Recent advances in robotics technologies have opened multiple opportunities for the use of robots to support various activities of our daily life and to interact with humans in different ways. In such contexts, it is crucial to identify potential threats related to physical human-robot interactions and to assess the associated risks that might affect safety and dependability. Because of the complexity of human-robot interactions, rigorous and systematic approaches are needed to assist the developers in: i) the identification of significant threats and the implementation of efficient protection mechanisms to cope with these threats, and ii) the elaboration of a sound argumentation to justify the level of safety that can be achieved by the system. To fulfil these objectives, we believe that risk analysis should be carried out based on system models as soon as possible in the development process and hence provide elements to reason about system safety using a structured argumentation. The risk analysis method HAZOP-UML presented in this paper is a guided method to identify potential occurrences of harm, their causes and their severity. The results from risk analysis are then used as input for safety case construction in which we structure an argument about system safety. This process is illustrated by a case study on a robotized rollator.