  This paper presents VORRT-COLREGs, a hybrid technique that combines velocity
obstacles (VO) and rapidly-exploring random trees (RRT) to generate safe
trajectories for autonomous surface vessels (ASVs) while following nautical
rules of the road. RRT generates a set of way points and the velocity obstacles
method ensures safe travel between way points. We also ensure that the actions
of ASVs do not violate maritime collision guidelines. Earlier work has used RRT
and VO separately to generate paths for ASVs. However, RRT does not handle
highly dynamic situations well and and VO seems most suitable as a local path
planner. Combining both approaches, VORRT-COLREGs is a global path planner that
uses a joint forward simulation to ensure that generated paths remain valid and
collision free as the situation changes. Experiments were conducted in
different types of collision scenarios and with different numbers of ASVs.
Results show that VORRT-COLREGS generated collision regulations (COLREGs)
complaint paths in open ocean scenarios. Furthermore, VORRT-COLREGS
successfully generated compliant paths within traffic separation schemes. These
results show the applicability of our technique for generating paths for ASVs
in different collision scenarios. To the best of our knowledge, this is the
first work that combines velocity obstacles and RRT to produce safe and COLREGs
complaint path for ASVs.
