Deepwater Intercontinental Gas Pipeline

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Research nears deepwaterintercontinental gas pipeline 12/05/2011Richard Freeman Tata SteelCorby, UK

Research into the collapse and buckling of deepsea pipelines and the UOE method of line pipe production aremoving the industry closer to being able to build and operate intercontinental natural gas lines.

Currently, intercontinental transport of natural gas typically occurs via LNG tanker, not pipeline. The movement ofnatural gas by ship, however, requires a variety of steps—liquefaction, loading, transit, regasification—not required ofpipeline transit. A proposed pipeline linking India to the Middle East across the Arabian Sea, for example, wouldeliminate the need for these steps while at the same time satisfying some of India's growing appetite for natural gas.

Current alternative onshore pipelines, originating in either Turkmenistan or Iran, would transit either or both Afghanistan and Pakistan en route to India, with attendant political and security concerns. A subsea intercontinental

pipeline, however, would present many technical challenges.

Deepwater pipelines

As subsea pipelines are taken to greater depths (generally down to 1,000 m), external pressure may eventuallybecome great enough to buckle and collapse the pipe. The pipe is particularly vulnerable during installation when thepipeline has no balancing internal pressure and the bending during lay (Fig. 1) further stresses its axial fibers.



Despite the apparent simplicity of this failure case, pipe collapse is a complex phenomenon involving plastic andelastic instability. The exact pressure at which the pipe fails is a function of the material properties of the steel, thegeometry of the pipe, and its overall surface topography. Local variations in strength and shape lead to an inherentunpredictability of the exact pressure that would lead to catastrophic failure, adding further uncertainty andcomplication.



Engineers can use theoretical principles to estimate collapse pressure within a specific pipeline and much work hasbeen done to evaluate these principles through full-scale testing. Exploration, however, now demands pipelineperformance at such depths and with pipe of such large diameters (an inherent weakness against hydrostaticpressure) that current, conservative expressions and testing regimens form potential barriers to advancement ofpipeline technology.

An offshore pipeline bringing natural gas into India from the Middle East would need to have sufficient diameter (26-

28 in. OD) to transport gas economically. The pipe's route, however, would reach water depths of nearly 3,500 m(Fig. 2), requiring WT of more than 40 mm for those diameters.

This WT-OD combination is at the edge of current economical pipe-forming capabilities and would requireconstruction of a specialized fleet of new lay vessels. Any WT reduction would yield significant material, installation,and logistic cost savings when considered along the total 1,100-km route.

Reducing WT by 2.5 mm, for example, would yield savings in material alone of about $80-100 million. A WT reductioncould also be the difference between lay-barge capability and not being able to lay the pipe at all.

Optimizing WT

Two simple mechanical relationships, elastic collapse and plastic collapse, describe current generally held failuremodes.



Elastic collapse defines a region where OD is large compared to WT. The resultant flexible geometry allows thecylinder to deform and fail while the material is behaving elastically (Equation 1).





The material's elastic modulus (E) and thickness-diameter ratio (t/D) govern pipe failure in this instance.

A pipe thick in relation to its diameter, however, doesn't flex enough to become elastically unstable. Instead, themounting pressures cause the material to deform plastically. The material hardens as it deforms, eventually losing itsability to resist collapse and failing. This is referred to as plastic collapse (Equation 2).

Both of these treatments are simple to derive, but neither accurately describes what occurs at the point of failure.Pipe failure involves a subtle but complex blend of these failure modes (Fig. 3), with factors like shape also playing apart.

Equation 3, from DNV-OS-F101,1 seeks to blend these formulas to provide a continuous transition from one mode tothe other using shape as a transitioning parameter.

US design guideline API 1111 and classical treatments by Timoshenko

1

and others contain similar expressions.

Each formula, however, yields a different value of t/D, leaving open the question of which is correct and which bestdescribes the mechanics as the cylinder becomes unstable and fails catastrophically.

Analytical mechanics

Development of finite-element packages removed the need for long, complex formulas and delivered increasinglyaccurate treatments of particular design problems, allowing perfect design of individual components.



Finite-elements packages cannot, however, readily evaluate parameter variations intended to optimize systemdesign. Each change has to be reevaluated through the software and its effect quantified.

Newton's Second Law of Motion (F = ma) shows that a vehicle increasing in mass requires more force to generatethe same acceleration. A formula for the prediction of collapse similarly describing what happens to the pipe at thepoint of failure would allow examination of how material property and shape variations in the pipe and shape could beadjusted to maximize performance. Such a formula would increase pipeline operators' confidence, ensuring material

and project integrity should the representation be proven.

Researchers completed the bulk of work addressing deepwater pipeline collapse in the early 1990s during the firstevaluation of an Oman-to-India pipeline (OGJ, July 5, 1993, p. 22). Pipes manufactured by the UOE process (U-ing,O-ing, and Expanding), generally the most economical option for subsea applications, received particular attentionregarding their collapse capacity (Fig. 4).

The Oman-to-India team subjected more than 20 pipes manufactured in Europe and Japan to rigorous testing,including construction of a hyperbaric chamber capable of simulating service at 3,500-m water depth. Researchersplaced all pipes from the various manufacturers in the chamber, pre-bent them to simulate installation strain andrecorded the pressure at which instability and collapse occurred, collating and comparing the results. The pipes'collapse pressure averaged 15% lower than expected with the DNV formulation,

3 a discrepancy attributed to

reduction in compressive strength due to cold forming of the material, the Bauschinger effect.

The Bauschinger effect is not fully understood but adheres to this general principle: When a material experiences atensile stress, it first generates an elastic strain and eventually begins to yield, at which point the relationship betweenthe stress and the induced strain becomes non-linear and the material begins to transform plastically (Fig. 5). Duringplastic transformation the resistance of the material to strain reversal also lessens. Removing tensile stress and thenapplying the opposite, compression, causes compressive yield to occur earlier.





The theoretical equation under development describes the transformation of the pipe shape through the elastic andplastic transition by building on Gerrard's work, which looked at collapse using the actual shape of the stress straincurve as defined by the tangent modulus, instead of using Young's modulus with an assumed value for the yieldstrength. This approach allows the new equation to incorporate the strain hardening of the material through theRamberg-Osgood relationship.

The technique demonstrates good correlation with experimental results and work is underway to add more complexity

to the formula to examine the effects of non-homogeneous materials. Once all of the factors driving collapse areunderstood researchers will attempt to re-simplify the equation for incorporation into design standards. Insight intohow shape can influence the onset of collapse could also lead to potential manufacturing improvements.

References

1. Det Norske Veritas, Offshore Standard DNV-OS-F101, "Submarine Pipeline Systems," October 2010.

2. Timoshenko, S.P., and Gere, J.M., "Theory of Elastic Stability," 3rd Edition, McGraw-Hill, New York,1961.

3. Aamlid, O., Collberg, L., and Slater, S., "Collapse Capacity of UOE Deepwater Linepipe," OMAE 2011Ocean, Offshore, and Arctic Engineering, Rotterdam, June 19-24, 2011.

4. Stark, P.R., and McKeehan, D.S., "Hydrostatic Collapse Research in Support of the Oman to India GasPipeline," Offshore Technology Conference, Houston, May 1-4, 1995.

5. Slater, S., Aamlid, O., Devine, R., Hernandez, D., and Swanek, D., "Qualification of Enhanced CollapseCapacity UOE Deepwater Linepipe, OMAE 2011 Ocean, Offshore, and Arctic Engineering, Rotterdam, June19-24, 2011.

The author

Richard Freeman ([email protected]) is manager, product and market development,at Tata Steel, Corby, UK, working in the oil and gas pipeline sector. He has also served as development engineer forTata Steel specializing in line pipe development and flow assurance systems. He holds a first class honors degree inmechanical engineering (1991) from the University of Leicester. Freeman is a member of the Chartered Institute ofMechanical Engineers.

Manuscripts welcome

Oil & Gas Journal welcomes for publication consideration manuscripts about explorationand development, drilling, production, pipelines, LNG, and processing (refining,petrochemicals, and gas processing). These may be highly technical in nature andappeal or they may be more analytical by way of examining oil and natural gas supply,demand, and markets. OGJ accepts exclusive articles as well as manuscripts adaptedfrom oral and poster presentations. An Author Guide is available at www.ogj.com, click"home" then "Submit an article." Or, contact the Chief Technology Editor([email protected]; 713/963-6230; or, fax 713/963-6282), Oil & Gas Journal, 1455West Loop South, Suite 400, Houston TX 77027 USA.

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