Calculo De Fatiga De DP

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 may occurs when drillstring is crooked and rotated.

The material is indeed characterized by S-N curve also

called Wöhler curve where stress amplitude (S) is given

versus the maximum allowable number of cycles (log Nf),

(Fig. 2). Failure is likely to occur when the working number

of cycles is equal to the allowable number of cycles Nf. Other

representations are the Haigh diagram (Fig. 3) [6] or the

Bending in buckled area

Drilling in Rotary

Bending

Bending in dogleg in buckled area

8

Endurance limit

Fatigue with corrosion

Log cycles to failures (N)

Fatigue without corrosion

Stress

amplitude S

Tensile strength

Yield strength

Re0.2% Rm

Failure point for

a static tension test

Average stress

amplitude

(tensile stress)

Reversed stress amplitude

(bending stress)

Failure point

for a fully

reversed

bending

test without

tension

Figure 2

S-N Curve.

Figure 3

Haigh diagram [6].

O Vaisberg et al. / Fatigue of Drillstring: State of the Art

Goodman diagram [7]. Cyclic stress amplitude is given

versus average stress.

Loads applied on the drillpipe should be known in order to

determine both permanent and cycling stresses. A calculation

methodology is presented in the next section. Using the previous

parameters, life duration can be estimated as detailed in

Section 1.3. Nevertheless, vibration effects will be neglected

as a first approach and we assume that they should be avoided

as much as possible. Anyway, the Institut français du pétrole

(IFP) is presently studying the effects of dynamic vibration

behavior of a complete drillstring using a finite element

software where large displacement and friction effects are

taken into account. For further details, refer to Section 6.

However, this simplified approach is empirical and lacks

the physical basis necessary to consider the fatigue as a

progressive and history dependent phenomenon. While

working, microscopic cracks come out in the structure. Those

cracks tend to gradually increase until their length is large

enough to create the drillpipes failure: washout or twist-off

may occur. Each step, initiation, propagation and failure can

be modeled as shown in Section 1.4.

In the present section, corrosion will be ignored as it is

described in Section 4.

1.2 Drillpipe Stresses

Most of the papers are based on Lubinski works [7, 8].

Regarding modified Goodman diagram, the reversed bending

stress is the cycle stress amplitude, which is given

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