The demand for energy from developed and developing economies of the world isdriving the search for energy resources to more challenging environments. Theexploration and exploitation of hydrocarbons now requires the drillbit to hit pay zonesfrom drillships or platforms that are located on water surfaces below which is, possibly,in excess of ten thousand feet of water above the sea bed. From Brazil, to the Gulf ofMexico and the Gulf of Guinea on the western coast of Africa, hitherto unfamiliar, butnow common, concepts in the drilling parlance such as ultra-deep drilling (UDD), ultraextended-reach drilling (uERD) and slimhole drilling, are employed to reach andproduce reservoirs which a few decades ago would seem technologically impossible toproduce.This is expected to exert tremendous demands on the physical and mechanicalproperties of the drillstring components. Limiting factors for reaching and producing oiland gas resources hidden very deep in the subsurface are both the capacity of thedrilling rig to support the weight of the drillstring, which in some instances can beseveral kilometres long, and the bending, tensile and impact stresses the string has towithstand in well trajectories that are getting both longer and more tortuous.Associated with this increased well depths and complex well trajectories is theprohibitive cost penalty of a failed drillstring. The in-service failure of drillstrings hasalways been an issue in the industry long before the wells become this deep andcomplex. The global oil and gas industry estimates the cost of string failure to be inexcess of quarter of a billion dollars annually.Researchers are continuously looking for ways to design against string failure andimprove the level of confidence in drillstrings. Defect-tolerant design, tooljoint geometrymodification and surface coldworking are just a few of the ideas that have gainedmileage in this effort. Others that are now in consideration are the use of nonconventionalmaterials such as aluminium and titanium alloys for drillstringcomponents. More novel, still, is the use of a combination of two materials - one ‘softer’than the other to form a hybrid string of two materials of unequal moduli of elasticity.This is done to make the string lighter, reduce stress concentration factor at theconnections and place fatigue resistant materials in areas of high well bore curvature.In this work a computational technique in the form of two-dimensional finite elementanalysis is used to develop a robust model of a drillstring connection and to analyse thestresses on the model of a threaded connection of standard drillstring tooljoint madefrom alloy steel. Further comparative analyses were undertaken on models ofdrillstrings made from a newly developed drillstring material for ultra-deep drilling, theUD-165, aluminium and titanium alloys and, finally, on hybrid drillstrings made from twodifferent materials of unequal moduli of elasticity.The aim is not only to develop and validate a better method of computational drillstringanalysis but also to use the model to investigate and suggest areas of optimisation thatwill benefit industry especially in the areas hybrid strings.
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