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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.

Acknowledgements
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).

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

posted by The Rogtec Team @ 15:40 

2 Comments:

 The Rogtec Team said...

such a usefull studycase of pipeline.
especially for those who studying in the same orientation course program.

i used to learn pipeline and offshore structure at my college, so..
if you don't mind,(subscriber),..
would you mind to send me another paper/journal/news/studycase,tec that related to pipeline..

sincerely yours..
Mr. Yuangga

28 April 2009 10:38  

 derek said...

I'm currently doing a thesis for a university/company titled the "fitness-for-purpose" assessment of damaged subsea pipelines using FEA from data provided during in-line pipeline inspection and photogrammetric techniques, if anyone else is doing something similar or think they can help me in this area, please do drop me an email, dezzy67@hotmail.com.

21 December 2009 07:39  

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