Modelling thickness limits transition joints between pipes of different thickness

Transcription

1 Institute of Materials Engineering TITLE: Modelling thickness limits transition joints between pipes of different thickness AUTHOR: Michael Law DATE OF REPORT: November 2008 APPROVED BY: W Payten ISSUED TO: APIA ANSTO REPORT NUMBER: R08M115 APIA project: APIA08-01 Australian Nuclear Science and Technology Organisation Institute of Materials Engineering Private Mail Bag 1, Menai, NSW 2234, Australia Telephone

2 Executive summary Thickness limits for welded joints between pipes of different yield strengths and thicknesses Executive Summary When two pipes of different strength and thickness are joined, the transition region is thinner than the permitted pressure design wall thickness of the thick weaker pipe. Currently the grade difference allowable by AS has a maximum yield strength ratio of 1.5. Finite element modelling and analysis led to the following conclusions:- Transitions can withstand high pressures (> 114% SMYS of the strong thin pipe material) with all grade combinations because of the support of the stronger pipe ( bridging ) Though bridging is significant under pressure loadings, the same joint under axial load gets very little support and fails below the SMYS of the thin pipe. Compressive loading (or the compression side of a bending load) significantly reduces the pressure capacity. An analytical method of assessing transition joints under pressure was developed and validated against FEA models. Recommendations The YS/thickness ratio restriction can be lifted. As premature failure may be caused by compression/bending loads, the following is currently suggested:- 1. No restriction on the transition WT/ grade ratio limit provided that the pipe is not subject to any bending or additional axial stress (including temperature effects). 2. If these conditions are not met, then either: (a) Existing WT/ grade ratio limitations apply, OR (b) Restrictions on the WT/ grade ratio limit can be lifted provided that stress analysis shows that maximum total axial stress limits are between -10% or +60% SMYS. 3 Under displacement controlled bending higher bending strains may be achieved stably. These must be demonstrated by project specific materials testing and modelling. 4 Testing of pipes with widely differing grades should be undertaken to validate this work.

3 Introduction When two pipes of different strength and thickness are joined, the wall of the thicker pipe may be machined so that there is a tapered transition in wall of the thicker (lower strength) pipe. The transition region is thinner than the permitted pressure design wall thickness of the thick weaker pipe. This apparent lack of strength is acceptable because of the support from the full thickness/strength sections on either side, particularly in the thin strong pipe. Currently the grade difference allowable by AS and ASME B31.8 has a maximum yield strength ratio of 1.5. If the 1.5 thickness ratio can be safely increased then it will save both time and cost in completing many pipeline projects. The project objective is to develop an understanding of the thickness limits for joining pipe manufactured from different yield strength and develop an understanding of how the support provided by the full thickness of each pipe permits the reduced thickness of the tapered section of the lower strength pipe to support the pressure and other loads. AS has the following requirement for the design of welded joints between pipes having different pressure design thicknesses: In the case of the thicker component, the thickness for design internal pressure shall not be greater than 1.5 times the nominal thickness of the thinner component. Figure 1 Weld preparation from AS δ tp = nominal wall thickness of inner pipe and δ dp = thickness for design internal pressure. Note that the thickness is not limited, just the design thickness, which is depends on the grade. Thus the grade ratio between the thin and thick pipe is limited to 1.5.

4 Pipe grade combinations which cannot currently be joined are noted with grey highlight (table 1). Table 1 Yield strength ratio (based on SMYS) for various pipe combinations Thick pipe or fitting grade Thin pipe grade B X42 X46 X52 X56 X60 X65 X70 X80 base X70 base X65 base X60 base X56 base X52 base X46 base X42 base In this work, joints not acceptable under AS are termed unacceptable ; thin will refer to the thin walled high strength pipe, and thick will refer to lower strength thicker pipe or fitting.

5 Literature Review George H, Rodaburgh E Tests of Pups Support Bridging Effect Pipe Line Industry, October 1959, pp This paper introduced the concept of the bridging effect, where the stronger or thicker material on either side of the taper supports the section of under-strength material in the taper. For a design factor of 0.72 using the X52 YS and thickness, the MAOP is MPa. The design thickness at 72% SMYS for the grade B pipe is 18.92, close to the actual Grade B thickness of 19.05mm. Tapers of 4, 8, and 16:1 were contained in an end-capped pipe. The pipe burst in the X52 pipe at MPa (157% SMYS). Table 2 Dimensions and properties for pipes OD (mm) t (mm) YS (MPa) UTS (MPa) YT ratio Grade B X An FEA model of the test vessel, using true stress/true strain data based on Table 2 predicted failure at 41.29MPa, adjacent to the 1:16 taper. In testing it occurred at 97% of this value, in the plain wall adjacent to the 1:4 taper. This indicates that material property and/or wall thickness variation may have occurred. The testing was inconclusive in one regard as none of the transitions failed. They recommended maximum taper of 1:16 based the measured strains in the tapers after testing. No specific recommendation of grade ratios was included in this paper. In the current project this test was modelled with FEA as a pressure vessel with the tapers used in the test (4:1, 8:1, 16:1) and also with additional tapers up to 128: Burst pressure (MPa) Taper results Thin X52 pipe Thick B pipe Taper (x:1) Figure 2 Burst pressure and taper ratio The finite element modelling showed that the tapers did not affect the burst pressure with tapers of less than approximately 32:1 for the grade combination tested.

6 Zhu X, Leis B Plastic collapse assessment method for unequal wall transition joints in transmission pipelines J. Pressure Vessel Technology, November 2005, V127, pp This paper ignored the taper and looked at failure in the pipe or fitting only, and proposed that failure would occur in the pipe when: - UTS 2 /UTS 1 > t 1 /t 2 If the fitting and pipe were designed to the same pressure and design factor, no grade combinations pass this criterion unless the fitting is made thicker. The taper is not assessed by this method. Table 3 possible pipe and fitting combinations Thin Pipe Grade Thick Pipe Grade UTS 2 /UTS 1 t 1 /t 2 x80 B X X X X X X X X70 B X X X X X X X65 B X X X X X X60 B X X X X X56 B X X X X52 B X X X46 B X X42 B The paper modelled only 30 o tapers (1.73:1), not the more common 4:1 taper. Longer tapers move the failure location to the taper area. This paper indicated that the controlling property was the tensile strength.

7 Cosham A, Hopkins P, Macdonald K (2007) Best practice for the assessment of defects in pipelines Corrosion Engineering Failure Analysis 14, 2007, pp Though this paper discussed common defects in pipelines, the following points are significant in assessing the tapered understrength section of the transition as a defect, as is done later in the current project:- The longitudinal extent of a corroded area is the most important parameter for the burst strength under internal pressure loading. The circumferential extent has a small influence on the burst strength, but the effect is sufficiently small to not need considering. However, the circumferential extent must be considered if external axial and/or bending loads are present. The effect of tensile external loads is generally small, whilst compressive loads can cause a significant reduction in burst pressure. No difference between the behaviour of internal and external corrosion has been noted in full scale tests or FEA (as pipelines are thin-walled geometries).

8 FEA Modelling All transitions with grade differences > 1.5 were modelled with D/t ratios of 35, 55, and 100. The fitting thickness was the design thickness. All tapers were at 4:1. Cases were analysed for pressure, and for axial loadings. Pressure cases - restrained pipeline Failure pressure (% SMYS) X80-B X80-X42 X80-X46 X80-X52 X70-B X70-X42 X70-X46 X65-B X65-X42 X60-B D/t=35 D/t=55 D/t=100 Figure 3 Failure pressure (% SMYS in the thin pipe) All joints failed > % SMYS in the taper. The lowest failure pressure in all cases was where grade B material was used on the thicker pipe, some of this effect is due to the greater length of taper required due to the greater thickness of the thick-walled pipe, and some is due to the low strength of the grade B pipe at the thin end of the taper. As there were not significant changes with D/t ratio, all possible grade combinations (Table 1) were modelled at a D/t ratio of 55 (figure 6).

9 120.0 YS ratio > Burst pressure (% SMYS) X80 base X70 base X65 base X60 base X56 base X52 base X46 base X42 base B Thick pipe grade Figure 4 Burst pressure (w.r.t the thin pipe), D/t =55, grade combinations with YT ratios > 1.5 are marked. All had burst pressures of between and % SMYS. The differences between acceptable and unacceptable grade combinations allowable under AS are small (Table 4) Table 4 Burst pressure comparison YS ratio < 1.5 YS ratio > 1.5 Maximum Minimum Average

10 Axial load cases These were subject to axial load ( force controlled loading ) with no internal pressure, as this gives the lowest failure strains (for example X80 - grade B section, D/t = 35 fails at 0.215% strain at 80% SMYS internal pressure, 0.185% strain at zero pressure. All bar one (X42 to B grade) failed at >79% SMYS axial stress (of the thin pipe), at a general strain > 0.15% strain, and all failed in the taper (figure 7). The failure stress and strain can be taken as straight tension, or as the tension component of bending. There is only minimal support from the stronger sections (unlike in the pressure case). Figure 5 X60 B grade transition approaching collapse at 97% SMTS axial load showing a maximum plastic strain of 21 % at the taper, with a general strain of 0.2 %.

11 120.0 YS ratio > Axial stress (% SMYS) X80 base X70 base X65 base X60 base X56 base X52 base X46 base X42 base B Thick pipe grade Figure 6 Axial failure stress (%SMYS of the thin pipe) 0.30 YS ratio > Axial strain % X80 base X70 base X65 base X60 base X56 base X52 base X46 base X42 base B Thick pipe grade Fig 7 Axial failure strain (average strain in thin pipe).

12 Table 5 FEA results with high YS ratio joints in bold Main (thin) pipe grade Thick pipe grade YS ratio Burst (% SMYS) Axial stress (% SMYS) Axial strain % 80 B B B B B B B B All pipes had adequate burst pressure, with little difference between the acceptable (YS ratio <1.5) and non-acceptable joints (YS ratio >1.5). The non acceptable joints showed lower failure stresses under axial load. The lowest axial failure strains were in the acceptable X42 to B grade combination.

13 Failure position Under internal pressure in a restrained pipeline, with low YS ratio combinations, failure generally initiates in the weaker pipe. As the YS ratio increases, the length of understrength material becomes significant and the failure location switches to the taper, with little change in burst pressure (fig 16). Figure xx Failure position under pressure in X70-X46 joint is in the taper (at % SMYS), in X70-X60 joint, failure is initiating in the thick X60 pipe (at % SMYS). Under axial tension, failure initiates in the taper for all cases (fig 17). X70 X46 axial X70 X60 axial Figure xx Failure position under axial tension is in taper for both X70-X46 and X70-X60 joints.

14 Combined axial plus pressure loading Models of X80-B transitions with a 4:1 taper and D/t ratios of 35 and 55, and an X70- X56 transition and a length of plain X70 pipe, both with a D/t of 55, were subject to a range of axial stress and internal pressures. The failure envelopes show typical features of transition joint behaviour. In a restrained pipeline, the transitions can withstand very high hoop stresses (> 114% SMYS) due to the support of the stronger sections (the bridging effect). Tensile axial stresses increase burst pressure up to a certain point (< 100% SMYS), then the burst pressure drops rapidly. Compressive axial stresses significantly reduce the burst pressure. In unacceptable joints (i.e. X80 B) buckling initiates at approximately 20% SMYS compression (depending on the D/t ratio), as seen by a step in the failure envelope. The buckling may be initiated by offset between the pipe centrelines. In acceptable joints (i.e. X70- X56) buckling is not seen. The X70 pipe (red line) envelopes all other joint behaviour. The X70-X56 joint is almost identical under axial tension, and shows a slightly lower failure pressure under axial compression. The X80-B joints are similar at axial loadings from -20% SMYS to +75% SMYS. X80-B Dt35 X80-B Dt55 X70-X56 Dt55 X70 pipe Dt Hoop stress (%SMYS) Axial stress (% SMYS) Figure 8 Failure envelopes in terms of SMYS of the thin pipe

15 Analytical method using the notch concept The tapered section of weak pipe can be modelled as a circumferential metal loss defect of equal strength pipe. To assess this analytically the following procedure was used, based on the defect assessment method in AS The end support or bridging effect is modelled by the Folias factor. To assess this analytically the following procedure was performed. The thin end of the pipe was converted to an equivalent thickness at the yield strength of the thick pipe (which is equal to the design thickness). The tapered notch in the equivalent thickness pipe is transformed into a rectangular defect of the same length and depth (figure 9). Effective taper length Design thickness Equivalent thickness at lower grade d 2c Figure 9 Equivalent thickness model Using the defect assessment method in AS , the defect is analysed by use of the Maxey equation and the Folias factor to assess the strengthening effect of end support ( bridging ). This gives the burst pressure. The procedure is as follows:- 1. Convert the thin pipe to an equivalent thickness of thick pipe (multiply t thin x YS thin /YS thick). The defect depth d = t 2 -t 1, where t is the design thickness, subscript 1 is the thin pipe and subscript 2 is the weak pipe 2. The taper is 2c 3. Calculate the Folias factor (M) for the flaw using c, R, and t

16 4. Calculate the yield elevation factor for a restrained pipeline [1] where F = ( x 2 x 2 v + x v 2 x 2 + xv + x + 1) and x = D inner / t 5. The failure hoop stress is calculated by σ fail. hoop d (1 ) t2 = YS * F * d (1 ) Mt 2 6. The burst pressure is calculated using the Barlow equation with the inner diameter of the thick pipe. σ P = fail. hoop ( ID2) * 2* t 2 The results for 9 pressure cases are shown below; the analytical method is generally conservative and close in value to FEA results (between 0.97 and 1.01 of the FEA prediction) Notch method result/ FEA result X80-B X70-X42 X65-B D/t=35 D/t=55 D/t=100 Figure 10 Comparison of analytical and FEA cases

17 Material properties above SMYS In most cases the YS will be above SMYS. The lower grade pipe is unlikely to be coated, and coating low grade material doesn t tend to raise the YS significantly, whereas the higher grade pipe is likely to be coated, and coating may raise the YS significantly (by more than 50 MPa). The joint modelled was X70 B, the modelling had 2 sets of material properties for the B grade: - B at SMYS (241 MPa) and SMYS + 20 MPa (261 MPa). There were 3 sets of material properties for the X70:- X70 at SMYS (483 MPa), SMYS + 40 MPa (520 MPa), and coated SMYS + 80 MPa (560 MPa). Cases were modelled with pressure and also with axial load. Table 6 Failure stress Pressure Axial B SMYS B SMYS+20 B SMYS B SMYS+20 X70 SMYS X70 SMYS (+40) X70 coated (+80) Failure stress (% SMYS) Pressure B SMYS Pressure B SMYS+20 Axial B SMYS Axial B SMYS X70 SMYS X70 SMYS+40 X70 coated (+80) Figure 11 Failure pressure Increased yield strength increased the failure stress. The higher grade B material had a moderate effect on burst pressure (average 3.5 %) and a smaller effect on axial stress (average 1.6%). An increased X70 YS increased burst pressure more (6.3% for 520 MPa YS, and 11.0% for 560 MPa YS) and less for axial stress (2.4% for 520 MPa YS, and 2.7% for 560 MPa YS). Yield strengths above SMYS result in small increases in the failure stress. Though the increased YS can not be used in design, use of SMYS value is conservative in assessment as any increases in the YS of either component increase the strength of the joint.

18 Fatigue Different thickness pipes will generally have an offset in their centrelines; this will cause a bending stress which can enhance fatigue crack growth. One fatigue code (Recommended practice DNV RP C203 - Fatigue design of offshore steel structures) recommends the use of external tapers and a constant ID, as the stresses are much higher at the inner wall. Figure 12 Suggested use of external tapers from DNV RP C203 - Fatigue design of offshore steel structures While the use of steep tapers such as 1:4 is recommended for pressure and axial cases, these enhance bending stresses and fatigue crack growth. Pipelines subject to fatigue require longer tapers such as 1:10 or more to minimise the bending stresses. If possible these transitions should be on the outer wall with the pipe having equal internal diameters where fatigue is a critical issue. Taper on inside or outside of pipe The previous section on fatigue design suggests that it is preferable to have any transition occurring on the outside of the pipe for fatigue life improvement. Modelling of X80 to B grade joint with D/t ratios of 35, 55, and 100 with equal IDs was undertaken to assess any effects. Table 7 Burst pressure for inside and outside tapers Inside Outside Inside / Outside D D D The burst pressures were similar for these cases. This supports the comment in the Cosham [2] paper reviewed earlier that there was little difference between internal and external defects.

19 Effect of weld metal strength The strength of the weld metal (matched or overmatched) is significant in defect tolerance. The effect of weld metal strength on the behaviour of the transition was analysed in transition joints. The case modelled was of an X80 to grade B transition with a 1:4 taper. The weld metal was either equal in strength to the X80 pipe, or 50 MPa stronger. The stronger weld metal increased the burst pressure of the transition minimally (25.86 vs MPa). The axial load case was internally pressurised to 80% SMYS, overmatched weld metal slightly increase the axial failure strain (1.46%, vs. 1.40%). It appears that weld strength is not a significant factor in failure of the transition joint.

20 3D modelling A section of X80 pipe (D/t = 55) joined to thick Grade B pipe with a taper of 4:1was subjected to bending, with zero, 40, 80, 100 and 110% SMYS internal pressure. Pressure vessel or restrained pipe??? The bending load was applied both as displacement controlled and also as force controlled load. All pipes failed at the tension side of the bend (figures 13, 14, 15). Force controlled loading lead to lower failure strains while larger strains can be stably achieved under displacement control. Specific analysis would be required to demonstrate this. Table 8 - Maximum bending strain (%) at failure. Internal pressure (% SMYS) Displacement controlled bending Force controlled bending Maximum bending strain % Load control Displacement control Internal pressure % SMYS Figure 13 Bending strain at failure under displacement control

21 Bending Figure % SMYS pressure, bending, and von-mises stress Bending Figure % SMYS pressure, bending, plastic strain, failure on tensile side

22 Conclusions Transitions can withstand high pressures (> 114% SMYS of the strong thin pipe material) with all grade combinations because of the support of the stronger pipe ( bridging ) Under internal pressure, the failure location in YS<1.5 cases is in the thick pipe, and moves to the taper as YS>1.5, with little change in burst pressure. Though bridging is significant under pressure loadings, the same joint under axial load gets very little support and fails below the SMYS of the thin pipe. Joints unacceptable by AS have the lowest axial failure stress. In all axial cases, failure is in the taper. Compressive loading (or the compression side of a bending load) significantly reduces the pressure capacity. Higher bending strains can be achieved under displacement control; to make use of these would need to be demonstrated by project specific materials testing and modelling. An analytical method of assessing transition joints under pressure was developed and validated against FEA models. The use of the 1:4 taper is satisfactory, shallower tapers produce longer sections of lower strength material and shorter tapers may have issues with fatigue, piggability, and inspectability Weld metal strength has little effect on the strength of the transition. The position of the taper, inner wall or outer wall has little effect on the strength of the strength of the transition. Yield strengths above SMYS for either component result in small increases in the failure stress. Thus assuming SMYS is conservative. Recommendations It appears that the YS ratio restriction can be lifted. As premature failure may be caused by compression/bending loads, the following restriction is recommended:- 1. No restriction on the transition WT/ grade ratio limit provided that the pipe is not subject to any bending or additional axial stress (including temperature effects). 2. If these conditions are not met, then either: (a) Existing WT/ grade ratio limitations apply, OR (b) Restrictions on the WT/ grade ratio limit can be lifted provided that stress analysis shows that maximum total axial stress limits are between -10% or +60% SMYS. 3 Under displacement controlled bending higher bending strains may be achieved stably. These must be demonstrated by project specific materials testing and modelling. 4 Testing of pipes with widely differing grades should be undertaken to validate this work.

23 References 1 M Law, G Bowie, L Fletcher Poisson s Induced axial stresses in restrained pipelines: their effects on yielding and subsequent plasticity Journal of Pipeline Integrity, V4 N1 (2005) pp Cosham A, Hopkins P, Macdonald K (2007) Best practice for the assessment of defects in pipelines Corrosion Engineering Failure Analysis 14, 2007, pp