A numerical modeling procedure was developed, using the finite-elementsimulator ABAQUS/Standard, to predict the local buckling and post-bucklingresponse of high strength pipelines subject to combined state of loading. Thenumerical procedures were validated using test data from large-scaleexperiments examining the pure bending and local buckling of high strengthlinepipe. The numerical simulations were consistent with the measuredexperimental response for predicting the peak moment, strain capacity,deformation mechanism and local buckling response well into the post-yieldrange.
A parametric study on the local buckling response of high strength plainpipelines was conducted. The influence of pipe diameter to wall thickness ratio(D/t of 40, 60 and 80), pipe segment length to diameter ratio (L/D of 3.5, 5, 7and 12), yield strength to tensile strength ratio (Y/T of 0.7, 0.8 and 0.9) andinitial geometric imperfections on the local buckling response was examined.The loading conditions included internal pressure and end rotation. Mechanicalresponse parameters examined included moment-curvature, ovalization, localstrain and modal response.
The five Arctic regions of Russia, Alaska, Norway, Greenland and Canada holda tremendous potential for both discovered and undiscovered reserves of Oil andGas. The USGS estimates 160 BBO, and 1,670 TCF of natural gas reserves in theArctic, with most of these reserves being located offshore. The Arctic region,however, presents its own unique challenges; extreme low surface temperatures,a highly fragile environment protected by strict regulatory controls, very highcost of operations - and of failure prevention, and a narrow weather window tooperate. Combined, these challenges leave no room for complacency whileplanning an Arctic drilling campaign. The drilling tubular risks are mainlyaccentuated during transportation, storage, and surface handling in thepermafrost region. In order to drill safely and reliably in such harsh surfaceconditions, ordinary drillstring solution is neither considered safe norreliable due to the adverse and unpredictable effect of extereme lowtemperatures on mechanical properties of steel. VAM Drilling has successfullydeveloped and deployed proprietary Arctic steel grades that deliver a greatcombination of strength and ductilitly at temperatures as low as -60°C(-76°F).
Marshall, P.W. (Department of Civil and Environmental Engineering, National University of Singapore) | Sohel, K.M.A. (Department of Civil and Environmental Engineering, National University of Singapore) | Liew, J.Y. Richard (Department of Civil and Environmental Engineering, National University of Singapore) | Jiabao, Yan (Department of Civil and Environmental Engineering, National University of Singapore) | Palmer, A. (Department of Civil and Environmental Engineering, National University of Singapore) | Choo, Y.S. (Department of Civil and Environmental Engineering, National University of Singapore)
There is a wide range of offshore structures which may be constructed byeither steel or concrete materials to be used in the arctic region, such assteel tower platforms, caisson-retained islands, shallow-water gravity-basecaisson, jack-up structures, bottom-founded deep-water structures, floatingstructures, well protectors, seafloor templates and breakwaters. One commonfeature of these structures is that they must be able to resist the highlateral forces from the floating ice and transmit these forces to thefoundation. This study explores the use of Steel-Concrete-Steel (SCS) curvedsandwich system for arctic caisson structures. SCS sandwich system, whichcombines the beneficial effects of steel and concrete materials, has promisingbenefits over conventional plates and stiffeners design and heavily reinforcedconcrete design because of their high strength-to-steel weight ratio and highresistance to contact and impact loads. Shear connectors have been proposed toprovide bonding between the external steel plates and high-performancecementitious core materials. Finite element analyses and large-scale testresults showed that SCS sandwich panels without mechanical bond enhancement arevulnerable to interfacial shear failure and impairment of structural integritywhen subject to shrinkage and thermal strains, accidental loads, and impact.The proposed SCS sandwich system features mass-produced mechanical shearenhancement and/or cross-ties. It can reduce structure complexity, particularlyin the number of weld joints which are prone to fatigue, hence increasingservice life, cutting down the cost of fabrication, and reducing the manpowercost to operate, inspect, and maintain the structure in the long run.Considering local ice load, the punching shear and shell bending strength ofthe SCS sandwich composite shell is studied experimentally. Test results showedthat the SCS sandwich panels, which are designed using the ISO ice load, arecapable of resisting the localized contact and punching loads causedthereby.
Pipeline systems are integral components of the system infrastructure forthe transport of hydrocarbon resources. In arctic and harsh environments, thesepipelines may be subject to large deformation geohazards. Pipeline/soilinteraction events are often examined using a structural pipe/spring model.This approach does not account for more realistic soil constitutive behaviour,soil deformation mechanisms and effects of soil load transfer on pipelinemechanical response. This paper examines pipe/soil interaction events duringoblique lateral-vertical soil movements using plane strain finite elementanalysis. The results from this study provide a technical framework to assessthe effects of geotechnical loads on buried pipelines, highlight key parametersinfluencing soil yield envelopes, and identify soil failure mechanisms foroblique pipe/soil interaction events that can be used in the design of buriedpipelines for large deformation geohazards. The results may be used tobenchmark more complex loading events, such as coupled ice keel/seabed/pipelineinteraction, that has limited physical basis for validation.
The development of arctic resources requires wells to be drilled, cased, andcemented through permafrost. Permafrost presents unique challenges, especiallyto cementing operations, requiring a cement system with the capability toperform in the subfreezing permafrost environment. The performance required isthat the cement provides isolation, exhibits low heat of hydration, and setswith sufficient strength to provide casing support. There are also specifictesting requirements detailed in API recommended practices.
In the polar region, there are several approaches used in the design of cementsystems. The approaches used in Russia, Canada, and USA (Alaska) areillustrated. The design considerations take into account local conditions andrequirements and use knowledge from cementing practices employed in thedrilling industry.
It is important to understand the current cementing practices in use withinthe arctic region. This will allow future improvements as more developmenttakes place and the resources become exploited.
A two-year-long field study was conducted by ConocoPhillips Alaska, Inc. andPND Engineers, Inc., at Kuparuk in Alaska's arctic North Slope region. Thestudy verified that pipe piles can be directly driven into predrilled pilotholes in frozen ground, without requiring thermal modification of thepermafrost. The traditional "drill-and-slurry" method of permafrost pileinstallation involves hanging piles in an oversize hole and backfilling theannulus with a sand/water slurry. Vibratory driven pile installation isconsiderably more efficient with large benefits in installation time, expenses,and safety. Both methods require an adequate adfreeze bond for pileperformance. The objective of this testing program was to determine whetherdespite great benefits in installation efficiency the vibratory driven pilewould perform adequately. Twelve 12.75-inch-diameter steel pipe piles wereinstalled in permafrost in the Alaskan arctic; eight piles were installed inice-rich sandy silt and four piles were installed in a frozen gravel soil.Piles were loaded in tension for six different durations ranging from five daysto six months at loads varying from 35 kips to 145 kips. At the completion oflong-term testing, the test piles were unloaded, rested, and then loaded tofailure to characterize the adfreeze short-term strength. Pile load anddisplacement were continuously recorded with electronic displacement and loadtransducers. Subsurface soil temperatures were also monitored. Collected datawas used to characterize long- and short-term pile velocity as a function ofload and adfreeze temperature. Experimental results were compared to currenttheoretical and empirical performance.
Reel-laying is a fast and cost-effective method to install offshore pipelines. During reel-laying, repeated plastic strain is introduced into the pipeline which may, in combination with ageing, affect strength and ductility of the pipe material. The effect of reel-laying on the pipe material is achieved by small- or full-scale reeling simulations followed by mechanical testing according to corresponding standards. In this report an appropriate test setup for full-scale reeling simulation is presented. The fitness-for-use of the test rig is demonstrated by finite element calculations as well as by full-scale reeling simulations on different pipes of various grades. Plus, small-scale reeling simulations with subsequent ageing and mechanical testing are performed on the same pipe material. A comparison of results from mechanical tests after small- and full-scale reeling simulations is given. Additionally results from collapse tests on pipes after full-scale reeling simulations are presented, and the influence of repeated bending of the pipe on its collapse behavior is discussed.
Two main concepts are normally used for laying offshore subsea pipelines. In the S- and J-lay method a pipeline is fabricated on the deck of a lay barge by welding individual lengths of pipe as the pipe is paid out from the barge. The pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string. The S- and J-lay method requires skilled welders and their relatively bulky equipment to accompany the pipe-laying barge crew during the entire laying operation; welding must be carried out on board and often under adverse weather conditions. Further, the S- and J-lay method is relatively slow, with even experienced crews laying only few miles of pipe a day. This can subject the entire operation to weather which can cause substantial delays and make working conditions quite harsh.
Bienen, Britta (The University of Western Australia) | Gaudin, Christophe (The University of Western Australia) | Cassidy, Mark J. (The University of Western Australia) | Rausch, Ludger (University of Applied Sciences) | Purwana, Okky A. (Keppel Offshore & Marine Technology Centre)
This paper establishes the undrained capacity of a circular skirted mat under uniaxial horizontal and moment loading, respectively, and presents the combined vertical, horizontal and moment (VHM) capacity envelopes for a novel concept for foundations that combines a skirted mat with a suction caisson. This foundation concept enables self-installation and preloading of the footing. Specifically, this research explores the effect of the central caisson on the failure mechanisms and the resulting VHM capacity through finite element analysis. The results demonstrate that the central caisson more than doubles the horizontal capacity while moderately increasing the capacity in the vertical and moment loading directions.
Combinations of vertical load 4V 5, horizontal load 4H5 and moment 4M5 are typically applied to foundations in the offshore environment, due to platform self-weight, wind, waves and current. The industry increasingly embraces the use of VHM interaction surfaces to describe foundation capacity rather than (semi) empirical modifications to the classical bearing capacity theory that assumes predominantly vertical loading characteristics of onshore applications. These failure envelopes are affected by the footing shape, the embedment and the soil shear strength profile. Skirted foundations are often used in shallow waters. The skirt, which may extend up to 0.5 diameters below the mudline, is used predominantly to increase the horizontal capacity of the foundation. Recent research established the combined vertical, horizontal and moment (VHM) capacity of skirted foundations, although the research excluded the combined load capacity of circular skirted mats. Table 1 provides an overview of the available solutions for cohesive soils with uniform or nonuniform strength profiles. These numerical studies are supplemented by experimental results, including those shown in Cassidy et al. (2004) and Kelly et al. (2006). Note that only the most recent study explicitly modelled the skirts on strip footings.
For burst design, engineers routinely assume that the casing annular space is filled by a fluid equivalent. This assumption ignores mechanical resistance provided by solid cement. Some studies addressed this shortcoming by modeling the cement sheath as a solid with elastic failure criteria. Prior work used cement elastic modulus and Poisson's ratio to classify cement as "ductile" (soft) or "brittle" (hard). In the current study, numerical results from finite-element analysis (FEA) indicate that casing burst resistance is increased by the presence of the cement sheath. This study focuses solely on improvement offered by the cement sheath to casing burst resistance and ignores consequences of cement failure on overallwell integrity. Comparisons are provided for casing burst resistance, assuming various backup profiles. These include fluid hydrostatics, solid cement matrix (both elastic and plastic response), and cement as "loose" particles. The fluid hydrostatics include mud weight in hole, cement-slurry density, mixed-water density; normal pressure (saltwater column), and actual pore pressure. Calculations show that these fluid profiles are conservative when used as burst-resistance backup. Original cement-slurry density is least conservative. Because well designers are familiar with fluid profile backup assumptions in casing burst design, recommendations are provided to approximate cement behavior as particles with a fluid profile. This allows ease of calculation and is consistent with current practice. Guidelines are provided to explicitly calculate the enhanced casing burst resistance caused by the particulate cement.
Li, Yanghui (Dalian University of Technology) | Song, Yongchen (Dalian University of Technology) | Liu, Weiguo (Dalian University of Technology) | Yu, Feng (Dalian University of Technology) | Wang, Rui (Dalian University of Technology) | Nie, Xiongfei (Dalian University of Technology)
To acquire more knowledge about the mechanical properties of gas hydrate-bearing sediments and assess the longterm stability of the strata, samples of clayey sediments containing methane hydrate were prepared and tested for their mechanical properties under various conditions by triaxial compression. The effects of temperature and confining pressure on the mechanical properties of methane hydrate-bearing sediments were analyzed. And a strength criterion considering the influence of temperature was developed for methane hydrate-bearing sediments. It agrees well with the experimental data, it and can be taken to predict the shear failure strength of methane hydrate-bearing sediments under subzero conditions.
Gas hydrate has attracted global attention due to its widespread occurrences and large potential as an energy resource (Brown et al., 2006; Glasby, 2003). It generally exists in the regions of Arctic permafrost and submarine continental margins where there are acceptable P/T conditions (MacDonald et al., 1994). Additionally, the naturally occurring hydrates are always associated with large quantities of gas trapped underneath, with the hydratebearing strata acting as seals for trapped free gases. Gas may release abruptly during mining and well drilling, which can lead to blowouts and well-control problems (Cameron et al., 1990; Vanoudheusden et al., 2004). Characterizing the mechanical properties of gas hydrate-bearing sediments will provide a scientific basis for the evaluation of the safety of the structure and the stability of the reservoir. Up to now, the mechanical properties of gas hydrate-bearing sediments have not been fully understood. Durham et al. (2003) and Stern et al. (1996) indicated that the mechanical properties of methane hydrate are very different from those of water ice. Winters et al. (2004, 2007) studied the strength and acoustic properties of laboratory-formed methane gas hydrate. However, the analyses of the mechanical properties of gas hydrate-bearing sediments under various conditions are qualitative.