A South American pipeline project that traverses the Amazon jungle and the Andes Mountains to the Pacific coast originates in the Camisea fields of eastern Peru, which are among the world's largest natural gas (NG) reserves, holding approximately 11 trillion cubic feet of NG and 600 million barrels of associated natural gas liquids (NGL). To access these reserves, a $500-million (USD) project was led by Pluspetrol Peru Corporation S.A. (Pluspetrol) that included construction of two pipelines — one for NG (714 km) and the other for NGL (540 km). As construction of the pipelines neared completion, work focused on endpoints such as the fractionation plant where NGL would be processed into commercial products including propane, butane, and condensates.

Figure 1. The Leyla Steel Barge with spud legs that carried a 180-ton excavator to dredge an under-sea canal. (photo courtesy of Pluspetrol Pero Corp).

Servicios Industriales de la Marina (SIMA Peru S.A.), 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 a sub-sea piping canal in Pisco Bay. The company used finite element analysis (FEA) software from ALGOR, Inc. to verify the barge and meet a challenging design and fabrication schedule.

Using the software, SIMA designed a safe barge requiring less steel than had been originally quoted and approved by the customer. SIMA was able to deliver a fabrication drawing of the barge's spud legs to its shop less than one month after obtaining the FEA 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 plant is now transporting NGL products through the sub-sea piping to a loading platform for export on ships.

The barge, named Leyla, was designed and manufactured at the SIMA shipyard in Callao, Peru, and was designed as a steel platform with three spud legs of hydraulic operation (with independent drive and lift), similar to an offshore 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. The three spuds of the Leyla barge, which were 20 meters in length, provided a self-elevating and stable work platform. The barge carried a 180-ton Hitachi EX 1800-II excavator, which lifted soil from the sea bottom at a maximum depth of 16 meters and deposited it onto other barges for transport.

The primary engineering challenge in designing the barge was to ensure it would withstand the high-load operating conditions expected while in continuous service. The service loads of the excavator, as well as ocean wave surge, wind, and weights — including ballast and deck equipment — were considered. 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 0.7 meter also were considered. While all of the high-load zones were important, the spud legs were of particular concern since they are very large and tall structures and had to work 24 hours a day under high loads. The load on the three spud legs was 340 tons.

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 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.

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 8-mm-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.

One complicating factor in modeling the spud legs concerned the constraints. Not all of the spuds' bearing points work simultaneously, so if all three spuds are driven into the sea bottom, the boundary conditions in the three bearing points are pinned — Tx, Ty and Tz constrained. There is a 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. The critical load combination was determined, and SIMA found it could reduce the spud plate thickness from 19 to 12.5 mm for 9 meters 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 12.5-mm-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.

Using FEA, the engineers learned how the behavior of a structure can vary regarding loads and constraints, and why it is necessary to understand the physical-mechanical phenomenon that controls the model in order to apply the correct loads and constraints. The company plans to use the software for future projects such as steel bridges, pressure vessels, fishing vessels, barges, and mechanical components.

This work was performed by SIMA Peru S.A., using FEA software from ALGOR, Inc. For more information, contact Julie Halapchuk of ALGOR at 412-967-2700, ext. 3029; e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it. ; or visit www.algor.com