Ngày 25 tháng 04 năm 2014

Ocean Waves – A Nightmare for Offshore Structures

You may have heard about the grounding of an Alaskan oil rig in January, 2013. The 28,000-tonne rig was pushed toward the shore by waves up to 35 feet and winds up to 62 mph, dragging its main towing vessel and a tug behind it. There have been several such oil rig incidents over the past few decades. The below image shows the failure of an another oil rig platform due to extreme wave forces. A huge wave hitting the offshore platform leads to high wave impact loads that can eventually result in significant platform damage and collapse. These incidents can cause fatalities and damages that can cost hundreds of millions of dollars.
An image provided by the U.S. Coast Guard, shows the conical mobile drilling unit Kulluk owned by Royal Dutch Shell aground on the southeast side of Sitkalidak Island, Alaska, January 1, 2013.

Courtesy Esri DC Development Center Blog

Why do these offshore structures fail even though they are designed for wave loading? A main reason is that sometimes the wave height exceeds the design wave height with the occurrence of a tropical storm or hurricane. Another reason is that old structures are designed to a lower environmental criterion and have lower strength characteristics. So there is a need for detailed analysis of wave loading for extreme design wave conditions for the existing and new structures.
Of course, experiments can be performed to assess wave loading but these are too expensive and most often not accurate as these are performed on model scale tests. There are different wave estimate methods but CFD is more accurate with the cost of computational time.
ANSYS CFD offers solutions to these problems with various wave modeling capabilities that have been added over the past few releases (inbuilt capability to model the regular waves in all the wave regimes — Airy Wave, Fifth Order Stokes, Fifth Order Cnoidal/Solitary and inbuilt capability to model long- and short-crested irregular waves using wave spectrum — Pierson-Moskowitz, Jonswap and TMA).
Computational fluid dynamics can be used effectively to estimate the wave loads and analyze the wave run up for different wave heights, wave directions, deck heights, local geometries, etc. Twenty seconds of simulation (one wave period of 20m wave) of an oil rig analysis was completed within a day time on 24 CPUs. Since CFD simulation scales almost linearly with the number of CPUs, this simulation can be completed in a few hours if more CPUs are used.
In this analysis, wave run up depended on the rig structure and it was more than the wave height. The upward and downward vertical load on the deck was found to be higher than the horizontal loads for that oil rig structure, which suggests considering the vertical load along with the horizontal load in the structural analysis. This CFD solution was presented at the Offshore Technology Conference (OTC) 2013.
Animation: 20 m wave hitting on an oil rig platform with deck height 13m

Horizontal load on the deck
Vertical load on the deck
If you’d like more information on this topic, my colleague Gilles Eggenspieler created a great tech tip recently that gives a step-by-step demo of using ANSYS Fluent to predict hydrodynamic forces and green water phenomena of an oceanic wave hitting an offshore platform.

Attending OTC 2014 on May 5-8 in Houston?

Stop by booth 5001 to view product demonstrations and speak with industry experts about how engineering simulation solutions from ANSYS are applied to a range of key applications in offshore, subsea, and unconventional oil and gas projects. Here’s a link to the papers we’ll be presenting.

 author :Lubeena Rahumathulla

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