optimal dredge fleet scheduling within environmental work...

Optimal Dredge Fleet Scheduling within Environmental Work Windows Dr. Ned Mitchell

Research Civil Engineer ERDC Coastal and Hydraulics Lab Dr. Heather Nachtmann University of Arkansas

4th Biennial TRB-CMTS Research and Development Conference National Academy of Sciences Washington, D.C. June 21st, 2016

Innovative solutions for a safer, better world BUILDING STRONG®

Research Collaboration Research discussed today stems from a

collaboration between U.S. Corps of Engineers (USACE) and University of Arkansas

Research support facilitated through the Maritime Transportation Research & Education Center (MarTREC), the Mack-Blackwell Institute and the USACE Research and Development Center (ERDC)

U.S. Department of Transportation and

USACE R&D funds are being leveraged to enable an implemented tool for improved dredge fleet scheduling


Current Dredge Fleet Scheduling Navigation projects are contracted at the USACE District-level Dredging vessels (government or private industry) are

generally selected by awarding the contract to the lowest-cost bid that meets the demands of each individual project ► Availability ► Capability ► Environmental work windows

In recent decades, overall dredging costs have steadily increased even as total quantities dredged have remained stable.


Scheduling Optimization Concept • Schedules are uncoordinated • Potential for inefficient dredge plant

itineraries over course of dredging year • Results in higher dredge mobilization costs

• While accounting for Project-level requirements and environmental work windows, schedule dredging so as to minimize mobilization costs.


Dredge Scheduling Constraints 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.

Full costs of these environments

restrictions cannot be quantified without considering the system-level interdependencies across projects.


Optimization Approach Objective function

► Maximize the volume of cubic yards dredged over a finite time horizon

Model constraints ► Each job is satisfied by at most one dredge vessel (for now) ► Total cost incurred by any assignment scheme cannot exceed the total

budget; cost incurred at each project cannot exceed Presidential Budget amount

► A dredge vessel may not work at full capacity on a project during any applicable restricted period or while it is traveling

► A dredge vessel may not work on more than one job at any time ► Completion time of each project occurs before the end of the planning

horizon


Constraint Programming Approach Interval design (decision) variable

► For each possible project and dredge vessel assignment ► Less complexity in decision variable space

• No time index in decision variable definition • Combines start, end times as well as duration and usage rate in one variable

Global constraints including ► Alternative: Assignment ► ForbidExtent: Environmental work windows ► TransitionDistance: Sequence dependent setup times ► Intensity Function, Cumulative, Pulse, Startmin, Startmax, Sequence

Interval Variables etc.

IBM CP Optimizer software


South Atlantic Division Implementation

Project-level Data Requirements: • Work window dates • Unit costs ($/CY) • Daily production rates (CY/day) • Min. CY requirement • Budget amount • Distance Matrix

• 123 Total dredging sub-projects (roughly 90 considered per run)

SAW – 13; SAC – 20; SAS – 9 SAJ – 48; SAM – 33

• 63 environmental windows/restrictions

• 31 distinct dredges identified

- (12 hoppers; 5 mechanical; 14 pipeline)

- Subset of 8-10 used to test feasibility


Sample Output: Single Dredge Plant

Env. restricted periods


Implications for Dredging Program

• Fleet scheduling model provides a quantitative way to evaluate the relative cost-effectiveness of various approaches to O&M dredging program execution.

• It also serves as a starting point for exploring the most

promising candidate groups of projects for regional contracting.

• Helps quantify the relative constraining effects created by environmental work restrictions roadmap for targeted research


South Atlantic Potential Savings

Model mobilization cost assumptions: - $500 - $1000 per mile for hoppers, depending on size - $1000 - $1500 per mile for pipelines, depending on size - $800 - $1200 per mile for mechanical dredges - 500-mile distance for first dredge assignment

NAV + FRM Dredging Total $

Mobilization Cost %

Mobilization Cost Total

South Atlantic Division Baseline $250M 16% - 21% $40M - $52M

Fleet Scheduling Model Results $250M 5% - 8% $12.5M - $20M

Total Potential Mobilization Cost Savings: $20M - $40M


FY16 West Coast Hopper Fleet Planning


West Coast Hopper Fleet Planning

Scheduling Model Output


Gulf of Mexico Application Using USACE District dredging

reports from recent WEDA meetings and direct correspondence with project managers, model data was compiled for: ► Dredging requirements for 32

projects ► 12 environmental work windows ► Hoppers and pipeline dredges

considered

Dredge production rates and project unit costs were assumed (for now).


Sensitivity Analysis Model can be re-run with varied inputs to reflect:

► Relaxed or lengthened environmental restricted periods ► Changes in size and production rates of available fleet ► Various budget scenarios

Resulting changes in model output provide insight into significance of underlying parameter(s).


Impact from Relaxing Restricted Periods

Results indicate that the jobs with larger dredging requirements relative to the duration of the work window act as scheduling “bottlenecks” in the overall dredging program.


Problem Extensions and Modifications

Extensions: Partial Dredging During RPs Variable Job Sizes Multiple Trips to the Same Job Multiple Dredges work on the Same Job

Modifications: Different Operation Rates for Job/Dredge Pair Mob/Demob Cost Based on CY Dredged (west coast only) Downtime for Dredges Mob/Demob Cost Based on Travel Distance


Optimal Dredge Fleet Scheduling within Environmental Work Windows

Ned Mitchell, Ph.D. U.S. Army Engineer Research and Development Center

[email protected] 601-529-9005

USACE Team: Dr. Corey Winton, ERDC-ITL Mr. Mark Cowan, ERDC-ITL Ms. Jackie Kieser, SAJ Dr. Clay McCoy, SAJ

Univ. of Arkansas Team: Dr. Heather Nachtmann Dr. Chase Rainwater Mr. Fereydoun Adbesh Dr. Ed Pohl

mailto:[email protected]

optimal dredge fleet scheduling within environmental work...

Documents