With the expectation of hull girder asymmetry and corresponding shift in elastic neutral axis resulting from collision damages and other forms of structural deteriorations, the interaction of vertical and horizontal hull girder capacities become quite significant in the assessment of ship structural safety. This paper therefore extends the application of a previously proposed interactive-numerical probabilistic based methodology for structural safety to assess the hull girder ultimate strength reliability of a damaged ship by means of a user-defined numerical framework. Hull girder capacity is calculated using the NS94D ultimate strength code, which is based on the Smith’s progressive collapse method. The resulting deterministic responses have been interactively linked to the NESSUS probabilistic framework so that the reliability of the damaged hull girder is predicted using an implicit limit state function defined based on a transformation of coordinates to appropriately account for any shift in the neutral axis. Random deviations of the constituent variables are directly applied to calculate the ultimate strength deterministic responses, thereby circumventing the need to characterize any correlated strength variable, which is at best subjective. The conventional approach of characterizing ultimate strength by an assumed coefficient of variation and distribution type was found to be conservative in predicting structural safety of ships relative to the proposed method. Application of the interactive-numerical technique for structural reliability is therefore considered significant for problems involving correlated random variables with unknown statistical characteristics. The method is being considered to predict the safety of cracked hull girders by accounting for the residual strength and further load bearing capabilities of deteriorated and adjacent elements.

This paper describes the implementation of the Advisory Hull Monitoring System (AHMS) onboard the existing Bonga FPSO (Nigeria) during operations and production. AHMS has been developed for FPSOs in the Monitas Joint Industry Project as a fully automated system, which explains and advises on the fatigue lifetime consumption of the hull of FPSOs. The explanations and advice offered are based on a comparison between the design and the actual predicted lifetime consumption by replacing the design parameters including environmental and operational conditions with the measured data. The system differentiates between the contributions of environmental and operational conditions as well as hydro-mechanic and structural responses. The AHMS system comprises hardware and software for smart data gathering and processing. AHMS hardware includes strain- type sensors on deck and inside the Water Ballast Tanks (WBTs) and/or void spaces and interfaces with external systems including the Computer Loading Instrument (CLI), Gyro and metocean system. AHMS has generally been installed onboard newly built floaters including the Usan FPSO (Nigeria), Clov FPSO (Angola), Ichthys FPSO (Australia) and Moho Nord FPU (Congo). Being in operation since 2005, the Bonga FPSO has lived 14 of its 20-year design life. Given the constraints inherent in her design, deployment of the AHMS for the FPSO's hull fatigue life monitoring therefore presented unique installation challenges to overcome as would be expected for ageing brownfield assets. To add to this challenge, the installation was carried out during production and so required strict adherence to the stringent safety requirements of Simultaneous Operations (SIMOPs) on a live plant. This paper describes in detail, the AHMS hardware, the complexity and challenges of their installation for the Bonga FPSO and highlights lessons learned for typical brownfiled retrofit of this nature. OCTOPUS MONITAS, the software of the AHMS system for the smart data processing, calculates onboard fatigue lifetime consumption of the hull and explains the differences against design predictions. Methodology of the software is herein described, and the first set of measurements taken from the Bonga FPSO as well as preliminary results produced by the software are similarly presented.

Delivering full service life performance for mooring systems of Floating Production Storage & Offloading assets (FPSO) has been a frustrating challenge to operators across the industry. Remaining strength and fatigue assessment on degraded top mooring chains of the Bonga FPSO and Single Point Mooring (SPM) loading Buoy has been investigated as part of an in-house Bonga Asset Preservation Program. Both facilities are located approximately 120 km off the coast of Nigeria in the Gulf of Guinea operating in tropical waters just North of the Equator, where top chain links have been subjected to accelerated deterioration from Sun Corals and other forms of Microbiologically Induced Corrosion (MIC). These phenomena have led to overall corrosion rates being slightly above general design requirements, but more importantly to formations of large pitting on several sections of the top chain links. Remotely Operated Vehicles (ROV) assisted inspections, chain link cleaning and underwater 3D photogrammetry have allowed capturing the surface geometry of representative degraded chain links of the mooring lines to provide detailed input data for further analyses. Reverse engineering has been performed via Finite Element Analysis and fracture mechanics methodologies using the scanned geometry of selected highly exposed critical links to estimate the residual strength and fatigue life performance of the degraded links relative to their original design criteria. To evaluate the potential impact of cracks on the capacity of degraded chains relative to a reference link, crack tip Stress Intensity Factors have been computed at worst case stress-raising pits and parametric analyses using varying initial crack sizes have been performed to calculate the number of years for the cracks to propagate to critical sizes. A baseline for benchmarking the strength, fatigue and crack growth behaviour of the degraded links investigated has been provided by analysing non-degraded and uniformly corroded links after 12 years of service with projection to end of service life capacity. The paper provides a comprehensive application of numerical methods for assessing the fitness-for-service of the chains and recommendations on in-situ performance integrity management by circumventing the need to retrieve chain samples for testing.