Subhub Tidal Platform Hits the Water

Subhub moored at the D1 quayside showing her waterline and draft marks; Image: QED Naval

Scottish engineering consultancy QED Naval said that its Subhub tidal power platform was lifted into the water earlier this month.

The lift was managed and supported by Doyle Shipping Group who conducted all stevedoring operations and provided boat support.
They contracted Kavanagh Cranes to perform a dual lift using a 350t and 500t cranes.

The barge used to support the subsea manifold was provided by Cuan Marine. The barge also supported the umbilicals used to provide the remote control of the Subhub platform used in this unique method of installing and recovering tidal turbines from the seabed.

Finally, the barge provided the tow operators with easy access to the emergency tow line and anchor.

The clever bit about Subhub is it’s fundamentally stable in the dive condition when submerged and the descent can be controlled using trim tanks easily. It is responds quickly to subtle changes in the ballast system so it can be installed safely or recovered in minutes over a wide range of tidal and wave conditions, QED Naval explained.

The Subhub’s draft was measured at 3.8m when it was launched without the cross beam and turbines fitted. This is a little lower than expected but indicates that the stability analysis is conservative. The platform is said to be very stable in this condition and with the guardrails fitted shows how safe access to the top deck allows marine operations to be performed whilst at sea.

There is full access to the tidal turbines above the waterline whilst in the transit or maintenance condition along with the PTO, electrical transmission equipment and connectors from the top deck.

Integration of Cross Beam and Tidal Turbines

The cross beam including the central turbine was fitted to the Subhub whilst afloat to minimise the overall weight of the lift. These essential parts of the payload were fitted in under 30 minutes using locating pins to provide the exact position to fit the cross beam to the foundation structure with the bolts.

The outboard tidal turbines were fitted in under 15 minutes and provides a useful check of the Subhub’s stability again by performing an inclining experiment which showed the draft levels on the port and starboard sides.

Validation of Stability

The final draft mark of the Subhub with all the systems fitted came to just under 4.0m draft. The trim of the vessel was completely level and there was no list. This was pretty remarkable since there are significant differences in the structure between the port and the starboard sides, QED Naval said.

The port side includes all the Power Take-Off and transmission equipment and additional structure to secure it in position. This all needs to be counter balanced with ballast on the starboard side. Therefore the results of the waterline checks shows stability assessment was performed to a high standard by the QED Naval design team.

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