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Applications of Clump Weights in Offshore Mooring Systems

Mooring systems keep offshore structures in place by withstanding all environmental loads as currents, winds and waves. To achieve this, moorings need a proper design but as well efficient maintenance. Material is of critical importance , the lack of reliability of the station keeping system can threaten the entire viability of the project.

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Mooring systems keep offshore structures in place by withstanding all environmental loads as currents, winds and waves. To achieve this, moorings need a proper design but as well efficient maintenance. Material is of critical importance , the lack of reliability of the station keeping system can threaten the entire viability of the project.

Besides in the oil and gas industry for stabilisation of vessels, rigs and other offshore systems, mooring installation have been recently widely used in the renewable energy sector. The fast development of the offshore wind industry requires more and more innovative and various solutions. Several projects are currently developing floating wind technologies, most efficient in important water depths. Here a robust and reliable mooring systems is as well of crucial importance to ensure the floater stability.

The mooring system is mainly composed of drad anchors or suction piles, chains and / or synthetic / nylon ropes and connection systems such as shackles or connectors. It can also feature buoyancy modules, clump weights and different connection elements (H links, Y links...). The mooring system is most of the time connected by connectors, fairlead and / or chain stoppers.

The continuous exploration of the offshore sector due to the increasing demand for energy and the growth of deeper and more complex subsea installations, challenges the mooring sector by requiring safer, cheaper, more resistant and easier to install and maintain solutions.


Offshore Mooring System: Essentials and Importance

Even though all moorings have similar components, they differ a lot in design, since most of them have been specifically tailored to the project and depend on the bathymetrie and the metocean data but as well on the design of the platform. Chain links (the most common choice for permanent moorings in shallow water up to 100 m), cable and/or rope, connectors and an anchor of some type are the most essentials components. Environmental factors as wind, waves and currents determine the choice of the different components and their dimensioning.

The most common types of mooring systems, deployed until now in different projects include catenary, taut leg, semi-taut, spread, single point and dynamic positioning, which actually doesn’t use moorings but computer to control the position of the vessel. Some Mooring systems deployed in the oil and gas industry can be coupled as well to a dynamic positioning system.

Despite the wide range of mooring solutions available for the oil and gas market, the fast development of the marine renewable energy industry and especially of the floating offshore wind sector induces a number of new challenges and concerns. Cost reductions, shallower waters, smaller mooring area and limited footprint outline the main difficulties.

Professionals are aware of all these challenges and are trying hard to develop moorings with a better design that can provide a simple and long-term solutions to meet all these new requirements at reasonable cost. Clump weights offer a great opportunity for a better product. In this case clump weights can be coupled to some of the traditional mooring designs to contribute to the restoring forces applied by the mooring system on the platform. Floatgen, for example – France’s 1st floating offshore wind turbine has chosen the design with the clump weights as the most reliable mooring solution. Currently, other companies with project commissioned or in commissioning FID (Final investment decision) are also considering this solution.


Cast Iron Clump Weights for Better Offshore Performance

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As a trusted partner of the Marine Renewable Energy sector, FMGC is committed to driving the industry’s development through the creation of innovative ballast solutions. The clump weights were created out of the need for cost efficient, safe and reliable solution.

FMGC clump weights are made of cast iron EN-GJL-200 with density : 7,1 and are designed to offset the vertical forces against the anchor and restrict the movements of the floating structure by contributing to the restoring forces. They are available in two models:

  • The “distributed” configuration is a set of medium-sized clump weights, distributed over a segment of the anchor line. This configuration optimizes the effectiveness and the cost of the solution thanks to the proportional recovery of the loads and the most accurate dimensioning.
  • The “mutualized” configuration consists of one clump weight, attached to one specific point of the anchor line. This configuration neutralizes some of the impact of wind and wave and thus limits the tensions on the anchoring line.

FMGC provides a customized design of all clump weights based on the target weight, the chain line diameter and the available installation means.

Consult FMGC studies on the behavior of cast iron in marine environment

The team of FMGC can provide any project with a set of clump weights that have been properly dimensioned and designed to fulfill the exact needs of the project. We offer as well all the numerical simulations and can easily scale a test which can ensure the proper behave of the clump weights and the product’s abilities to withstand the loads applied.

Until now the product has been considered part of the moorings of the following installations:

Clump Weights for floating wind turbine moorings

Considered as one of the most reliable offshore technologies to harness large wind resources in deep water, floating offshore wind turbines require highly reliable mooring solution ready to withstand the loads applied by the environment.

Clump Weights for wave power system moorings

Wave Energy Converter is a technology that is currently at a low development level however its developments are promissings and can be a part of a renewable energy mix.. Designing a mooring system for a Wave Energy Converter is a challenging process because of the highly non-linear action of very high waves entering relatively shallow waters. Clump weights can add in this case additional stability and thus ensure the performance of the system.

Clump weights for floating tidal stream moorings

The technology for tidal stream devices has not yet settled on a design concept and there are many ongoing developments. However, here as well foundations and installation methodology are a key area and highly resistant mooring would be of essential importance.

Clump weights for Offshore Aquaculture

Offshore aquaculture is a growing marine farming sector where fish farms became bigger due to the high demand. Since the farms are becoming bigger and heavier, the installations require dedicated mooring systems. Clump weights can be successfully applied here as well.

The fast development of the MRE industry and the higher volume of offshore wind projects, in particular of floating wind installations are expected to fuel the offshore mooring sector. Their design variety requires innovative and customized moorings solutions.

For some years clump weights have been deployed by the oil and gas industry but faced many issues and failures which has led to the bad reputation of the technology. FMGC has seen this as an opportunity and taking into account the past has created a better solution, meeting the challenges of the sector and improving the performance of all involved devices at reasonable cost.