Time required to back the dredge down and reposition (reset) for each cut

c. When moved for passing vessels, the dredge was usually relocated to

enable it to keep dredging with as little downtime as possible rather than

standing by for vessel traffic. The decision to relocate to dredge another

position was influenced by the number of deep-draft vessels and the

times between passings. If just one vessel was going to pass, the dredge

could move aside, then immediately reoccupy the same digging position,

but if several vessels were going to pass within a short time, then the

dredge would start digging in a new location.

On 11 June when the Beachbuilder was dredging the high spots in the nega-

tive ranges (total time of 10 hr 20 min), it took 55 min to reposition the dredge

six times (average of 9.2 min per move), and 5 min of downtime was due to a

delay from hopper dredge maneuvering. This resulted in a vessel delay

percentage of approximately 9 percent.

During the total demonstration duration (192 hr), a total of 8 hr was logged

as delay due to vessel traffic (4.2 percent of the total). During the last five

demonstration days (after relocating to Reach 2) when the crew had become

more familiar with this dredging method and dredging operations were more

routine, the logged vessel delay time (2 hr 29 min) consisted of 2.1 percent of the

total 117 hr available.

Time required to back the dredge down and reposition (reset)

for each cut

The time required and distances traveled to back down and reposition the

dredge for successive cuts were calculated from the resets identified in the daily

dredge logs by station and range. These resets' start and stop positions, linear

distances, respective times, and transit speeds are shown in Table 3. Entries that

included additional tasks completed along with the reset (i.e., add pipe, clean

dustpan, etc.) were excluded from these calculations due to the intent to calculate

an average transit speed based solely on reset time. Reset speeds ranged from

9 ft/min to 198 ft/min. The average reset speed for 37 resets was 74 ft/min, with a

median of 55.5 ft/min. The total time required to conduct these maneuvers was

5 hr and 20 min. One reason for the wide range of speeds is that on some of the

setbacks the floating hose required more repositioning by the tugs. Three resets

and respective times were identified from the daily dredge report that included

additional tasks, i.e., adding shore pipe that increased the total time for resets to

7 hr 10 min.

Cross-channel maneuvering capabilities

The ability of the dredge to move (laterally) across the channel is a major

element in analyzing this type of dredge's operational feasibility regarding

navigation safety. Table 4 lists 14 individual lateral moves made by the

Beachbuilder during the demonstration. These lateral moves are described by

respective date, start/stop times and positions, linear distance traveled by the

dredge, move time, and transit speed. The time used to calculate transit velocity

Chapter 4

Dredging Operational Characteristics Analyses