Experience that DeliversExperience that Delivers

Advanced post-processing of FE Analyses: 3D sub-modelling for subsea pipelines

Graham Viecelli

AOG Focus on TechnologyPerth Convention & Exhibition Centre, 20th February 2014

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Sub-modelling for pipelines

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• Identify the peak stress & strain (tensile and compressive) plus stress ranges at the girth weld – governing location for design

• Account for:– All potential temperature and pressure loads during installation, pre-start-

up, operation and decommissioning– Coatings, field joints, CRA clad or liner material– In-line structures (TEE, WYE, reducer, valves, anchors etc) – External factors (pipe-soil interaction, trawling interaction, counteracts,

other pipelines, anchors, dropped objects)– Any other case-specific issues that influence the pipeline structural

response

Subsea pipeline design

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• Simple geometry – a pipe is just a cylinder after all!

• Homogeneous material – only the steel pipe is structural

• Flanges, TEE’s, WYE’s, buckle arrestors, reducers only affect the submerged weight

• Stress and strain concentration can be accounted for later by applying factors

Design assumptions

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Numerical representation of a pipeline

Element features• Based on Timoshenko ‘shear flexible’

non-linear beam theory

• Consists of 2 nodes with 6 degrees of freedom, a single integration point and at least 8 section points around the pipe circumference

• Fully validated element response against numerous experimental tests within the limits of non-linear beam theory

PIPE31H beam element

Nodes

Integration PointSection Points

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3Illustration of a PIPE31H beam element indicating the location of the integration point and section points

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• Simple geometry – a pipe is just a cylinder after all!– What about the weld? Field joints? Misalignment or ovality?

• Homogeneous material – only the steel pipe is structural– What about CRA liner/clad, thick insulation or CWC?

• Flanges, TEE’s, WYE’s, buckle arrestors, reducers only affect the submerged weight– Thick, dense structures are also incredibly stiff…

• Stress and strain concentration can be accounted for later by applying factors– True, but what about ovalisation or local deformation

› Will high bending strains affect the fluid flow? › Can you pass a pig through the pipe under design operating conditions?› Do conservative SCF / SNCF influence the ECA results?

Design assumptions

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3D pipeline design problems

Typical design approach• Assume simplified geometry

- Full cohesion between layers- No weld geometry at field joint

• Assume simplified loads- Pure axial or bending, not both- Internal pressure on pipe wall

only, end cap force only applied at ends

• Ignore operational configuration- Initial conditions include; straight

pipe, virgin material

Calculating strain concentration

Or wrinkling of lined pipe

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How can sub-modelling help?

BMW Crash testing• An initial analysis is performed

including the whole car

• Many simplifications are included to reduce the model complexity and analysis run-time

• Smaller sections are analysed using sub-modelling to confirm the behaviour in the global analysis

• Even the passengers can be modelled!Taken from “Migration of Crash

Simulation Software at BMW”, 2005

Global model/analysis

Sub-model

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How can sub-modelling help?

Airbus A380 design and testing• The design process for this aircraft

represented a step-change in design challenges

• Experimental testing was prohibitively expensive – design relied heavily on numerical modelling

• Multiple analysis levels (global models and sub-models) were used to obtain the aircraft structural response

Airplane components

Taken from AIRBUS Central Entity, 2005

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1. Using a coarse model of the pipeline, perform a ‘global’ analysis(The global analysis defines the overall structural response of the pipeline)

2. Smaller sections of pipe may be analysed using sub-models – each sub-model is ‘driven’ by the output of the global analysisThe sub-model is a more accurate representation of the pipe section, and may include additional materials and/or geometry as required.

3. Multiple sub-model analyses may be performed on the same global analysis

4. A sub-model analysis can also be used as a global analysis

Sub-modelling overview

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Sub-Model analysis sequence

• 3D sub-model is driven by the output from the global analysis:o Nodal displacements and rotation

oro Nodal forces and moments oro Combination of the above

• Additional details may be added:o Geometric featureso Additional materialso Local loadso In-line structures

Glo

bal p

ipel

ine

mod

el

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Sub-model advantages & limitations

• Advantages:o Greater accuracy in stress and straino Include geometric and material interactionso Can be performed many times on the same global analysiso Computationally cheap

• Limitations:o The global analysis must be an adequate representation of the structural

response (i.e. the stiffness of the sub-model should reasonably match the same section in the global model)

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Sub-model applications

Design considerations• Initial global analysis defines the

overall pipeline structural response

• First sub-model allows 3D visualisation of the stress and further sub-modelling

• Second sub-model captures the weld geometry and material properties

• Third sub-model can recreate test specimens for model / material calibration and testing

Multiple sub-models

Sub-model 3

Global analysisSub-model 1

Sub-model 2

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Sub-model applications

Figure illustrates the stress distribution 12m either side of the apex of a lateral buckle

Design considerations• Stiffness of the CWC is captured in

the global analysis

• Field joints are also included as regions devoid of concrete.

• No need for SNCF!! Strain output from global analysis already captures increase due to presence of coatings.

• 3D analysis computationally ‘cheap’, analysis time for single load cycle < 1.5 hours.

Apex of a lateral buckle with CWC

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Sub-Model AnalysisGlobal Analysis

Animation illustrates the strain distribution 12m either side of the apex

Apex of a lateral buckle with CWC

Sub-model applications

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Sub-model applications

Figure illustrates the stress distribution 3m either side of the apex of a lateral buckle

Design considerations• Misalignment not included in global

analysis.

• Stress concentration can now be based on known (as built) pipe geometry under design conditions; can compare with analytical calculation in DNV-OS-F101.

• 3D analysis computationally ‘cheap’, analysis time for single load cycle < 10 minutes.

Pipe-joint misalignment

Pipe-joint misalignment

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Applications

Design considerations• Sub-model can be used to

assist in buckle arrestor design.

• Increase confidence in the ability of a buckle arrestor to halt propagating buckles under a variety of design conditions.

Buckle arrestor design

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Installation case study

Global analysis Sub-model analysisModel overview

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Installation case study

Global analysis Sub-model analysisInitiate a local buckle

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Installation case study

• Local buckle initiated in the sub-model only

• Sub-model boundary conditions from the global analysis provide support to the pipeline during this unstable process

• Buckle propagation measured to be faster than 200 m/s!

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Installation case study

Global analysis Sub-model analysisBuckle propagation ends near the stinger

Propagating

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Installation case study

• Cut view of the pipeline as (approximately) viewed from the end of the stinger

• The local buckle propagates up the catenary, eventually running out of steam as it approaches the stinger

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Installation case study

Global analysis Sub-model analysisBuckle arrestor stops buckle progression

Propagating

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Installation case study

• As viewed downstream from the buckle arrestor

• The local buckle propagates down the catenary and along the laid pipeline until it reaches the buckle arrestor. Propagation is then halted by the buckle arrestor – a satisfactory design!

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• Sub-modelling is an established method for obtaining highly accurate results from finite element analyses

• Sub-model analyses are fast and stable, and offer the most efficient method for modelling a 3D pipeline under all operational loads

• Wood Group Kenny has developed an ABAQUS GUI plug-in capable of sub-modelling pipeline FEA. Model development is fast, accessible to non-FEA engineers and capable of performing many different pipeline design analyses.

AcknowledgementsDavid Timmins, Rotger Jost, Terry Griffiths & Andrew Rathbone

Further questions:graham.viecelli@woodgroupkenny.com

Summary