  With the introduction of autonomous technology into our society, e.g.
autonomous shipping, it is important to assess and assure the safety of
autonomous systems in a real-world context. Simulation-based testing is a
common approach to attempt to verify performance of autonomous systems, but
assurance also requires formal evidence. This paper introduces the Assurance of
Digital Assets (ADA) framework, a structured method for the assurance of
digital assets, i.e. novel, complex, or intelligent systems enabled by digital
technologies, using contract-based design. Results are shown for an autonomous
ferry assurance case, focusing on collision avoidance during the ferry's
transit. Further, we discuss the role of simulation-based testing in verifying
compliance to contract specifications, to build the necessary evidence for an
assurance case.
