Lenguaje y Convención

The series o f tests carried out in Chapter 3 form a part o f a study programme at UCL on the effect o f CP o f high strength steels under variable amplitude loading. The first phase o f the study was on tubular joint, carried out by Etube [4.5]. The second phase was for the thick plate specimens, which forms part o f Chapter 3 and the third was for the long life fatigue corrosion tests on thin T-butt welded plates. Further third series of tests were completed for Long Life Test (LLT) at UCL on thinner T-butt welded plates to study the fatigue behaviour at lower stress range region. The second and third phase make up the HSE Offshore Technology report [4.8].

The objective of LLT was to provide long life corrosion fatigue data for high strength steels welded plate. A series o f tests was carried out using DILLIMAX690E-Z15, a high strength Jack-up steel with yield strength o f 690 MPa, which was available from the second phase. The plate thickness was 16mm and Dillinger Hutte supplies the grade in a different composition to the 85mm plate, which were used for the flush ground welded plates. The ACPD technique was used to monitor crack growth behaviour.

The seven T-butt-welded components were made from 16mm thick flame-cut ground plate, and contained full-penetration welds at the attachment plate. The plate dimension is shown in Figure 4.11 and the dimension o f the test set-up for the four point bending rig is shown in Figure 4.12. A spectrographic metallurgical examination of the steel is shown in Table 4.4 and the mechanical properties are given in Table 4.6.

The tests were performed under four-point bend fatigue loading, using a JOSH Variable Amplitude loading sequence. The specimens were tested in air, using a mean frequency 2Hz and in the seawater environment, using two levels o f CP at -800mV and -1050mV. These seawater tests were performed at a mean frequency of 0.2Hz.

Fatigue results for other high strength steels were obtained from Lindley et al [4.9]. The work was carried out on higher strength steels with yield strength o f 470 MPa. The specimen tested were 50mm thick T-j oints with fillet welded, load carrying, full penetration welds. The joint was subjected to in-plane bending, with the base plate

Chapter 4 Analysis Work on Fatigue Test Results

restrained under constant amplitude. The fatigue results are displayed in Figure 4.13. These are then compared to the results from Chapter 3, where Figure 4.14 is for the air data and Figure 4.15 is for the seawater data.

4.3.1 Discussion on Comparison Study

S/N results for the Long Life Test (LLT) are shown in Table 4.3. The S/N plot o f the results is presented in Figure 4.16. The results from the LLT study are combined with the earlier work on thick T-butt plates from Chapter 3 and are shown in Figure 4.17. The plots for the seawater tests with CP at -SOOmV and -1050mV are shown in Figure 4.18 and Figure 4.19 respectively.

Table 4.4 shows the chemical composition of the material used in part five and this must be compared with Table 4.5 for the 85mm thick plate used for fabricating parent and flush ground welded plates. The material mechanical properties for the 16mm LLT plates are shown in Table 4.6 and for the 85mm plates in Table 4.7. It is evident that although the strength levels are very similar, the composition o f the two supplied steels are clearly different. It is normal practice for the supplier to change the composition with plate thickness.

For the LLT study, three plates were fatigue tested in air, one of which was a run-out. The air fatigue results fit in nicely to the earlier air tests conducted on thicker plates of similar strength material as shown in Figure 4.17. The S/N curve shows that the data for both compositions is very close to the P curve (Class F) mean curve.

Figure 4.18 shows the long life data for a CP o f -800mV and also the data from other VACF tests. It would seem that the behaviour for both steels is similar, and all results are better than the BS 4360 50D P Curve (Class F) CP design line. Figure 4.19 shows the long life data for a CP of -1050mV and also the data from Chapter 3. In this case the data for long life is closer to the P Curve (Class F) CP design line whilst from the previous study the data lies higher than the air and CP design lines. Overall, the number of tests carried out was limited and more tests are required to establish the behaviour at the lower stress region.

C hapter 4 Analysis Work on Fatigue Test Results

The fatigue results obtained for higher strength steels at UCL were also compared to results from other published study. However, there was limited data available for high strength steel plate. Lindley [4.9] carried out a study on the effect o f different CP level on high strength steels with yield strength around 470 MPa. The 50mm thick T plates were fatigue tested under constant amplitude in-plane loading. The data are plotted together with the fatigue results from Chapter 3 as shown in Figure 4.14 and Figure 4.15. The S/N curves show the fatigue results to be in good agreement even though they are for different material and under different loading condition. Figure 4.14 show that the air data from Lindley has slightly better fatigue performance than that tested in this thesis. For the seawater result as shown in Figure 4.15, both the -850mV and the overprotection (-lOOOmV and -1050mV) are within the same cluster. The interesting point though is the performance o f the steels for free corrosion, where the result is comparable to that of -850mV and better than -1050mV at high stress range. However, it also shows that at lower stress range, the steels perform better with CP for -800mV and -lOOOmV than that o f free corrosion. Thus CP is at its most effective at lower stress range. The result at lower stress region also show the slope to less steep and did not fit in with the LLT study, but this could be due to the effect of variable amplitude. Under constant amplitude at lower stress range, the welded joint is affected by the fatigue limit, where the effect might be less under variable amplitude loading.

Overall, the results show that the steels perform satisfactorily as all the points in the S/N curve are comparable to the mean BS436 50D mean line. This compare well with the work carried out for SE702 tubular joints. As stated by Etube [4.5] tubular joints fabricated from SE702 grade steels are as good under fatigue loading as the conventional fixed platform steels. The fatigue testing studies with overprotection carried out on SE702 type steels at UCL has shown not to be susceptible to hydrogen embrittlement.