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Grimsby Pontoon System

Following the completion of a number of offshore and marine projects, we were approached by Grahams Construction to carry out the detailed design for 3 restraint booms that would make up part of a floating pontoon system in Grimsby. Several other companies had tried to complete the design but had struggled, Eadon’s approach of fully understanding the problem before beginning the task enabled us to simplify the fatigue spectrum and thus solve the key design sticking point.

Each steel boom, which weights around 43 tonnes and is 34 metres in length, is designed to anchor the 2400 m2 pontoon in place and is articulated at both ends to allow for tidal movement. They also feature a lightweight walkway to allow for access to the dolphins, which act as fixing structures for the whole pontoon.

Key facts

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Location

Grimsby

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Client

Associated British Ports

(ABP)

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In Partnership with

Grahams

Among the key challenges in the design was the severe fatigue loading to which the booms were subjected as a result of wave action. The specification for the structure had used a very conservative method for deriving the fatigue spectrum, resulting in the required steel thicknesses being excessively high. However using the raw time history data from the wave modelling that had been performed, Eadon Consulting were able to use a cycle counting algorithm based on the rain-flow method to derive a much less onerous spectrum. This reduced the effect of the fatigue loading sufficiently that it was no longer the critical load case for the design of the sections, and some careful detailing at the connections resulted in a structure with a high anticipated service life. The clevis fabrication was fastened to a concrete corbel on the pontoon by an array of post-tensioned Macalloy bars to withstand the fluctuating loads.

3D model of boom pivot area with walkways and handrails

An additional issue was ensuring that the pontoons could be installed easily. The steel pin assembly connecting through the bearing at each end weighed over a tonne, and to maximise the life of the bearings it needed a very close fit. Careful design was required to allow these pins to be inserted safely and easily on a pontoon that moves with the waves. Ultimately the final three pins were installed successfully within a single six hour window.

While seemingly straightforward structures, the design of the booms presented a number of difficult problems and required the consideration of a number of complex interacting components. However, Eadon Consulting were able to produce an efficient design that is safe to install and maintain. The pontoon installation was completed in 2013 and continues to withstand constant tidal movement and arduous operational loading cycles.