Selantic Drop Test Accupak/VE Mechanical Event Simulation

NORWEGIAN ENGINEERS PERFORM DROP TEST SIMULATION TO DETERMINE LIMITS OF PROTECTION NET FOR NEW ELF OIL RIG

Engineers at Selantic Industrier A.S. performed a drop test simulation with ALGOR's Accupak/VE Mechanical Event Simulation software to determine the limits of a protection net for a new ELF offshore oil platform currently under development in the North Sea, off of the East Coast of the UK. The nets will be positioned at the top of two legs to prevent objects from falling into the leg trusses and damaging pipes, which transport oil from the ocean floor.
Artist's rendering courtesy of Technip-Geoproduction of Paris, France.

December 4, 1998, Pittsburgh, Pennsylvania - Advances in computer-aided engineering technology are enabling engineers of all disciplines to create more complex, detailed finite element models that realistically simulate the behavior of interacting systems. Recently, engineers at Selantic Industrier A.S. in Agotnes, Norway used Accupak/VE Mechanical Event Simulation (MES) software for Virtual Prototyping from Pittsburgh-based ALGOR, Inc. to simulate the interaction between a small steel container and a dropped object protection net -- a virtual prototype test that previously could only be performed physically in a laboratory.

Selantic engineers were asked by Technip-Geoproduction of France and McDermott of the UK in partnership with ELF Exploration UK PLC to develop a new protection net design for a "jack-up" oil rig. This platform is currently under development in the North Sea off of the East Coast of the UK. A protection net will be situated within the trusses of two of the platform's three triangular-shaped legs. Each net will be attached to a circular steel ring located just above the leg.

Loading cranes located directly above the legs will transport containers carrying supplies between the platform and ships daily. The dropped object protection net will prevent falling objects from damaging oil-carrying risers, which are mounted inside the legs and transport oil from the ocean floor. The engineers needed to restrict the net's maximum deflections while ensuring it will withstand stresses created upon impact, both in the net as well as in the supporting structure.

This artist's rendering shows the main oil platform and two satellite drilling rigs. Pipelines on the ocean floor will transport oil to the main platform where it will be processed and then carried ashore via separate export lines.
Artist's Rendering courtesy of ELF Exploration UK PLC of Aberdeen, Scotland, UK.

The MES Approach

Lars Bjoland, technical manager of Selantic Industrier and an ALGOR customer for over seven years, is no stranger to ALGOR's finite element analysis (FEA) software; however, the protection net project was his first attempt at using ALGOR's Accupak/VE MES capabilities.

"In the beginning of the project, my colleague performed some rough hand calculations to determine the best approach for the net design," Mr. Bjoland said. "We could not make any solid conclusions from his work because it was just too vague. Performing the calculation manually would be impossible."

The next logical step was to turn to FEA. Mr. Bjoland originally modeled the net using Superdraw III, ALGOR's precision finite element modeling tool, and replaced the falling object with nodal forces acting directly on the net. Mr. Bjoland was unsatisfied with this for two reasons: calculating the correct loading was complicated and time consuming, and he wanted to simulate the springboard effect that occurs when a falling object deflects off of the net. Modeling the container and applying known physical properties such as its dimensions and mass, the height from which it falls and gravity acceleration enabled Mr. Bjoland to realistically simulate the interaction of the container and the net within a short time frame.

To set up the MES, Mr. Bjoland added a container to the existing protection net model, which is made of truss elements with three degrees of freedom. A rope would exhibit resistance only when pulled outward like a cable when the object strikes the net. As the net rebounds, the ropes would push together without resistance; thus, Mr. Bjoland did not need to consider bending moments.

Mr. Bjoland placed the 5,000-kg steel container model, measuring 2 by 2 m, approximately 12.6 m above the net. The net had three sides, each 16-m long, and was terminated in each of the three corners with fully constrained boundary conditions. He specified gravity acceleration for the container. Mr. Bjoland designated contact elements between the surfaces of the container and the net to enable complete interaction, including the transfer of inertia from one object to the other. Then Mr. Bjoland specified the duration of the simulation because he wanted to determine the behavior of the system over time. After specifying material properties of steel for the container and of aramid, a synthetic fiber, for the ropes, Mr. Bjoland processed the nonlinear elastic material model with Accupak/VE.

Analysis Results and Modifications

According to Mr. Bjoland, the initial deflections exceeded Technip's failure criteria of 2.6 m, the distance to critical objects underneath the net. Mr. Bjoland performed several variations of the analysis, dropping the container at the center, at one corner and along the edge of the net, to confirm the results before conferring with his client.

The initial analysis results of the mechanical event simulation showed that the net would exceed the maximum allowable deflection. Selantic engineers modified the net design based on these findings by substituting a different material type and adding more termination points at the edges of the net.

"After the first set of analyses, we were able to determine that the current net design would fail under the extreme loading from the impact," Mr. Bjoland said. "Without Accupak/VE, we would have been required to make a prototype to get the same conclusion."

Technip revised the requirements and asked Mr. Bjoland to perform a modified set of analyses. The new net design replaced the aramid ropes with ropes made of a high-performance fiber HMPE (High Molecular Polyethylene), a lightweight fiber rope that is ten times stronger than steel and exhibits the best elasticity and breaking strength. In addition, the design required nine termination points at both the corners and along the edges compared to the previous design's three termination points at the corners. Three of the points will be adjustable in order to pre-tension the net.

The modified set of analyses revealed much more reasonable deflections throughout the net and satisfactory material dimensions, but showed higher stresses than expected at the termination points. According to Mr. Bjoland, the stresses at these points were within the acceptable range of stresses; therefore, the model did not require further modification. In addition, the simulation showed that the net would be stretched permanently under the maximum loading. However, this is not a concern because each net will be replaced after a single drop accident, which is very unlikely to happen. To visualize the net behavior, .avi files created from the ALGOR simulation were converted to VHS format and presented to the customer on videotape.

The modified simulated event based on analysis results revealed both deflections and stresses within the allowable limits set by the platform manufacturer. These ALGOR analysis results will decrease the time and cost of physical prototype testing by eliminating the need for full-scale laboratory tests.

Physical Prototype Testing

Mr. Bjoland anticipates conducting small-scale prototype tests to confirm the simulation results. This testing will replace full-scale testing that would have been necessary if he had not used Accupak/VE. This translates into a decrease in the amount of time and cost of materials needed in the course of physical prototype testing.

"If we had not used Accupak/VE, we would need to do full-scale testing to find the deflection and termination forces," Mr. Bjoland said. "We also would have been required to perform additional physical testing to determine the design modifications that we made after the initial mechanical event simulation."

Physical prototype testing will begin in early 1999 with the platform becoming operational in the year 2000. Mr. Bjoland expects to use Accupak/VE for future projects. "Engineers always need to be open to new ways of solving problems to get the best results," Mr. Bjoland said. "If we hadn't been open to a new method of modeling in this case, we would not have been able to show the behavior of the net as quickly or inexpensively."

Construction of the ELF jack-up rig is underway at the Barmac Construction yard near Inverness, Scotland. The rig will be built onshore, as one complete unit, and then towed into position at sea where its legs will be lowered. On contact with the seabed, its deck will be jacked-up above sea level.