RESEARCH CONCERNING THE LOCAL LOSS OF STABILITY UNDER EXTERNAL PRESSURE AND TENSION OF OIL INDUSTRY TUBULARS

Transcription

1 ГОДИШНИК НА МИННО-ГЕОЛОЖКИЯ УНИВЕРСИТЕТ СВ. ИВАН РИЛСКИ, Том 48, Св. I, Геология и геофизика, 25 ANNUAL OF THE UNIVERSITY OF MINING AND GEOLOGY ST. IVAN RILSKI, Vol. 48, Part I, Geology and Geophysis, 25 RESEARCH CONCERNING THE LOCAL LOSS OF STABILITY UNDER EXTERNAL PRESSURE AND TENSION OF OIL INDUSTRY TUBULARS Vlad Ulmanu, Dragos Gabriel Zisopol, Andrei Dumitresu, Ciprian Niolae Trifan Petroleum-Gas University, Ploiesti 68, Romania ABSTRACT. Two of the most important loads whih an deisively affet the resistane apaity of asing, tubing and submarine pipelines are the external pressure (ausing the ollapse phenomenon) and the tension, espeially in high pressure wells and when installing deep water sea lines. This paper presents the researh ativities based on the investigation of the external pressure (ollapse) phenomenon, inluding the effet of the axial tension, for perfetly irular tubes in order to define the design methodologies and riteria for assessing the resistane apaity of oil industry tubulars. For this purpose, the tests have been performed on small sale pipe speimens, based on the similitude law, and the results have been ompared with the alulation formulae usually applied to assess the pipe resistane apaity to ollapse with tension. ИЗУЧАВАНЕ НА ЧАСТИЧНИТЕ ЗАГУБИ НА САТБИЛНОСТ ПРИ ВЪНШНО НАЛЯГАНЕ И НАПРЕЖЕНИЕ ПРИ ТРЪБИ ЗА ПЕТРОЛНАТА ИНДУСТРИЯ Влад Улману, Драгос Габриел Зисопол, Андрей Думитреску, Циприан Николае Трифан Университет за петрол и газ, Плоущ 68р Румъния Introdution An important load whih an deisively affet the resistane apaity of oil industry tubulars is the external hydrostati pressure. Under the effet of suh pressure, often ombined with tensile and/or bending loads, the loal bukling (loss of stability) phenomenon an our leading to the ovalisation followed by flattening of tubulars. Suh phenomenon is of ruial importane for asing and tubing (mostly in high pressure wells), and for submarine pipelines during the installation phase (when the pipeline is empty), espeially in deep waters. The present tendeny of oil industry to move towards the exploitation of deeper and deeper oil wells and the installation of deep water submarine pipelines (the present world reord water depth for suh sealines is 25 meters) led to inreased requirements regarding the ollapse resistane apaity of pipes for asing, tubing and sealines. In suh ontext, the researh ativities desribed in this paper aimed to investigate the loal bukling phenomenon for perfetly irular tubes under the ombined effet of external pressure and axial tension by performing some tests on small sale models, based on the similitude law. In the past, the authors have performed a series of experimental results to investigate the ollapse phenomenon under external pressure only, inluding the effet of the pipe initial ovality (Dumitresu, 998; Zisopol, 2; Dumitresu and Zisopol, 24; Zisopol and Dumitresu, 24). In the future, the researh will be ontinued by investigating the most important fators affeting the pipe loal bukling 235 phenomenon (geometrial imperfetions of pipes for oil industry tubulars, mainly the initial pipe ovality; pipe material anisotropy, level of residual stress, et.) and also the effet of bending loads on suh phenomenon. Review of previous results regarding loal bukling of perfetly irular tubes Along the years, various researhers proposed a series of alulation formulas, based on theoretial models and/or test results, to evaluate the ritial external pressure at ollapse, p, for perfetly irular (nominally round) pipes and to asses the influene of the axial tension, N, on suh value. The main problem emerging from these studies was that the loal bukling mehanism differs essentially with the value of the ratio between the pipe outside diameter, D, and the pipe wall thikness, t. For great values of suh ratio (D/t > 35), ollapse (loal bukling under external pressure only) ours by means of an elasti flattening, before the pipe material reahes its yield strength. For small values of the D/t ratio (under 5...2), typial for instane for deep waters submarine pipelines, ollapse will take plae in the plasti field. Finally, for D/t = , the pipe failure mehanism is muh more omplex an elasti-plasti ollapse will take plae. In ase of elasti failure of a perfetly irular tube, the ritial value of the external pressure (the so-alled elasti ollapse pressure) is given by the following equation (Langner, 99):

2 p 2E = pe = 2 3 ν / ( D t), () where E is Young s elasti modulus of the tube material, and ν is Poisson s oeffiient. For tubes with thiker walls, for whih a plasti ollapse will our, the ritial external pressure value is dependant on the pipe material harateristis. Suh value an be assessed either as the external pressure value for whih the maximum irumferential stress reahes the yield strength or as the pressure value for whih the entire transverse setion of the tube plastiizes. If onsidering the thin-wall tubes theory, whih assumes a onstant value of the irumferential stress - σh - aross the tube wall thikness, both variants above lead to the same value of the ritial pressure (the so-alled plasti ollapse pressure): p = p F = 2σ t/d, (2) where σ is the minimum speified yield strength (SMYS) of the pipe material. In the transition zone between elasti and plasti ollapse, haraterised by omparable values of pressures pe and pf (for D/t = ), a gradual passage is atually taking plae from the elasti failure mehanism to the plasti one. As a onsequene, the simplest alulation method for the ritial pressure in suh ase is to assess the value of p as the minimum between the values of pe and pf. However, suh assessment leads to ollapse pressure values greater than the ones obtained as test results. Due to this reason, different alulation relationships have been proposed for a perfet irular tube, presented by Langner (99) and Dumitresu (998). After investigating these relationships and omparing them with our experimental results (Dumitresu, 998; Zisopol and Dumitresu, 24), we have reahed the onlusion that the best results are obtained using the following Shell relationship, proposed in 975: p = p E p F (p E 2 + p F 2 ) -/2. (3) Equation (4) above has been developed for the ase of a perfet irular tube (no geometrial imperfetions, material anisotropy, et. have been onsidered). That is not the ase in pratie, as a pipe is always affeted by suh imperfetions and espeially by ovalisation. The equations developed to inlude the effet of initial ovality of a tube on the ritial ollapse pressure values have been investigated in our previous work (Dumitresu, 998; Zisopol, 2; Dumitresu and Zisopol, 24). Our onlusion has been that the relationship proposed in 98 by de Winter, imposed by the most reent internationally reognized Code dediated to submarine pipelines (DnV, 2; BSI, 993), muh used worldwide, leads to the best results: (p - p E ) (p 2 - p F 2 ) = p p E p F δ D/t, (4) where δ is the initial ovality of the pipe with a minimum reommended value of %. 236 In this paper, both equation (3) and (4) with δ = % - have been used to asses the value of p. If, in addition to the external pressure, a tensile load is applied to the pipe, its resistane to loal bukling dereases signifiantly. Suh redution is governed by the ratio between the axial tensile stress, σl, and σ. This ratio is atually equal to the ratio between the axial tension, N, applied to the pipe and the axial fore orresponding to yielding of the entire pipe setion, given by the following equation: N F = π (D t) t σ. (5) Various researhers tried to aount for the axial tension influene on the ritial ollapse pressure, p. The most reommended and used method (espeially for submarine pipelines) is based on the von Mises ombined stress theory and onsists of adjusting the predited ollapse pressure value for axial tension (Dumitresu, 998). Aording to this method, the yield strength value, σ, is adjusted by multiplying it with the following orretion oeffiient: α σ L σ L = σ, (6) 2 4 σ where theσl/σ ratio an be replaed with. The adjusted value of σ will be used to alulate the plasti ollapse pressure using equation (2), and then the ritial ollapse pressure, using equations (3) or (4). This last value an be ompared with the one alulated in the absene of the axial tension. Another method to aount for the axial tension effet is to use an interation formula inluding the ratio between the ritial pressure when the axial load is present, p, and the ritial ollapse pressure in the absene of the axial load, p, and the ratio. After investigating several suh formulas, Zisopol (2) reahed the onlusions that the best results are obtained for asing and tubing if using the following equation (developed initially for oiled tubing): p N + =, (7) p N F As it an be easily observed, in all equations presented above the ritial pressure value, p, depends only on the D/t ratio, and therefore the similitude law an be applied to study the pipe ollapse phenomenon. As a onsequene, tests an be performed on small diameter pipe speimens who an be onsidered small sale models of large diameter pipes. Based on the statement above, a pipe loal bukling testing faility (under external pressure and axial tension) has been designed and onstruted. An image of the testing faility is shown in figure 2, while its sheme is inluded in figure 3. The pressure hamber is shown in figure.

3 Fig.. Pressure hamber of the testing faility The testing devie an develop a maximum hydrostati pressure of bar and a maximum axial tension of kn, while the outside diameter of the pipe speimens an be 6 mm or, in ase the lids (see fig. 3) are hanged, 32 mm. The minimum required length of the pipe speimens is 5 mm. Fig. 2. Loal bukling testing faility Fig. 3. Loal bukling testing faility sheme Experimental results regarding loal bukling of perfetly irular pipes The tests performed aimed at studying the loal bukling phenomenon under external pressure and axial tension for tubes that an be onsidered perfetly irular (haraterised by very low values of geometrial imperfetions). The tests have been performed using 24 steel speimens, whih were taken from 6 seamless pipes (4 speimens from eah pipe). The main harateristis of these pipes are shown in Table. The yield strength and the ultimate tensile strength values of the speimens materials have been verified in eah ase by performing tensile tests on pipe samples, aording to API methodology (API, 98). 237

4 Table Speimens Charateristis No. D/t Ratio Material E235 EN297/ MoCr STAS 3478 Yield Strength [MPa] Ultimate Tensile Strength [MPa] The pipe speimens have been mahined both outside and inside in order to obtain very small values (under. %) of the initial pipe ovality. Moreover, the pipe eentriity values, measured by utting the speimens after testing, have been found to be suffiiently low (under %) in order not to have any pratial influene on the ritial ollapse pressure obtained during testing. Based on the above, it has been onluded that the 8 pipe speimens used for testing an be onsidered as perfetly irular tubes. For eah pipe tested, one speimen has been used to determine the ritial ollapse pressure in the absene of the axial fore (N=), while the other 3 have been firstly tensioned to an axial tension orresponding respetively to 4%, 7%, and % of NF, given by equation (5). The experimental values of p (obtained for N=) have been ompared with the results obtained using equations (3) or (4), ombined with equations () and (2). A good agreement has been observed between experimental and theoretial results. The experimental values of p in the presene of an axial tension have been ompared with the results obtained using equation (7) or equation (6), ombined with equation (2) and with equation (3) or (4). The three variants of theoretial results have been plotted as a dependene p/p = f (). The urves obtained, together with the test results are synthesised in figures 4-9 for eah pipe used to obtain the investigated speimens. Figures and show some of the test speimens after testing, while figures 2 and 3 show some of the test speimens setioned in the ollapsed zone. If omparing experimental test results with alulated values, a good agreement has been observed, with the exeption of the tests for whih N=NF, due to the random fators affeting the pipe loal bukling behaviour whih annot be inluded in the theoretial models. The three alulation methods used to asses the ritial ollapse pressure in the presene of an axial tension have given very lose results with the exeption of pipes with D/t ratios lose to 3. In these ases, the alulation methods based on equation (6), developed espeially for the submarine pipelines, are reommended as suh D/t ratios are typial for these pipelines. The post-ollapse onfiguration of a perfetly irular tube, i.e. with negligible geometrial imperfetions, have been also analysed during the tests program. P/P. Fig. 4. Results for E235 steel, D/t=28 P/P. Fig. 5. Results for E235 steel, D/t=9.33 P/P. Fig. 6. Results for E235 steel, D/t=

5 P/P. Fig. 7. Results for MoCr steel, D/t=27.6 Fig.. Collapsed test speimens made of E235 steel P/P. Fig. 8. Results for MoCr steel, D/t=9.6 P/P. Fig.. Collapsed test speimens made of MoCr steel Fig. 9. Results for MoCr steel, D/t=2.24 Fig. 2. Setioned test speimens made of E235 steel Fig. 3. Setioned test speimens made of MoCr steel 239

6 If onsidering the pipe transverse setion, two typial onfigurations have been identified, as follows (see figs. 2 and 3): - an ovalised pipe onfiguration, with the pipe ovality inreasing with the D/t ratio value, finally reahing an 8 - shape; suh onfiguration orresponds to the theoretial one, espeially in the ase of plasti ollapse; - a total flattening of the pipe speimen, harateristi for an elasti ollapse. Conlusions The tests performed, even if using a relatively small number of pipe speimens (24), allowed for an evaluation of the alulation methods proposed by various researhers in order to haraterise the loal bukling phenomenon under external pressure and axial tension for the ase of perfetly irular tubes (haraterised by small values, well below the allowed ones, of their geometrial imperfetions). If omparing the alulation methods onsidered to asses the influene of the axial tension on the ritial ollapse pressure for pipes without geometrial imperfetions with the test results, it an be onluded that these three methods an be used in the same measure to evaluate suh influene. The only exeption has been observed for pipelines with D/t ratios lose to 3, for whih the alulation methods based on equation (6), developed espeially for the submarine pipelines, are reommended. Referenes BSI, 993. Code of Pratie for Pipelines (BS 8) Part 3. Pipelines Subsea: Design, Constrution and Installation, British Standards Institution, London. DnV, 2. Offshore Standard OS F-: Submarine Pipeline Systems, Det Norske Veritas, Hovik, Norway. Dumitresu, A Studies on Improving Resistane and Stability Charateristis of Subsea Pipelines, Ph. D. Thesis, Petroleum-Gas University of Ploiesti. Dumitresu, A., D. G. Zisopol. 24. Influene of Initial Ovality on Loal Bukling Under External Pressure (Collapse) of Oil Industry Tubulars Experimental Results. In: Oil Gas European Magazine International Edition of Erdöl Erdgas Kohle, Hamburg, 3, 3, Langner, C. G. 99. Introdution History and Review of Collapse. In: Proeedings, Seminar on Collapse of Offshore Pipelines, Amerian Gas Assoiation. Ulmanu, V Oil Tubular Material. Tehnial Publishing House, Buharest. Ulmanu, V., D. G. Zisopol, C. N. Trifan Theoretial Experimental Researh onerning the Influene of the Exploitation Charateristis on the Coiled Tubing Life Time used in the Oil Industry, 3335/ CNCSIS Grant, TD Type between Petroleum-Gas University of Ploiesti and the Ministry of Eduation and Researh. Zisopol, D. G. 2. Researh Conerning the Coiled Tubing Manufaturing used in Petroleum-Gas Industry, Ph. D. Thesis, Petroleum-Gas University of Ploiesti. Zisopol, D. G., А. Dumitresu. 24. Experimental Results for Loal Bukling Under External Pressure (Collapse) of Perfetly Cirular Tubes. In: Oil Gas European Magazine International Edition of Erdöl Erdgas Kohle, Hamburg, 3, 2, API, 98. Bulletin 5C3 on Formulas and Calulations for Casing, Tubing, Drill Pipe and Line Pipe Properties, 3 rd edition, Amerian Petroleum Institute, Dallas. Reommended for publiation by Department of Drilling and Oil and Gas Prodution, Faulty of Geology and Prospeting 24