“World’s Largest Offshore Caisson Breakwater”
“World’s Largest Offshore Caisson Breakwater”Two Putzmeister placing booms meet the complex construction demands of massive 12-caisson breakwater for first LNG terminal along Pacific Coast of Americas
STURTEVANT, WI (May 22, 2008) – Constructed 14 miles (23 kilometers) north of Ensenada, Mexico, the largest offshore caisson breakwater of its type in the world came to life through a combination of special concrete construction techniques that utilized innovative Putzmeister placing boom technology.
The enormous breakwater was constructed for the first liquefied natural gas (LNG) receipt terminal along the Pacific Coast of the Americas as part of the $875 million Energía Costa Azul project. The project was designed to help the Mexican state of Baja California meet its demand for natural gas, which is expected to almost double by 2010. Currently, this region imports its natural gas from the U.S., which faces increasing energy consumption yet dwindling supplies.
LNG is natural gas cooled below -260º F (-162º C) and condensed into a liquid that occupies 600 times less space than in its gaseous state. This enables shipment in cryogenic tankers from remote locations to the new Mexican terminal, where LNG ships can dock and unload into two above ground storage tanks. Each tank will store the liquid until it can be vaporized back into natural gas and moved via pipeline to customers.
Although the entire LNG terminal project was highly involved, a 2,150-foot (652 meter) long breakwater posed especially complex concrete placing and logistical challenges. Tackling the major task head on was a strategic combination of Putzmeister equipment, including separate placing booms and high pressure trailer-mounted concrete pumps. The high performance units placed concrete for 12 huge caissons. The caissons were of two sizes – each 82 feet (25 meters) tall, 125 feet (38 meters) wide and either 152 feet (46.25 meters) or 225 feet (68 meters) long.
The 12 gigantic caissons, each about the size of an eight-story office building, were towed out to sea and sunk to form a specially constructed breakwater. Since no natural harbor exists off the deep coastal waters of Costa Azul, this breakwater would protect LNG ships from being damaged by forceful ocean waves when unloading at a specially constructed 820-foot (250 meter) long berthing pier.
To design and construct the breakwater, the joint venture of China Harbour-Costain was awarded the $170 million contract. The breakwater construction tasks were divided within the joint venture, and the Costain Group PLC (Costain) of London was responsible for the specialized concrete work of the caissons. Costain, an international engineering and construction group founded on over 140 years of experience, refers to the challenging project in their company news magazine as “a world first, as no one has ever put such massive sized caissons in the Pacific Ocean.”
“It certainly took a great deal of ingenuity to develop the most efficient plan for concrete placement during caisson construction,” says Bob Liebermann, special products sales manager at Putzmeister America, Inc. “The strategic arrangement involved placing concrete for the caissons within a specially created casting basin, then floating them to a jetty to finish the concrete placing process, and finally towing them 14 miles (23 kilometers) out to sea for placement.”
Size matters in equipment selection
“This is the longest caisson breakwater ever constructed within the deeper waters along the West Coast of the Americas,” says Paul Bowers, deputy project manager at Costain. “Although various construction technologies utilized on this project were tested before, the overall combination of all these special techniques while on such a large scale basis obviously complicated the process.”
To construct the giant caissons, the first step was the creation of a dry dock or casting basin. A massive 330-foot by 553-foot (100 meters by 170 meters) cavity was excavated to a 40 foot (12 meters) depth, while a 30-degree slope angled to its 165-foot by 409-foot (50 meters by 125 meters) bottom area. This provided ample space to build two caissons at a time.
For concrete placement, the contractor chose two Putzmeister MXKD 38/42 separate placing booms for their long 125-foot (38-meter) horizontal reach. The placing booms were utilized with two large 400 meter/ton crane lattice towers, which were mounted on either foundation anchors or base plates, depending on their location in the casting basin or jetty.
With the help of the extremely high capacity cranes already on site, the two placing booms and two towers were cost-effectively moved between five mounting locations – four within the casting basin and one in the jetty. The tower cranes easily picked and moved the placing boom together with its power pack and pedestal. With the crane’s 10.5 metric ton capacity at 150 feet (46 meters), it could also lift the entire placing boom tower with all tower sections connected.
“Four steps in the lifting and reconnection process were eliminated with every move, as no boom counterweight was needed with the placing booms selected for this job or with any conventional placing boom model from Putzmeister America,” says Liebermann. “This significantly improves speed and efficiency.
“The contractor also benefited from the extra reach of our larger MXKD 38/42 placing boom model and avoided additional mounting locations in the casting basin because the boom and tower arrangement provided effective coverage for the pours,” adds Liebermann.
Three Putzmeister concrete trailer pumps were also chosen for the project, notably for their higher outputs in pumping the caisson bases at a faster pace. Therefore, on site were a BSA 1407-D and two BSA 1409-D models, capable of maximum outputs of 93 and 119 cubic yards an hour (71 and 91 cubic meters an hour) respectively, and high pressures up to 1,537 psi (106 bar). Powered by turbo-charged 157 horsepower (115 kilowatt) Deutz diesel engines, the pumps delivered a highly abrasive concrete via five-inch (125 millimeters) pipeline stretching over 150 feet (46 meters) distances.
As almost every pour required both concrete placing booms and two trailer pumps to be simultaneously utilized, the equipment had to perform with supreme reliability. “We selected Putzmeister products based on their market reputation, and the equipment performed to our expectations on this demanding project,” says Martin Orrells, general manager at Costain.
“Putzmeister was quick to respond to our queries with the locality of technical personnel in California and the availability of equipment when we needed it,” says Bowers. “Plus, the manufacturer’s distributor in Mexico had a service rep on site during the initial pours, which offered an added comfort factor.”
To ensure peak machine performance, Construmac, S.A., an authorized Putzmeister distributor for over 15 years, provided maintenance and service support of the concrete placing equipment. Established in 1976, Construmac is headquartered in Naucalpan with nine branch locations throughout Mexico.
Step by step
Each caisson required six concrete pours, four in the casting basin and two in the jetty. Initially, the entire base of a caisson was formed and placed at a three-foot (one-meter) concrete thickness. Then, one half of the caisson was pumped via a self-climbing slip form approach to a 45 feet (14 meters) height. Overall, the process required five to seven days of continuous pumping, with total outputs averaging 30 to 70 cubic yards per hour (23 to 54 cubic meters per hour).
The slow, methodical process was limited by the raising of the forms to accommodate concrete set times, not by the pumps’ output or concrete availability. The remaining other half of the caisson was slip formed to the same 45 feet (14 meters) height, and all steps repeated in a similar manner to create the second caisson alongside the first one.
Resembling a honeycomb, the hollow concrete caisson featured deep cells, each about 20 feet square (6 meters) with a 2.5 feet (762 millimeters) wall thickness. Each short caisson was five cells wide and up six cells long; and the longer caissons were five cells wide and nine cells long.
Prior to their journey to the jetty, the caissons were capped with 10-inch (254 millimeters) thick pre-cast slabs set in place by a crane. This was followed by a monolithic pour by the Putzmeister equipment to add another 10-inch (254 millimeters) top layer of concrete as a protective seal.
Maintaining an aggressive schedule, construction of the casting basin gate was finalized while the first two caissons were also being built. The gate utilized the same forms as the caisson but at a six cell width, which meant a narrow clearance for moving the large caisson out of the casting basin.
Floating from casting basin to jetty
Floating the caissons out of the casting basin was a complex endeavor. First, all concrete placing equipment had to be relocated to a special lay down area. This removed the equipment from the path of the floating caissons and also avoided equipment contact with the corrosive salt water when the casting basin gate was opened.
An eight to 10-day cycle started, which involved a day to flood the casting basin, a day to remove the casting basin gate and two days to move the caissons out during high tide. A tug boat in combination with cast-in-place winches and bollards controlled the caisson’s movements to prevent damage to the sides of the gate seal area. Then, a day of repositioning the casting basin gate and its seals were required, followed by two days of pumping water out of the casting basin. The remaining cycle time was spent cleaning up and preparing for construction of the next two caissons.
Two at a time, the caissons were floated to a jetty about 100 yards (91 meters) away and flooded in preparation for the next step. A placing boom and tower were repositioned to place concrete for the turret, which is the upper 36-foot (11 meters) section of each caisson. This portion will remain above water when finally positioned at sea.
The final 14 miles
When the turret was placed with concrete and appropriately capped, each short caisson weighed approximately 35 million pounds (16,000 metric tons) and each long caisson approximately 52 million pounds (24,000 metric tons). They were towed by tugs 14 miles (23 kilometers) up the coast, using GPS for precise positioning.
Once in place, butterfly valves, carefully positioned in the caissons, were opened to flood and sink the caisson on a specially prepared granular bed. The cells were then filled with sand to further ballast them and displace the water. Once ballasted, each short caisson weighed a hefty 127 million pounds (58,000 metric tons) and each long caisson weighed over 189 millions pounds (86,000 metric tons). The 12 caissons were further interlocked together to form the one-third mile long breakwater, which weighed over an astounding 1.76 billion pounds (800,000 metric tons) in total.
Started in January 2005, construction of the caissons consumed over 105,000 cubic yards (80,000 cubic meters) of a high strength concrete. Cemex, headquartered in Monterrey, Mexico, produced the 50 N/mm² concrete mix from two on site batch plants during construction of the breakwater. Each batch plant was capable of 75 cubic yards an hour (57 cubic meters an hour) outputs.
Cemex also supplied the concrete for the LNG tanks, which although large, only required about 8,000 cubic yards (6,000 cubic meters) for each of the two tanks. Core locks, positioned in front of the caissons to help break up the waves, added an additional 19,500 cubic yards (15,000 cubic meters) to the project’s total concrete usage.
Delivering the gas
The 400-acre terminal, including pipeline, comprises over a $1 billion capital investment in Baja California, Mexico, with commercial operations commencing in early 2008. The new LNG terminal is capable of annually receiving 7.6 million tons of LNG from loading terminals around the world and processing one billion cubic feet of natural gas per day. Designed to provide additional natural gas to northern Mexico, it may also offer a supply alternative to California and southwestern U.S. markets.