Currently, the 30 nautical mile (nm) minimum longitudinal separation standard is applied on oceanic routes under the Automatic Dependent Surveillance-Contract (ADS-C) environment. However, the periodic position report should be provided at least every ten minutes in Japanese airspace based on previous safety analysis. The position report is usually sent via satellite connection, so less frequent position reports would reduce connection costs by airlines. Since the previous safety analysis estimates the risk of collision conservatively, this paper proposes a refinement of the collision risk model by considering the dependency between two closely separated aircraft. The result shows that the periodic position report interval can be extended without infringing safety.