This is the first of three companion papers which examine the elastic buckling and collapse of laser-welded sandwich panels with an adhesively bonded core and uni-directional vertical webs. In compression parallel to the webs the face sheet in between two webs can be treated as a long rectangular plate supported at the sides and resting on a continuous elastic foundation. Using a generalised form of the Kirchoff equation the plate buckling load is evaluated for a Winkler and Pasternak foundation, and for a foundation model where the core is characterised by an elastic halfspace. Although the elastic half-space model is most sophisticated in the way that the decay of transverse stress through the core is described, it requires that the buckling load is calculated iteratively and is therefore not suitable for engineering applications. In contrast, the Pasternak foundation model is able to demonstrate the effect of different boundary conditions at the laser weld and different core properties by the variation of a simple clamping factor and two foundation moduli. By changing the clamping factor the model can be used to describe plate buckling as well as wrinkling, and by varying the foundation moduli it is possible to distinguish between symmetrical and anti-symmetrical buckling modes. Evaluation of the buckling solution shows that for low modulus core materials the plate buckling strength is determined only by the boundary conditions at the laser weld, while for high modulus materials anti-symmetrical wrinkling dominates. To improve the accuracy of the solution for non-standard configurations the foundation moduli are calibrated using the elastic half-space model and finite element results as a reference. It is found that by using a smaller Winkler foundation modulus the accuracy and consistency of the Pasternak model are improved considerably. However, the results also show that for current sandwich configurations the proportional limit of the face plate material is usually reached well before elastic buckling occurs. All experimental work is therefore delegated to the third paper, which deals with extension of the Pasternak foundation model into the elasto-plastic regime and non-linear finite element modeling.

This is the second of three companion papers which examine the elastic buckling and collapse of laser-welded sandwich panels with an adhesively bonded core and uni-directional vertical webs. In the first paper it has been shown that the critical load in compression parallel to the webs can be accurately predicted by using a Pasternak foundation to model the core and by using a simple clamping factor to describe the boundary conditions at the laser weld. In this paper the same model is adapted to predict the critical load in compression normal to the webs. As the face plate is now short and wide rather than long and narrow as before, a 'constrained wrinkling' approach is outlined which recognises that the face plate is limited in its ability to wrinkle naturally and can only deform in certain mode shapes. The clamping factor and the effective wave length are derived for different buckling mode shapes and boundary conditions at the laser weld, and the constrained wrinkling load is determined by evaluating the buckling load for successive modes and simply picking the smallest value. It is shown that for low modulus core materials the constrained wrinkling load is equal to the bare plate buckling load and for high modulus materials the solution converges toward antisymmetrical wrinkling, as has been found for the case of compression parallel to webs. The constrained wrinkling load agrees well with finite element results but as before the proportional limit of the face plate material is usually reached well before elastic buckling occurs.

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This is the last of three companion papers which examine the elastic buckling and collapse of laser-welded a sandwich panels with an adhesively bonded core and uni-directional vertical webs. By evaluation of the buckling stress in the first two papers it has been found that the buckling stress in compression parallel and normal to the webs typically reaches the proportional limit of the face plate and web material well before elastic buckling occurs. Hence, this paper presents an extension of the buckling model into the elasto-plastic regime, with the aim to determine the ultimate (local) strength of the sandwich and to allow experimental verification of the results. Using tangent modulus theory to 'plasticise' the elastic buckling model, the ultimate strength is evaluated for a sandwich configuration with high-strength steel face plates and a broad range of core moduli. The critical load predicted by the inelastic buckling model agrees well with non-linear finite element results and experimental values obtained from compression testing.

A high-fidelity FPSO Structural Digital Twin (SDT) based on Reduced Basis Finite Element Analysis (RB-FEA) coupled with inspection data and physical sensor measurements (advisory hull monitoring system) is presented to demonstrate a complete FPSO "digital thread" that combines operational data feeds, detailed structural analysis based on as-is asset condition, and automated structural integrity reporting. This lays the groundwork for a philosophical shift for asset lifecycle management by enabling the use of "as-measured" conditions in lieu of assumed "design-conditions" for a more accurate, and robust understanding of asset health. We demonstrate the deployment of this methodology for the Bonga FPSO and discuss the value that it brings during day-to-day operations.