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Abstract Ageing offshore assets are increasingly being retained beyond their original nominal design lives due to tie-backs, incremental developments, and changes in CP depletion forecasts. For many assets, life-extension decisions depend on whether external corrosion control remains effective for the intended extended service period. CP retrofit design is commonly performed case-by-case using new-build guidance, inspection data, and engineering judgement, noting there is no dedicated, fully prescriptive international standard that covers CP retrofit design across offshore asset types. For in-service assets, this often drives over-conservative retrofit scopes because the structure is already polarised, calcareous deposits and biofouling change electrochemical response, installation access is limited, and remaining anodes may still provide meaningful protection margin. This paper presents a practical multiphysics modelling led workflow that integrates inspection and CP survey data with three-dimensional electrochemical simulation to (a) quantify the true current demand of the in-service asset, (b) estimate the residual contribution of existing CP systems, and (c) optimise retrofit configurations by simulation, including remote and suspended concepts that minimise subsea intervention. "Calibration" is used to constrain uncertain inputs (coating breakdown, effective polarisation curves, electrical continuity, and current density assumptions) until model predictions reproduce measured field data within defined tolerances, using evidence such as ROV potentials, anode depletion measurements, and rectifier and reference electrode data. Representative case histories demonstrate the method across: (1) floating production assets, where recoverable suspended ICCP modules are optimised for CP protection of the hull (2) fixed subsea structures, where examples of multi-structure coupling, mutual interference and shielding driven under protection is modelled and identified and corrective retrofits are prescribed using targeted local anodes or distributed anode-string concepts; and (3) pipelines, where 3D CP modelling is used instead conventional analytical attenuation models to quantify end-of-life risk and evaluate low-intervention retrofit options. Across the representative studies, the modelling-led approach reduced retrofit anode capacity requirements by approximately 30% versus conservative redesign, without compromising CP potential compliance, while reducing offshore installation burden by focusing on fewer, higher-impact interventions.