Mooring differences between oil and gas structures and marine energy converters

Floating wind turbines
Floating wind turbines

Marine Energy Converters (MECs) and Floating Offshore Wind Turbines (FOWTs) have, to date, been installed as individual prototypes, though arrays are in the early planning stages in Europe and Asia. As a result, standards for these installations are in early development. With regards to moorings, design requirements are currently based on oil & gas mooring standards and practices. While there are similarities in some of their mooring processes and designs when compared to oil and gas structures, many design drivers are unique to renewables.

Similarities between oil & gas structures and renewable structures:

  • Primary Goal – Station keeping without disturbing production or energy extraction.
  • Mooring Equipment – Chain, wire, rope, anchors, anchoring, and associated jewelry are required.
  • Mooring Design Considerations – Installation requirements, weight of the asset, soil, metocean conditions, dynamic analysis, mooring components type and size.

Differences between oil & gas structures and renewable structures:

  • Vessel Motions – Oil & Gas Industry Floating Production Units (FPUs) are single rigid body floating structures with motions better understood than the typical renewables structure. These units may have several interacting moving parts that induce cyclic loads in different frequency ranges than those experienced by rigid floating bodies. For example, wind/tidal turbines have additional thrust and torque loading that needs consideration. Wave energy converters are often designed to resonate at periods close to predominant wave periods, of which is undesirable for oil and gas production.
  • Analysis – A mooring engineer designing the mooring system for an oil & gas asset will try to minimize offsets. The opposite may be desirable for MECs and FOWTs, as some required range of motion may be necessary for optimal power production. The MEC device dynamics will then require different mooring design analysis tools.
  • Environment – Harsh ocean conditions with strong wind, strong waves and strong currents are a pre-requisite for MECs and FOWTs in order to maximize energy production, whereas they are not the norm for oil & gas assets.
  • Anchor Selection – It is common for MECs and FOWTs to be installed in shallow water with hard soil or sand. In addition, some MEC designs require vertical resistance for power capture operations and this can create a significant anchor uplift requirement. The combination of hard soils and high uplift loading requirements significantly reduces anchoring options.
  • Use of Synthetic Rope – While common in oil & gas, the use of polyester rope may be limited in the case of MECs in shallow water due to sunlight exposure or marine growth. More data is needed to evaluate how synthetic ropes would fare when used with MEC devices.
  • Installation Vessels – While anchor handling vessels (AHVs) have been used to install some oil & gas FPU mooring systems, it is likely that the use of AHVs will be more prevalent in the renewable energy sector as they are less expensive and more economical.

Summary

Much of the knowledge previously acquired in the oil & gas industry is useful and applicable to mooring MECs and FOWTs, and provides an initial basis for mooring design for these structures; however, further review is necessary for assessing their suitability for these projects. Since few full scale Marine Energy Converters have been installed at this stage (early 2017), industry standards are in early development or first stage publication. To learn more, join us at the Offshore Technology Conference for a presentation on this topic.

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