Engineers Use ALGOR to Design Liquids Berth at Major Australian Port
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Madsen Giersing, an Australian consulting engineering firm that specializes in marine engineering and construction solutions, was commissioned by Barclay Mowlem, one of Australia's leading building and engineering companies, to design a new Bulk Liquids Berth at Dampier, Australia, one of the country's largest ports. The berth or dock consists of four independent berthing and mooring dolphins (or groups of piles-see Figure 1) and a 490-meter-long access trestle or bridge that leads out to a 37.4 by 34.3-meter loading platform, from which up to 65,000 Dead Weight Tons (DWT)[1] tankers are to be loaded. The company had previously used ALGOR for a variety of projects, including for the modification of a 240-ton ship-lifter, the analysis of tie-rods for sheet pile walls and the analysis of an out-of-vertical jacking box on a drill rig. Given such past success, Madsen Giersing turned to ALGOR for the analysis of a trestle structure subject to marine conditions such as wind, waves and earthquakes, as well as the heavy-duty loads of the giant tankers.
Modeling and Initial Analysis of the Bulk Liquids Berth Trestle
Working with a team of engineers, including General Manager, Peter Madsen, and Senior Engineer, Jay Macintyre, the company began by determining an initial concept for the geometry of the trestle. Given the basic design concept, Lasse Madsen, Structural Engineer, was responsible for the detailed design of the access trestle, the largest part of the berth. It provides support for the product pipelines and allows vehicular access from the shore to the loading platform.
Lasse Madsen used the Superdraw 2- and 3-D sketching, modeling and meshing tools within ALGOR FEMPRO to draw the various parts of the structure. The trestle consisted of headstocks, girders, walls, pipes, piles and pile braces, all consisting of beam and plate elements with material properties of reinforced concrete for the headstocks, prestressed girders for the roadway and structural steel pipes for the piles. The piles, aligned in a series of "pile bents," or rows of capped piles, support the roadway plates on which the vehicles travel.
Madsen used the Australian Standards AS1170 to estimate the dead, live, wind and earthquake loads and the Austroads Standards to estimate the traffic-induced loads. In order to cut analysis time, he made four copies of the model and applied the load cases separately. Because the structure was designed to only handle the imposed loads within the linear material range, all of the loads were static.
Iterating to a Final Design
After the initial model and analysis, Madsen noticed that the Trestle 8 and 9 pile bents (T8 and T9), the last bents of the trestle, were highly stressed. To reduce these stresses, he added an extra pile to each pile bent and optimized the locations and rakes (slant) of the six piles. He then reconsidered the various load combinations. Subsequently, pile bent T5 had attracted increased loads. To compensate, Madsen increased the rake of the T5 piles, thus reducing the stiffness of the bent to absorb the stresses. As he added further detail to the model, he accounted for the fact that no piles could be driven at bents T1 and T2 due to the rock formations at these locations. Therefore, he replaced the raked piles with cross-braced vertical columns and mounted these directly onto the rock. He then defined the rock as a fixed boundary condition (see Figure 1). At bent T5, due to deeper rock formations, the piles were unable to be driven into the soil at the length required to obtain a fixed support. Therefore, he had to model the structure based on more superficially-driven piles. He did so by using spring elements to simulate the stiffness of the soil over the length of penetration of the piles. As such, he was able to determine and account for the new stresses in the structure due to the lack of pile penetration.
Based on continued design iterations and analyses, Madsen was able to determine the size and thickness of the steel piles and the amount of reinforcement needed in the concrete headstocks. He used the results capabilities of ALGOR to determine the area of the maximum deflection or displacement. This was important as there was a strict criterion on the allowable deflections due to the ability of the product pipes to handle the movement.
Results
Using ALGOR, Madsen was able to model the global structure of the access trestle and obtain accurate stress responses for the structural members. Madsen determined the thickness of the piles and headstocks and the allowable distance between piles within each pile bent. "The software analysis allowed us to include fewer piles in the structure, which are very expensive and time consuming to install. Thus, using ALGOR, we saved a considerable sum of money for our client," said Madsen.
Asked if Madsen Giersing will use ALGOR in the future, Madsen replied: "Absolutely. We will continue to use ALGOR for all of our marine engineering projects."
