Case History of Drillstem Failures Offshore West Africa
- R.G. Sweet (Conoco U.K. Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1992
- Document Type
- Journal Paper
- 55 - 58
- 1992. Society of Petroleum Engineers
- 4.1.4 Gas Processing, 1.2.5 Drilling vibration management, 1.1 Well Planning, 1.10 Drilling Equipment, 4.2.3 Materials and Corrosion, 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating, 2 Well Completion, 4.2 Pipelines, Flowlines and Risers, 1.6 Drilling Operations, 1.6.1 Drilling Operation Management, 1.11 Drilling Fluids and Materials, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.4.1 BHA Design
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A high incidence of drillstem (or drillstring) failures was experienced by Conoco Inc. during drilling of exploration wells offshore West Africa. A data base of failure information was used to identify quickly where the majority of the problems occurred. Almost all failures were related to fatigue and corrosion. A study resulted in several recommendations that included improved practices in drillstem selection, quality assurance, and corrosion control. After the recommendations were implemented, a significant reduction in failures was recorded. This increased operating efficiency, reduced drilling costs, and improved safety.
During 1987-88, five vertical wells were dried off the coasts of Gabon, Angola, and the Congo by four drilling contractors. The well depths ranged from 8,500 to 13,000 ft [2591 to 3962 m]. All wells were drilled with saltwater/polymer mud systems. Conventional pendulum and packed-hole rotary drilling assemblies were used. Drilling conditions sometimes included abnormal pressures, hard rock sections, and H2S or C02 - An unusually high number of drillstem failures occurred during drilling of these wells.
A database of the drillstem failures was compiled and studied. Table 1 summarizes this information, and Figs. 1 through 3 show the trends. A total of 66 failures was recorded for the five wells; this is an incidence of 13.2 per well. At the time of this study, the average cost per failure was $20,000 U.S.
The failure data were reviewed by drilling engineering and operations personnel for causes and possible solutions. Advice waalso obtained from several recognized experts in the service and manufacturing sectors of the drilling industry. Several practical recommendations were developed that could be implemented by the area drilling manager for the local conditions. Many of the recommendations were simply reinforcements of existing industry practices that are sometimes neglected.
Discussion of Failures
Figs. 1 through 3 were used to identify the major problem areas quickly. It was decided to focus corrective action on the major problem areas because this provided the greatest opportunity for problem areas because this provided the greatest opportunity for improvement.
General. Overall, the drillstem failures were distributed as follows (Fig. 1): the bottomhole assembly (BHA) accounted for 77% of the failures and the drillpipe had 23 %. Failure distribution by hole size was (Fig. 2) 33 % for 17 1/2 in. [444.5 mm]; 53 % for 12 'A in. [311.2 mm]; and 14% for 81/2 in. [215.9 mm].
A discussion of the predominant failures for the specific drillstem components in the data base follows.
BHA. Failure distribution for components with the BHA was (Table 1) 47 % for drill collars, 16 % for jars, 16 % for subs, 10 % for bits, 6 % for heavyweight drillpipe P), and 6% for stabilizers.
Drill Collars. Eighty-five percent of the drill-collar failures oc-curred in 6 5/8in. (168.28-mm) API regular box connections onominal 8-in. [203.2-mm] drill collars. These failures were mostlcracks and related washouts in the back of the box, near the last engaged thread. Many of the connection failures in the rotary subs and a few in the drilling jars also occurred here. This area is notorious for failure because it is exposed to stress concentrations a mud corrosion that ultimately cause fatigue failures.
General observations were made on the relative location of all the failed drill collars. A slight majority of the failures occurred in the upper half of the drill section; many of these were adjacent to the jars. Most of the drill failures in the bottom section occurred near the transition between the 8- and 9 1/2-in. [203.2- and 241.3-mm] steel drill collars and in the nonmagnetic drill collars.
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