Optical sensor monitors fog for San Francisco Bay mariners


A new fog sensor installed in San Francisco Bay and in other waterways nationwide is the latest tool to provide mariners with real-time information on whether it is safe to leave the dock and transit fog-bound waters.

That new sensor, called the FS11 Vaisala sensor, is seen as crucial to containership traffic in San Francisco Bay, where 3,500 ships travel yearly.

That is no small matter there given the collision potential in San Francisco Bay which was underscored in January 2013 when the oil tanker Overseas Reymar struck a tower of the San Francisco-Oakland Bay Bridge after unloading crude at a Royal Dutch Shell refinery.

“Since the Overseas Reymar we have put into practice a best practice within our region that if the fog density is less than a quarter mile, you do not leave the dock,” said Lynn Korwatch, chair of the Harbor Safety Committee in Oakland. “The sensor provides the information of when visibility is less than a quarter-mile so ships know not to leave the dock.”

Korwatch said the Reymar collision — and the 2007 collision when Cosco Busan hit the Bay bridge, spilling 53,000 gallons of crude oil — would have not been prevented by the sensor given the ships were underway. However, she said that the sensor readings may have kept them at the dock.

The sensors will likely be installed at other locations in San Francisco Bay, said Korwatch, including one at the upper reaches of the bay near the Benicia-Martinez Bridge at the Carquinez Strait.

The sensor was recently integrated into National Ocean Service’s Physical Oceanographic Real-Time System (PORTS) in San Francisco Bay. It is expected to give pilots and other mariners an indication of visibility conditions at the entrance to the inner and outer harbor areas in advance of arrival or departure from Oakland, said Chris Peterson, chief wharfinger at the Port of Oakland.

“This only measures visibility at that exact location,” said Peterson. “We hope that if this system shows value that the Coast Guard will look to install more sensors around the Bay Area.”

Indeed more sensors are planned, said Darren Wright, PORTS program manager with the National Oceanic and Atmospheric Administration (NOAA). Wright said NOAA has been working in tandem with the Harbor Safety Committee and the San Francisco Bar Pilots Association as well as the Coast Guard in identifying 10 to 12 additional sites that would be good candidates for sensors.

And their use will likely become widespread as part of the PORTS system nationwide. Two sensors are already in use in Alabama’s Mobile Bay, and sensors are destined for Jacksonville, Fla., Narragansett Bay and Chesapeake Bay, said Wright.

“The ability to make decisions prior to experiencing adverse or favorable conditions has increased safe and efficient navigation in areas that have a PORTS system,” said Wright.

“Economic benefit studies in four PORTS locations have shown a 50 percent reduction in groundings, and millions of dollars in annual benefit through more efficient navigation.”


The introduction of the sensor is welcomed by the bar pilots who work in San Francisco Bay.

“It’s a good addition to all of the other ways of telling us about the visibility around the bay,” said Bruce Horton, a San Francisco Bay bar pilot.

“Until now we’ve used either other pilots on ships or those around the general area, or tugboats traveling in the area to get information,” Horton said. “We’d call on them and get a pretty good idea of what the visibility was.”

Horton said that the weather has not been foggy enough as of late to assess the full value of the sensor. However, the bar pilots have already used the sensor to assess when to leave the dock, he said.

“More than anything, this may be a better tool for the Coast Guard,” said Horton. “That way they have a better idea of what’s going on, because it is more instantaneous.”

Horton said captains are not supposed to move from safe anchorage or a pier in seven critical maneuvering areas in San Francisco Bay unless there is more than a half-mile visibility.

“If you’re moving already and fog sets in, it’s up to you whether to try to anchor somewhere,” Horton said. “We’ve transited in the middle of San Francisco Bay with visibility less than a quarter-mile, because we have to get to a safe anchorage. You cannot stop (just) anywhere in the bay.”

For containerships destined for the Port of Oakland or leaving it, the technology will prove useful, he said. Many micro-climates are encountered in San Francisco Bay.
“You can have clear visibility in Oakland, but no visibility when approaching the entrance to the Oakland Estuary,” said Horton.

That entrance is a critical area for bar pilots because of the cross currents pilots must contend with passing through it.

Horton welcomes the eventual use of the sensor in other areas of San Francisco Bay, especially at the Carquinez Strait, where the Benicia-Martinez Bridge and a Union Pacific bridge await passage.

“In Benicia, they’re looking at putting it near the Union Pacific railroad bridge — which is a very narrow transit,” said Horton. “That’s one of the critical maneuvering areas where you’re not supposed to move if you have less than a half-mile visibility, unless you get caught in fog and you have no other option other than to transit.”

The FS11 sensor is made by Vaisala, a Finnish environmental measurement company. The sensor looks like a high-tech version of bull horns mounted on a long base. Its technology works when fog and other atmospheric particles pass between a transmitter which shoots a beam toward an optical receiver. The more the beam is scattered, the more the presence of fog and lower visibility at that location, said Janne Räsänen, Vaisala product manager.

“The sensor is particularly advanced in that it can compensate for optics contamination,” he said.

To do so, the sensor detects the amount of contamination in its sensors and makes corrections. The sensor has wider applications than shipping, said Räsänen. At present the sensor is widely used at airports as a visibility measurement and runway visual range assessment tool. The sensor is installed at airports in the U.S., Germany and South Africa, said Räsänen.

By Professional Mariner Staff