Subsea rock installation is widely applied in the offshore industry and utilized for a wide range of purposes including but not limited to: pipeline protection, scour protection, insulation of pipelines, upheaval buckling prevention and seabed preparation. Tideway Offshore Solutions is specialized in subsea rock installation and currently operates three state-of-the-art fallpipe vessels. Their vessel 'Flintstone' makes use of an innovative closed fallpipe system to provide high accuracy subsea rock installation. The presence of rocks in the water column of the fallpipe increase the density of the mixture in the fallpipe. The density difference between the mixture in the fallpipe and the density of the surrounding sea-water results in a water level drop in the fallpipe. To keep this water level drop within acceptable limits extra water is added to the fallpipe system which accelerates the fallpipe flow. The accelerated fallpipe flow can result in high outflow velocities of the rock mixture at the fallpipe exit. High outflow velocities of the rock mixture can eventually result in increased impact velocities of the rock particles on the seabed. Increased impact velocities of the rock particles on the seabed can lead to unsatisfactory rock berm shapes resulting in the need for remedials. To have their fallpipe system perform as efficient as possible Tideway Offshore Solutions was interested in possible measures to reduce the outflow velocity of the fallpipe which resulted in this thesis. In the first part of this thesis different concepts, that could potentially reduce the outflow velocity of the fallpipe, are generated and conceptually analyzed. The information acquired from this analysis is used as input for a multi criteria analysis that resulted in the selection of the most promising concept, the use of a deflector. The deflector will act as a flow deflector at the fallpipe exit thereby decreasing the impact velocity of rock particles on the seabed. In the second part of this thesis a complete three-dimensional computational fluid dynamics (CFD) analysis is performed on the fallpipe outflow with and without deflector. The CFD program used to simulate these situations is ANSYS Fluent. The simulations for both cases are performed for two different turbulence models, the k – ε and k – ω SST turbulence models, the distance from the fallpipe exit to the seabed is varied as well and a range of deflector angles and dimensions are simulated. The fluid flow velocities obtained from the CFD analysis are used as input in a MATLAB model to compute the rock particle trajectories in a two-dimensional plane. Combining the rock particle trajectories and their velocity components it is possible to compute the impact velocities of the rock particles on the seabed. The results of the trajectory model set up in MATLAB showed that a substantial decrease in impact velocity of the rock particles on the seabed can be achieved by using a deflector at the fallpipe exit.

Over the last decade, the demand for offshore power cables has increased significantly due to, in particularly, the emergence of offshore wind. Despite this growth, industry-wide rules and guidelines to analyse fatigue during cable installation operations are yet to be developed, even though fatigue can become an important parameter in the installation design.  In similar offshore operations, e.g. installation of pipelines or umbilicals, fatigue is assessed based on fundamental fatigue theory like rainflow counting, S-N curves and Miner's rule and therefore these principles form the basis of analysis in this research. However, submarine power cables consist of numerous layers of different materials that result in complex cable cross-sections. The fatigue behaviour of the cross-sectional elements due to external loads has been investigated to determine a maximum stand-by time during installation operations. In this regard, a cross-sectional analysis was performed to establish the stress-strain response of the individual cable components to the global cable deformations. For bending behaviour, the analysis was shown to be consistent with existing test data. However, this type of data is scarce and the analysis was based on a limited sample size. Moreover, geometrical cross-sectional data of submarine cables is rarely provided by suppliers and hence several assumptions were made in the analysis. For future work, it is recommended to develop in-house test data of cables such that the cross-sectional model can be accurately verified and improved. The cross-sectional stresses were implemented in a fatigue assessment model, in which the local stresses were calculated based on the output of global modelling software OrcaFlex and subsequently analysed with rainflow counting and Miner's rule. The model was applied to three cable types and it was found hat lead, used for cable sheaths, is the critical cable component in terms of fatigue. When no lead is used in the power cable, the conductors are the critical component. Furthermore, for mild loading scenarios, i.e. waves with significant wave heights Hs≤1.5 m, the model yields components infinite life for all cable components. For higher load cases, i.e. Hs≥2.5m, the model showed that fatigue mitigation measures are required to stay within the fatigue budget. Several mitigation methods were implemented, from which it was found that increasing the layback length of the cable combined with adjusting the vessel heading to favourable conditions most effectively reduces fatigue damage as all load cases with Hs=2.5 m resulted in maximum stand-by times of tsb≥ 125days. However, for higher loading scenarios, i.e. Hs=4 m, all researched mitigation measures were insufficient to stay within fatigue budget. For future work, it is therefore recommended to improve the mitigation measures such that severe load cases can be survived. Lastly, the wave conditions were simulated as JONSWAP waves. These simulations are time-consuming and therefore not many loading scenarios were modelled. It is recommended to research methods to approximate wave conditions with regular waves, as the duration of OrcaFlex simulations will decrease exponentially. An increase in number of load cases will result in more accurate limit states for the cable components.