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With a clearance of about 226 feet between Mean Lower Low Water (MLLW) and the span underside of San Francisco’s Bay Bridge, there’s usually plenty of room for the world’s biggest ships to pass through on their way to the Port of Oakland. But when one of those ships is loaded with three of the world’s tallest container cranes, maybe there’s not enough room . . . or maybe there is. The job of deciding fell to Dave Murtha, the Port’s chief surveyor.
A Tall Enough Bridge?
The cranes in question are "SuperPostPanamax" and they’re monsters. PostPanamax ships are too big for the Panama Canal, and as more are built, ports around the world are installing cranes that can accommodate them. In this case, the cranes being delivered are wide enough to reach across vessels carrying up to 22 Sea-Land style cargo containers side by side. Of more concern to Murtha was their height: 253 feet. When loaded on a ship big enough to carry them, this would exceed the Bay Bridge’s clearance.
The crane’s designers knew this, and planned to cut and fold the cranes shortly before passage was attempted. But this still left plenty of uncertainty. To be sure he was making the right call, Murtha would have to precisely equate tidal elevation values and NAVD 88, determine absolutely the Bay Bridge clearance, and verify the total height of ship and cranes. And just to complicate matters, he would have to do it all in realtime; the San Francisco Bar Pilots, who oversee large vessel operations in the Bay, wanted verification of sufficient clearance as the cranes approached the Bay Bridge. The Bay Bridge, incidentally, is known to be several feet shorter than the Golden Gate Bridge, so Murtha’s work would automatically confirm that the cranes would pass under the Golden Gate.
"The catalyst for this realtime GPS crane height survey was an e-mail I received from Marti Ikehara, NGS’s state geodetic advisor to California," explains Murtha. "The e-mail included a link to a video message about air gap sensors, a relatively new type of sensor typically installed on bridges. They use microwaves to measure the distance from the lowest point on a structure to the surface of the water."
Because they can dynamically measure the effects of tide level, steel temperature, and vehicle load, an air gap sensor would have been a good way for Murtha to confidently determine bridge clearance. "But unfortunately," he says, "it was quickly determined that an air gap sensor could not be added to the Bay Bridge in time for the crane’s scheduled arrival in March 2010."
But Murtha had another idea, one that made use of his extensive experience with leading edge survey techniques. "Since RTK GPS methods are now being used to measure elevation profiles of airport runways," he says, "it didn’t seem like a big stretch to adapt RTK methods to verify load clearance. I told people in my organization that I could measure the height of the cranes as they approached the bridge. Eventually my claim got passed on to the San Francisco Bar Pilots, and they were very interested in having me provide that information."
Laying the Groundwork
Airport runway profiles can be post-processed and remeasured if necessary, but given the inertia of giant cargo vessels, there would be no second chances to remeasure as the cranes approached the bridge. Providing realtime information for this high visibility project required painstaking preparation for several reasons. For example, Murtha knew he needed a backup plan. "Redundancy was a very important part of the survey plan," he says. "Two different RTK rovers would be used at the top of the load of cranes, one using cellular modem communication equipment, and the other using a spread-spectrum radio modem."
The cellular modem could access a Leica GRX1200 Pro permanently installed at the Port’s headquarters. This receiver is also part of RTKMAX, a subscription real-time network operated by Haselbach Surveying Instruments. But for reliable radio link RTK, he needed a base station with line-of-sight from both the Golden Gate and Bay Bridges. "The levee on the west side of Treasure Island was the perfect location," says Murtha.
Work was already underway to verify the Port’s reference station and relate it to tide station values. "I included the Port’s reference station in a GPS control survey which I am submitting to the NGS. The control survey was mostly conducted in June 2009 using four Leica ATX1230GG units. Additional vectors focusing on height differences were measured in August 2009 using three ATX1230GG units on fixed-height poles. This control survey consists of more than 100 vectors between one CORS site, three Bay Area CGPS sites, and eight passive NGS stations [CGPS is a continuous GPS station with data available from the California Spatial Reference Center-CSRC]. It also includes several miles of Second Order, Class Two differential leveling conducted in June 2009 with a Leica DNA03 digital level and a calibrated pair of Wild GPCL3 Invar rods. Four different tidal bench marks were part of this control survey including two from the Alameda tide station which is the primary tide station for all secondary tide stations in the East Bay."
To supplement this work for the crane height survey, Murtha planned a static control survey with two objectives: establish the needed base station location and elevation on Treasure Island, and relate local tide datums to NAVD 88. He included six stations in the final network:
• a tidal benchmark near the Golden Gate Bridge (and the reference for the longest continually operating tide station in the United States– uninterrupted tide readings go back 150 years)
• the Port’s headquarters reference station in Oakland
• a tidal benchmark on Yerba Buena Island
• the new station on Treasure Island
• two points used in the Central Coast Height Modernization Project in 2007 (CENCHM2007).
Murtha’s static survey procedure was exceptionally thorough. "Using three Leica units along with the two active sites, this network was measured in four sessions of 45 minutes measured over two days (two sessions per day). All occupations used fixed-height poles, and all vectors were measured twice with a four-hour difference in satellite geometry. The CENCHM2007 heights and the height of the Port reference station determined by the 2009 control survey agreed with each other very well. Before each static occupation, three minutes of RTK data were collected to be used later for a calibration to the adjusted coordinates of the control network. The RTK measurements conformed to NGS guidelines for class RT1, except that only one base station was used."
With control firmly established, tide related to available benchmarks and NAVD 88, and the Treasure Island station set, Murtha could move on to additional tasks in this challenging project: verifying Bay Bridge clearance, and verifying crane height above the deck of the transport vessel.
Tricky Measurements on the High Seas
In 2000, when a shipment of PostPanamax container cranes was delivered to the Port of Oakland at the Navy’s former Fleet Industrial Supply Center (FISCO) in Oakland, Port personnel measured the Bay Bridge’s mid-span clearance by trigonometric leveling methods. This time Murtha used RTK to establish a spot elevation on the upper deck of the bridge, then used a Leica TCRP 1201 total station to transfer elevation from that point to a magnetically mounted prism target t
hat was visible from the upper deck and from the base of the nearest suspension tower pier.
Then, in what must have been a fun day in the field, Murtha took a boat to the pier and set up his total station. Two Caltrans employees, certified to climb on the bridge, used safety harnesses and belaying equipment to set another prism directly on the bridge’s bottom chord. Murtha was able to confirm a clearance of 226 feet above MLLW.
The three cranes, standing their full 253 feet tall, arrived at Drake’s Bay (north of San Francisco) on March 12, 2010, loaded on the Zhen Hua 15, a tanker with a specially modified low deck. While anchored at Drake’s Bay, the crew of the Zhen Hua 15 spent three days folding over the crane apexes. On March 14th, Murtha traveled by boat to the crane vessel to verify the final crane height, and to set GPS antenna mounts at the top of the middle crane.
It turned out to be another exciting day in the field. Says Murtha, "After boarding the vessel, the crew of the Zhen Hua hoisted our equipment up to the boom level of the crane, which is about 180 feet above the deck of the vessel. Ordinarily people go up and down these cranes in an elevator, but since the cranes had no electrical power, we had to climb the metal stairs to reach the boom level. There are also metal stairs that go all the way up to the top of the apex, but since the apex had been folded over more than 70 degrees, the stairs were much more difficult to climb. At this point we switched out of our floatation vests and into safety harnesses and clip lanyards, then proceeded to climb to the top of the crane. We went up the stairs, but since they were leaned over so far, we put our feet on the lower hand rail and our hands on the higher hand rail. Think of a jungle gym 200 feet in the air slowly rocking back and forth with the waves.
"Once we got to the top we set ourselves to the task of setting up the GPS antenna mounts. I had modified two old tripods by removing the metal points and replacing them with threeinch diameter disk magnets attached to the tripod legs by metal hinges. Since tripods are excellent for setting up over non-level surfaces, I figured these modified tripods would be the best way to setup the antenna mounts."
With antenna mounts in place, Murtha and his crew returned to the deck to make total station measurements. The rolling of the deck ruled out the use of the vertical compensator (says Murtha, "I could see the bull’s eye bubble moving back and forth"), so turned it off and took a series of measurements intended to define the deck plane and crane height above deck. Back in the office, he "performed a classic seven-parameter, three-dimensional coordinate transformation," which confirmed what the crew’s engineers had told him–the cranes had been lowered even more than planned, and should clear the bridge with about ten feet to spare.
The Big Day
With apexes lowered and vital measurements confirmed, the transit was set for March 16th.
Since big vessels like the Zhen Hua 15 don’t move with the ocean’s swells, and smaller boats do, boarding is always an adventure, especially since the crane vessel had ballasted down to only five feet of free-board in order to be as low as possible for the transit under the bridges. In this case, large smooth swells made things very difficult and on the first boarding attempt the fenders of the pilot vessel overlapped the Zhen Hua’s deck and ripped out some or the hand rail. Murtha and crew switched to a smaller rescue boat and finally managed to get onboard with just a soaking.
That accomplished, the Port of Oakland employees climbed once again to the boom level, donned safety harnesses, and climbed to the top of the center crane. Even with all the checking and rechecking, it was still a tense moment. "We got there just a few moments before the Zhen Hua reached the Golden Gate Bridge," says Murtha, "and we were happy to see it pass under with what looked like fifteen feet of clearance."
Murtha had previously confirmed that the cell and radio links were effective on the bay, so when he set up both receivers he didn’t suffer any surprises. He put his equipment into stakeout mode and started gathering data. "We hadn’t yet reached Alcatraz, so we were still more than three miles away from the Bay Bridge, and I was able to tell the pilot that we had nine feet of clearance. I called him again when we were between Alcatraz and Treasure Island, and he called me once more when we were much closer to the Bay Bridge to confirm the clearance values.
Shortly after that I realized I could see the bottom of the bridge, so I called him on the radio one more time and said, `I can see the bottom of the bridge. We’re definitely going to clear it!’"
Brad Longstreet is a freelance writer who specializes in construction and surveying. Dave Murtha is Chief Surveyor for the Port of Oakland, California. He graduated from Fresno State University in 1994 with a BS in Surveying Engineering (now called Geomatics Engineering).
A 1.617Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE