Safe disposal of dredged material in an environmentally sensitive environment



Stefan Aarninkhof, Senior Engineer, Hydronamic, Royal Boskalis Westminster NV; & Arjen Luijendijk, Senior Researcher in Hydraulic Engineering, Deltares, The Netherlands


Studies have shown that the certified reserves at Qatar’s North Field currently stand at more than 25 trillion cubic meters of natural gas. Large-scale investments in LNG infrastructure enable ongoing growth of the country’s annual LNG production, which is expected to reach 77 million tonnes per annum this year.

In this context, Qatar petroleum (QP) has decided to extensively expand Ras Laffan Port with the development of a major GTL terminal (Figure 1), and Ras Laffan Industrial City. The new port will accommodate around 225 million tonnes of products per year – more than double its present capacity. The first stage of the works commenced in 2005 and covered the large civil marine work related to the engineering, procurement, installation and construction for dredging, reclamation and breakwaters.

The approximate quantities involved were:
• 20 million m3 of hard rock dredging with cutter suction dredgers.
• 27 million m3 of sand reclamation from offshore borrow areas.
• 16 million tonnes of rock from Qatar for breakwater construction.
• 7 million tonnes of rock from overseas for breakwater construction.

These large-scale dredging and reclamation activities were inherently associated with the release of fine excess material (because of cutter spill and overflow losses during barge loading), resulting in the accumulation of fine material in the new port area. This material had to be removed. As it was not suitable for filling purposes, it had to be disposed in an offshore disposal area. Numerical models were used to demonstrate that dredging and disposal operations could safely be carried out without violating environmental requirements.

This article adopts the Ras Laffan case to demonstrate the capability of present-day numerical models to provide realistic simulations of sediment plumes and – equally important – the applicability of such complex techniques in dredging practice through innovative interpretation of model results.

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