How to make new installations just float into place

As facilities in the energy industry become ever larger, new ways must be found to install equipment. Trelleborg technology is helping to ensure that new installations just float into place.

One consequence of the world’s ever-increasing demand for energy is that energy producers must venture into locations that are increasingly difficult to access.

These locations have special challenges – among them, the construction of facilities in extremely adverse environments. And economies of scale sometimes mean that things get so big that they can no longer easily be handled by conventional means.

That’s what is happening with offshore oil rigs. Conventionally, the jacket – the structure on which an oil rig stands – is secured to the seabed, and the deck, or topside, is lowered onto it by a heavy lifting crane. But with topsides now often weighing more than 30 000 t, it is often no longer possible to use cranes.

This is where the floatover installation method comes in. The technology is used regularly for oil rigs in shallower waters and is now being used further offshore. The floatover method involves a barge manoeuvring the topside between the legs of a jacket. Then the barge takes on seawater as ballast, sinking slightly so the topside is installed in place on top of the jacket.

Trelleborg supplies the leg mating units (LMUs) and the deck support units (DSUs) that make this possible. In the mating process, LMUs – steel structures with elastomer pads – take up the static and dynamic loads of the topside as well as the horizontal forces arising from wave conditions. DSUs perform a similar function between the topside and the barge’s deck support frame during transit. During mating, decompression of the DSUs has to match compression of the LMUs as the two colossal units slowly join together. Sway and surge fenders protect the substructure and barge from damage.

The LMUs – one for each leg of the tower – are designed separately, using nonlinear finite element analysis to match data on expected load and movement. This is to achieve what J.P. Chia, Engineering Manager at the Trelleborg Offshore & Construction engineered product operation in Singapore, calls “the required nonlinear spring stiffness for this crucial task.”

The challenges are enormous. The recent Wheatstone project off Western Australia involved a topside weighing 37 000 t, the heaviest ever moved into place with LMUs. Another project required LMUs designed to deal with waves of up to three meters – larger than floatovers had coped with before. In both cases, the LMUs had to be designed individually for the different circumstances.

Now Trelleborg is looking into using the same technology for wind power. Offshore wind farms far from the coast need offshore substations to convert DC power to AC power before the power is transferred to land. These substations can weigh as much as 22 000 t and Trelleborg is working to offer floatover technology to place them on offshore jackets.

“Traditional transport and installation methods in the shallow-water offshore oil industry cannot cope with the size and weight of larger topsides,” says Julian Wee, Managing Director at the Trelleborg Offshore & Construction engineered product facility in Singapore. “We are predicting a similar trend in the offshore wind industry, as a floatover is more efficient, faster, safer and more cost-effective.”

So as the hunt for more energy continues, Trelleborg is helping to harness conventional sources more effectively, as well as making possible the next big leap into alternative energy.

Adapted from a press release by David Bizley

Published on 02/02/2015

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