EXPERIENCE WITH TDS

ERDC TN-DOER-I2

July 2000

vessel form (displacement curves). Draft measurements are usually taken with at least two pressure

sensors as shown in Figure 2, one mounted forward and one mounted aft on the underside of the

vessel (Rokosch 1989). These sensors measure the pressures (proportional to depth) experienced at

the underside hull locations.

EXPERIENCE WITH TDS

European Experience. The European TDS system (TDSS) was developed by the Dutch

Ministry of Transport, Public Works and Water Management (Rijkswaterstaat) for maintenance

dredging in navigation channels of the Port of Rotterdam. Its development was initiated to replace

the bin measurement method called the half-sphere and centrifuge that had been used previously as

a payment basis. This method involved lowering a half-sphere (designed to float in a material density

of 1,200 kg/m3) into the hopper at predetermined locations to determine the volume of material with

a density over 1,200 kg/m3. The additional hopper material above this measured interface was

sampled halfway in the remaining dredged material column, and the percentage of solids volume

was determined in a centrifuge. Although improved technology has allowed hopper dredges to

dredge material with densities over 1,200 kg/m3, this method of payment measurement did not

provide the contractor with an incentive to dig anything over 1,200 kg/m3 (van der Gouwe and Blok

1993). The TDSS determines, presents, and records the amount of TDS onboard the hopper dredge

on a continuous basis and per dredging cycle. The data collection system comprises three elements:

the dredge cycle data (including delays), the hopper load per dredge cycle, and positioning data in

real time (van Oostrum and van Rijn 1989).

TDSS field trials were conducted from 1985 to 1987, and the first dredging contract using it as a

payment basis was executed during the winter season 1988-1989 (Ottevanger and van Rijn 1992).

The experiences with this prototype system proved that the TDDS was a reliable and accurate

method for determining the amounts of dry solids dredged and transported (Rijkswaterstaat 1990).

Since that first contract, the system has been used in Rotterdam as a payment basis and has been

continuously modified to improve measurement accuracy and efficiency.1 Measurement of the

weight of solids in the hopper is one of the most effective methods for evaluating the amount of

dredging work being done. TDSS has undergone lengthy testing aboard various types of trailing

suction hopper dredges, and various studies (Rokosch 1989; Rullens 1993; Rullens, dAngremond,

and Ottewwanger 1994; and van der Gouwe and Blok 1993) have found the following:

TDSS requires no manual actions.

TDSS is more precise than the half-ball and centrifuge method.

TDSS is objective since it eliminated errors due to human fatigue caused by routine actions.

TDSS becomes an incentive for the contractor to improve performance.

The TDSS used in 1993 measures production to an accuracy of approximately 8 to 10 percent.

Personal Communication, 22 February 1999, Henk Keyser, Civil Engineer, Ministry of Transport, Public

Works and Water Management (Rijkswaterstaat) North Sea Directorate, The Netherlands.