Dredge Fleet Scheduling Optimization and Sensitivity Analysis Kenneth Ned Mitchell, PhD ERDC Coastal and Hydraulics Laboratory Dr. Heather L. Nachtmann University 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 Navigation 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. http://www.theresilientfamily.com/wpcontent/uploads/2012/01/ratchet-effect.png http://coryandjennifer.net/corymorse/blog/wpcontent/uploads/2011/01/gr0102traffic-lights-011.jpg
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 Average Difference Between Winning Bid and Gov't Est. 15.00% 10.00% 5.00% 0.00% -5.00% -10.00% -15.00% -20.00% -25.00% -30.00% -35.00% relates to dredging costs in model. -40.00% 1 2 3 4 5 6 7 Number of Bidders
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 Laboratory Dredging Innovations Group (DIG) Deputy Director Kenneth.N.Mitchell@usace.army.mil