The nature of the cable installation market for offshore windfarms is changing, as are the contractors, installers and trenching equipment they use
In the past, cable installation was usually the preserve of dedicated contractors, but increasingly engineering, procurement, construction and installation (EPCI) contractors are taking a much more active role in the market, with companies such as Boskalis and Van Oord now owning and operating their own cable installation assets.
With this growing interest in taking on cable installation has come what SMD managing director - equipment Paul Davison describes as a much more proactive attitude to maximising the efficiency and reducing the cost and risk of cable installation.
“This has led to them adopting a performance-oriented and technology-led approach to cable installation,” says Mr Davison. “With that has come demand for more powerful, reliable trenching technology and systems that can provide much greater control over installation, and a requirement to actively monitor the cable installation process.” Increasingly contractors want to measure as-laid tension and ensure cable bend radius is not exceeded while having a visual representation of the trench, he says.
The offshore wind industry has experienced a number of cable-related issues that Mr Davison believes are driving contractors to reduce risk by burying cables ever-more deeply. This means trenchers need to be capable of excavating deeper and deeper trenches. “Burying cable to a depth of 3 m has become the norm,” says Mr Davison. “But in some cases, we are definitely seeing over-burial because of anxiety about the cost of remedial work should it be required.”
The trend Mr Davison identified towards deeper burial means trenchers have grown in size and in capacity. “For several years 1,000 hp was typical for a trencher, but next-generation units of 1,600-2,000 hp have become the norm,” he tells OWJ.
Osbit director Robbie Blakeman agrees with Mr Davison that EPCI contractors and installers want more and more data about the installation process. “They want to know more and more,” he says, noting that trenching for offshore wind has also had to evolve because cables themselves have changed. So too, he says, has the type of seabed into which trenches are cut for offshore wind projects. In many early projects, the material a trencher was cutting into was relatively easy.
The fact that soil types are more variable means what Mr Blakeman describes as a ‘Swiss army knife’ trencher is less likely to be the best tool for a job. “The choice of trencher should always be driven by the soil type it is going to be working in,” he tells OWJ. He notes that upcoming projects in French waters require trenching in hard rock for which not all trenching assets would be well-suited.
Things are also changing in the way cables are laid on some windfarms, Mr Blakeman explains. He cited the example of the Kriegers Flak offshore windfarm in Denmark, where developer Vattenfall decided to pre-trench inter array cables before the foundations for the turbines were installed. Doing so enabled it to use a towed tool. “There has long been a concern in the industry that ploughs damage cables, but if you can use a plough, that is the fastest way to get a cable into the ground,” he says.
He cites the example of a single-pass plough the company developed for Global Marine Group for the Vattenfall project as an example of the potential effectiveness of this approach, if conditions allow. The multi-function, pre-lay and backfill plough was designed to clear boulders and pre-trench to a depth of 1.7 m. It has a sophisticated surveillance suite and can be reconfigured into backfill mode while ensuring the safety of the cable.