Cutter Suction Dredge Productivities in Semi-Protected Harbors

Abstract

Dredging in semi-protected harbor areas is difficult for large cutter suction dredges, which cannot work under the action of waves. However, only this type of dredge can cut hard bottom materials. A case study was carried out in which net times of operation and productivities were defined for a large cutter suction dredge working in a semi-protected area, as a function of the monthly nearshore wave climate inside the harbor. A numerical model was utilized to predict wave conditions in the working area and dredge behavior under these waves was estimated. Possible scenarios of productivity for dredging hard bottom materials were obtained, and the best working season of the year was defined for different lengths of breakwater construction and levels of protection at the specific harbor site. INTRODUCTION A cutter suction dredge is hydraulic dredging equipment that has the ability to excavate most materials and pump directly to a disposal site. These dredges work almost continuously and most are not self-propelled. They can cut hard bottom materials and dredge some types of rock without blasting but have limited capability in rough weather as cutter suction dredges cannot work under the action of waves and swift currents. Large cutter suction dredges are defined here as the ones with power on the cutter drive of at least 3,000 horsepower (HP). Examples of this type of equipment can be found in the register Dredgers of the World, published by Oilfield Publications Limited. Dredging in semi-protected harbor areas is difficult for these dredges as they can only work in calm waters with limited wave action. A case study is presented as an example of planning the net times of operation and productivities for large cutter suction dredge working in a semi-protected area, as a function of the monthly nearshore wave climate inside the harbor. The information and data used herewith is not to be regarded as site-specific but only as example in order to aid the presentation of the methodology. The numerical model MIKE21 was utilized to predict wave conditions in the working area and dredge behavior under these waves was estimated. Potential productivity for dredging hard bottom materials were obtained, and the best working season of the year was defined for different lengths of breakwater and levels of protection at the specific harbor site. 1) Sr. Coastal Engineer, MWH, 175 West Jackson Blvd., Chicago, IL, USA, 60604-2814. 2) Sr. Marine Engineer, Marconsult, R. Fritz Feigl 41, Rio de Janeiro, RJ, 22750-600, Brasil. 3) Coastal Engineer, MWH, 175 West Jackson Blvd., Chicago, IL, USA, 60604-2814. Copyright ASCE 2004 Dredging 2002 ENVIRONMENTAL CONDITIONS The example project site and the initial dredging areas are shown in Figure 1. The port is located on the Southern Atlantic Ocean and takes advantage of the existence of a break in the shoreline reef that borders that stretch of coastline. This site is particularly challenging because of the large tidal range (extreme range of 9.5 m) and the rapid changes that occur in water depth at the edge of the reef. As the reef structure itself is irregular, the bathymetric contours are also highly irregular near the port area. Figure 1. Site layout and dredging areas Winds at the site can be quite strong (exceeding 65 km/h) and are predominantly from the southwest, west, and northwest. Waves generally approach the site from the north, northeast, and east directions. Typical wave conditions are represented by a significant wave height (Hs) of 3 m and a peak wave period (Tp) of 10 s. Extreme waves are typified by Hs = 7.5 m and Tp = 15 s. During the period February 1998 to April 1999, wave data were collected by others using directional waveriders at two locations: offshore and near the port and this data were made available to the authors. The soil condition is considered to be hard substrate. From boreholes located in the vicinity of the dredge area, it was found that the surficial layer of the reef is a moderately weak limestone, with unconfined compressive strength of 6 to 12 MPa, and in-situ density of 1,900 to 2,100