Calculo De Fatiga De DP
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Fatigue of Drillstring: State of the Art
O. Vaisberg1, O. Vincké1, G. Perrin1, J.P. Sarda1 and J.B. Faÿ1
1 Institut français du pétrole, division Mécanique appliquée,1 et 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex - France
e-mail: olivier.vaisberg@ifp.fr - olivier.vincke@ifp.fr - gilles.perrin@ifp.fr - j-paul.sarda@ifp.fr - j-baptiste.fay@ifp.fr
Résumé—Fatigue des tiges de forage : état de l’art — La rupture des tiges de forage est un problème
coûteux dans l’industrie de ce secteur. Bien que de nombreux spécialistes se soient penchés sur ce point,
la fréquence des ruptures demeure toujours importante. Les ruptures par torsion ou par tension restent
toutefois limitées, car les causes en sont connues et peuvent être aisément corrigées, en revanche, les
ruptures par fatigue sont plus difficilement appréhendées.
Le présent article se propose d’établir un état de l’art sur la fatigue des tiges de forage. La prédiction et
les calculs d’un dommage de fatigue sont ici abordés selon la méthode simpliste du cumul de dommage
(somme de Miner) mais aussi de façon plus complexe, par l’utilisation des éléments de la mécanique de
la rupture. Les méthodes d’inspection et leurs limitations sont discutées, des recommandations sont
également émises. Par ailleurs, des tests de fatigue sont mis en oeuvre face au risque humain ou
environnemental. Cette étude précise les conditions de chargement, la fréquence des essais, le nombre et
la taille des éprouvettes. Nous rappelons les effets d’un environnement corrosif ainsi que leur prévention,
bien que ce sujet ne soit pas l’objet principal de cet article. Le dernier chapitre résume les différentes
façons d’améliorer les tiges de forage. À ce titre, il aborde la géométrie, le design des connexions, les
propriétés de l’acier telles que la résilience, le rechargement dur des tool-joints et l’inspection des
garnitures de forage.
Mots-clés : tige de forage, garniture de forage, fatigue, rupture, calcul de dommage, inspection, tests de fatigue, effets de l’environnement,
corrosion, amélioration, connexions vissées, tool-joints.
Abstract —Fatigue of Drillstring: State of the Art— Failure due to fatigue is a very costly problem in
oil and gas industry. Many investigators have previously addressed this problem, but its frequency of
occurrence is still excessive. Torque and tension can be correctly predicted but computations of fatigue
duration are still approximate.
Regarding the fatigue failure of drillstring, this paper summarizes the state of the art. Prediction and
calculation of fatigue duration are stated, including both history of the simplified approach based on
Miner’s rule and a few elements of the fracture mechanics theory. Existing inspection methods, their
limitations and further recommendations are provided. Moreover, the fatigue tests are performed when
human life and environment may be at risk. The loading conditions, the test frequency, the number and
the size of test specimens are given. Environmental effects such as corrosion are recalled. Prevention and
inhibitors are mentioned. Last chapter focuses on enhancement of drillstring. Drillpipes geometry
improvement, connections re-design, steel properties such as toughness, tool-joints hardfacing and
inspection of drillpipes are discussed.
Keywords: drillstring, drillstem, drillpipe, fatigue, failure, damage calculation, inspection, fatigue tests, environmental effects, corrosion,
improvement, threaded connections, tool-joints.
Oil & Gas Science and Technology – Rev. IFP, Vol. 56 (2001), No. 6
INTRODUCTION
Failure due to fatigue is a very costly problem in oil and gas
industry. Although many investigators have previously
addressed this problem, its frequency of occurrence is still
excessive.
Drillstring failure occurs on 14-percent of all rigs and the
resulting downtime costs roughly $106 000 per event [1, 2].
A survey of all drilling problems reported worldwide over a
15-month period shows that 36-percent were due to stuck
pipe. Stuck pipe cost estimates for the worldwide drilling
industry range as high as $250 million for this period [3].
Hill has analyzed 76 drillstring failures from 1987 to 1990
on three continents [4]. These incidents are costly because of
the loss of rig time, tubular goods and even the well in some
time. In 1992, one in seven wells are concerned. Failure
causes can be estimated as follows.
– Fatigue is the main cause in 65-percent of the failures and
has a significant impact in 12-percent.
– Combined excessive tension and torque give failures in
13-percent of the cases.
– Low toughness of material is mentioned for only 8-percent
of the failures.
The same conclusion is issued in [5] where 73-percent of
inspected drillpipes were defective because of fatigue cracks.
Torque and tension are correctly estimated but fatigue is
still an approximate skill.
Mechanical stresses in drillstem, environmental and
unusual conditions, such as corrosive mud, horizontal well,
etc., should be predicted as accurately as possible in order to
define the best drillstring assembly and then reduce fatigue
failure. Planning an inspection program, before and while
drilling is an important step. Monitoring results while drilling
and tripping should be compared with theoretical models.
The first two points are developed in Section 1 and 2.
Fatigue tests, presented in Section 3, are necessary to have a
better understanding of both steel and equipment behavior.
Environmental conditions are listed in Section 4 but this is
not the main subject of this paper. Section 5 focuses on
improvement on drillstring, on manufacturing methodology
and on material properties. Anyway, state of the art,
limitations and improvement will be underlined.
1 FATIGUE CALCULATIONS
1.1 General
Fatigue damage is due to the reversed variations of the
stresses, such as those induced when the drillpipe rotates in a
curved section of a wellbore. Rotating a buckled pipe may
also lead to rapid fatigue failure (Fig. 1). Fatigue troubles can
be estimated from the number of cycles associated with the
amplitude of the stress cycles.
Figure 1
Fatigue
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