The authors present the study design and main findings of a quasi-experimental evaluation of the learning efficacy of the Serious Game (SG) 'Hazard Recognition' (HR). The SG-HR is a playable, two-level demonstration version for training supervisors who work at oil and gas drilling sites. The game has been developed with a view to developing a full-blown, game-based training environment for operational safety in the oil and gas industry. One of the many barriers to upscaling and implementing a game for training is the questioned learning efficacy of the game. The authors therefore conducted a study into the game's learning efficacy and the factors that contribute to it. The authors used a Framework for Comparative Evaluation (FCE) of SG, and combined it with the Kowalski model for Hazard Detection and the Noel Burch competence model. Four experimental game sessions were held, two involving 60 professionals working in the oil and gas industry, and two with engineering students and consultants. Relevant constructs were operationalized and data were gathered using pre and post-game questionnaires. The authors conclude that the SG-HR improves players' skills and knowledge on hazard detection and assessment, and it facilitates significant learning efficacy in this topic. The FCE proved very helpful for setting up the evaluation and selecting the constructs.