A novel subsea shuttle tanker (SST) concept was proposed as a cost-effective alternative to subsea pipelines and tanker ships for liquid CO2 transportation between a source facility and a subsea well. The SST will be deployed to transport CO2 to marginal subsea fields with an annual CO2 storage capacity of around 1 million tonnes. A baseline design was recently developed by the authors to support research work aimed at assessing large and ultra-efficient subsea cargo drone technology. One crucial aspect is the development of SST's operation envelope, i.e., the safe depth versus speed regions. The development of this envelope entails comprehensive and detailed studies of SST's dynamic load-effects under all expected operating scenarios which in the early concept development phase can be performed using suitable computational models. In this technical brief, the initial development of such a model is unveiled. This fully coupled 2D planar model considers the most relevant load-effects which are from hydrodynamics, hydrostatics, and control surface induced loads. The most important features of the model such as the derivation of hydrodynamic derivatives and model verification are also discussed. As an example, this model is then used to study the depth control problem which is a key aspect in the determination of the safety operational envelope. The results show that unsuitable control schemes that do not look ahead in the trajectory lead to undesirable results. In contrast, a feed-forward heading control method achieves a good and fast control response.