Safe disposal of dredged material in an environmentally sensitive environment

Introduction

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.
 

Safe disposal of dredged material in a sensitive environment

To guarantee safe disposal of excess material at sea, careful selection of a disposal site is of paramount importance. The Environmental Impact Assessment for the Ras Laffan Port Expansion project had demonstrated that the near-shore coastal zone (with water depths less than 20m) and the waters to the southeast of Ras Laffan are the most sensitive locations in terms of biological productivity, fisheries and ecological habitats. Offshore disposal at water depths above 20m is thus preferred.

The sand mining area JV4 is located at 19km northwest of Ras Laffan Port, at water depths of 19m to 25m (Figure 2a). As a result of the extraction of approximately 6 million m3 of material for the present port expansion, it offers sufficient space to accommodate the anticipated 3 million m3 of excess material from Ras Laffan Port. Hence no reduction of water depth would occur. An extended environmental study, carried out prior to the start of the sand mining operations, revealed that the seabed in the JV4 area was mostly covered with soft material and that benthic communities were not particularly rich. This observation applied to the full sand mining area.

Consequently, it could be concluded that local ecological sensitivity for the JV4 area was low by nature. To avoid further disturbance in other, pristine areas, it was decided to select JV4 as the primary disposal location. The sand mining activities in JV4 were subject to environmental requirements to minimize possible environmental impacts to surrounding waters. These requirements stated that during dredging the concentration of suspended solids was not allowed to exceed a depth-averaged limit level of 30mg/l on an environmental boundary surrounding JV4 (Figure 2b).

To verify whether these requirements were met, the suspended solids concentration (SSC) was measured on a daily basis, at 21  locations along the environmental boundary. SSC measurements were carried out by lowering and subsequently raising a calibrated YSI turbidity sensor through the water column. This yields a vertical concentration profile, which was averaged over depth. Owing to the relatively large distance to the dredging operations, vertical concentration profiles were found to be virtually depthuniform. A proposal was offered to apply the same environmental requirements to the execution of the disposal activities as were used earlier during the sand mining operations. To obtain permission for the start of the disposal works, authorities demanded a demonstration that the disposal operations could be carried out without exceeding SSC limit levels at the environmental boundary, under all possible current and weather conditions. A state-of-the-art numerical model was used to do so.
 

Model prediction of plume dispersion

In order to evaluate the dynamic plume created by the process of jet release by trailing suction hopper dredgers, and the subsequent descent and collapse, a computational gr id of an existing hydrodynamic model for the Ras Laffan region was refined in and around the JV4 area.
 

Approach

The work method for the removal of unsuitable fine material from Ras Laffan Port foresees the use of trailing suction hopper dredgers (TSHD). After sailing to JV4, this fine material is disposed of by opening the bottom doors of the TSHD. This yields a fluid-like jet of fine material that rapidly descends to the seabed [e.g. Van Rijn, 2005]. The bulk behavior of this watersediment mixture is important, rather than the settling velocity of the individual particles [Winterwerp, 2002]. After impact upon the bed, the sediment load will radially flow away from the point of impact over the bed as a density current in the lower 15to 20 percent of the water column. This phase is characterized by rapid dissipation of energy and settlement of material. The process of jet release, descent and collapse is generally referred to as the ‘dynamic plume’ [e.g. Spearman et al., 2007]. While the fine material jet descends through the water column, part of the material gets eroded from the outside of the bulk load (slurry jet) and suspended in the surrounding water (entrainment). After impact on the seabed, re-suspension of fine material occurs from the near-bed density current, caused by turbulence-induced upward mixing at the upper surface of the mud layer.

Stefan Aarninkhof, Senior Engineer, Hydronamic, Royal Boskalis Westminster NV; & Arjen Luijendijk, Senior Researcher in Hydraulic Engineering, Deltares, The Netherlands
Edition: Edition 47

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