Rodriguez-Sanchez, Jose Efrain; (1999) Fatigue crack repair for offshore structures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A fatigue crack repair concept based on crack removal by cutting is presented. The aim is to extend the fatigue life of cracked welded connections. The concept developed presents an option to repair fatigue cracks particularly for structures installed in deep waters. Thus, the application of inspection techniques and cutting procedures such as Electrochemical Machining (ECM) more suited to Remotely Operated Vehicle (ROV) deployment are addressed. Repairs of large fatigue cracks require the use of subsea clamps. A major problem is ensuring that load on stud-bolts is maintained. A second study considered the use of Alternating Current Stress Measurement (ACSM) for stress measurement. Results from a study of stress analysis of the repair geometry using the finite element method are given. Recommendations for repair profiles were developed based on the analysis and experimental results. An experimental testing programme was conducted considering welded T-butt and butt specimens. The numerical stress analysis was validated experimentally and conclusions to the application of ROVs and cutting techniques were made. The repair geometry was machined on the specimens using ECM, disc cutting and ROV grinding thus, pros and cons of each technique were identified. Application of compressive residual stresses by shot peening was briefly investigated and some observations are given. Analysis of experimental data was made for the determination of fatigue life extension after repair. Analysis of the experimental crack shape evolution data was made to propose a crack shape design curve to determine stress intensity factors. Predictions of fatigue cracking after repair using fracture mechanics analysis are made and compared to the experimental results. Observations of ROV performance on crack repair and stress measurement on bolts for tightening clamps were obtained from full scale trials. This allowed an assessment of ACSM for deep water applications. Finally, conclusions and recommendations for future work are given.

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