Maintenance is fundamental for the safe and profitable operations of many critical assets, including gas networks. The inherent complexity of these assets and budgetary constraints pose significant challenges to the decision-making related to maintenance management, which requires trading-off among conflicting objectives while respecting technical and normative constraints. The difficulty of such decision-making stems from the incomplete knowledge of the technical parameters, operating conditions and degradation states of the components. Here, we address this challenge by proposing a risk-based maintenance framework for supporting decision makers in selecting maintenance plans that are optimal with respect to the objectives and constraints considered. We apply Robust Portfolio Modeling (RPM) to identify those maintenance decisions that are most effective in reducing the severity and likelihood of failures in the gas network. RPM allows us to handle partial knowledge on the objective values and on the preferences of decision-makers. We lay down the complete steps of the framework, including the quantification of the likelihood of failures and their consequences for the population and for the gas network operation. The framework is demonstrated on the high pressure natural gas pipeline network of Great Britain. The results reveal that if there are no spatial constraints on the budget allocation, maintenance actions are focused on some critical zones, e.g. Scotland and the southernmost part of England. Instead, if predefined maintenance budgets are divided among the different areas of the network, the selected maintenance projects are sub-optimal and the risk reduction achieved over the maintenance horizon is smaller.