Online Structural Integrity Monitoring of Fixed Offshore Structures

Abstract

Abstract Natural frequency monitoring of offshore platforms is a valuable tool forstructural integrity management. Online structural integrity monitoring assumeseven greater significance when platforms are highly susceptible to damage dueto environmental wave loading and fatigue processes. The implementation of anonline monitoring system allows rapid detection of member failures therebyminimizing the possibility of exposure to a high risk of platform collapsebetween inspection intervals and improving the structural reliability. Thispaper illustrates an application of online structural natural frequencymonitoring applied to a fixed North Sea jacket structure and describes how itis used within the context of structural integrity management. Introduction Structural failure of offshore platforms can lead to loss of life, loss ofasset and environmental pollution, and this risk needs to be reduced to anacceptable level. Monitoring of structural integrity of offshore fixedplatforms is essential for ensuring their safety in service. Structuralmonitoring measures the structural response with damage being identified by achange in the measured response such as natural frequency (Roitman et al. 1992, Sanderson et al. 2002), flexibility (Sunder and Ting 1985), accelerations(Hillis and Courtney 2011) and vibration modes (Li et al. 2008, Mangal et al.2001). The UK Health and Safety Executive (HSE) published a state of the artreview on the offshore structural monitoring techniques (OTO 97 040 and OTO 98046). Many offshore fixed platforms are approaching or have exceeded their originaldesign life, with a good proportion seeking life extensions. Maintainingstructural integrity of aging offshore structures is an ever increasingchallenge. Consequently, there is a growing interest in techniques that onlinemonitor the state of a structure. The assessment of remaining life can beenhanced with knowledge of the structural performance during a period of time, resulting from the use of online monitoring. These can provide both acost-effective solution to integrity management, as well as providing acontinuous output on the state of the structure. The implementation of anonline monitoring system allows for instantly detecting member failures therebyminimizing the possibility of exposure to an elevated risk of platform collapsebetween inspection intervals and improving the structural reliability. Onlinestructural integrity monitoring assumes even greater significance whenplatforms are highly susceptible to damage due to environmental wave loadingand fatigue processes. This paper illustrates an application of naturalfrequency based online structural integrity monitoring applied to the CNRInternational-operated Ninian Southern Platform, which is sensitive to fatiguedamage, and describes how online structural monitoring is used within thecontext of structural integrity management.