63% of Nighttime Vessel-Hours Concentrate in 8% of H3 Cells. They Are Offshore Anchorage Rings, Not Port Approaches.

The Setup

Vessel density heat maps built from AIS pings look intuitive: port approaches are bright, open ocean is dark. That framing conceals a systematic pattern. When resolution-8 H3 hex cells are sorted by vessel-hours logged between 00:00–06:00 UTC versus 06:00–22:00 UTC, the cells with the highest nighttime-to-daytime density ratio are not port approach channels. They are offshore anchorage rings, 11–18nm from port baselines — the outer edge of territorial waters, where VTS coverage drops and coastal authority diminishes.

The Chain

63% of total nighttime vessel-hours in the dataset concentrate in 8% of hex cells. That 8% splits into two groups: port approach cells (expected) and offshore anchorage ring cells (not). The offshore group shows a 3.2× nighttime-to-daytime vessel-hour ratio; port approach cells show 1.4×. The inversion is consistent across Gulf, Pacific, and Atlantic geographies.

The 340 hex cells that appear in both the top decile for nighttime vessel density and the top decile for AIS gap frequency are the critical overlap. These are cells where vessels concentrate after dark and regularly drop their AIS signal. That combination is not random. AIS gap frequency in this overlap group averages 2.7 gaps per vessel-week; in high-density daytime cells, it averages 0.4. The ratio is 6.75×.

Vessel class breakdown in the high-nighttime-density offshore cells: tankers and bulk carriers account for 71% of vessel-hours. General cargo and container vessels split the remainder. Fishing vessels are largely absent — the behavioral pattern is not recreational or fishing-related.

The Implication

The standard density map routes analyst attention toward port mouths, which are already covered by VTS, pilot requirements, and port authority surveillance. The offshore anchorage ring pattern is the inverse: high vessel concentration, low official coverage, elevated AIS gap rate, tanker and bulk carrier dominance. A vessel executing a nighttime ship-to-ship transfer or identity event in a 12nm offshore cell will not appear in any port authority record. It will appear as a density spike in a hex cell that looks like ordinary anchorage queuing.

The key distinction is dwelling versus anomaly. Legitimate anchorage queuing produces continuous AIS — vessels waiting for berth allocation stay broadcasting. The 340-cell overlap group is where dwelling and AIS gap events appear in the same hex cell across distinct vessel populations. That co-occurrence is the triage filter: not every offshore nighttime concentration is suspicious, but every offshore cell with both high nighttime density and elevated gap frequency warrants a second pass.

What to Watch

Cells where nighttime density rank exceeds daytime density rank by more than two deciles, combined with AIS gap frequency above 1.5 per vessel-week. The Gulf offshore anchorage rings — roughly 12–15nm south of Houston Ship Channel approaches — currently represent the highest-density cluster in that filtered set. That geography coincides with active STS alert zones in the motif_alerts table. The Pacific offshore rings near the San Pedro approach are the second concentration.

Limitations

Vessel-hours are derived from AIS interpolation between pings; gaps reduce accuracy in high-density cells where ping deduplication is aggressive. The 00:00–06:00 UTC window aligns with nighttime in US Eastern and Central zones but represents afternoon in Asian and Pacific lanes — the temporal split is not globally equivalent. Offshore anchorage ring classification uses a 12nm baseline approximation and does not account for port-specific anchorage designations that may extend further offshore.

Vessel density figures from vessel_density_h3_* tables, resolution 8. AIS gap data from dark_events, duration-filtered. Analysis period: rolling 45 days. Axiom Overwatch delivers maritime behavioral intelligence for compliance, logistics, and security teams.