This book introduces the development of the digital twin of the marine infrastructure in Norway, which will be a significant scientific and operational achievement for the industry, making efficient and safe offshore operations possible. It enables data exchange safely and easily between different sub-systems, modules, and various applications. A complete digital twin ship will be presented in details. Thus, the twin ship can provide an integrated view of the ship's various physical and behavioral aspects in different stages, and allow simultaneous optimization of functional performance requirements. In addition, it enables advanced control and optimization, e.g., creating more reliable prediction for flexible objectives (time, output, emissions, fuel consumption), and executing day-ahead and long-term planning for operations. More importantly, several related applications and case studies are presented in the end to confirm the effectiveness of the digital twin system. The research work is not only interesting for academia, also for industry.