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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.

The objective of this paper is to present recent work in the development of a method for the strength assessment of offshore jacket structures subject to storm loading that is consistent across all global offshore regions. Recent developments in the application of response based metocean analysis together with the establishment of a standardized structural reliability model has resulted in an assessment method that can be applied in a consistent manner to all fixed jacket structures in all global regions. Applying this method in the “assessment” stage of the SIM process, as published by API and ISO, would aid global standardization of both the assessment methods and the performance criteria. The assessment methods have evolved over the last 3 years and have been subject to application and testing on several projects involving the assessment of existing offshore jacket structures. They have proven to be capable of discriminating between the differing extreme environments and differing evacuation and unmanning procedures that occur in different regions while being compatible with existing approaches to structural integrity. The paper will describe the standardized structural reliability model and provide examples of load factors for pushover and pushdown analysis for fixed steel offshore platforms that have a storm load capacity that is sensitivity to topside load and wave-in-deck load.

Many fixed steel offshore platforms in the Arabian Gulf are situated in shallow water and have a relatively high proportion of dead loading to environmental loading. As such their capacity is governed by the combined effect of extreme environmental loading and vertical dead loading. Reliability-based assessment methods normally consider the probability of the lateral capacity being exceeded by the extreme environmental loading. This assessment is typically performed by considering the pushover capacity of the platform compared with the regional environmental hazard curve. Allowance is made for uncertainty in the capacity and load model when evaluating the reliability. In the presence of significant dead loading, and for some ultimate failure modes, the sensitivity of pushover capacity to dead load plays an important role in the reliability analysis. This paper describes the methodology for reliability analysis against the extreme load hazard and which can be implemented into future assessment codes. Particular attention is focussed on the sensitivity of structures to high levels of dead load and how this may be incorporated into reliability analysis. The development of acceptance criteria for reserve strength ratios (RSRs) which depend on regional hazard curves and dead load is also described.

Offshore Structures are typically designed with a target end of field life between 20 and 30 years. Many offshore Structures remain in service while exceeding their designed life which requires robust methods for evaluating and maintaining target risk levels through regular sub-sea inspections. Risk based inspection plans are generated based on strength and fatigue analysis (typically S-N based fatigue assessment) which involves a number of uncertainties which are inherent to the fatigue process. These uncertainties inevitably lead to inconsistencies between analysis predictions and the outcome of underwater inspections. Although the inspection findings are acknowledged, there is generally not much effort made to make use of the inspection findings to optimise future inspection plans. This paper applies reliability updating methods to optimise inspection plans (inspection intervals) using fatigue reliability based on the results of in-service inspections. This methodology forms part of the work requested by ADMA-OPCO during the customisation of Atkins Fleet Management System (FMS) for the application to their Offshore Structures fleets. It has been reviewed and adopted by ADMA-OPCO to replace existing qualitative methods. The methodology works towards optimising their inspection efforts and cost to maintain acceptable target risk levels. The methodology is based on evaluation of the SN fatigue reliability since the SN Curves are implemented in the fatigue design methods outlined in International Standards. A probabilistic model for fracture-based reliability calculation is developed by calibration of initial crack size to match SN based fatigue reliability. This model enables utilisation of the Bayesian updating techniques which allow incorporation of the inspection results in the calculations. Environmental data from Umm Shaif Field (ADMA-OPCO) have been utilised in the calculations along with fatigue design SN Curves from the API RP 2A international standard. The calculations target the acceptance probability of failure levels of a critical component (hotspot) with a target design life of 30 years. In the event that no fatigue cracks are identified through in-service underwater inspections, the existing inspection plans can be revised utilising the developed methodology to optimise the inspection intervals, leading to cost saving and optimisation of the inspection plan. Major in-service inspection campaigns have been executed by ADMA-OPCO where no fatigue cracks were identified. Other operators within the region have also concurred that no fatigue cracks were observed during in-service inspection campaigns, which indicate that the methodology proves effective in optimising inspection costs within the region.