The role of marine growth on the performance and survivability of dynamic subsea cables

Project Description:

The Project

The aim of this project is to better understand how marine growth or biofouling impacts the functionality and vulnerabilities of dynamic subsea power cables (dSPCs) used in floating wind and marine renewable energy technologies. These technologies are expected to play a major role in fulfilling societal and governmental objectives to decarbonise electricity generation. However, success in the sector is partly dependent on removing economic uncertainties associated with these new technologies. An important concern is the impact of marine growth on the survivability of dSPCs. While cabling for an offshore wind farm accounts for around 9% of overall cost, dSPC failures may account for 75-80% of the costs of insurance claims on these projects. Such failures are costly to repair and may result in a significant loss of revenue due to disruption in power supply.

Dynamic subsea power cables, necessary to transmit electricity from floating devices to the seabed, are vulnerable to fatigue due to exposure to cyclic wave and tidal loads in the water column. The resulting structural failure is caused by lift and drag forces from the dynamic environment which are exacerbated by marine growth. Compared with static cables, the design of dSPCs is still under development as implications of operation in an exposed, dynamic environment continue to be assessed. Replacement of cables and components is costly in terms of materials, vessel-use, and operational ‘down-time’ of the turbine/farm.

The settlement and growth of marine organisms on submerged infrastructure (biofouling) will increase loading on dSPCs and may affect hydrostatic properties. Coupled with exposure to extreme hydrodynamics forces in the inherently energetic, resource-rich environments targeted for ORE deployments, dSPCs are highly vulnerable to fatigue failure. Existing studies on marine growth on offshore infrastructure, including dynamic mooring systems, are mostly restricted to the Oil and Gas (O&G) sector and inferences from these mooring systems to dSPCs are limited. Biofouling of dSPCs, and the subsequent impacts, are likely to differ considerably from existing studies because: (i) different component materials are used, with substrate being a major factor in the settlement and growth of marine organisms; (ii) installations are expected in ‘data poor’ regions without detailed knowledge of growth rates and species of biofouling organisms; (iii) heat and electromagnetic fields generated during cable operation will affect marine growth; and (iv) dSPC functioning is more complex with greater vulnerabilities than mooring structures. Standards and guidelines used in the ORE sector provide broad generalisations on marine growth, typically informed by surveys of large O&G structures, but these will have limited application for smaller diameter structures such as cables.

The objectives of this studentship are: to identify and assess impacts unique to these technologies, explore risk and economic consequences, highlight mitigation strategies and knowledge gaps, and to develop novel approaches to in situ ground-truthing of models used to predict cable behaviour.

A multi-disciplinary approach will be adopted to gather and quantify information on targeted knowledge gaps of the impacts to dSPCs from biofouling. The following topics will be addressed in this project: identification of marine growth specific to dSPCs and associated components; assessment of potential mitigations, including the latest antifouling strategies; characterisation of the impact of marine growth on hydrodynamic and structural response of dSPCs, with focus on fatigue life prediction; and appraisal of economic impacts and risks to assess installation, operation and maintenance costs of ORE farms.

This will be achieved through assessment of literature and investigation of current data on biofouling from floating offshore platforms, fixed offshore wind farms, O&G installations, etc. Monitoring methodologies to gather in situ fouling data will adapted to novel opportunities to study smaller diameter infrastructure in hydrodynamically energetic conditions. Specifically, bespoke frames containing infrastructure samples (including different materials relevant to the sector) will be deployed and periodically retrieved for sample collection based on a system developed by Want et al. [2021].

Boat based work will be conducted by the Primary Supervisor with possible opportunities for the student to engage in training necessary to work at sea. Opportunities to ‘piggy-back’ marine operations with other research projects to allow additional sampling points or to extend the period of data collection will be pursued. Community composition analysis will use PRIMER techniques. Opportunities to receive additional in situ data from ORE installation surveys will be pursued and may allow the inclusion of thermal and electromagnetic effects on biofouling generated by ‘live’ cables to be investigated. OrcaFlex and ANSYS/UFLEX software may be used in modelling the impact of marine growth on cable structural response and predicted fatigue life.