Subsea Rock Installation vessels cover pipelines, which transport oil from oil and gas fields. This is done by means of a flexible fallpipe until depths of 1000 meters. Around the Shetland Islands, the workability of the vessels at two specific project locations is compromised by the conditions in the ocean. In this research, data from a 3D hydrodynamic model and logged information from the vessels has been analysed and combined to find an explanation for these problems. At the second location, for tidal flow to the east or east-northeast in the bottom and middle part of the water column, it is highly likely that the working activities have to be interrupted. Moreover, if the value of the turbulent vertical eddy viscosity exceeds 0.028 m²/s, the chance on non-working conditions increases with 70%. For the other project location, these kind of correlations have not been observed.

The reach of the Magdalena River around the city of Barrancabermeja experiences large issues concerning its navigability since the construction of the Yondó Bridge. The purpose of this study is to investigate possible causes of this poor navigability and to come up with an improvement to the current situation. The influence of bars and scour holes around bridge piles are seen as hypothetical problem causes. A bar mode analysis shows that, for the examined river section, the river contains one alternate bar over the years. It is plausible that the construction of the bridge has induced changes in flow conditions in such way that the original alternate bar started to erode and eventually totally disappeared. Therefore, it is highly possible that the construction of the bridge forms the main reason for the shift of the thalweg. The scour holes around the piers of the Yondó Bridge that were approximated by the empirical method of Melville and Coleman. These scour holes are incorporated in the Delft3D-model to assess their influence by adapting the initial bed elevation profile. From the model simulation it turns out that the presence of the holes does result in more erosion in the vicinity of the bridge.

Three possible solutions (null-solution, groynes and guide bunds) have been weighed using Multi-Criteria Analysis. Based on this analysis the guide bunds appeared to be the most suitable solution. The structure was implemented in the Delft3D-model and some additional simulations proved that the effect of this structure on the hydro and morphodynamic conditions in the river is twofold. First, the guide bunds improve the distribution of the flow over the cross section of the river. More flow is forced through the right side and indeed the flow velocities turn out to be higher at that location. Moreover, the flow velocities on the left side decrease, as expected. However, the structure has an opposing effect on the cumulative erosion and sedimentation. More sedimentation takes place at the right side of the channel, whereas the left side of the channel gets deeper. It can be concluded that the best way to improve the situation in Barrancabermeja, is the construction of a guide bund structure in the vicinity of the Yondó Bridge. However, more detailed (physical) model tests should be performed to gain better insight in the effect of the guide bunds.