Maritime Technologies

Offshore-Ring System

To improve the offshore cultivation of marine macroalgae under harsh environmental conditions a specific carrier device – the offshore ring system (see Fig. 1) – has been designed (since 1994/95) and successfully been tested on Laminaria saccharina in the North Sea, 3.5 nautical miles NE off the coast of Helgoland. The macroalgae grew well on the newly invented ring-structure, which seems to be beneficial not only in meeting rough weather conditions (strong current velocities and wave heights), but also in permitting easy handling when compared to other constructions.

The tested offshore-ring prototype consisted mainly of a 1 cm strong polyethylene tube of 11 cm in diameter that was welded to an outer ring-structure of 5 m in total diameter (Fig. 2). A 3 cm steel cable was inserted into this outer polyethylene ring for buoyancy purposes. Using metal cuffs, carrier ropes were radially clamped between the outer polyethylene ring and an inner central guide ring to build an anchorage to the actual culture lines. In a cobweb manner, 80 m of inoculated and pre-cultivated culture line were fixed between the inner and outer rings, using lacing cord to attach the culture line to the carrier ropes. Towards the bottom, a crow’s foot, connected to the same metal cuffs as the carrier ropes, was employed to tighten the cobweb to a common mooring system (concrete blocks) via an additional centre guide ring (see Fig. 2). Upwards, the whole ring structure floated between 1.2 and 1.5 m beneath the water surface, which was achieved by outbalancing the down-weight of the inserted steel cable of the outer polyethylene tube with a supplementary buoyancy system. The latter consisted of a central buoy of 300 kg buoyant force that was linked via a third centre guide ring and a reversed crow’s foot to the same metal cuffs holding the carrier ropes and the lower crow’s foot (see Fig. 2). Thus, both (upper and lower) crow’s feet were linked to the same metal cuffs that were as well connecting the outer- and inner ring of the cobweb. Through the additional centre guide rings, chafing of the mooring and buoy lines could be avoided.

This construction allows adjustment of the culture lines to an appropriate depth between 1 and 1.5 m, which is important to avoid physical harm through water motion as well as PAR and UV damage of the young plants, while simultaneously providing sufficient light intensity for photosynthesis.

Apart from that, ring management is rather simple as it can be assembled onshore with the respective culture line and harvesting or sampling can be accomplished by heaving the entire ring up via a ships crane or even by hauling the whole structure onshore.