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Plot of the month: Wave interaction with a tension leg platform


Contributed by Dr. Sundar Prasad, Senior Analyst, Coastal & Ocean Engineering Group
Sandwell Engineering, Inc., Vancouver, B.C.

The crest of a wave reaches the forward legs of a platform in the figure below. Data visualization is a key component in post processing results from Sandwell’s numerical models. Tecplot quickly allows them to represent large quantities of data in a form that can be understood easily. This layout shows how the platform disturbs the water surface in its vicinity, which helps determine how high the deck of the platform should be located so it is not impacted by wave crests.

About the Engineer

Dr. Sundar Prasad works for Sandwell Engineering, Inc. in Vancouver, Canada. He is a Senior Analyst in the Coastal and Ocean Engineering Group. The primary focus of his work at Sandwell is to provide their structural engineers with data on the effects of ocean wave interaction with offshore and coastal structures such as oil platforms and breakwaters. Typically, this involves the use of both numerical and experimental methods, depending on the complexity of the problem.

Numerical Methods

Numerical methods have the advantage of lower cost, increased flexibility, and faster turnaround times from problem definition to results. Since their needs are very specific, and are not fully addressed by commercially available software, Sandwell has written their own in-house Wave3D code to model wave interaction with large offshore platforms. The accuracy of the code has been verified against results from experimental tests in wave basins.

Wave Interaction with a Tension Leg Platform

Our plot of the month represents a number of ways in which results from a simulation of wave interaction with a tension leg platform can be visualized. A wave of 12.8 m height with a period of 9.9 seconds was generated within Wave3D. The large plot shows an instant in the simulation when the crest of the wave reaches the forward legs of the platform.

Shown in the bottom left of the layout are color contours of the maximum water surface that was reached during the course of the simulation (usually consisting of about eight to ten waves passing by the platform). It is clear that for the chosen wave direction, the region just forward of the gap between the front legs sees amplification of the wave crests.

Dr. Prasad’s group also needs to determine the forces experienced by the platform in waves. The bottom right plot displays the maximum hydrodynamic pressure reached during the simulation on the body of the platform. This information allows structural engineers to determine how the hull plates (and internal structural bracing components) on the platform will perform under different operating conditions.

How They Do It

Sandwell’s Wave3D has specific subroutines written to write files in a format that can be directly read into Tecplot. All the data in the plot is based on four-node quadrilateral finite-elements. The data is supplied to Tecplot as a list of node numbers and X, Y, Z coordinates and connectivity data for the elements. In the case of the pressure contours, a pressure is also associated with the location of a node on the platform model. The data is imported as an ASCII file into Tecplot which then generates a plot based on a predefined layout. Dr. Prasad has several layouts depending on the nature of the data in the plot.