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Manufacturing

On the Job: Painstaking Design, Simulation Bring Leyla to Life

14 Feb, 2005 By: Cadalyst Staff Cadalyst

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Shipbuilders use Algor FEA to create dredging barge for natural gas pipeline

When undersea piping was needed to complete a South American pipeline project that traverses the Amazon jungle and the Andes Mountains to the Pacific coast, ALGOR FEA (finite element analysis) software was chosen to help get the job done quickly and economically.

The pipeline originated in the Camisea fields of eastern Peru, which are among the world's largest natural gas reserves, holding approximately 11 trillion cubic feet of natural gas and 600 million barrels of associated natural gas liquids, according to the Camisea Project. To access these reserves, a $500 million project was led by Pluspetrol Peru that included constructing two pipelines -- one for natural gas (714 km) and the other for natural gas liquids (540 km). As construction of the pipelines neared completion, work focused on endpoints such as the fractionation plant in Pisco, where natural gas liquids would be processed into commercial products including propane, butane and condensates.

Servicios Industriales de la Marina, also known as SIMA Peru, a state-owned corporation and shipyard serving the Peruvian Navy, was commissioned by Pluspetrol to build a steel barge that could carry a 180-ton excavator and related equipment for digging an undersea piping canal in Pisco Bay. SIMA used ALGOR FEA software to verify the barge and meet the challenging design and fabrication schedule.

"With ALGOR, we designed a safe barge requiring less steel than had been originally quoted and approved by the customer," said Eddy Cordova, SIMA structural projects chief designer. "We were able to deliver a fabrication drawing of the barge's spud legs to our shop less than one month after obtaining ALGOR software. All fabrication drawings were delivered in less than two months, and the manufactured barge was delivered to the customer within four months." The canal was dug, the piping installed and the Pisco plant is now transporting natural gas liquids products through the subsea piping to a loading platform for export on ships.

Applying FEA to Ship Design
SIMA has a long history tracing back to 1845 with the founding of the Bellavista Naval Factory, the first in South America. Today, SIMA has three shipyards and a labor force of 1,200 for shipbuilding and repairs, steelwork, electronic systems and safety and defense. "We had been modeling our designs with [Autodesk] Mechanical Desktop and AutoCAD, but without FEA," said Cordova. "We decided to get FEA software because it would allow us to quote and design new projects faster, optimize our existing designs and design steel structures of less weight compared to previous manual designs."

After comparing available FEA packages, SIMA chose ALGOR. "We chose ALGOR over NASTRAN because of confidence in ALGOR's meshing capabilities and over SAP2000 because ALGOR's expansive analysis capabilities were better suited for our range of applications," said Cordova.

Due to the tight schedule of the barge project, the FEA software had to be easy to learn. Cordova quickly learned to use ALGOR through keystroke-specific tutorials, two Spanish-language distance-learning courses, online documentation and customer service. "I had used ALGOR several years ago," said Cordova. "With ALGOR's latest FEMPRO user interface, the current version is even easier to use."

Engineering Considerations
The barge, named Leyla, was designed and manufactured at the SIMA shipyard in Callao, Peru. SIMA designed the barge as a steel platform with three spud legs of hydraulic operation (with independent drive and lift), similar to an off-shore oil rig. "A spud is a sharp-pointed vertical post that can be forced by power through a socket or bearing to anchor a barge or platform into the ocean floor," explained Cordova. "The three spuds of the Leyla barge, which were 20m in length, provided a self-elevating and stable work platform."

The barge carried a 180-ton Hitachi EX 1800-II excavator. Cordova said, "The excavator lifted soil from the sea bottom at a maximum depth of 16m and deposited it onto other barges called 'ganguiles' for transport. All barges used for the project were designed and fabricated by SIMA."

The primary engineering challenge in designing the Leyla barge was to ensure it would withstand the high-load operating conditions expected while in 24/7 service.

"The service loads of the excavator as well as ocean wave surge, wind and weights, including ballast and deck equipment, were considered," said Cordova. In addition to the excavator's 180-ton weight, its service loads were calculated at 30 to 66 tons, depending on the sea height. The ballast weight was 210 tons and the deck equipment was 60 tons. The effects of a maximum wind speed of 28 knots and maximum wave height of 0.7m were also considered.

"All of the high-load zones were important," said Cordova, "but the spud legs were of particular concern because they are very large and tall structures and had to work 24 hours per day under high loads." The load on the three spud legs was 340 tons.

Cordova designed the spud legs, spud houses and high-load zones of the barge. The general structure of the barge was designed by Domingo Mussio, SIMA engineering department chief. Mechanical and structural CAD draftsmen Gerson Silva and Eduardo Flores used AutoCAD software to create shop drawings of the barge and spud legs. The project was overseen by Victor Pomar Calderon, SIMA Callao chief.

"The general structure of the barge was designed in compliance with the American Bureau of Shipping: Rules for Building and Classing Steel Barges," said Cordova. "High-load zones, including below the excavator, spud legs, spud houses, structural spud bearings and interior bulkheads of the spud zone, were designed using allowable stresses from the AISC Manual of Steel Construction."

Simulating the Leyla Barge
SIMA created finite element models of the general barge structure as well as several components in high-load zones including the spud helmet, bulkheads and spud leg structure. Linear static stress and critical buckling load analyses were performed to determine how the design would withstand the expected loading conditions. "The goals of the ALGOR analyses were to cut the design cost by making the barge in less time, ensuring safety and reducing the steel quantity required by optimizing the design," said Cordova.

The model of the general barge structure consisted of four parts: the main platform (including the shell plate, deck plate, bottom and bulkheads), modeled with 8mm-thick plate elements; supports, modeled with truss elements; spud legs, modeled with beam elements; and spud helmets, modeled with truss elements and used to apply loads to the spud legs. "We used beam elements to model the spud legs in order to get the maximum normal forces and bending moments," said Cordova.

One complicating factor in modeling the spud legs concerned the constraints. "Not all of the spuds' bearing points work simultaneously," explained Cordova. "If all three spuds are driven into the sea bottom, then the boundary conditions in the three bearing points are pinned -- Tx, Ty and Tz constrained. However, there is the possibility of one of them slipping horizontally over a rock or stone on the ocean bottom. In this case, only two spuds would be pinned and the third would have only vertical translation constrained."

Static stress analyses with linear material models were performed for several different load combinations and constraints, which varied the number of spuds pinned, the excavator position, the wave position, the sea height and the wind direction. "In this simulation, the most important results were the beam and truss stresses and deflections," said Cordova. "We determined the critical load combination and found that we could reduce the spud plate thickness from 19mm to 12.5mm for 9.0m of the spud length while maintaining structural integrity. This significantly reduced the amount of steel required to manufacture the spud legs."

The detailed model of the spud leg structure was then modeled with 12.5mm-thick plate elements and included transversal stiffeners (or diaphragms), which were omitted from the general structure model. A critical buckling load analysis was performed, which verified that the spud leg would not buckle.

"The ALGOR models of the spud legs, spud boxes, spud houses and the high-load zones below the excavator were used for fabrication drawings," said Cordova. "This challenge could not have been met on time without ALGOR finite element software. Using ALGOR FEA allowed us to design the high-load zones of the barge very fast and analyze different load combinations, which gave us a better understanding of the structure's real behavior during operation. This helped us to improve the design by getting structures that weighed less, such as the spud legs. Without ALGOR FEA software, designing and fabricating the barge would have taken more time and material."

Lessons Learned and Future Applications
Cordova revealed, "I learned many things from using FEA on the Leyla barge project. For example, I learned how the behavior of a structure can vary regarding loads and constraints. It's necessary to understand the physical-mechanical phenomenon that controls the model in order to apply the correct loads and constraints. This software is a powerful tool, but it does not replace engineering knowledge."

Cordova concluded, "We plan to use ALGOR FEA for future projects such as steel bridges, pressure vessels, fishing vessels, barges and mechanical components."