Variable frequency drives (VFD) and subsea (umbilical) cables are frequently used in electrical submersible pump (ESP) systems for offshore platforms. There are two basic system configurations for ESP systems; VFD can be installed on the platform and the motor is connected to it through an umbilical cable, and VFD and the motor are installed closely and they are connected to the system through an umbilical cable. In this thesis, the pros and cons of each configuration are mentioned, but the focus is on the system with a VFD controlled motor through a long umbilical cable. A 36-pulse VFD is studied. Since multilevel VFDs have high frequency harmonics, high frequency modeling of the umbilical cable is used, and skin effect is also taken into consideration in the cable. The effect of the interactions between the umbilical cable and high frequency harmonics on the motor terminal voltage is explored.

In the expansion of offshore sustainable energy systems, there is growing pressure on the environment and permit processes and the accumulation results in much higher total risk for accidents of future assets. Anticipating the problems at the design stage and improving verification is likely to increase energy development and reduce costs. This thesis explores offshore DST (Decision Support Tools) and risk verification of subsea cable assets.For subsea cables, a statistical method is proposed utilizing measurement data together with shipping traffic data (AIS) to estimate the environmental risk and risk of accidents of installed cable assets. This should partially solve issues of improving design using more data and surveys and utilizing mechanical and sensor-specific characteristics to improve the confidence and burial estimation, contrary to today’s methodology. The implication of the two studies of cable burial risk assessment techniques and verification shows how a developed methodology can solve issues for verifying the integrity of an installed asset. Putting our methodology into practice involves many challenges.  For the marine Decision Support Tool (DST) and sustainable energy development, to estimate potential savings if permit processes would be shorter and less burdensome without degrading the quality of the EIA (Environmental Impact Assessment). A method is proposed to model various scenarios of effective savings from the development of a DST to reduce costs spent on EIA permitting by the offshore energy developers. The study of the implication of the marine EIA DST shows a quantifiable estimate of the savings potential for permit processes for sustainable offshore development, and results indicate a need for optimization of DST development, which can be an essential factor in its implementation and success.

Addressing the decommissioning issues is vital to ensure a sustainable and effective process of such an essential part of the project life cycle. While there is a set of good practices and regulations that govern most of the decommissioning activities, the cable decommissioning issue is still in a big debate and often left abandoned at the seabed due to environmental concerns, as justified by most developers. This paper is aiming to understand the environmental and economic consequences of cable decommissioning. The available research papers and reports that are dealing with cable decommissioning issues have been reviewed. The cables are often decommissioned using similar methods to installation. However, there are no regulatory obligations to removing the cables in most countries. Cable installation will be associated with environmental impacts, but they are considered to be negligible. Additionally, Recycling cables’ copper is beneficial in both aspects environmentally and economically as copper prices are on the rise. A comparison between the ESs and decommissioning programs in a number of OWFSs have been conducted to understand the justification used for abandoning the cables. Most of the decommissioning reports have considered cable decommissioning to cause “considerable damage to seabed ecology”. However, that contradicts what was found in the ESs, where the impact level was considered negligible and anticipated to be similar to installation. It was unclear whether the abandonment of cables was driven by environmental considerations or not. A case study has been selected to compare cable and monopile decommissioning costs and the contribution of each component to the total decommissioning cost, including possible revenue generated from recycling. It was found that the cost-benefit of cable decommissioning is incomparable to monopile decommissioning as the latter is very costly, and the possible residual value is insignificant when compared to cables. Moreover, it is possible that the total cable decommissioning cost to be largely offset by the revenue generated from copper resell. Additionally, the cable decommissioning total cost can be almost paid by recycling cables if copper prices increase in the near future.