Computational Fluid Dynamics Predicts Air Gap and Wave Impact Loads of Offshore Structures
The content for this article was contributed by Anup Paul & Chris Matice of Stress Engineering Services1.
The calm-water air gap under offshore platform decks is an important design parameter and is determined by the required minimum air gap in extreme design conditions. For structures like semi-submersibles and tension leg platforms, the prediction of the minimum air gap and the probability of deck impact events are challenging.
Animation 1: Dynamic response of a spar with a 12 m wave.
Steep waves show significant non-linear behavior and wave amplification due to interaction with the platform legs that must be accounted for in air gap design. In the event of a negative air gap in harsh environments, prediction of the resulting deck impact loads becomes important. As oil and gas production moves into deeper waters floaters are required and deck height is limited by weight and stability requirements. Accurate prediction of deck clearance to the free surface and the deck impact loads is critical in predicting the performance of these structures under harsh environments.
Computational Fluid Dynamics
Computational Fluid Dynamics (CFD) methods are widely applied across a range of industries to examine fluid flow and heat transfer behavior. CFD in combination with the Volume of Fluid (VOF) model can be used effectively in the prediction of air gap and the wave impact loads on offshore platforms. The VOF method can be used to accurately predict the free-surface shape and non-linear wave behavior. For floating systems, CFD can be coupled with FEA to predict the dynamic and structural response of the platform during wave impact.
Wave Interaction of a SPAR Platform
Figure 1: Dynamic response of SPAR
Figure 1 shows the dynamic response of a SPAR to 10- and 20-meter waves. Both waves have a 20-second period and are generated using a linear wave boundary condition. The SPAR is modeled as a rigid body with 6 degrees of freedom at the center of mass. Figure 2 shows the vertical displacement of the SPAR center of mass. Figure 3 shows the horizontal force on the SPAR due to wave interaction.
Figure 2: Vertical displacement of SPAR
Figure 3: Horizontal forces on SPAR
Wave Impact on a Gravity Based Structure (GBS)
Figure 4: Wave impact on GBS
Figure 4 shows the wave impact on the deck of a gravity based structure (GBS). The mean water depth is 151.1 meters with an initial air gap of 21.7 meters. The incident wave has a height of 40 meters and a period of 17 seconds. Figure 5 shows the horizontal and vertical forces on the GBS due to the wave impact on the top deck. The spikes in force correspond with the initial impact of the wave on the front of the GBS and the secondary impact on top of the deck as show in Figure 4.
Coupled with Stress Engineering Services other deep-water design and assurance programs, the ability to compute air gap, in-deck wave loading and structural response to these extreme events provides our clients with the opportunity to obtain an assessment of their deep-water structures with a new level of detail and the flexibility to rapidly evaluate proposed design changes.
Figure 5: Force history of GBS due to wave impact on deck
1 Stress Engineering Services (SES) is an engineering consulting company providing design, analysis and testing services to clients in the upstream and downstream oil and gas industries, as well as a broad range of other process and manufacturing industries. SES is an employee-owned services company with 36 years of experience helping clients solve their toughest technical problems.
Anup Paul is an Associate with SES, specializing in fluid dynamic analysis of structures, products and processes.
Chris Matice, Ph.D., P.E. is a Principal with SES and heads their Process Technology Group, specializing in fluid dynamic and structural evaluation of plant and equipment.