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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.

Technical Challenges
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 Stress based design codes are no longer applicable at these high temperatures, and the solution is to design such pipelines using a limit state methodology.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.

Viable Solution
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.

Cost Savings
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:

• Innovation: Novel ideas, lateral thinking, and original ideas;
• Advanced Numerical Tools: 'Bespoke' software, path dependent, highly non-linear;
• Competency: Range of backgrounds, highly qualified, years of experience.

These factors allow you to drive down to an optimised solution, and experience from a project is then feedback into the next one, and this allows projects to deliver and evolve. By knowing what you are doing, through the use of analysis tools and experience, 'added-value' can then be given to a project. It is the combination of all three elements that allows design solutions to be developed, optimised, and then achieve cost savings.

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

Engineering
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;

• Upheaval Buckling;
• Lateral Buckling;
• Single pipe and PIP response;
• Reeling Analysis;
• Pipe/Soil interaction;
• Expansion/Span Analysis;
• Ice Scour/Pipe/Soil interaction.

These tools allow the simulation of the pipeline response and the prediction of possible buckles in the pipelines, as shown in Figure 1. This is particularly important for high temperature pipelines.


The use of 'Simulator' during the design stage allows Limit State Based Designs, and allows the following to be undertaken;

• Change and optimize the design;
• Undertake a range of sensitivities;
• Simulate Pipeline response, displacements and expansion;
• Obtain forces, moments and stress/strain.

The design can be iterated and, through the adoption of limit states, the design can be optimized resulting in possibly significant financial savings.

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:

• Submerged Weight;
• DNV Load Controlled Utilization (if required);
• Axial, Lateral Movement;
• Effective Axial Force;
• Axial and Hoop Stress;
• Von Mises Stress;
• Bending Moment;
• Plastic Strain, Buckling Curvature.

The FE elements used are PIPE31H, which are the hybrid formulation pipe elements within ABAQUS/Standard. These elements are selected, as they are particularly well suited to modeling long, slender pipelines with better convergence behaviour than the standard pipe elements.

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.



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

Labels: Advanced finite element analysis tools, Arctic conditions, modelling, Pipeline Analysis, Russia

posted by The Rogtec Team @ 11:30 

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