Periodic Reporting for period 1 - MoWE (Mooring of floating wave energy converters:numerical simulation and uncertainty quantification)
Période du rapport: 2017-10-01 au 2019-09-30
The main goal of the MoWE is to obtain cheaper solutions and design procedures for mooring systems of wave energy converters (WECs), to lower the cost of ocean wave energy.
For this we applied an approach based on numerical uncertainty quantification. We used (i) an uncertainty quantification (UQ) toolbox based on generalised polynomial chaos coupled to the mooring dynamics solver "MooDy", and (ii) performed detailed numerical investigation on the influence on snap-loads on the mooring design. This design approach was compared with the traditional deterministic approach (using safety factors).
Our work demonstrated that deterministic design methods for mooring systems lead to solutions with probabilities of failure 10 times smaller than required by regulations, a clear over-design that can be improved. Additionally, we also demonstrated that the normal drag coefficient is the most important hydrodynamic parameter influencing snap and peak loads. The soil friction coefficient is the most important ground parameter, damping the propagation of internal waves within the cable, especially in high frequency cable motions. These results provide inform designers about the critical mooring design parameters.
For this we applied an approach based on numerical uncertainty quantification. We used (i) an uncertainty quantification (UQ) toolbox based on generalised polynomial chaos coupled to the mooring dynamics solver "MooDy", and (ii) performed detailed numerical investigation on the influence on snap-loads on the mooring design. This design approach was compared with the traditional deterministic approach (using safety factors).
Our work demonstrated that deterministic design methods for mooring systems lead to solutions with probabilities of failure 10 times smaller than required by regulations, a clear over-design that can be improved. Additionally, we also demonstrated that the normal drag coefficient is the most important hydrodynamic parameter influencing snap and peak loads. The soil friction coefficient is the most important ground parameter, damping the propagation of internal waves within the cable, especially in high frequency cable motions. These results provide inform designers about the critical mooring design parameters.
Research activities
The coupling between MooDy and WECSim was validated in two ways: i) by comparing WECSim-MooDy simulations, with simulations performed using WECSim coupled to MoorDyn (a Lumped Mass Method mooring code) and; ii) using WECSim-MooDy to reproduce physical model experiments. In both cases, WECSim-Moody showed good results.
Uncertainty quantification tools (UQLab) were successfully coupled to external codes for dynamics of moored structures (WECSim, MooDy and Orcaflex), used in different tasks of MoWE. We identified the Quantities of Interest (QoI) for UQ in mooring systems. These QoI were highly dependent on the case study and, so, their importance had to be analysed carefully in each case.
In a collaboration with the team at AAU working on the project “Mooring Solutions for Large Wave Energy Converters” (MSLWEC), we selected a mooring system of a floating WEC for a reliability study, using our approach based on numerical UQ. The results were published in a conference paper
We analysed in detail the influence that uncertainty in mooring system properties has in the dynamics of mooring cables and of WECs, focusing on the following:
- tension in the cable
- snap loads
- power production.
The results concerning power production were published in a conference paper; the results concerning cable tension were submitted to a journal (Marine Structures, Elsevier) and are pending review; the results concerning snap-loads are being described in a manuscript to be submitted for journal publication with the next two months (Ocean Engineering, Elsevier).
Training activities
The research fellow attended the following training activities:
- “Uncertainty Quantification” at DTU, Copenhagen,;
- “JCSS Advanced Course on Systems Risk Modelling and Analysis in Engineering Decision Making”, at AAU (37.5 h);
- “Basic Course in Higher Education Pedagogy”, (14 h, 2 ECTS);
- “Basic Course in Problem Based Learning, the AAU model”, (14 h, 2 ECTS).
- AAU's Adjunktpædagogikum - University pedagogy for assistant professors (10 ECTS).
Together with Claes Eskilsson and Jens Peter Kofoed, the fellow prepared a career development plan. This plan includes 1 and 5 year goals, training needs, expected research outcomes and networking opportunities.
Networking activities
In a collaboration with Caitlyn Clark (a visiting PhD student at AAU from OSU, Oregon, USA), we started a preliminary investigation regarding synergies between of floating offshore wind turbines and WECs. The results were presented in a conference paper. Another collaboration was started with the Division of Water and Environment of AAU, studying the impact of mooring systems of wind and wave energy parks on the water quality. A publication regarding an early appraisal is scheduled to be submitted before the end of 2019.
Dissemination activities
Two dissemination activities were planned: one at the Partnership for Danish Wave Energy (Denmark) in November 2017, and one at the Blue Business Week (Portugal) in May 2018. The participation at the Blue Business Week replaced with a participation at the Business2sea (Portugal) a Industry-Academia event for networking, because the first event terminated. The participation at the Partnership for Danish Wave Energy was not possible: in 2017 it occurred before the start of the research contract, and in 2018 it did not happen.
The Fellow signed up to for Danish Natural Sciences Festival. Participation on this event happens by request, where companies, schools, book a lecture or demonstration from any of the registered researchers. Unfortunately, the presentation prepared by the research fellow was not booked.
Important project milestones were announced in the Twitter and LinkedIn accounts of the Fellow. Dissemination through LinkedIn was the most successful, because the results were communicated directly to a community that has research interests similar to those of the Fellow.
The coupling between MooDy and WECSim was validated in two ways: i) by comparing WECSim-MooDy simulations, with simulations performed using WECSim coupled to MoorDyn (a Lumped Mass Method mooring code) and; ii) using WECSim-MooDy to reproduce physical model experiments. In both cases, WECSim-Moody showed good results.
Uncertainty quantification tools (UQLab) were successfully coupled to external codes for dynamics of moored structures (WECSim, MooDy and Orcaflex), used in different tasks of MoWE. We identified the Quantities of Interest (QoI) for UQ in mooring systems. These QoI were highly dependent on the case study and, so, their importance had to be analysed carefully in each case.
In a collaboration with the team at AAU working on the project “Mooring Solutions for Large Wave Energy Converters” (MSLWEC), we selected a mooring system of a floating WEC for a reliability study, using our approach based on numerical UQ. The results were published in a conference paper
We analysed in detail the influence that uncertainty in mooring system properties has in the dynamics of mooring cables and of WECs, focusing on the following:
- tension in the cable
- snap loads
- power production.
The results concerning power production were published in a conference paper; the results concerning cable tension were submitted to a journal (Marine Structures, Elsevier) and are pending review; the results concerning snap-loads are being described in a manuscript to be submitted for journal publication with the next two months (Ocean Engineering, Elsevier).
Training activities
The research fellow attended the following training activities:
- “Uncertainty Quantification” at DTU, Copenhagen,;
- “JCSS Advanced Course on Systems Risk Modelling and Analysis in Engineering Decision Making”, at AAU (37.5 h);
- “Basic Course in Higher Education Pedagogy”, (14 h, 2 ECTS);
- “Basic Course in Problem Based Learning, the AAU model”, (14 h, 2 ECTS).
- AAU's Adjunktpædagogikum - University pedagogy for assistant professors (10 ECTS).
Together with Claes Eskilsson and Jens Peter Kofoed, the fellow prepared a career development plan. This plan includes 1 and 5 year goals, training needs, expected research outcomes and networking opportunities.
Networking activities
In a collaboration with Caitlyn Clark (a visiting PhD student at AAU from OSU, Oregon, USA), we started a preliminary investigation regarding synergies between of floating offshore wind turbines and WECs. The results were presented in a conference paper. Another collaboration was started with the Division of Water and Environment of AAU, studying the impact of mooring systems of wind and wave energy parks on the water quality. A publication regarding an early appraisal is scheduled to be submitted before the end of 2019.
Dissemination activities
Two dissemination activities were planned: one at the Partnership for Danish Wave Energy (Denmark) in November 2017, and one at the Blue Business Week (Portugal) in May 2018. The participation at the Blue Business Week replaced with a participation at the Business2sea (Portugal) a Industry-Academia event for networking, because the first event terminated. The participation at the Partnership for Danish Wave Energy was not possible: in 2017 it occurred before the start of the research contract, and in 2018 it did not happen.
The Fellow signed up to for Danish Natural Sciences Festival. Participation on this event happens by request, where companies, schools, book a lecture or demonstration from any of the registered researchers. Unfortunately, the presentation prepared by the research fellow was not booked.
Important project milestones were announced in the Twitter and LinkedIn accounts of the Fellow. Dissemination through LinkedIn was the most successful, because the results were communicated directly to a community that has research interests similar to those of the Fellow.
The project developed an approach to design mooring systems combining reliability analysis and numerical UQ. Using this approached we demonstrated that mooring systems designed using the safety factor method might be over-designed: their probability of failure is about ten times lower than what is required for offshore Oil and Gas structures, which is already too restrictive for renewable energy structures.
Another important result is that the hydrodynamic coefficients used to model the fluid forces on a cable might not be as important as currently thought. While the normal drag coefficient can have a considerable influence in the tension, the added mass and the tangential drag coefficients have very limited influence. As a consequence, it is not necessary to perform extensive physical/numerical modelling, or literature review, to obtain the hydrodynamic coefficients of mooring cables; only the drag coefficient needs to be known accurately. Also, Morison’s formula might not adequately describe the fluid-cable interaction, and might need to be tuned or replaced by better approaches.
When there is interaction between a mooring cable and the sea-floor, the floor has a somewhat reduced influence in the tension. Applying considerable changes to the ground parameters, does not result in significant changes to the tension time-series. The most important ground parameter seems to be the tangential damping, which damps shock wave propagating along the cable, reducing peak loads. Interestingly, it most important in high velocity, high frequency motions, which is when snap loads tend to occur.
Our results allow designers to better focus their efforts in the design of mooring cables. We demonstrated that mooring systems can be made cheaper and also which of the parameters are more significant in their study. This can have a cascading effect for society. WEC developers will be able to use less demanding regulations than the current ones (those of the offshore oil and gas industry), lowering the cost WECs. This will, in turn, facilitate development of commercially viable WECs. This might contribute to increasing the share of renewable energy use, and sustain a highly skilled industry in Europe, to support wave energy business.
Another important result is that the hydrodynamic coefficients used to model the fluid forces on a cable might not be as important as currently thought. While the normal drag coefficient can have a considerable influence in the tension, the added mass and the tangential drag coefficients have very limited influence. As a consequence, it is not necessary to perform extensive physical/numerical modelling, or literature review, to obtain the hydrodynamic coefficients of mooring cables; only the drag coefficient needs to be known accurately. Also, Morison’s formula might not adequately describe the fluid-cable interaction, and might need to be tuned or replaced by better approaches.
When there is interaction between a mooring cable and the sea-floor, the floor has a somewhat reduced influence in the tension. Applying considerable changes to the ground parameters, does not result in significant changes to the tension time-series. The most important ground parameter seems to be the tangential damping, which damps shock wave propagating along the cable, reducing peak loads. Interestingly, it most important in high velocity, high frequency motions, which is when snap loads tend to occur.
Our results allow designers to better focus their efforts in the design of mooring cables. We demonstrated that mooring systems can be made cheaper and also which of the parameters are more significant in their study. This can have a cascading effect for society. WEC developers will be able to use less demanding regulations than the current ones (those of the offshore oil and gas industry), lowering the cost WECs. This will, in turn, facilitate development of commercially viable WECs. This might contribute to increasing the share of renewable energy use, and sustain a highly skilled industry in Europe, to support wave energy business.