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Global Offshore Pre-Lay Plough (PLP240) | Taking the next step in Cable Laying

December 2019

Global Offshore’s new Pre-lay Plough gives unrivalled single pass capability to minimise operational risk and subsea cable installation time. This is the next generation of subsea ploughs.

The Pre-lay Plough reduces the cost of offshore wind by minimising the operational risk and the time required to install subsea cables. The system offers unrivalled single pass capability delivering boulder clearance and pre-trenching up to 1.7m in a single run, leaving a boulder cleared swathe and a ‘backfill-ready’ open trench with segregated spoil.

The Challenges of the Current Cable Laying Market

The challenges of laying and burying an increasing number of ever more sophisticated and fragile power cables further offshore and in difficult seabed conditions, is becoming a pressing issue that is driving technological innovation.

Accelerations in the growth of offshore transmission infrastructure, especially in cable installation at offshore wind farms and the cable interconnector market, means that there is an urgent need to lower costs and increase the reliability of installation.  Therefore the need to find cost effective and efficient solutions delivered by the right assets and personnel, is crucial.

THE NEXT GENERATION OF SUBSEA PLOUGHS

Scoping the Problem

In the renewables market, windfarms are increasingly located further offshore and subsequently we are finding that the seabed has more boulders, boulder clays and glacial tills that standard ploughs find difficult to navigate and effectively work in.

Larger sites and longer cable routes mean that surveys are more expensive to carry out, resulting in reducing data sets, increased uncertainty and requiring tools deployed to be more robust and flexible than ever.

Current subsea cable ploughs are designed to work in a seabed consisting mostly of sands and clays, where the seabed is well known and understood.

The existing process of pre-trenching cable routes and backfilling the seabed, whilst not new, has been refined over the last ten years, but clients are now looking for a solution that will tackle the more challenging environments, achieving burial depths with greater reliability whilst reducing project time and cost.

Global Offshore recently secured a contract with Vattenfall covering inter array cable installation, burial, testing and termination at the 72 turbine Danish Kriegers Flak site, located in the Baltic Sea. The seabed in this location has boulders and heavy clay which, with current technologies, requires time consuming multi-pass boulder clearing and trenching. The implication of using existing ploughing approach are economic and environmental, taking too long and costing too much, whilst producing large amounts of CO2 and NOx.

The Global Offshore innovation team looked at overcoming these issues by designing a new plough for pre-lay trenching.  This new plough would have to be able to clear boulders, surface and subsurface, whilst simultaneously cutting a graduated “Y” shaped trench and depositing the spoil heap to the edges of the trench separate from and inboard of the boulder pile.  This allows the required pre-cable installation intervention on the seabed to be done in a single pass, saving time, costs, fuel and emissions.

The cable is then laid inside the trench, where, depending on conditions and risk profile it may either be left as-is or subject to operations to achieve a required depth of cover.  Depth of cover can be achieved through a combination of backfilling operations to deposit the spoil mounds back into the trench, or by using the Q1400 trenching systems to jet the cable within the trench itself.

Working with Partners

The contract for the plough development and build, was awarded to Osbit who are a world-class specialist bespoke equipment engineering company, delivering solutions for the global oil & gas, subsea and renewable sectors. In collaboration with Global Offshore’s engineering and subsea teams, as well as its first confirmed client, Vattenfall, the pre-lay plough concept was tested using 3D modelling and analytical software to understand its operational ability. This enabled the team to not just model the final design, but to address redundancy issues ahead of time in the design stage, such as robustness and manual overrides in the event of hydraulic failure.

Delivery and Testing of the New PLP240

The plough, named PLP240, has now been built and has recently completed dry dock testing in Blyth. The next steps will be wet testing using a dedicated vessel with an adapted scissor frame attached to the ships A-frame. The plough will be in the splash zone, mid-line water column, and on the seabed, where they will deploy the PLP240 over a range of seabeds. Once trials are finished, the plough will be deployed on the Vattenfall Kreiger Flak project to start in 2020.

Key PLP240 Stats

  • Global Marine Group’s pre-lay plough, the PLP240, will operate in waters up to 500m deep to facilitate a single mode solution for single pass route clearance.
  • Boulders are removed to the area outside the potential back fill spoil and pre-cutting of a “Y” profiled trench up to 1.7m depth for post cable lay project requirements.
  • It has a 200t continuous pull with the ability to remove all boulder from a trench up to 1000mm in size, with “Y” trenches of bases 500mm wide 700mm depth – Y @ 45 degrees in soil types up to 600kPa.

Boulder clearance corridor will be 10m – 16m wide with depth control available in all modes

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