Recommended Hot-Spot Stress Design S-N Curves for Fatigue Assessment of FPSOs

S J Maddox, TWI

Published in the International Journal of Offshore and Polar Engineering (Paper also given at 10 ^th International Offshore and Polar Engineering Conference (ISOPE), Stavanger, Norway, 17-22 June 2001)

Abstract

FPSOs are critical structures in which there is a high risk of fatigue cracking and therefore comprehensive design guidance is required. An approach based on the hot-spot stress is expected to be the most suitable for treating welded joints in which weld toe cracking is the likely mode of failure. As part of a recent JIP addressing the fatigue design of FPSOs, available hot-spot S-N data obtained from weld details relevant to FPSOs, including results obtained as part of the JIP, were evaluated as the basis for hot-spot stress design S-N curves.

Introduction

The hot-spot stress approach has been in routine use in the design of steel tubular structures for over 25 years (HSE, 1995). However, the approach is not yet fully developed for application to plate structures like ships. Two issues still need to be addressed:

• The definition of the hot-spot stress and how it is derived from stress analysis of the structure.
• The choice of hot-spot stress design S-N curves.

Both issues were addressed in a major Joint Industry Project (JIP) 'FPSO - Fatigue Capacity', which included extensive FEA and fatigue testing of typical FPSO weld details (Lotsberg 2001). The present paper addresses (b), partly on the basis of fatigue test results obtained in the JIP (Kim 1999) but mainly from relevant published fatigue test data for a wider range of structural connections and dimensions.

A basis for hot-spot stress design S-N curves

Another significant publication was Partenen (1995). This presents a very extensive database (180 results) of hot-spot stress S-N data obtained over many years in Finland from specimens of around 10mm thickness. The document provides a case for adopting FAT 100 as the hot-spot stress design S-N curve for plates up to 10mm thick. Assuming the plate thickness correction (t _ref/t) ^0.1, this would translate to FAT 90 for t = 25 mm. Unfortunately, the actual test data were not presented in the document and therefore they cannot be evaluated in the same way as the other published information.

Although hot-spot stresses were determined by FEA in some cases, this was not generally the case and greater reliance was placed on measured stresses. Therefore, in the present assessment the fatigue test results were evaluated in terms of hot-spot stresses estimated from measured strains, FEA results only being used in one case, (Dahle, 1997), when measured stresses were not quoted. An advantage of using measured stresses is that allowance is included for any secondary stresses in the test specimens, for example due to misalignment, which would not be included in FEA of an idealised geometry. Consequently, scatter in the fatigue data should be reduced. However, against this it needs to be borne in mind that the resulting hot-spot stress S-N curve will be less conservative as a design curve than one based on lower-bound FEA estimates of the hot-spot stresses, as used in the evaluation of fatigue data from the present JIP by Fricke (2001).

In most cases it proved possible to determine the IIW extrapolated hot-spot stress and the stress 0.5t from the weld toe. There was not usually sufficient information to determine the third hot-spot stress, by extrapolation from stresses 0.5t and 1.5t from the weld toe, but where possible this was also estimated.

Preliminary assessment of individual test series

In a preliminary assessment of the available data each set of results was analysed for comparison with the proposed FAT 90 design curve. S-N curves were fitted by regression analysis (excluding unbroken specimen results) assuming a slope m = 3 to facilitate comparison with the design curve. The lower 95% confidence intervals (approximately two standard deviations of log N below the mean) were also determined. Table 2 summarises the resulting differences between the FAT 90 mean and design curves and the fitted S-N curves. They are compared on the basis of the ratio of the fatigue strength given by the data to that obtained from the FAT90 curve at the same life. Where possible, this is shown for the experimental data expressed in terms of both the IIW hot-spot stress range and the stress 0.5t from the weld toe. A ratio of less than 1.0 means that the experimental value is below FAT 90, indicating that FAT 90 would not be safe.

Table 2 Comparison of published hot-spot stress S-N data and FAT90 mean and design curves

It will be seen that almost every set of data supports FAT 90 used in conjunction with the IIW hot-spot stress range. One exception, the corner gussets tested by Kawono (1993), involved virtually the same test specimen as Yagi (1992), which did satisfy FAT 90. The small difference may reflect the difficulty of estimating the IIW hot-spot stress from the information provided. The other case where the FAT 90 design curve was unconservative, Dahle (1997), was one in which the lower 95% confidence limit was particularly low because of the few very high fatigue lives (which were obtained under constant amplitude loading). Thus, again it would seem reasonable to overlook these results. In some cases FAT 90 appears to be over-conservative, but it will be noted that this was not the case for the structural components, apart from the hopper corner structure tested by Kim (1999). However, this case was somewhat surprising in that the test results were consistent with published data for failure from a transverse weld toe when expressed in terms of nominal stresses, which were well defined. A possible explanation is that the hot-spot stress has been overestimated. Certainly there was some doubt about the validity of the strain measurements. The results used to determine the hot-spot stress in the present evaluation were those which were closest to the FEA stress distribution. However, much lower stresses were also measured and they would have led to lower hot-spot stress estimates and much closer agreement with FAT 90. In view of the importance of the hopper corner detail to FPSOs, it is planned to investigate these results further in Phase II of the JIP.

Turning to the use of the hot-spot stress 0.5t from the weld toe, it will be seen that more results indicate that FAT 90 is un-conservative. Thus, FAT 80 may be the better choice for use with this hot-spot stress. The main problem was associated with the structural corner connections. A feature was that the stress gradient approaching the weld toe was relatively high, with the result that the stress 0.5t from the weld was significantly lower that the IIW hot-spot stress. Examination of the stress distributions provided in the papers by Yagi (1992), Kawono (1993) and Marquis (1995) indicates that a better estimate of the IIW hot-spot stress, for use with the FAT 90 design curve, would be the stress 0.3t from the weld toe.

Finally, from the few cases in which the hot-spot stress could be estimated using the '0.5t/1.5t' extrapolation method, it was up to 5% lower than the IIW value for plate specimens, but up to 12% lower for structural corner joints, reflecting the high stress gradient referred to above. Assuming that these differences were typical, it would seem that FAT 90 is still suitable for simple details, but FAT 80, 12% lower fatigue strength, would be required for structural components incorporating high stress gradients. However, note that it was only possible to estimate the '0.5t/1.5t' hot-spot stress in four of the investigations considered and so this conclusion is tentative.

Thus, the published data highlight the problem of estimating the hot-spot stress for situations in which there is a high stress gradient approaching the weld, as in most of the structural components. It seems that the IIW '0.4t/1.0t' extrapolation method produces hot-spot stresses that satisfy FAT 90, but FAT 80 may be more appropriate if the hot-spot stress is assumed to be the stress 0.5t from the weld toe. Few data were available to check the '0.5t/1.5t' extrapolation method, but those available also favoured a lower design curve than FAT 90 for situations of high stress gradient.

In practice, it would be unrealistic to relate the choice of stress analysis method and design curve to the stress gradient since it is not known in advance. A better approach is to relate the S-N curve to the method of stress analysis regardless of the stress gradient. On the basis of the data considered here, this would be reasonable for the IIW (FAT 90) and 0.5t (FAT 80) definitions of the hot-spot stress. However, the distinction is less clear for the '0.5t/1.5t' extrapolation method and downgrading to FAT 80 for all cases could be over-conservative. Further consideration of the data in terms of the two hot-spot types (a and c) helps to resolve this issue.

Analysis of available S-N data

Type (a) hot-spots

Figures 5(a) and (b) include the FAT 90 and FAT 80 mean and design S-N curves respectively, and the mean and 95% confidence intervals fitted to the data (neglecting results for unfailed specimens) by regression analysis. In both cases, the results from plate specimens, all of which consisted of plates with longitudinal fillet welded attachments, are in close agreement. The results obtained from structural components are more widely scattered, especially when expressed using the hot-spot stress 0.5t from the weld toe ( Fig.5(b)), but generally agree with those for plate specimens. Based on the fitted mean S-N curve and 95% confidence intervals, the results in Fig.5(a), in terms of the IIW hot-spot stress range, show that FAT 90 is conservative and would actually support FAT 100. This margin is sufficient to encompass the differences between hot-spot stresses estimated by the IIW and '0.5t/1.5t' extrapolation methods. Thus, FAT 90 should still be applicable for both extrapolation methods. In contrast, due to the low estimates of the hot-spot stress in the structural corner joints using 0.5t, as discussed earlier, FAT 80 is appropriate. Indeed, the lower 95% confidence interval actually coincides with the FAT 80 design curve.

Thus, the total database for type (a) hot-spots supports the adoption of FAT 90 for hot-spot stresses estimated by either of the two extrapolation methods, and FAT 80 for hot-spot stresses 0.5t from the weld toe.

Fig.5b) Fatigue test results for type (a) hot-spots, expressed in terms of the hot-spot stress 0.5t from the weld toe

Type (c) hot-spots

The remaining results are presented in the same way as the type (a) hot-spot data in Figs.6(a) and (b). It will be seen that there is close agreement between all the results, with little scatter. The mean S-N curve fitted to the results expressed in terms of the IIW hot-spot stress range ( Fig.6(a)) is slightly lower than the FAT 90 mean, while the lower 95% confidence interval is just above the FAT 90 design curve, due to the low scatter. Thus, the results provide reasonable support for FAT 90. The results in Fig.6(b), expressed in terms of the hot-spot stress range 0.5t from the weld toe, are compared with FAT 80. In this case, the mean S-N curve virtually coincides with FAT 80 mean, but the lower 95% confidence limit is significantly higher than FAT 80 design. In fact, it lies just above FAT 90 design. However, results below the confidence limit lie close to or on the FAT 80 design curve and therefore FAT 80 seems to be appropriate.

Fig.6b) Fatigue test results for type (c) hot-spots, expressed in terms of the hot-spot stress 0.5t from the weld toe

The three sets of results which did not agree with those in Fig.6 are plotted in Fig.7. Also shown are the 95% confidence intervals enclosing the results in Fig.6. The results obtained by Dahle (1997) are only available in terms of the IIW hot-spot stress and therefore they are only shown in Fig.7(a). Similarly, those from Dexter (1993) are only available in terms of the hot-spot stress 0.5t from the weld toe and they are only included in Fig.7(b). However, the overall picture is the same in both figures, with the extra sets of data generally lying above the confidence intervals enclosing the remaining hot-spot type (c) results. Higher results might be expected from Dahle, since the stress ratio was -1 and the fatigue crack grew in a shell bending stress field. Similarly, the fatigue cracks in Dexter's specimens grew in shell bending stress fields and therefore higher fatigue lives are to be expected. This might also be an explanation for the hopper corner structure data from Kim. Thus, in every case there are aspects of the results that could explain why they were higher than expected on the basis of the results given in Fig.6. From the practical viewpoint, the proposed FAT 90 for use with the IIW hot-spot stress or FAT 80 with the 0.5t hot-spot stress are conservative for these extra data.

Fig.7b) Exceptionally high fatigue test results for type (c) hot-spots, expressed in terms of the hot-spot stress 0.5t from the weld toe

Hot-spot stresses determined by extrapolation from 0.5t and 1.5t lie between the other two. However, since both lower 95% confidence limits in Fig.6 lie above the FAT 90 design curve, it will probably be reasonable to assume FAT 90 for use with this hot-spot stress. Against this argument, it will be recalled that the use of hot-spot S-N curves based on measured stresses may be unconservative when used for design with calculated stresses. Therefore, caution is needed before accepting the conclusions drawn. The lack of calculated hot-spot stresses for the published data provides an important limitation on their use for defining hot-spot stress design curves. Thus, their value would increase if, in future, the specimen geometries could be analysed using the FEA procedures recommended in Fricke (2001). Relevant work would form part of Phase II of the JIP.

Conclusions

• Individual sets of fatigue data support FAT 90 as the hot-spot stress design S-N curve when used in conjunction with the IIW definition of the hot-spot stress. The same is true for the other two definitions of the hot-spot stressconsidered for simple weld details, but FAT 80 would be more appropriate for structural components incorporating high stress gradients approaching the weld toe.
• However, there was limited support for the more useful solution of adopting FAT 90 for hot-spot stresses obtained by either of the extrapolation methods and FAT 80 for hot-spot stresses 0.5t from the weld toe.
• Evaluation of the results was hindered by the wide scatter in some cases and the lack of data expressed in terms of all three hot-spot stress definitions. However, support for the more practical solution comes by combining the dataand considering them in terms of hot-spot type (a) or (c).
• Fatigue data from beams with cope-holes also support the use of FAT 90 but only in conjunction with a different definition of the hot-spot stress (extrapolation from 4 and 10mm from weld toe).
• The stress 0.3t from the weld toe proved to be a better estimate of the IIW hot-spot stress than 0.5t.
• Further work is needed to analyse the results in terms of hot-spot stresses calculated using FEA, and to address type (b) hot-spots, welds on plate edges.

Acknowledgements

References

Andrews, R. M (1996): The effect of misalignment on the fatigue strength of welded cruciform joints. Fatigue Fract. Engng.Mater. Struct. Vol.19. No.6, pp755-768

BSI (1993): BS 7608:1993 Code of Practice for Fatigue Design and Assessment of Steel Structures. BSI, London.

Dahle T (1997): 'Fatigue behaviour of a railway component - application of new design criteria and TIG remelting techniques', in 'Welded High-Strength Steel Structures', Proc. 1st North European Engineering and Science Conference (NESCO I), EMAS Publishing, Cradley Heath, West Midlands, p.519-526.

Dexter R J, Tarquinio J E and Fisher J W (1994): 'Application of hot-spot stress fatigue analysis to attachments on flexible plate', Proc. 13 Intl. Conf. 'Offshore Mechanics and Arctic Engineering', ASME, New York, p85-92.

DNV(1998): Classification Note No.: 30.7. Fatigue Assessment of Ship Structures. Det Norske Veritas, Oslo.

DNV (2000): Recommended Practice RP203. Fatigue Assessment of Offshore Structures. Det Norske Veritas, Oslo.

Fricke W (2001): 'Recommended hot-spot analysis procedure for structural details of FPSOs and ships based on round-robin FE analysis', ISOPE, Stavanger, June.

Hobbacher, A.(1996): Fatigue Design of Welded Joints and Components. IIW, Abington Publishing, Abington, Cambridge

Huther I, Lieurade H P, Sayki N and Buisson R (1996): 'Fatigue strength of longitudinal non-load carrying welded joints. Use of the hot-spot stress approach', in ' Fatigue of Welded Components and Structures', Proc. 3rd Intl. Spring Meeting, Les Editions de Physique, Les Ulis Cedex A, France, p.57-64

Kawono H and Inoue K (1993): 'A load approach for the fatigue strength evaluation of ship structure', in ' Fatigue Design', Proc. Intl Symposium, Mechanical Engineering Publications Limited, Bury St Edmunds, p.95-108

Kim, W. S (1999): 'Fatigue Tests of Typical Welded Joints'. Hyundai Heavy Industries Co., Ltd. Ulsan (unpublished)

Koskimaki M and Niemi E (1995): 'Fatigue strength of welded joints in three types of stainless steel', IIW Doc. XIII-1603-95.

Lotsberg, I 'Background and overview of the FPSO-Fatigue capacity JIP', to be presented at OMAE, Rio, June.

Maddox S J (1997): 'Developments in fatigue design codes and fitness-for-service assessment methods', IIW International Conference on Performance of Dynamically-loaded Welded Structures, WRC, New York, pp22-42.

Maddox S J and Manteghi S (1993): 'The application of fatigue design rules to large welded structures', TWI Report No. 5587/22/93, Dec.1993 (unpublished).

Marquis G B and Kahonen A (1995): 'Fatigue testing and analysis using the hot-spot method', VTT Publication No. 240 'High Cycle Spectrum Fatigue of Welded Components', P O Box 2000, Fin-02044 VTT, Finland.

Miki C and Tateishi K (1997): 'Fatigue design of cope hole details in steel bridges', Int. J. Fatigue, Vol.19, No.6, p.445-455.

HSE (1995): 'Offshore Installations: Guidance on Design, Construction and Certification'. HSE. London.

Partenen T and Niemi E (1995): 'Collection of hot-spot S-N curves based on tests of small arc-welded steel specimens', IIW Document XIII-1602-99.

Yagi J and Tomita Y (1992): 'Definition of hot-spot stress in welded plate type structure for fatigue assessment (Report 1),' J. Society of Naval Architects of Japan, Vol.169, June, p311-318 (IIW Doc XIII-1414-91).