Dredge Fleet Scheduling Optimization and Sensitivity Analysis

Kenneth Ned Mitchell, PhDERDC Coastal and Hydraulics Laboratory

Dr. Heather L. NachtmannUniversity of Arkansas Dept. of Industrial Engineering

Mack-Blackwell Rural Transportation Center

USACE Dredging Program• Hundreds of projects dredged

every year to maintain navigable depths.

• Costs increasing, even though cumulative dredged volumes are not.

At least $X in Avg. annual O&M

funding

# of Navigati

on projects

$10M 34

$1M 190

$100k 448

$10k 669

$1k 797

Source: Dredging Information System, USACE Institute for Water Resources

Dredge Fleet Scheduling• USACE dredging jobs are

contracted at the District level.

• Industry and Corps-owned dredges are selected based on cost, availability, and ability to execute project dredging requirements.

• Dredging at any one project must be completed during local environmental windows.

Dredge Fleet Scheduling• Thousands of project-level

environmental restrictions for dozens of species, applied incrementally through the decades; easy to implement, hard to undo.

• Cumulative effect of these scheduling constraints has contributed to significant overall increases in USACE dredging mission execution costs.

• Costs of these environments restrictions cannot be quantified without considering the system-level interdependencies across projects.

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Fleet Scheduling Optimization

• Constrained resource scheduling problem• Daily scheduling over a one year planning

horizon• Objective function

– Minimize total number of active dredge days (dredge time + travel time) to dredge all projects

• Model Constraints– Each project assigned to a single dredge– Dredge can only be assigned to one project at a time– Dredge may not work on a project during any

applicable restricted period

Fleet Scheduling Optimization

• Optimal nationwide assignment and scheduling of dredges to navigation projects is interesting, perhaps even helpful…

• BUT, it is sensitivity analysis that will help answer persistent, long-standing questions concerning:– Critical environmental windows for targeted

R&D– Next-generation dredge fleet characteristics– Sequencing of projects with multi-year

dredging cycles

Initial Fleet Scheduling Model• Dredging Information System (DIS) used to

parameterize dredges and project dredging requirements: http://www.ndc.iwr.usace.army.mil/data/datadrgsel.htm– Each dredge given a daily production rate (CY/day)– Each project given an average annual dredging

requirement (CY)

• USACE Threatened, Endangered, and Sensitive Species Protection and Management System used to establish project-level scheduling constraints: http://el.erdc.usace.army.mil/tessp/

Fleet Scheduling Sample Problem• 32 dredge jobs of varying cubic yardage

requirements located throughout the U.S.; distance matrix for travel times

• 10 dredges with varying daily production rates• 4 restricted period scenarios evaluated for initial

sensitivity analysis to show proof of concept:– Baseline restricted periods (from TESSPMS)– 10% across-the-board restricted period

relaxation– 15% relaxation– 25% relaxation

Goal is to demonstrate that relaxation of environmental restrictions can lead to significant, quantifiable, system-level efficiency gains for USACE dredging program.

Baseline scenario results

10% relaxation results

25% relaxation results

Sensitivity Analysis Results• Simplified preliminary

model nonetheless captures system-level efficiency gains.

• Model output can be analyzed to pinpoint specific restrictions with largest impact on overall dredging costs.

Next Steps• Increase model size and sharpen

parameter values to be more reflective of real-world conditions

• Include dredge-type specific environmental restrictions, as well as stochastic aspects (ie temperature thresholds).

• Incorporate dredging contract cost analysis (Linkov, et al., 2012) so that number of available dredges directly relates to dredging costs in model.

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Dredge Fleet Scheduling Optimization and Sensitivity Analysis

Questions?

Kenneth Ned Mitchell, Ph.D.U.S. Army Corps of Engineers – Research and Development Center

Coastal and Hydraulics LaboratoryDredging Innovations Group (DIG) Deputy Director

Kenneth.N.Mitchell@usace.army.mil