Wave Loads on Underwater Protection Covers

Abstract

Submerged structures in shallow water can be exposed to large hydrodynamic wave- and current forces. An estimate of the total external loads will be important for the design and robustness of the structures. The aim will also be to achieve more correct estimates in accordance with the physical loads. In the present work, wave forces on submerged protection covers over offshore pipelines in shallow water have been investigated. Protection covers have usually a small dry weight. In some cases, it can last some time before stabilizing gravel is dumped on the covers and the covers will in the mean time be freely exposed to the hydrodynamic forces. In order to investigate the shallow water forces, two simplified protection covers were studied with both a numerical CFD analysis and performed experiments in a wave tank. A rectangular and a half circular geometry was chosen for the simplified covers.

A Numerical Wave Tank (NWT) modeled with Computational Fluid Dynamics (CFD), can be used to make similar analysis as performed experiments in a physical wave tank. The use of numerical analysis can reduce both cost and time in comparison with physical experiments. In the present work a two dimensional numerical wave tank was investigated with the numerical CFD software ANSYS CFX. Further the numerical wave tank was used to investigate the dynamic wave forces on the simplified protection covers. Numerical wave elevation and numerical forces was compared with a theoretical approach. A modeled numerical piston or flap wavemaker gave reasonable wave elevation and was found usable to produce numerical waves. A comparison with results by Silva et al. (2010) gave almost similar results. Time series of wave elevation was found to be in good agreement with both theory and experiments. Numerical wave forces on the simplified covers gave good results compared with linear theory. Theoretical horizontal added mass coefficient was found to Ca,1= 1,53 for the rectangular cover and Ca,2= 1,145 for the half circular cover. Similar in CFD the horizontal added mass coefficients was found to be Ca,1= 1,4 and Ca,2= 1,18 respectively. The vertical forces from CFD was a small value larger than the theoretical linear vertical Froude-Kriloff forces.

The experimental tests was performed in the student wave tank at MARINTEK in Trondheim. Wave elevation from two wave probes and vertical force from three force transducers was measured. The wave elevation was compared with both numerical and theoretical wave elevation. The damping along the tank length was found to be similar as in the CFD analysis of around 1%/m. Due to different experimental setup and numerical modeling the vertical forces was not comparable. Non dimensional vertical forces and overturning moment was found from the performed experiments. Largest forces was measured on the rectangular cover. Largest non dimensional forces and overturning moment was found for the half circular cover.