The demand for energy from developed and developing economies of the world is
driving the search for energy resources to more challenging environments. The
exploration and exploitation of hydrocarbons now requires the drillbit to hit pay zones
from 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 of
Mexico and the Gulf of Guinea on the western coast of Africa, hitherto unfamiliar, but
now common, concepts in the drilling parlance such as ultra-deep drilling (UDD), ultraextended-
reach drilling (uERD) and slimhole drilling, are employed to reach and
produce reservoirs which a few decades ago would seem technologically impossible to
produce.
This is expected to exert tremendous demands on the physical and mechanical
properties of the drillstring components. Limiting factors for reaching and producing oil
and gas resources hidden very deep in the subsurface are both the capacity of the
drilling rig to support the weight of the drillstring, which in some instances can be
several kilometres long, and the bending, tensile and impact stresses the string has to
withstand in well trajectories that are getting both longer and more tortuous.
Associated with this increased well depths and complex well trajectories is the
prohibitive cost penalty of a failed drillstring. The in-service failure of drillstrings has
always been an issue in the industry long before the wells become this deep and
complex. The global oil and gas industry estimates the cost of string failure to be in
excess of quarter of a billion dollars annually.
Researchers are continuously looking for ways to design against string failure and
improve the level of confidence in drillstrings. Defect-tolerant design, tooljoint geometry
modification and surface coldworking are just a few of the ideas that have gained
mileage in this effort. Others that are now in consideration are the use of nonconventional
materials such as aluminium and titanium alloys for drillstring
components. 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 the
connections 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 element
analysis is used to develop a robust model of a drillstring connection and to analyse the
stresses on the model of a threaded connection of standard drillstring tooljoint made
from alloy steel. Further comparative analyses were undertaken on models of
drillstrings made from a newly developed drillstring material for ultra-deep drilling, the
UD-165, aluminium and titanium alloys and, finally, on hybrid drillstrings made from two
different materials of unequal moduli of elasticity.
The aim is not only to develop and validate a better method of computational drillstring
analysis but also to use the model to investigate and suggest areas of optimisation that
will benefit industry especially in the areas hybrid strings.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/7752 |
| Date | 11 1900 |
| Creators | Salihu, B. M. |
| Contributors | Brennan, Feargal |
| Publisher | Cranfield University |
| Source Sets | CRANFIELD1 |
| Language | English |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
| Rights | © Cranfield University 2011. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright owner. |
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