Why are your dynamic cables behaving like this?

Moving offshore wind turbines into deeper waters will tap vast new areas of potential wind energy. But moving dynamic systems into an even more dynamic environment comes with its challenges, not least when it comes to dynamic cable monitoring, protection and management.

If there’s one thing we’ve learned from the past three decades of building offshore wind farms, it’s that the offshore environment is highly dynamic. There are a multitude of constantly shifting factors that we need to consider, above and beneath the waves, from leading edge erosion to the protection and management of power cables.

So what will happen when we evolve our wind turbines from fixed structures into floating structures in deep water? Put it another way, what happens when we introduce a dynamic system into an even more dynamic environment? One answer is that we get more yield and access to more capacity. Another answer is that we open up a new world of dynamic and environmental complexity.

There is a lot that the industry has already learned, the hard way. We’re talking better understanding of cable protection systems, cable depth of burial and free spans. I think it’s fair to say that the industry slightly underestimated the impact of the subsea environment in these areas. We have come a long way.

A new focus on dynamic cable monitoring

But, as we move turbines from static foundations onto dynamic platforms and into even more dynamic environments, there will be even more to learn, particularly around cable behaviour, protection and management. In these environments, cable behavioral patterns will differ, even within a localised area, due to small variations across the subsea environment, as well as variations in catenary designs. Export and cable arrays will now be exposed to potentially damaging factors such as:

• Hydrodynamic drag forces,
• Touch down point migration
• Cable compression
• Protection sleave movement
• Platform induced motion
• Vortex-induced vibrations

The challenge is to understand these forces so that we can resolve or counteract them, which becomes more important as our individual assets – reaching and going beyond 15 MW – represent an ever-greater proportion of our project. While we can build models, they only get us so far without having to build in large error margins due to the level of uncertainty involved in a design. So, we need real-world data.

Learning at the floating wind demonstration phase

At the demonstration stage, real-world dynamic cable monitoring data will allow us to enable, teach, and verify our models. But as we move into commercial-scale operations, it will help to reduce our error margins and increase safety, efficiency and yield – reducing project risks and long-term costs. Longer term, it will provide assurance of fatigue life and system performance and enable us to predict and remedy faults before they cause system outages, helping to rationalize maintenance regimes and be more confident when it comes to field life extension.

Five decades underwater positioning and monitoring

With our underwater acoustic and inertial systems, twinned with smart analytics, we’ve been helping operators understand how their underwater infrastructure is influenced by dynamic subsea environments for decades.

In fact, at Sonardyne, we have been positioning and monitoring underwater infrastructure since it was founded in 1971. We’ve been there, from the very first tension leg platform, Hutton, in 1983, through to complex deepwater floating production, storage and offloading vessel mooring and riser systems. We’ve learned how to integrate a combination of wireless subsea technologies to create a subsea integrated network that gives operators a full, real-time picture of dynamic behaviors. These range widely, from pipeline buckling and blow-out preventer inclination through to vortex-induced vibration of risers, including automated alerts and warnings when systems move out of set parameters.

Marine risers – a behavioral equivalent to power cables, delivering energy in its liquid or gaseous form to the surface – are often subject to significant forces through the water column. By monitoring them with an integrated suite of wireless subsea acoustic sensors, operators can better calculate the fatigue life of the subsea infrastructure and monitor for any events deemed outside their operating envelope. This could be points deemed most susceptible to cause catastrophic failure, such as the tether and anchor line and the touchdown point. Exactly the same can be done for floating offshore wind infrastructure.

SMART technology for subsea monitoring and analysis

Our SMART technologies have everything you need to measure cable and ancillary system, mooring lines and subsea environment behaviour, whether that’s using a SMART transponder to measure high-frequency vibration at the bend restrictor or seabed mounted inverted positioning system to measure the position of a slow oscillating dynamic cable. SMART stands for Subsea Monitoring, Analysis and Reporting Technology. That’s also because they’re easy to interface with third-party sensors, for example, a strain gauge to monitor load or tension on either the cable ancillary system or mooring lines.

The SMART transponders will report back to our seabed-mounted SMART technology hub that supports edge analytics, so you can manage the information you want, when you need it, not just a mass of data. Most of our instruments are capable of long-term deployment (i.e. up to at least 10 years, with our existing batteries) and support both on-demand data retrieval or send alerts when a threshold or pre-programmed metric is hit.

Building a bigger picture with current data

To build a wider, holistic picture, we would also recommend gathering underwater environmental data, using acoustic Doppler current profilers (ADCPs). These can also be integrated into our wireless network. Leaving ADCPs on the seabed and harvesting data throughout the entire wind farm lifecycle will lead to larger data sets that enable more accurate simulation and predictive modelling of the subsea environment.

Scalable dynamic cable monitoring systems to suit all sites

However, we see that not all floating offshore wind farm sites are born equal. Topography and metocean characteristics, such as water depth, current regimes and seabed characteristics, will differ between arrays. Where there’s a greater confidence in the characteristics at one site or more stable weather patterns, you might want to “spot” monitor one or two turbine locations early in your project life, to verify and build confidence your existing models for the entire farm. Where there’s a higher level of uncertainty or environmental risk, such as, northern North Sea, offshore Brazil or in the Gulf of Mexico, where there are complex current and eddy regimes, you may want a more comprehensive monitoring system or even to monitor throughout field life to better predict and manage cable behaviour. The options are easily scalable and tailored to individual sites.

If you want to learn more about the dynamic behaviour characteristics of the dynamic cables in your floating offshore wind farm by gathering real-world data, contact our team.