Many companies traditionally supporting the oil and gas industry are carefully observing the progress and potential of the renewable energy sector. Currently, much of this interest is focused on offshore wind; however, marine energy converters (MECs) are making headway as potentially viable options in the renewable energy industry.

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. When compared to the oil and gas industry, floating renewable energy structures are in their infancy, with standards in early development. This nascent industry offers potential for investment and collaboration, which may in turn lead to feasible products.

One opportunity for collaboration with the knowledge within the oil and gas industry is in the area of mooring design and installation of the MECs and FOWTs. For the most part, there are many similarities in the design requirements and processes for both industries; however, there are design drivers unique to renewables. Mooring designers within oil and gas have the practical experience and expertise to guide and influence the MEC and FOWT mooring design, and to help plan a successful installation for optimal operational performance.

Similarities between oil and gas structures and renewable structures

The similarities between the mooring design of renewable energy devices and floating offshore oil and gas facilities tend to lend weight to using current oil and gas mooring standards.

For both industries, the primary goal of the mooring system is to maintain station keeping without disturbing production or, in the case of renewables, energy extraction. Likewise, mooring equipment component types are similar for both industries. Line profiles commonly consist of chain, wire, synthetics or a combination of the previously listed component types. Associated jewelry and the need for anchors also are required and consistent for any moored system.

In addition, the same theoretical design spiral exists for both industries and plays a part in the final mooring design. This includes mooring design considerations regarding installation requirements, soil properties, metocean conditions, dynamic analysis, mooring component types and size.

Differences between oil and gas structures and renewable structures

Due to the unique behavior of renewable energy devices, there is a learning curve required to fully understand how to properly model and design the mooring for these devices. Mooring designers and renewable device designers are constantly learning and trying to compensate for these differences.

One case in point addresses vessel motions and analysis. Floating oil and gas facilities are typically single rigid body structures, while renewable devices often have several interacting moving parts that induce cyclic loads. For example, traditional wind/tidal turbine rotors induce thrust and torque loading due to their rotating blades, while wave energy converters often are designed to resonate at frequencies close to predominant wave periods. The additional loading and operational behavior are not factors conventionally addressed in oil and gas mooring design that must be considered in renewable mooring designs.

With regard to environment, MECs and FOWTs are typically moored in shallow water (less than 152 m [500 ft]) with harsh metocean and/or wind conditions to maximize energy production. These conditions may also limit anchor selection and installation. Shallow-water site locations can have hard soil or sand, which also limits anchor selection. Location, strong waves, wind or current also can limit installation windows and available installation vessels.

Moving forward

Improved collaboration between mooring designers and MEC/FOWT designers will help accelerate the learning curve with mooring design and installation of these structures. However, mooring designers traditionally involved with oil and gas must also work closely with device developers to properly consider the unique complexities of these structures. Working together can allow assessment of suitability and feasibility for these projects to move forward.

A paper on this topic, contributed by InterMoor, will be presented at the Offshore Wind Energy Technical Program on May 4 at OTC in room 606.

Offshore