Abstract

A numerical prediction system for sea ice and spilled oil has been developed specifically for the Okhotsk Sea. Our method enables the prediction of the behavior of oil spilled in an ocean with sea ice present using PCs to obtain high resolution results; a one-week forecast of spilled oil behavior can be obtained in a few hours of computation. First, sea ice behavior is computed for the entire Okhotsk Sea (grid size: 16 km×16 km). Second, appropriate results such as ice concentration and velocities are set as boundary conditions for high-resolution (grid size: 4 km×4 km or 2 km×2 km) computations of areas of interest. Third, predictions of sea-ice and spilled oil behavior are computed. We used a Distributed Mass/Discrete Floe (DMDF) model for sea ice computations that can predict the behavior of spilled oil on both open water and ice-covered seas. The DMDF model combines the advantages of both a continuum model and a discrete element model: the shorter computation times found in continuum models as well as being able to express the discrete nature of sea ice. In addition, this combined model can treat a larger number of floes in much shorter computation times than previously developed discrete element models.

Introduction

In cold ocean environments with sea ice present, oil-spill cleanup is technologically difficult since spilled oil remains under/between sea-ice cover. Up-to-date information of spilled oil drift is indispensable for the development and implementation of an effective response. In particular, shorter computation times would be advantageous for timely implementation of oil-spill cleanup procedures, e.g., deployment of oilspill equipment, protection of fisheries, and rescue of wildlife. Sea ice is generated every winter in the Okhotsk Sea and drifts to the coastal area of Hokkaido, Japan's northernmost island. Within this environment, the Sakhalin Oil & Gas Project, undergoing continuing development offshore of Sakhalin Island, has greatly increased oil transportation in the Okhotsk Sea. Any oil spilled in this area may drift to the coastal areas of Hokkaido and cause damage to the marine environment and economy of this area. The risk of oil spill incidents requires a system for anticipating, simulating, and monitoring oil spills. Figure 1 shows the affected region, including Hokkaido, Japan, and Sakhalin Island.

The Engineering Advancement Association of Japan (ENAA) started a six-year program, "A Study to Predict Spilled Oil Behavior in the Okhotsk Sea Under Sea Ice Conditions," in 2003 that was sponsored by the Ministry of Economy, Trade, and Industry (METI) of Japan. The University of Tokyo (Prof. Hajime Yamaguchi) and Hokkaido University (Prof. Kay I. Ohshima) joined the project to work on the numerical modeling of the ice-spilled-oil rheology and the ocean circulation, respectively, of the Okhotsk Sea.

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