Acting systems aim at refining high-level actions into executable commands, while managing access to resources, possible failures, or any other unpredictable situation. Improving the trust on autonomous robots also requires to have a formal model of acting, and the capability to perform some analysis on this model. In this paper, we present ASPiC, an acting system based on the modeling of robot's skills using a specific control-flow Petri net model. The skills can then be combined using well-defined operators to build a complete plan that refines a high-level action. Some properties are guaranteed by construction, while others can be verified on the resulting plan model. ASPiC is finally applied to an area protection mission by an autonomous surface vehicle.