PSI - Issue 38

Hamza Abbad El Andaloussi et al. / Procedia Structural Integrity 38 (2022) 238–250 239 Hamza Abbad El Andaloussi, Luc Mouton, Firas Sayed Ahmad, Xabier Errotabehere, Stéphanie Mahérault-Mougin, Stéphane Paboeuf/ Structural Integrity Procedia 00 (2021) 000 – 000 2 1. Introduction Structural maintenance of offshore units comes with safety, technical and economic challenges for oil companies and operators. Floating, Production, Storage and Offloading platforms (FPSO) are a good example: these offshore floating platforms are permanently moored units where safety standards and production stakes are extremely high. Unlike standard ships that can go to dry- dock for “crop and renew” operations, FPSOs maintenance shall be done in situ: in this case, “hot work” techniques may yield to high risky simultaneous operations and/or production disruption. FPSO operators have therefore been looking for “cold work” non -intrusive solutions that guarantee safe and economical hull repair. From the present literature review and writer experience, COLDSHIELD technology constitutes today the first solely bonded solution for the reinforcement of FPSO hull that is approved by Classification Societies for the reinforcement of corroded flat plating, even in the most loaded area submitted to hull girder strength. This technical approval has been possible by overcoming the technical limitations that are inherent to standard bonded reinforcements (installation control, design & strength, durability). Among them, strength prediction is possible with the presented solution as it removes stress singularities commonly observed on bonded reinforcement, and thus, allows to use a conventional stress-based approach [1] [2] . Establishment of a robust and reliable stress criterion was mandatory in order to be able to develop an industrial FPSO hull repair solution. Indeed, this new solution has been implemented in the past two years in offshore West Africa, notably in the frame of a deck repair, with several areas located at midship. This application constitutes one of the most severe application in this field, as the deck plating is subject to significant loading cycles that are transferred to the bonded reinforcement. Indeed, FPSO hull can be represented as a girder that withstands cyclic bending loadings due to waves (called WBM: Wave Bending Moment) and to crude oil loading/offloading cycles (called SWBM: Still Water Bending Moment). As such, the definition of a predictive fatigue design tool is needed to address this kind of application and is part of the compulsory steps for the justification of the “permanent” status of the reinforcement (in opposition to the “temporary” repairs). This article is focused on the fatigue behaviour of COLDSHIELD reinforcement, and details how a regulatory fatigue design tool (S-N curve), convenient for industrial projects, is defined for this new bonded solution (whereas standard solely bonded solutions require to use complex energy-based approach). First, the position of the problem is set, and the reason of the absence of S-N curves for bonded assemblies is discussed. Then, the fatigue tests campaign led on the studied bonded reinforcement is presented: the experimental set-up is detailed, and the results of fatigue tests are presented. Finite element analyses are performed in order to convert fatigue loads into acting stresses in the bondline (substrate/adhesive interface). This numerical work, specific to this product and validated by Class Societies [1] [2] is key for the definition of a S-N curve. Finally, a statistical analysis of all these results is presented for the construction of a design S-N curve, and these findings are discussed. 2. Position of the problem Reliability is one of the most important points for structural bonded reinforcements. Among all parameters affecting reliability, we could summarize the main ones as manufacturing, surface preparation, installation, strength and ageing (including fatigue). The discussion of all these subjects being not possible in a single article, the present paper sets its focus on fatigue. Generally, during a fatigue test campaign, a set of specimens with a defined geometry withstands a high number of constant load range cycles until failure. The repetition of the same test on a given number of specimens at several level of load ranges enables the definition of a fatigue curve. The fatigue curve describes the number of cycles that would lead to the failure of specimens for each level of load ranges. It provides accurate data on one specimens geometry and only for this geometry. This is why steel industry has generated fatigue results on a large variety of fatigue details, which represent the most commonly used connections in steel structures (BV Rules [3] and guidance note such as BV NI611 [4], DNV Fatigue standards [5] and other steel structure fatigue standard).