This paper summarizes a risk framing and assessment exercise to support a major development decision in Azeri-Chirag-Gunashli field located in Azerbaijani sector of Caspian Sea, using at scale implementation of ensemble modelling technique. A multi-disciplinary team identified a comprehensive list of geological and mechanical uncertainties and their inter-dependencies that could impact the project outcome. 20000 subsurface scenarios with unique uncertainty parameter combinations, based on Monte Carlo approach, were modeled using reservoir simulation software. Match quality to the actual performance helped to select an ensemble of unique subsurface scenarios. With the non-subsurface uncertainties in the predictive stage, the ensemble was used to inform the investment case's incremental value range. The ensemble predicted a cloud of outcomes around the reference case with -25% and +12% uncertainty range. This cloud assumed the same (reference case) sequence and location of the wells. The team selected distinct scenarios from the cloud to communicate the risks on key project metrics. Incremental recovery and production ramp-up variation against significant uncertainties guided the selection of distinct downside and upside scenarios for economic evaluation to ensure the robustness of development decision. The initial cloud was considered to be unmanaged as the well locations were exposed to reservoir property, connectivity, sweep, and other uncertainties for a specific scenario in the ensemble. The team recognized the long duration of development activities and tangible subsurface learning opportunities from drilling through multiple reservoirs and other surveillances, particularly on the key uncertainties ("heavy hitters"). This observation was leveraged to manage uncertainties and make appropriate well location and sequence decisions. Overall uncertainty range slightly narrowed down to -20 % and +16 % while providing an appropriate downside and upside profiles for the investment case. Assessing the impact of key uncertainties on investment value helped refining surveillance and risk management plan with focus areas on well and facility design to ensure risk management measures are executable.