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Offshore Wind Journal

Offshore Wind Journal

Risk-based approach could help reduce cable installation costs

Thu 16 May 2019 by David Foxwell

Risk-based approach could help reduce cable installation costs
Nexans’ new cable-lay vessel Aurora is designed to work in conditions that would challenge other vessels

The cost of installing and maintaining cables for offshore windfarms could be reduced if companies throughout the supply chain adopt new approaches to the installation process, a leading figure in the sector believes

In a wide-ranging interview with OWJ, Nexans installation and engineering director Bjorn Ladegard said significant cost reduction could be achieved if the cost drivers in the cable burial and protection process were addressed using a risk-based approach.

“We need to look at the model the industry uses for cable protection,” said Mr Ladegard. “The difference in cost between burying a cable to 60 cm and 1 m are very different.

“A risk-based assessment of the level of protection required and the actual depth of burial required on a case-by-case basis would mean that burial depth could be optimised.” A risk-based approach would also provide a way to assess residual risk to the cable after burial operations, he suggested.

As Mr Ladegard also pointed out, the level of protection afforded to a cable also has a direct influence on the type of equipment used, which in turn has a direct influence on cost.

Leading players in the offshore wind market have echoed Mr Ladegard’s comments. The Carbon Trust recently developed cable burial risk assessment guidance for the preparation of cable burial depth of lowering specifications.

The Carbon Trust’s aim was to enhance risk management, improve conservative estimates of residual risk, and reduce installation and maintenance, and insurance costs. The guidance provides a risk-based assessment and enables burial depth to be optimised and residual risk to the cable after burial operations to a specified depth to be assessed.

Nexans is best known as an installer of subsea cables but as Mr Ladegard explained, in late 2018 the company also formed a dedicated team to support high voltage cabling and connectivity projects worldwide, bringing its experience and resources in cable fault detection, specialised offshore repair equipment and assets for cable repair into a dedicated inspection, maintenance and repair unit. Mr Ladegard said the aim was to provide “emergency response services, a 24/7 hotline and fast-track mobilisation.”

“The way that cables are maintained and repaired when necessary is changing,” he explained. Different asset owners have different approaches to responding to the need for maintenance and repair. A frame agreement or a call-off arrangement is one potential solution, said Mr Ladegard. Then there is the type of vessel and the tooling on vessels to consider. Maintenance work might not need the kind of very high spec vessel usually used for cable installation, but careful attention needs to be given to how it is fitted out.  

Mr Ladegard also described some of the features of the company’s new cable-lay vessel, Aurora, a steel-cutting ceremony for which took place in February 2019. The hull of the vessel is being built at Crist in Poland, with engineering, outfitting and system integration, equipment preparation, testing and sea trials at Ulstein Verft in Norway.

The ST-297 CLV was designed by Norwegian company Skipsteknisk and is intended for power cable laying, including bundle laying, cable jointing, repair, cable system protection and trenching. It will be delivered in 2021.

“We have put all of our experience into Aurora,” he said. “The vessel will have a large capacity cable handling spread including a 10,000-tonne split turntable with dual product lay lines. It will also have a  dedicated, enclosed cable splicing area offering a controlled environment for product splicing and termination work.

Designed to be equally suited to work in shallow water and in deep water, Aurora has DP3 dynamic positioning, and what Mr Ladegard described as “really excellent weather capability.” It also has plenty of installed power. “Aurora has more installed power than most other vessels that are available today,” Mr Ladegard told OWJ. “That will ensure the vessel can remain at work even in adverse conditions that would challenge other vessels.”

In the longer term, cable manufacturers and installers also have challenges to address in the floating wind segment and Nexans was recently awarded a contract to supply preterminated and factory-tested cables for the turbines and T-connectors terminating the dynamic inter-array cables for WindFloat Atlantic, a 25-MW floating windfarm to be installed off the coast of Portugal.

For the 66-kV project, Nexans has been selected to supply its Windlink preterminated factory-tested cable system, which provides a flexible connection inside the turbine between the transformer and switchgear. Nexans will also supply 66-kV T-connectors that will provide the inter-array cables another well-known cable company, JDR, is supplying.

In April 2019 The Carbon Trust announced the five winners of its dynamic export cable competition, which is part of the Floating Wind Joint Industry Project (Floating Wind JIP), which aims to accelerate and support the development of commercial-scale floating windfarms.

The competition, supported by BPP cables, was launched to address the lack of availability of high voltage dynamic export cables for transmitting power from windfarms to shore, which has been identified by industry as a potential bottleneck for deploying floating wind technology commercially. The objective of the competition is to ensure this necessary technology is a viable option for developers for commercial floating wind projects within the next 5-10 years.

The competition winners are Aker Solutions (Norway); Furukawa Electric (Japan); Hellenic Cables (Greece); JDR Cable Systems (UK); and Zhongtian Technology Submarine Cable Co Ltd (China).

Competition funding will support the design, initial testing and development of dynamic cables ranging from 130 kV to 250 kV to enable the efficient transmission of power from floating wind turbines to shore. The first phase of the project will conclude in March 2020. Results may then help to inform subsequent project phases to support the deployment of dynamic export cables across the industry.


Subsea  cable repair solution launched

AERSUB enables cable repair and maintenance to be undertaken in-situ, on the seabed

May 2019 saw Irish Sea Contractors launch a patented, subsea power cable repair solution, AERSUB, developed specifically to enable power cables to be repaired, in-situ, on the seabed. The company believes AERSUB will help to significantly reduce the cost of repairing power cables, potentially by as much as 50%, while cutting repair times in half.

The company also announced the development of a ‘subsea jointer’ training programme including a subsea jointing academy in County Wexford to train the personnel required to carry out the power cable repairs using the AERSUB, which has been successfully tested on a demonstration project in the water at Rosslare Europort.


Subsea plough can help reduce costs

Osbit Limited was due to deliver an advanced multi-function prelay and backfill subsea plough to Global Marine Group in May 2019. The company said the Scion 240 “reduces the cost of offshore wind by minimising the operational risk and the time required to install subsea cables.”

Osbit said the system “offers unrivalled single pass capability delivering boulder clearance and pretrenching up to 1.7m in a single run, leaving a boulder-cleared swathe and a ‘backfill-ready’ open trench with segregated spoil.”

The system is fully subsea adjustable and features an extensive surveillance suite for accurate route tracking and effective trenching. This extensive functionality is provided through Osbit’s control technology, which consists of an open source modular system incorporating Osbit Integrated Logistics Support asset monitoring, packaged in an ergonomic control cabin.

The Scion 240 can also be reconfigured into backfill mode, which uses the same control and surveillance suite to monitor the cable and trench profile, enabling reliable backfilling while ensuring the safety of the cable at all times.

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