The project entitled “Reducing mooring and cable costs through assessment of corrosion, wear, fatigue, and VIV in turbulent tidal flows” was funded by Nova Scotia’s Offshore Energy Research Association (OERA). The project was awarded to DSA in Sept 2017, and included several partners including Dalhousie University and Scotrenewables. The project was completed on January 15, 2019. The project was completed in Nova Scotia, and included the deployment of the SHiFT buoy in Minas Passage, as part of an experiment to better understand VIV and strumming in moorings and cables.
The wear, corrosion, fatigue, and VIV (vortex induced vibration) of moorings and cables for tidal energy applications is not yet well understood. Tidal environments contain turbulence, strongly directional currents, and large tidal elevation changes. Floating tidal platforms are moored in highly dynamic environments, and the unsteady hydrodynamic loading on the turbines and the structure is transmitted to its moorings and cables. Accurately predicting the lifetime of cables and moorings requires predicting platform motions and hydrodynamic loads, as well as hydrodynamic loading on the cables.
Vortex induced vibration (VIV), or vortex induce motion (VIM) will occur on bodies in flow when asymmetrical vortex shedding occurs, resulting in an oscillating lift force which can cause movement. Additionally, turbulence at a range of frequencies is present in tidal channels, resulting in varying drag loading.
Firstly, to better understand the role of turbulence on mooring loads, a numerical model previously created by DSA of the ecoSpray tidal energy test platform was augmented by adding synthetic turbulence input into ProteusDS using data from NREL’s TurbSim. The synthetic turbulence model was generated using the turbulence spectra recorded by an ADCP stationed near the platform. The mooring line loads from the numerical model were compared with load cells deployed on the ecoSpray platform. The comparison demonstrated that oscillating and stochastic loads caused by turbulence acting on floating platforms is significant and should be accounted for in numerical models of tidal platform’s mooring systems which are routinely used for mooring system design. The comparison also demonstrates that if the turbulence spectra is known, then a synthetic turbulence model developed using the TurbSim and ProteusDS software packages can be used to accurately predicting loads and motions.
Second, the project looked at the role of VIM on floating platforms motions, as these motions would affect mooring and cable loads. Computational fluid dynamic (CFD) software uses the Navier – Stokes equations combined with finite element discretization to solve complex fluid flow in either steady state or transient simulations. A CFD analysis was performed for the ecoSpray platform to determine whether CFD could predict VIM occurring on a floating platform. It was determined that no regular vortex shedding pattern could be seen in the simulation. This agrees with previous results that significant VIM is not occurring on the ecoSpray platform, as all the measured loads can be accounted for in the synthetic turbulence model outlined in section 4.1.2.
Third, the project was primarily focused on developing an experiment to determine whether VIV would occur on a cable suspended in a tidal channel, and if it did, whether that VIV could be predicted using traditional predictive methods based on theoretical derived equations or using modal analysis software (Shear7). The experiment was setup to represent a scaled version of a power umbilical deployed by Scotrenewables at EMEC. DSA developed a streamlined buoy that allowed for a low scope in the mooring, and would maintain a specific required tension that was likely to contain modes expected at full scale. DSA used accelerometers attached to the mooring line and a load cell to measure vibration in the mooring. The experiment was deployed for several weeks in the Minas Passage in Nova Scotia in November 2018. The data collected showed that VIV did occur in the tidal channel environment. The results indicated that the turbulence present at high frequencies (>1Hz) is not powerful enough to interfere with VIV or prevent its occurrence. The experiment also demonstrated that both the theoretically derived equations and the modal analysis software could predict the primary frequency of oscillation reasonably well, but neither could predict the exact modes that would occur. The experiment also demonstrated the effectiveness of accelerometer data loggers in measuring the frequency of cable vibrations.
To better assess corrosion effects in tidal passages, an assessment of codes and standards regarding corrosion on mooring lines was conducted. All offshore mooring standards considered in this review provide the same methodology to design for mooring line corrosion. The standards state that the diameter of the chain (or wire rope) must be increased based on a given corrosion rate. Other sources on corrosion were reviewed, from these studies, it can be concluded that the baseline rates of corrosion provided in the offshore standards are not applicable to these regions and baseline corrosion data must be established for this region. Once a baseline corrosion study has been completed, the rates of corrosion can then be factored into the design of moorings to ensure that the mooring maintains required strength over its lifespan.
The materials of each component in the experiment were measured to provide a baseline of corrosion data for the Minas Passage. However, due to the timeline of the project and experiment, the components were only deployed for 28 days. This is not a long enough period of time to develop a baseline for corrosion rates. The experiment provided qualitative results that demonstrated that painted surfaces were quick to lose paint and begin corroding immediately, whereas hot dipped galvanized surfaces were successful at slowing the corrosion process. The experiment demonstrated that zinc-plated cotter pins, which typically come with mooring shackles, will corrode very quickly and should not be used for deployments greater than 1 month in the Minas Passage. The cotter pins recovered from the experiment were heavily corroded.
Through the project, the state-of-the-art for mooring and cable load assessment was significantly advanced, as evidenced by:
- Development and validation of dynamic simulation using synthetic turbulence model: TRL 5 (technology development) → TRL 6 (technology demonstration)
- CFD analysis to predict VIM on tidal energy platforms: TRL 1 (basic principles) → TRL 2 (application formulated).
- Procedure and expertise to predict VIV occurrence: TRL 2 (technology concept formulated) → TRL 5 (Laboratory testing of system)
- Procedure and equipment to measure VIV: TRL 2 (technology concept formulated) → TRL 5 (Laboratory testing of system)
- Site and cable specific VIV testing: TRL 2 (technology concept formulated) → TRL 5 (Laboratory testing of system)
- Corrosion guidelines for industry: TRL 1 (basic principles) → TRL 2 (application formulated)