A few ideas for better offshore cabling

By Maryruth Belsey Priebe
IQPC GmbH

This article is part of Windpower Engineering & Development’s April 2017 issue. A complete digital version of the issue is here.

Despite the critical role cables perform at wind facilities, the devices are too often an afterthought during project planning. A push for faster wind-farm installations and lower project caps and operations costs means some components take less precedence over others in attempts to streamline project costs. While cables may fall to the bottom of the list for some onshore wind farms without much concern, the same does not hold true for offshore projects.

PEFLEX is a corrosion-resistant cable protection system used to protect freespan array and export cables on offshore wind farms. Its modular design enables quick assembly onshore or on a cable vessel. Once assembled, PEPFLEX is positioned from the wind turbine’s monopole to beyond the scour pit limit. (Photo: CIRCOR Energy)

Offshore developers in Europe have learned this the hard way. For instance, damage to cabling at both the Thanet and London Array wind projects have had British offshore wind-farm operators looking for ways to prevent such challenges.

At the 175-turbine, 630-MW London Array project in the outer Thames Estuary, the problem occurred during export cable laying. While install of the nearly 450 km of offshore cable took place, a part was damaged and required replacement when one leg of the installation rig pressed the cable into the seabed floor.

For the 100-turbine, 300-MW Thanet wind farm, about 12 km off the coast of Thanet district in Kent, England, a routine inspection found a kink in two export cables shortly after install. Repairs meant the wind farm could only operate at half power. The job also required conversion of a VBMS vessel (formerly, VSMC — a specialized offshore vessel supplier) to properly conduct the cable replacement because a chartered cable-laying vessel was unavailable on short notice.

Unlike onshore wind-farm installations, offshore cabling routes are more difficult to access, let alone repair. In fact, simply locating a fault can sometimes pose a challenge. Unexpected costs of specialized vessels (given the small number available worldwide with suitable capabilities) and wind technicians (who excel at work offshore) can add up quickly and impact a project’s ROI.

Another complicating factor is that intertidal operations for offshore wind-farm cabling have proven quite different than those typical in the oil and gas industry. Also, soil conditions vary greatly depending on the locale so one offshore wind project is not necessarily representative of another. Such factors require individualized vessel and equipment specialization, and an understanding of the marine ecosystems at each offshore site.

Protecting cables
The Thanet and London Array projects are two examples of offshore wind farms that have experienced unexpected cable problems, but they are not alone. Sweden’s 110-MW Lillgrund wind farm is another offshore project that stopped operation for nearly two months due to cable faults. Project operators had to send high-quality, underwater cameras down to locate and assess the problem.

At first they found that a cable section bent at an acute angle, and then operators identified another fault on a different point closer to the substation. Other offshore wind-farm operators have noted issues such as early cable erosion and insufficient protection against the current.

Tektube works differently than most cable protection systems by replacing the need for conventional steel J-tubes, which protect cables when they are connected to subsea foundations. The system’s pre-install capability lets it fit and secure onto cables onshore, which saves time on the installation vessel and improves efficiency of the overall operation. TekTube was successfully installed for the first time at the Westermeerwind near-shore wind farm in the Netherlands. (Photo: Tekmar)

Regardless of the cause, failures like these can severely impact a wind farm’s performance and revenue. An offline wind farm can cost upwards of $2 million a day, and then there are the added repair or replacement costs.

As such, one area garnering attention within the offshore wind-cabling sector is the production of better cable protection systems, and ideally one that can adapt to a cable already in use with little effort or added cost. One device now in use at a few offshore wind farms, including the London Array, is the PEFLEX subsea cable protection system.

PEFLEX protects subsea array cables and export cables against movement from wave action and turbulence through exposed monopole scour zones. It is intended to safeguard cables from impact, abrasion, fatigue from dynamic motion, and over-bending. It can also install without divers or an ROV to save additional project costs.

PEFLEX is equipped with a series of durable interlocking polyurethane vertebrae half sections interspersed with elastomeric sleeves. The system also uses J-tube and J-tubeless connections at the subsea entry point, and is clamped and fastened securely into place with Inconel strapping and banding.

Another example of innovation in offshore wind cabling protection is a compound cable solution called a two-in-one because it provides cable protection and replaces conventional steel J-tubes. J-tubes have been used in the oil and gas sector for many years as protection for power cables because they connect to a subsea foundation. The two-in-one is designed for jacket structures, gravity based foundations, and tripods.

One company, Tekmar, has designed the TekTube, which can install onshore and reduce offshore time and costs. TekTube can secure to a wind-turbine foundation when still on land, sealed in place, and then transported offshore to the wind-power site where it accepts the power.

According to Tekmar engineer Jack Simpson, saving install time is an important safety feature of this product. “Cabling represents about 7% of the cost of installing turbines, but historically 70% of the insurance claims — it is a very risky element.”

He added that by reducing risk, it is also possible to reduce installation costs. “This one product costs a quarter of the price of most other cable protection options. Installation is cheaper, too. This is a bit of a game changer in the industry.”

Other ideas for protecting subsea cabling include bend restrictors that limit the curvature of cabling, and bend stiffeners, which prevent over-bending during installation. Also, suppression strakes designed with computational fluid dynamics principles help prevent damage from vibration forces over free spans.

Smart cables
The durability of cable hardware is one way to ensure proper install and fewer cable faults and failures.  However, no device offers full assurance against cable damage, especially in harsh offshore conditions. But there is a product that gets one step closer. Smart cable is fast becoming capable of several things conventional cable is not.

Subsea cable health is a particular challenge for offshore wind developers and operators because of the harsh environment it must endure and work in. The ORCHIDS project (Offshore Renewable energy Cable Health monitoring using Integrated Distributed Sensor systems) is looking to enhance subsea cable monitoring capabilities by combining emerging optical sensing techniques to enable a smart cable management system. (Photo: European Marine Energy Centre)

For example, smart cables include capabilities such as continuous cable condition monitoring, pre-fault localization and detection, and the ability to send information to a web-based status monitoring system. These systems typically consist of advanced cabling hardware, sensors, control units, and control centers where all data points are collected, monitored, and analyzed.

Over the past couple of year, Tecnalia, a firm working with PDL Solutions and JDR Cable Systems and funded by the UK government, have been working together to develop smart cable technology for use in the UK offshore wind sector. Their goal is to reduce the levelized costs of offshore renewable energy.

The benefits of their smart cable technology include the ability to monitor and indicate where insulation degradation is taking place, test the function of cabling (under normal load and voltage, and without interrupting service), and pinpoint areas where the cable is failing, all with a high degree of accuracy.

More recently, Fraunhofer UK Research has teamed up with Synaptec (a developer of optical fiber networks to measure and monitor electricity network functions), and the European Marine Energy Centre, to develop a new way to address marine cable and electrical infrastructure reliability.

By means of the ORCHIDS project (Offshore Renewable energy Cable Health monitoring using Integrated Distributed Sensor systems), the team intends to enhance subsea cable monitoring capabilities by combining emerging optical sensing techniques that enable a smart cable management system for use during manufacture, transport, installation, through to end of life.

“Subsea cable health is a particular challenge for marine energy and offshore renewables due to the hostile environment in which they are placed and have to operate,” explained David Hytch, Offshore Renewables Specialist at InnovateUK. “Failure of cables can also lead to costly losses of revenue and hefty repair bills.”

While no offshore cable or hardware system provides full protection against faults, a combination of monitoring capabilities can significantly decrease downtime and speed repairs. In fact, now it is possible to proactively repair or replace cables at the same time turbines are scheduled for maintenance to increase wind-farm uptime and production. As the offshore wind industry matures, expect to see more ideas to make offshore cables more durable, cost-effective, and longer lasting. 

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