Engineering structures such as bridges and wind farms need to have a long useful lifetime. Structural Health Monitoring (SHM) via predictive simulations may be valuable towards lifetime extension. Digital Twins (DTs) are a suitable technology for such SHM, automated interventions, and providing analytical insights. In this paper, we present the Digital Twin as a Service (DTaaS), which is a collaborative online platform for management of DTs. This platform eases the task of building, using, and sharing DTs in a collaborative environment. The DTaaS provides real-time communication capabilities to link DTs with engineering structures, which helps with updating the models used in the DTs and thereby improving the prediction accuracy offered by SHM systems. The capabilities of the DTaaS are exemplified in the context of a wind turbine case study, in which a DT of the jacket-pile foundation is developed based on experimental data.