Most public transit fleets don’t include ferries, but some do. They’re a much cheaper way to move people, cars, and cargo across a body of water than building a bridge or tunnel.
Because it’s often featured in movies, the most famous is the New York City DOT’s 5-mile route between the boroughs of Manhattan and Staten Island. Running the service 24 hours a day seven days a week requires eight ferries. Halfway across the country, though, Texas DOT (TxDOT) has provided a similar service for almost as long. And, unlike the Staten Island Ferry, its ferries carry cars as well as people.
Since 1929, the TxDOT’s Houston District has provided free ferry service to and from unincorporated Port Bolivar and Galveston Island, a resort city that’s home to almost 50,000 people. At just under 20 minutes, the 2.7-mile trip saves motorists more than an hour of travel through Baytown and other communities north of Galveston Bay along the TX-146 highway. The ferry is the only way to cross the bay.
The entire area’s in a vulnerable location on the Gulf of Mexico, making the ferries critical to emergency response. Each carries about 500 passengers, 70 vehicles, and six crew members. When a hurricane threatens, the ferries are the fastest way island residents can flee to the mainland.
When built in 1996, the ferry work dock had four berths and TxDOT owned five ferries. Because at least one operates 24 hours a day, a fifth berth wasn’t necessary. But in 2011, the division bought a sixth ferry.
Meanwhile, years of wear and tear took its toll on the bulkhead and some of its mechanical and electrical equipment. Agency managers decided it was time to expand and modernize.
Not between a rock and a hard place, but close
Fitting more stuff into the same amount of space is always a challenge, and this job was no different. In addition to adding a fifth berth without encroaching into the ferry’s route or U.S. Army Corps of Engineers property to the north, TxDOT managers wanted their improved Ferry Maintenance Work Dock to withstand extreme weather.
In June 2014, they hired the Houston office of Lockwood, Andrews & Newnam Inc. (LAN), a planning, engineering, and program management firm, to assess waterside and landside infrastructure. This included an extensive evaluation of the bulkhead wall, concrete-paved uplands areas, and mechanical and electrical equipment. After reviewing the findings and recommendations, LAN was contracted to develop detailed engineering designs that would enable the work dock to function for the next 25 years.
The existing work dock consists of four floating, rectangular mooring structures that are laterally restrained with seven piles. Each consumes about 60 feet of space. To accommodate a fifth berth, the design team had to find at least 86.5 more feet of space.
New space-saving berthing and mooring structures
After an exhaustive analysis that included numerous manipulations of the existing layout, the team decided to replace the mooring structures with four monopiles equipped with an energy-absorbing “donut” fender.
The donut fender system features a nominal 9-foot-diameter fender placed over large, 5-foot-diameter steel piles. The fender comprises a polyethylene foam core that’s attached around a steel core. The foam is protected by a tough, wear-resistant, reinforced polyurethane skin.
The steel core is lined with low-friction bearings that allow the fender to rotate freely around the monopile and to rise and fall with the tide. The energy of an approaching ferry is absorbed through a combination of fender compression and pile deflection.
Also, electrically operated steel ramps, also called gangplanks, were designed with gradients to accommodate loaded and unloaded ferry heights above water level. The ramps prevent damage to the underside of low-profile vehicles whether they enter and exit the ferry at low or high tide.
In addition to netting about 90 feet, this solution enables technicians to service a ferry from any berth and gives TxDOT the flexibility to operate the maintenance facility’s waterside component.
It also saves money. Four massive mooring structures, each with seven piles, will be replaced with a system comprising four piles. Improvements will require less material and, thus, cost less to build.
Storm-resistant design
In September 2008, Hurricane Ike destroyed many structures on the Bolivar Peninsula and damaged the work dock. To ensure resiliency, the reconfigured facility must accommodate environmental loads and conditions, including those generated by hurricane conditions. TxDOT also wants to secure ferries at the facility during a hurricane instead of sheltering them at deeper seas.
The design team ran several models of the work dock and associated structures taking into account factors including wind and wave intensities and directions, currents, and ferry weight (ballasted or loaded). Given the unpredictability of these variables, reasonable estimates of bad weather were used to direct the design of each element or component.
For wind forces on the monopiles, an industry-standard 30-second design wind velocity was used to perform the mooring analyses, based upon the recommendations of the World Association for Waterborne Transport Infrastructure. This time interval allows full tension in mooring lines or fender compression to develop. Because the site is considered largely sheltered from the effects of large wave forces generated by passing ship and wind-induced wave action, the design team’s analyses assumed water heights of at least 15 feet.
Although Hurricane Ike wasn’t a heavy wind event, it generated a storm surge of more than 20 feet. When models and analyses were run, costs were factored in while considering the minimum threshold water heights for wave action. Ultimately, the design team decided on a minimum water height of 15 feet.
Building to accommodate budgeting
Along with functional and environmental considerations, two other mandates influenced project design.
The first was not interrupting service in any way during the upgrades. At least two berths must remain fully operational at all times. The second was providing enough flexibility for construction to happen within TxDOT’s annual budget allocation.
LAN observed maintenance activities for several weeks, particularly berthing and mooring sequences, talked to employees including two ferry captains, and studied data on the work dock’s physical space and technical requirements. Operational safety, especially during construction, was a key consideration throughout the project’s planning phase.
The team’s solution satisfies both mandates.
Construction is broken into four phases that can be done all at once or over multiple years in any order depending on operational need and/or funding availability. TxDOT can make all recommended improvements during one mobilization with one contract or spread construction over two or more years while keeping existing structures in use.
This approach creates the least disruption to daily ferry operations.
Battening down a new bulkhead
Bulkheads are waterfront retaining structures that stabilize the shorefront and retain fill soil. Because of the site’s physical constraints, the design of the new bulkheads required three different systems.
For instance, accommodating the additional slip required dredging the northern section of the site next to Army Corp property. However, soil stability and structural analyses showed that the increased water depth as a result of this dredging would compromise slope stability.
To guard against instability and avoid encroaching on Army Corp property, designers chose a cantilevered steel combi-wall bulkhead structure consisting of a series of steel pipe piles integrated with intermediate pairs of steel sheet piling. Because of its high stiffness properties, this type of bulkhead eliminates the need for any tiebacks that would otherwise be installed on lands of adjacent property owners. The tiebacks are steel rods that used to transfer the lateral loads from the bulkhead safely into the backlands.
Similarly, on the eastern side, a grouted tie-back anchor system was designed for one-third of its length. This became necessary to avoid removing recently installed infrastructure in the backlands. TxDOT had invested a lot of money on a newly commissioned bilge containment facility, so great care was taken to avoid disturbing the infrastructure around it. For the remaining two-thirds of the bulkhead, designers chose conventional sheet piling with anchored tiebacks, which was the more economical option.
The $22 million project went to bid in January 2017 and was awarded to Texas Gulf Construction Co. of Galveston. Construction should begin in April 2017 and take 18 months.
“This is a great opportunity to better serve our customers for years to come,” says Donald Marquise, maintenance supervisor/marine engineer for the Galveston-Port Bolivar Ferry. “The bulkhead design will improve the system that’s been in place for 20 years as well as preventative maintenance and repair capabilities.”
