◀ Back to projects overview P201305-004-IHC

 

Optimizing cable installation and operation, a life cycle perspective

To lower the levelised costs of energy of offshore wind farms, Capex and Opex resulting from power cable installation and inspection, repair and maintenance (IRM) need also to be reduced. Though 5 to 10% of the projects costs can be assigned to cable installation, this activity accounts for 70 to 80% of the insurance costs of a project. This FLOW project targeted on lowering these costs by a life cycle perspective covering:

• Installation (laying, connecting, burying): covered in work packages (WP) 1A & 1B
• Monitoring cable behaviour (predicting as well as measuring): covered in WP 2B
• Inspection, Maintenance, Repair: covered in WP 2A & 2B, 3
• Decommissioning: covered in WP 3

The project was initiated, executed and reported as an ‘umbrella’ project where each partner worked on their own Work Package (WP), covering a specific part of the overall project.

WP 1A - Cable Installation
By applying Lean methodologies (best practice in mature industrial production environments) during the development of inter array cable installation concepts, significant potential for cost and risk reductions was identified. By designing the installation method first and using logistic modelling tools in an early development stage two new concept designs, a carousel based design (fig. 1a) and a reel based design (fig.1b), were obtained including matching operational descriptions. As no suitable models were available a probabilistic logistic model needed to be developed in cooperation with Delft University of Technology (DUT). DUT is continuing research on this logistic model.

WP 1B - Cable Protection
Via gap analysis focussing on subsea power cable ploughs, technologies have been identified that could be implemented to improve the performance of ploughs. From this analysis it is understood that implementing a hydraulic hammer tool, an aggressive share, low friction coatings, jetting, subsea diver-less loading and improving operational procedures would all be beneficial. Ideas like a cable tensioner or vibrating share were ultimately not recommended due to risk of damage to the product.
Exclusive research and testing on trench cutting demonstrated that present capabilities of mechanical trench cutting for subsea vehicles can be significantly improved. Subsea cutting calculations have been developed, taking into account water pressure in the mechanical cutting process. This has been achieved through Discrete Element Modelling and laboratory testing. When combined with advances in chain technology this can results in a greater rate of trench cutting leading to reduction of operational cycle times.
A number of self-excited water jet nozzles which passively produce a pulsating exit flow were developed. Results of tests were encouraging, with varying degrees of exit flow oscillation detected for all nozzle configurations. Amplitudes of up to 20% of the mean were detected from the mathematical breakdown of the waveforms. The testing was conducted using rough concept designs for the nozzles, and hence there is potential to improve the results through further testing and optimization of the geometrical parameters.

WP 2A - Monitoring and predicting cable burial depth
A new robust and accurate algorithm has been developed for determining the burial depth of an offshore cable without the need for any marine operation. The principal idea is to utilize the fibre optic sensor, installed in most offshore electrical cables, to obtain detailed information about the state of the cable during operation. By analysing this information the burial depth can be determined with accuracy comparable to existing methods. The method has been successfully validated against data from the offshore wind farm Egmond aan Zee. It must be stressed that no additional sensors or equipment are required and that the methods can be applied in existing wind farms too.

The layout of in-field electrical cables is often based on the sea bed prior to installation. However, sea beds are mobile and develop in time due to the influence of currents, waves and the presence of the wind turbine foundations. In order to find an optimal cable layout, seabed changes for the entire lifetime of the wind farm should be taken into consideration. Such a cable routing optimization technique has been developed and validated for the Princess Amalia wind farm.
Furthermore, the migration of sand waves was investigated using the numerical hydro- and morphodynamic model, Delft3D. This is an important step towards prediction of morphodynamic developments in order to reduce the risk of cable failure. With further development it may be possible to numerically predict morphodynamic developments for the entire wind farm, which allow for better planning and ptimization.

WP 2B - Prediction of Power Cable Dynamics and Fatigue
Though power cables are supposed to be buried, sand migration and scour can remove their protective layer, leaving the cables exposed to adverse environmental conditions. One of the harassing environmental factors is current. Current induces forces on the cable, which causes it to respond dynamically. A high amplitude
response of the cable may lead to the reduction of its operational lifetime due to fatigue, and in the limit,
may even cause its failure. The final outcome of WP2B is an empirical model that predicts amplitudes and frequencies of vibration of the cable subjected to steady current. This information serves as input for fatigue calculations, which are used to assess whether exposed cables may have their lifetime reduced or not. This assessment will provide valuable information regarding the risk of failure of an exposed cable, and will dictate whether intervention is needed, thereby reducing costs associated with non-necessary re-burials.

WP 3 - Cable Inspection, Repair & Maintenance (IRM)
Based on analysis and evaluation of market data it has been concluded that there is currently a technology gap in the offshore renewable industry for subsea dredging equipment. A dedicated subsea vehicle, not only having the capacity for dredging but also manipulation arms to conduct complex and delicate handling operations, could fill this gap. Consequently it would be an ideal solution for completing IRM activities. All possible scenarios and functional requirements of the service providers were identified for the three modes of use of such an vehicle. Also likely tooling requirements were investigated resulting in the description of the functional requirements for a sub-sea vehicle with enhanced mobility. Such a sub-sea vehicle would allow the majority of IRM operations and decommissioning to be achieved. IHC envisages further research and development in this area. Existing products based on jet eduction can use excessive power and have a restriction in soil transportation passage that can lead to blockages. An annular eductor provides a constant flow to eliminate blockage problems and provides lower power consumption for an increase in performance. Calculations were developed to predict the required performance. Against the engineering theory model, a prototype annular eductor was designed and manufactured for testing. The prototype outperformed the theoretically predicted minimum suction velocity required for spoil removal by approximately three times. The next steps shall include further testing of the suction nozzle and spoil removal.

Overall conclusions
Different work packages contribute to the reduction of the Levelised Cost of Energy:

• WP Cable installation & protection: 0,69 %
• WP Monitoring and predicting of burial depth: 1,23%
• WP Prediction of power cable dynamics and fatigue: 0,1%
• WP Cable inspection, repair and maintenance: 0,03%.

Reduction of risks:

• Risk of capacity shortage of Cable Laying Vessel (CLV’s) capable of installing inter array cables for far-offshore wind farms (deeper water, further offshore and larger turbines) with low risks and against low cost is partly reduced by having concept designs available;
• Probabilistic modelling techniques enable f.i. contractors to include weather risks in a quantitative way when bidding for projects hereby reducing their project risks;
• The risk of cable failure can be significantly reduced;
• More insight into failure mechanisms of power cables which can reduce the risk of cable failure and lead to preventive mechanisms;