Deeper waters require fresh thinking on offshore turbine cranes

  • Jun 12, 2019
  • Recharge News

As the offshore wind industry continues to evolve and move into deeper waters, the supply chain must be prepared to adapt to meet new challenges. Effective and reliable lifting equipment on offshore turbines, which lift and lower materials from supply vessels, is an essential aspect of any installation to ensure they can withstand the harsh environments in which they operate.

Designing a crane for deeper-water offshore wind farms brings a new set of requirements and external factors to consider such as higher wave height and harsher winds. Offshore turbines in water depths of up to 30 metres are traditionally installed on monopile foundations and feature standard davit cranes with short fixed booms that extend over the sea just enough to lift and lower equipment as needed.

However, as offshore wind developments continue to move further away from the shore, jacket foundations with a wider footprint become the substructure of choice, increasing the horizontal distance from the turbine to the supply vessel, requiring a crane that can extend further with greater functionality.

Traditional davit cranes on offshore turbines generally have 2-4-metre booms with manual slew (horizontal extension) and no luffing function (extension of hooks up and down on ropes) as supply vessels can ‘bump’ next to the monopile structure. With jacket foundations, the crane requires a longer boom of 7.5-8 metres with full powered slewing, luffing and hoisting functions to ensure it can reach supply vessels from a greater distance, even in higher sea states.

All cranes have a deflection rate — an ability to flex so they can absorb the loading forces that come from lifting a heavy weight in a dynamic situation — and it is vital this is accurately determined to reduce the amount of impact on the crane. Ropes provide most of this deflection due to their flexibility, so hoist ropes must be fully optimised. This ensures they are strong enough to comply with the minimum safety requirements but not so strong that they become too rigid which would increase the dynamic forces affecting the specification of all other crane components.

Unmanned turbines also present the challenge of regular maintenance when compared to oil & gas rigs which are continuously manned. The opportunity to identify any deterioration in the structure is rare between annual maintenance visits so reliability is key in ensuring the crane is protected from wear and tear and the harsh environment it operates.

Cranes on offshore turbines are generally 15-25 metres above the water line which creates greater opportunity for corrosion to develop. Exposed steel can corrode at a rate of 0.2-0.7mm per year in this environment so a robust protection system is required.

Alternative materials such as nylons, glass-reinforced plastic and stainless steels are the most suitable option for long-term protection. A robust paint system also protects against corrosion, however, particular attention must be paid during manufacturing to safeguard against any contact or chipping during the assembly stage to prevent any breakdown of the coating that could lead to reduced corrosion resistance. For areas of the crane where stainless steel and general carbon steel are in contact, an insulator is effective in reducing the impact of accelerated galvanic corrosion which often occurs between these materials. Handrails and walkways can also utilise fiberglass opposed to steel to ensure long-lasting durability.

Due to lack of accessibility once deployed in-field, the reliability of cranes is imperative. To eliminate the chance of water ingress and material deterioration, a polytetrafluoroethylene (PTFE) coating protection is an effective solution when applied to steel components in combination with a marine sealant. For pinned connections, the biopolymer can be used to bond, coat and insulate the material — which is more resistant to cracking and chipping when compared to paint systems and is also less thick. Nylon can be utilised for the crane sheaves and spacers inserted between any moving components to provide wear protection.

Sparrows Group has taken all these aspects into account to when developing its new WindMaster luffing crane, which is designed and manufactured specifically for the offshore wind industry to be a reliable and effective lifting solution.

Stuart Smith is global design services manager at Sparrows Group, which provides products and services for the global energy industry.

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