EFFECT
OF
STRENGTH A.
LOADING OF
M.
FREQUENCY
MATERIAL
OF
ON
FATIGUE
LIGHT-ALLOY
DRILL
PIPES
UDC 669.539.8;620.194.8
Prishlyak
In the p r e s e n c e of a t t a c h m e n t s on s p e c i m e n s , the fatigue strength and c o r r o s i o n fatigue depend on the level of s t r e s s e s , m a t e r i a l of the s p e c i m e n s and a t t a c h m e n t s , and d e g r e e and t i m e of action of the e x t e r n a l e n v i r o n m e n t [1]. In the m a j o r i t y of c a s e s the drill pipe is a p a r t with an a t t a c h m e n t (coupling and tool joint). As f u l l - s c a l e t e s t s [2, 3] of s t e e l and e s p e c i a l l y l i g h t - a l l o y drill p i p e s (LDP) showed, intense c o r r o sion p r o c e s s e s o c c u r under the fitted p a r t s . In individual c a s e s b r e a k a g e o c c u r s even n e a r the edge of the tool joint and not just along the bottom of the s t r e s s r a i s e r (thread). And although m a n y studies [4-6] have b e e n devoted to investigations of the m a t e r i a l of LDP (alloy D16), including its fatigue with c o n s i d e r a t i o n of the loading f r e q u e n c y [5], they w e r e e i t h e r c a r r i e d out on smooth s p e c i m e n s or on s p e c i m e n s with a m e chanical s t r e s s r a i s e r . The p r o b l e m of the effect of the loading f r e q u e n c y on the fatigue of s p e c i m e n s of the m a t e r i a l of LDP is e s p e c i a l l y urgent, since in r o t a r y drilling of wells the speed of rotation of the drilling s t r i n g is as a rule 70-150 r p m and the pipe m a t e r i a l i t s e l f is subjected to c o r r o s i o n to a g r e a t e r extent than steel. Smooth s p e c i m e n s , s p e c i m e n s with steel and r u b b e r a t t a e h m e n t s , and s p e c i m e n s with a s t r e s s r a i s e r of radius p =0.1 m m w e r e subjected to p u r e r o t a r y bending t e s t s in drilling mud.* The design of the s p e c i m e n s and a t t a c h m e n t s , t h e i r m a n u f a c t u r i n g p r o c e s s , and m a t e r i a l of the a t t a c h m e n t s a r e p r e s e n t e d in [7]. The investigations e s t a b l i s h e d (Fig. l a and Fig. 2a) that a d e c r e a s e of the loading frequency in the 2800-100 c y c l e s / r a i n range l e a d s to a d e c r e a s e of cyclic fatigue life of the smooth s p e c i m e n s and s p e c i -
-JT-The ciriii[r,g
mud was the clarified drilling fluid used f o r drilling in the Dolina oil-field region of the I v a n o - F r a n k o v s k d i s t r i c t [2]
~e
E
a
b
&
6
p
2 N, million cycles
~
q/7 q 2
4
~ i
10 20 40
I lil]L
]
J._~. I I I I l i
t00 ZOO 400 T,h
~000
Fig. 1. Fatigue of smooth s p e c i m e n s (1, 2, 3) and s p e c i m e n s with a s t r e s s r a i s e r (4, 5) vs n u m b e r of c y c l e s (a) and t i m e of loading (b): 1 and 4) 2800 c y c l e s / r a i n ; 2 and 5) 730 c y c l e s / r a i n ; 3) 100 c y c l e s / m i n . P h y s i c o m e c h a n i c a l Institute, A c a d e m y of Sciences of the Ukrainian SSR, L ' v o v . T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika M a t e r i a l o v , Vol. 10, No. 4, July-August, 1974. Original a r t i c l e submitted August 16, 1973. 9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.
438
],!
/
2
J
0,2
g.#
/
2
N, million cycles
4
10
2
I
4,
~ISHI
~ I Ill~ll
10 20
~0
f00 200 400 '[h
fO00
Fig. 2. Fatigue of specimens with steel (1, 2, 3), and r u b b e r (4, 5) attachments vs n u m b e r of c y c l e s (a) and time of loading (b): 1 and 4) 2800 c y c l e s / m i n ; 2 and 5) 730 c y c l e s / r a i n ; 3) 100 c y c l e s / rain. mens with steel and rubber attachments, w h e r e a s f o r specimens with a s t r e s s r a i s e r the effect of loading frequency on fatigue life was p r a c t i c a l l y not noted in the range of 0.2-10 million cyctes. The fatigue life, estimated by the time to f r a c t u r e (Figs. lb and 2b), is much g r e a t e r in the case of a lower loading frequency than in the case of a high frequency for all investigated specimens. It is n e c e s s a r y to note that in the case of smooth specimens the frequency factor is displayed m o r e considerably upon a d e c r e a s e of the level of mechanical loads. This is apparently related with an i n c r e a s e of the intensity of the c o r r o s i o n - f a t i g u e p r o c e s s as a consequence of the gradual destruction of the p r o t e c tive film by the c o r r o s i v e environment. For specimens with steel attachments the effect of the loading frequency is m o r e considerable at inc r e a s e d levels of mechanical loads. This is probably related with the fact that on specimens with attachments the protective film n e a r the edge of the sleeve is destroyed much s o o n e r as a result of cyclic f r i c tion and the formation of juvenile s u r f a c e s with a m o r e negative electrode potential o c c u r s , and c o n s e quently the intensity of the c o r r o s i o n p r o c e s s on the s u r f a c e of duralumin is higher. The fatigue limit of specimens with r u b b e r attachments owing to the insignificant contact p r e s s u r e s is slightly higher than that of specimens with steel attachments, but the nature of the fatigue c u r v e s is the same as in the case of steel attachments. The absence of an effect of the loading frequency on the fatigue limit of specimens with a s t r e s s r a i s e r is related with the time of action of the c o r r o s i v e environment. F o r the same test time the fatigue limit of specimens in the case of a high frequency is noticeably lower than in the case of a low frequency. Thus,the fatigue limit of the m a t e r i a l of drill pipes (alloy D16T) is affected both by the time of action of the c o r r o s i v e environment and by the loading frequency. The effect of the loading frequency is the r e sult of the action of a n u m b e r of f a c t o r s which depend on the s t r e s s level, m a t e r i a l of the a t t a c h m e n t s , a n d desig-n of the s p e c i m e n s . LITERATURE 1. 2.
3. 4. 5. 6. 7.
CITED
G. V. Karpenko, R. G. Pogoretskii, and I. I. Kadar, F i z . - K h i m . Mekhan. Mat., No. 3 (1972). N . A . Severinchik, R. G. Pogoretskii, A. M. Prishlyak, and G. V. Karpenko, F i z . - K h i m . Mekhan. Mat., No. 4 (1971). A . M . Prishlyak, R. G. Pogoretskii, G. V. Karpenko, and N. A. Severinchik, F i z . - K h i m . Mekhan. Mat., No. 3 (1973). A . V . Karlashov and V. P. Tokarev, Fiz.-Khim. Mekhan. Mat., No. 4 (1971). A . V . Karlashov and V. P. Tokarev, Zavod. Lab., No. 7 (1967). G. M. Sarkisov and F. K. Semagin, NTS, s e r i y a Mashiny i Neftyanoe Oborudovanie, No. 4 (1966). A. Mo P r i s h l y a k , R. G. Pogoretskii, and V. A. Proniv, F i z . - K h i m . Mekhan. Mat., No. 2 (1974).
439