Development of an Automated Hopper Dredge Mechanical Ullage Sensor Measurement System as a Contract Payment Basis
Type:
Presented during:
WODCON XXII - Enhance the Harmony between Dredging and Ecology
Authors:
T.L. Welp, K.L. Garmire, T.J. Davidson, D.R. Crader, W.V. Gwin, and A.W. Braun
Abstract
"One of the U.S. Army Corps of Engineers (USACE) Portland District's (NWP) primary missions is the maintenance of federal navigation channels. A hopper dredge is contracted annually to help maintain several west coast navigation channels along the California, Oregon, and Washington Coast. This contract's main payment method is a unit price per cubic meter of material (predominantly sand) removed. The two main methods for determining payment are volumes measured by pre and post dredge hydrographic surveys and the other; bin measure. Pre and post dredge surveys work well for coastal and river areas without high currents or a lot of sediment movement, however in dynamic areas it doesn't determine an accurate volume that has been dredged. Bin measure is used to deal with rapidly changing areas or when there are multiple dredges working in an area. It involves the collection of soundings around the 'bin' or hopper of the dredge to establish an average value. This average value is then related to cubic meters of material within the hopper through the use of a certified ullage table (unique to each vessel) that relates the average sounding to the volume of material in the hopper. Historically, NWP had averaged ten manual soundings taken around the perimeter of the hopper. This method was both dangerous for the USACE and contractor employees that were required to take measurements 24 hours a day, 7 days a week in both fair weather and foul, and introduced human error into the measurement. In 2008 NWP, in conjunction with the U.S. Army Engineer Research and Development Center (ERDC) and the National Dredging Quality Management (DQM) program, began development of an automated hopper measurement system that would replicate hand soundings that involved the use of a technology utilized in measurement of saltwater-brine interfaces for industrial processing applications. This system's advantage was that it utilized a mechanical cable-weight system similar to the lead line manual measurement system and the durable sensors were manufactured to withstand harsh salt water environments typical of coastal bars. The manufacturer of this technology (UWT Level Control) worked closely with USACE to modify the sensors for application to hopper dredging. This paper describes the development of this measurement system that started in 2008 with two test mechanical ullage sensors installed on board the contract hopper dredge through today with NWP using the system as a dredging contract payment basis. The robust system has been validated by side-by-side comparisons with hand soundings. The capability to repeatedly and accurately measure the volume of material in the hopper at the push of a button on the bridge has given the drag tenders the ability to determine the quantity of material in the hopper for optimization of personnel safety and dredge production, allows for of volume production data consistent with historical hand measurement systems, and it reduces the need to have USACE and Contractor personnel on the open deck increasing the safety of everyone."
Keywords: Hopper dredge, production, ullage, contract payment