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  Offshore platforms have many uses including oil exploration and production, navigation, ship loading and unloading, and to support bridges and causeways. Offshore oil production is one of the most visible of these applications and represents a significant challenge to the design engineer. These offshore structures must function safely for design lifetimes of twenty years or more and are subject to very harsh marine environments. Some important design considerations are peak loads created by hurricane wind and waves, fatigue loads generated by waves over the platform lifetime and the motion of the platform. The platforms are sometimes subjected to strong currents which create loads on the mooring system and can induce vortex shedding and vortex induced motion (VIM). VIM can affect the platform as a whole and also the “risers” or pipes which reach from the platform to the ocean floor. Red Wing Engineering has an active program solving these and other problems associated with oil platform design.

Our recent work in offshore platform design has focused the use of computational fluid dynamics (CFD) to solve the problems of VIM and the loads generated by storms on tethered spar platforms and semi-submersibles. These platform types are often used in water to depths of 1,500 meters or more and are key elements in offshore exploration and drilling. In a typical spar platform design, the platform is a tall vertical cylinder held in place by mooring lines or tethers as shown in Figure 1. Note that the spar design shown in Figure 1 has spiral flanges or “strakes” to reduce vortex shedding and VIV in strong currents. Spar platforms can be quite large with diameters of 50 meters and drafts of 200 meters. The currents affecting oil platforms are often “sheared,” i.e. they exhibit significant changes in speed with depth. These currents typically feature a thin layer of warm low density seawater flowing rapidly over cooler higher density water which may be virtually motionless. Figure 2 shows the variation in velocity and temperature versus depth for a strong sheared current.


 
   
Figure 1.  Spar platform
 
     
   
Figure 2.  Typical current profile
 
 


Traditionally, the design of floating platforms was accomplished using semi-empirical methods in combination with scale model tests. These methods are well established but are sometimes difficult to apply because of non-linear effects and the difficulty of scaling small model results to very large structures. Red Wing Engineering, Inc. has developed CFD simulations of the tethered spar platforms acted on by waves and sheared currents individually or in combination. These simulations use the incompressible flow solver AcuSsolve™ developed by our research partner ACUSIM Software, Inc. AcuSolve™ is a highly accurate and fast flow solver that features arbitrary Langrangian-Eulerian (ALE) mesh movement technology, free surfaces, thermal transport, detached eddy simulation (DES), coupled rigid body motion and other features that are essential to open water wave and current simulations.

Figure 3 shows the surface geometry of a shallow draft spar platform with strakes. The animation of Figure 4 shows the motion of this platform in sea waves with a sheared current with the profile shown in Figure 2. In this animation, the waves are traveling from left to right as is the current. A constant temperature iso-surface (at 23 C) that divides the upper sheared current from the lower quiet seawater shows the current flow around the platform. This type of visualization can be used to gain understanding of the interaction of the platform with waves and current and also to visualize vortices formed behind the platform which can cause VIV. Note also the motion of the platform caused by the waves.

       
   
Figure 3.  Shallow draft spar platform with strakes
 
Figure 4.  Animation of platform with temperature iso-surface
 
 
 

Another view of the current and wave interaction is shown in Figure 5. In this animation, temperature contours are shown on a plane passing through the platform parallel to the wave and current direction. These contours show that the current pushes downward passing under the platform in the vicinity of the risers.


   
Figure 5.  Animation of platform showing temperature profile
 
 
 

More recent work on platform hydrodynamics has included studies of the effects of many appendages present in most platform designs such as that seen in Figure 6. These “appurtenances” can alter the fluid flow around the platform altering the vortex structures producing VIM. Red Wing Engineering is actively engaged in developing methods to include the effect of appurtenances in CFD simulations while keeping the computational cost low. This approach is described in the paper, “Modeling Vortex Induced Motions of Spars in Uniform and Stratified Flows”. A sample mesh is shown in Figure 7 along with predicted temperature contours around a spar in a sheared current.


   
Figure 6.  Appurtenances on spar platform
 
 
   
Figure 7.  Spar CFD mesh with appurtenances (left) and temperature contours around model in sheared current
       
       
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