value', and ultimately provide capital cost savings to a Project. The detailed factors for a successful project have been identified [Ref. 1] as the following:
Advanced Finite Element Analysis can be used to undertake global modeling of pipelines, and this allows the simulation and response to be obtained. This is a highly non-linear process, due to material non-linearity, large displacements, and pipe/soil interaction. This type of analysis can be used to undertake span analysis, lateral buckling and reeling analysis. Examples of these are described within the following sections of this paper. Non-linearities can be a particular issue when designing pipelines at high temperatures, stress based design can no longer be used, and a limit state based design is adopted.
Secondly, Finite Element Analysis can be used to undertake 'local' solid modeling of complex subsea components such as; bulkheads, flanges, field joints and spiral pipe. Constructing FE models efficiently, and quickly, is key and it allows design iterations to be efficiently undertaken to allow an optimized design to be achieved.
Thirdly, an 'integrated' approach to route selection, using 3-D software and stress analysis, to reduce pipeline length and minimise intervention is important. If the survey data, route selection and stress analysis can be undertaken quickly and efficiently, this will allow design iterations to take place in a cost effective manner. Time spent at this iterative design stage, when undertaken efficiently, could then have a significant financial saving in terms of the
Through advanced analysis tools, the challenges of deepwater and HP/HT can be addressed by integrating analysis tools with pipeline design methods, such as Limit State Based Design (LSBD). Pipelines are designed using this approach, and optimised wall thicknesses can be obtained, and this then allows significant financial savings, in linepipe costs, if undertaken correctly.
To accurately model and predict the ultimate failure of a pipeline requires looking at the limit states so as to gain an adequate margin of safety between the design loads and ultimate failure. The major target is to investigate the ultimate limit states, and a FE model is used to provide all of the pipeline response data as input for each limit state.
ADVANCED PIPELINE ANALYSIS AND DESIGN TOOLS
The key to undertaking complex designs of pipeline systems is to use advanced analysis tools. These analysis tools can undertake global modeling of pipelines, local modeling of subsea components, and micro modeling of pipeline welds. Examples of these different types of FE modeling is described in the following sections.
1. Global Modeling
The wide range of proprietary advanced FEA tools that allow the accurate prediction of pipeline responses, which has been developed, is called 'Simulator'. The FE engine is the commercial software ABAQUS [Ref. 2]. The models include elasto-plastic materials, 3-D route geometry, peak, and residual modeling of axial and lateral soil pipe forces. Pipe-in-Pipe (PIP) and single pipe models have been developed. Each model is fully checked and validated. Many of the models have been benchmarked against observed pipeline behavior. The 'Simulator' analysis is a static large deflection analysis and includes all relevant non-linearities such as large deflection and large rotations, elasto-plastic pipe materials interpolated over relevant temperature ranges, and non-linear pipe-soil interactions.
Tools have been developed that undertake the following design activities;
The use of 'Simulator' during the design stage allows Limit State Based Designs, and allows the following to be undertaken;
Detailed Description of 'Simulator'
The model runs using ABAQUS [Ref.2] and is designed to analyse the initial, prior to the moment of instability, and post lateral buckling behaviour, and expansion behaviour of straight, single pipe-in-pipe system flowline lying on a flat seabed. This model is applicable for shallow or deepwater condition and/or a HTHP PIP system. The modules can perform parametric studies if required, by simply changing the input parameters of the input script code.
Upon completion of a single analysis, the following results can be presented:
The friction between the pipeline and the seabed is one of the factors affecting the buckling performance. A friction model, that uses an ABAQUS user subroutine has been developed, and enables non-linear axial and lateral friction to be defined, as shown in Figure 2.
Figure 2: An Example of a Friction Model Adopted
The form of the friction-slip subroutine is similar in both axial and lateral directions. Starting at the origin O, the friction value starts to increase until a peak is reached at point A. Further slip is undertaken with decreasing friction values until a residual value of friction is reached at point B.
The seabed friction dominates the boundary conditions to the pipeline. However, connectors aligned with the pipeline at either end of the pipe are specified to simulate weak springs to remove any potential singularities before friction begins to act.
The global modeling of pipelines has been used on a number of Projects, and also used for undertaking detailed studies. Such studies include an investigation into 'Strain Localisation' [Ref. 9] and the analysis of 'Loadshare' components [Ref. 11]. Some global model examples are presented in the following sections.
Global Model Example: Pipeline Walking
The conventional expansion response of a short flowline involves a virtual anchor point close to the centre of the line and expansion from this anchor towards the ends of the flowline. After early start-up/shut-down, the cyclic expansion is of constant amplitude. Flowline walking can occur for short free-ended flowlines subject to a high thermal cyclic loading. If startup/shut-down cycles involve significant thermal gradients then axial ratcheting of the flowline can occur, with displacements toward the cold end. Over a number of cycles this movement can lead to very large global axial displacement with associated overload of the spool piece or jumper if any. This cumulative axial displacement is described as 'Pipeline Walking'.
The key to this phenomenon is the transient thermal profile developed during heat-up, as shown in Figure 3.
In high pressure flowlines the internal pressure is almost enough to mobilize the friction force over the whole flowline. For this reason the pressure in the analysis is kept constant. A typical pipeline walking response, for a number of start-up and shut-down cycles, is shown in Figure 4
Figure 4: Pipeline Walking Displacement Rachetting11:30 0 Comments
The result of pipeline walking is that the flowline can ratchet across the seabed, and hence overstress any connecting jumper, or structure, at the end of the flowline. The FE analysis tool allows the adequate simulation and prediction of this phenomenon so that design remediations can take place. The modeling of this pipeline walking effect would not be easily possible without such FE models.
Global Model Example: Integrated Reeling and Lateral Buckling Response
The reeling installation process of PIP systems, see Figure 5, produces residual loading in both the inner and outer pipes which need to be taken into account in any subsequent lateral buckling analysis. The residual loads could have a subsequent effect on the ultimate limit state capacity of the inner pipe, when temperature and pressure is applied, during the operational phase.
Figure 5: Key Components of the Reeling Process
A recent model has been developed that is a sequential integrated reeling and lateral buckling PIP FEA, which captures the full reeling history, and is then included in the operational analysis for lateral buckling [Ref. 3]. Figure 6 shows a typical PIP reeling response.
Figure 6: PIP Reeling Response
A full reeling analysis is particularly complex to undertake, due to convergence issues, but these issues have been addressed. Using this reeling module, results show that the effects of reeling should be taken into account for high temperature pipelines as it can reduce the ultimate loading capacity. A typical lateral buckling response is shown in Figure 7
Figure 7: Pipeline Displacement under Lateral Buckle
Global Model Example: Assessing Free Spans / Vortex Induced Vibrations (VIV) / Multi-Span Analysis
A FE model has been developed to undertake span analysis, in accordance with the latest version of DNV-RP-F105, 2006 [Ref. 4], which takes into account the complicated scenario of interacting spans. An initial assessment of the spans from the survey data is performed, followed by static and dynamic Ultimate Limit State (ULS) checks. A Vortex Induced Vibration (VIV) screening analysis is then conducted to determine maximum allowable span length limits for in-line and cross-flow directions under both current and wave conditions. Finally, a fatigue analysis is performed on the spans that exceed the allowable span length limits. A Finite Element Analysis (FEA) model is used in the analysis to determine natural frequencies, unit stresses and mode shapes. Figure 8 shows a typical mode shape for interacting in-line VIV. The results of the analyses provide efficient solutions to the field in terms of mitigation management for existing pipeline or new pipeline design.
Figure 8: Mode Shape – In-line
In the screening analysis, the onset screening criterion and fatigue screening criterion are used to applicable code requirements regarding acceptance criteria from DNV-RP-F105, 2006 [Ref. 4]. In the fatigue analysis, the fatigue life is determined using equations for both response and force models as defined in the Code.
A proposed methodology [Ref. 5], that has been developed, includes the following key areas: assessment of the field data, ULS check, screening analysis, fatigue analysis, and FEA modeling. The methodology has been used on real projects in various scenarios, yielding the following main conclusions: