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SSM Oil Spill Transport: San Juan County Vessel Drift and Response Analysis

Nuka Research and Planning Group, LLC, October 2020 – March 2021

The San Juan Islands are located at the confluence of the Strait of Juan de Fuca, Haro Strait, Rosario Strait, and Admiralty Inlet in the Salish Sea. San Juan County’s 428 islands are surrounded by major shipping routes linking Salish Sea ports to Pacific trade routes. The recent increase in ship traffic and increase in demand for shipping routes around the San Juan Island have resulted in heightened awareness and concerns over potential maritime accidents and damage to the precious natural resources from oil spills (San Juan County, 2018). The estimated economic and environmental short-term impact of the oil spill is from $84 to $243 million per 1-million-gallon spill of heavy fuel oil (Northern Economics, 2018). Therefore, San Juan County is exploring the effectiveness of an emergency response tugboats (ERTVs) to mitigate the risk of oil spills.
As part of a cost-benefit analysis for an ERTV, San Juan County sponsored this study (Robertson et al. 2021). ERTVs in this region are considered by many agencies as a high-priority mitigation measure. The primary function of an ERTV would be to rescue disabled ships before they ground in the confined waters around these islands. To evaluate the potential effectiveness of an ERTV to reduce the likelihood of a disabled ship grounding in these waters, the following two questions were posed.

  1. Throughout the study area, how much time may be available for an ERTV to arrive at a disabled ship before the ship grounds, considering winds and currents?
  2. Considering four focus areas around San Juan County, what is the probability that an ERTV could arrive before a ship drifting from the typical ship route grounds?

Figure 1. SSM’s predicted surface currents distribution at the Harro Strait on 1/1/2017 2:00:00 AM

San Juan County contracted Nuka Research and Planning Group, LLC, which partnered with the University of Washington Salish Sea Modeling Center to conduct a vessel drift and response analysis. This analysis first considers a vessel drift in open waters off the British Columbia coast. Then, using available wind and currents data, and a simplified vessel drift algorithm along with other inputs and assumptions, the analysis proceeds to estimate the probability that ERTVs from different locations could reach a drifting ship before it runs aground (Clear Seas, 2018).
The Zone of No Save (ZONS) model developed by Nuca Research for this type of analysis was adapted to incorporate currents as well as winds as critical drivers of vessel drift in the inland water environment of the Salish Sea. This was done using four years of hindcast hydrodynamic solutions (tide, currents, salinity, and temperature) from a high resolution (»100 m) version of the Salish Sea Model (SSM-OFS) provided by the Salish Sea Modeling Center (Khangaonkar et al. 2018, Premathilake et al. 2021). In addition, based on input from Project Contributors, the analysis was focused on the typical ship route through the four areas of interest and the probability that an ERTV could reach a disabled ship that began drifting from this route. The hourly unstructured grid SSM solutions, were interpolated to the at 6-minute intervals and 100mX100m structured grid cells for use with the ZONS model.
Figure 2. 50th percentile (median) drift times for a ship to ground Estimated time to grounding. For example, a ship transiting to Vancouver, Canada would drift at least 10 hours before grounding through the Strait of Juan de Fuca.

With this methodology, drift times to grounding were computed for marine waters over 10-m deep based on more than 6,500 model runs.  Similarly, ERTV rescue times were calculated for seven origin points around the region: Roche Harbor, Sidney, Delta Port, Victoria, Anacortes, Port Angeles, and Neah Bay. Fast, mid-range, and slow ERTV response times were determined from each origin.
Based on this analysis, an ERTV located in Roche Harbor or Sidney would have the best chance of arriving in time to rescue more than 80% of the cases modeled. An ERTV located outside the immediate area would have a lower probability of arriving in time to rescue a disabled vessel drifting from the typical ship route.

Project Highlights

Figure 3. Percentage of a vessel not yet grounded from Turn Point, and the response time that ERTV could rescue the vessel. ERTV origins from Sidney, Roche Harbor would arrive and save with more than 80% of disabled drifting vessels within 1.2 hours of mid response time.

Contacts and Project Team

Su Kyong Yun, Salish Sea Modeling Center
Tarang Khangaonkar, Salish Sea Modeling Center
Final Reports: Tim Robertson et al. (2021). Vessel drift and response analysis for the strait of Juan De Fuca to the Southern Strait of Georgia Retrieved from:
Public News:  Kimberly Cauvel. (2021). Rescue tug stationed in islands is best bet to avoid oil spills in San Juan – Gulf waters, study says. Retrieved from:


San Juan County. (2018). San Juan County Oil Spill Risk Consequences Assessment. Retrieved from:
Northern Economics, Inc. (2018). San Juan County Emergency Response Towing Vessel Cost Evaluation. Retrieved from:
Clear Seas Centre for Responsible Marine Shipping (Clear Seas). (2018). Vessel drift and response analysis for Canada’s Pacific Coast. Vancouver, BC. Retrieved from:
Khangaonkar T., A. Nugraha, W. Xu, W. Long, L. Bianucci, A. Ahmed, T. Mohamedali, and G. Pelletier. (2018). Analysis of Hypoxia and Sensitivity to Nutrient Pollution in Salish Sea. Journal of Geophysical Research – Oceans, 123(7): 4735-4761. doi: 10.1029/2017JC013650
Premathilake, L., T. Khangaonkar, A. Nugraha, S. Yun. (2021 in preparation). Basin-wide characterization of tidal currents in the Salish Sea – Data/model synthesis. Ocean Dynamics.
Robertson, T., B. Higgman, T. Khangaonkar, S. K. Yun, and S. Fletcher. April 2021. Vessel Drift and Response Analysis for the Strait of Juan de Fuca to the Southern Strait of Georgia. San Juan County, Environmental Resources Division, by Nuka Research and Planning Group, LLC in collaboration with the Salish Sea Modeling Center, University of Washington, Tacoma.

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