This paper focusses on the development of a tool for decision-making, tailored for personnel involved in complex Ship-To-Ship (STS) operations, to enhance the efficiency and safety of these operations. A step-wise approach has been selected. The first step includes specification, development and testing of the tool in a simulated work environment using full-mission simulators. In the second step the findings from application of the tool in the simulated work environment will be used to develop a prototype which will be tested during real life STS operations. This paper describes work done in the first of these two steps. During four iterations, a Graphical User Interface (GUI) has been made following Interaction Design (IxD) principles. The designs have been iteratively developed and tested by experienced ship's officers in a ship-handling simulator to clarify key information to enhance their Situation Awareness (SA) and decision-making process. In order to find indicators for performance, an initial performance test was carried out in a ship-handling simulator. The test indicates that a logic based Decision-Support System (DSS) can improve existing simulator-based training activities in STS operations. A Ship-to-Ship (STS) lightering operation typically involves two tankers manoeuvring in close proximity, at speeds in the range of 4-6 knots, in order to position alongside and commence cargo transfer. The operation often takes place out on the open sea, while the two ships are underway and sailing, as can be seen in Figure 1. One ship maintains speed and course, while the other ship slowly manoeuvres alongside. In an STS lightering operation the approach phase is critical in order to avoid steel-to-steel contact.