Murdock And Hunt/Leslie Use Computer To Design 'Compliant' TLP Coupling

Murdock Engineering Co., Dallas, Texas, and finite element engineering consultant Hunt/Leslie/ Associates have used a computer to design the flexible 'controlled yielding' coupling for the world's first deepwater tension leg oil platform (TLP).

The coupling is the critical connection between the platform hull and anchoring tubular lines that keeps the multimillion-dollar 53,500-ton TLP horizontal in severe North Sea weather.

The primary design requirement was to reduce the bending moment in the mooring system and protect the lines from fatigue. The thin tubular tethering lines replace the traditional rigid platform support structure. The system is designed to operate in all weather and operating conditions. In addition to minimizing bending moment, the final design had to support non-symmetric loads and allow angular deflection as the buoyant platform shifts with the current to create tension on one side of the coupling and compression on other, as well as vertical tension due to buoyancy. Using axisymmetric elements in the ANSYS finite element program, Hunt/Leslie decided redesign of the segmented retainer ring—the metal ring that holds the upper backflange—would be sufficient to meet North Sea application requirements.

The Murdock flexjoint system consists of alternating laminates of metal and rubber which serve as a pressure seal and load-carrying bearing. External loading is supported by controlled compression of the seal elastomer.

The bearing design can carry an axial tension load of almost 7 million pounds in each bearing at angles to 16.6°, control lateral platform motion and prevent heave or vertical motion. In heaviest seas, the coupling will allow lateral platform movement as much as 79 feet from its neutral position. Murdock and Hunt/Leslie used the ANSYS finite element engineering analysis computer program to evaluate stresses and displacements in the final design of the major metal flexjoint bearing structure. Hunt/Leslie found that including friction in gap elements between mating surfaces increased stress levels of a factor of 10 percent over frictionless analysis. Using ANSY's axisymmetric elements, Hunt/Leslie was able to complete the critical final bearing design several months prior to completion of the three-story-high test structure.

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