Hydrodynamic Wave Loading on Offshore Structures

Abstract

Abstract Fixed offshore structures are operated in shallow water depths up to 500 feet and are often subjected to huge wave loading. The huge waves destabilize the structures, which then can cause widespread damage to the local ecology, coastal towns, and the environment. Mitigating the impact of wave loading requires the accurate prediction of wave induced forces, which is used for the structural assessment. Determination of wave induced forces requires solution of two problems. The prediction of wave kinematics by using an appropriate wave theory and then the prediction of the pressure and viscous forces due to the wave impact loading. This paper focuses on the hydrodynamic wave loading of fixed offshore structures in shallow water. Accurate modeling of huge waves in shallow water is more challenging compared to deep water due to higher relative wave height (wave height to water depth ratio). In this paper, we propose a Computational Fluid Dynamics (CFD) solution using Volume of Fluid (VOF) method and fifth order solitary wave theory for modeling huge waves in shallow water with very high accuracy. This model is validated with experiment for the physical mechanisms in the wave loading of the structures such as wave propagation, run up and interactions. Finally, this model is used for the wave- in-deck analysis of a fixed offshore oil rig in shallow water. In this study, we use a 20m (65.6 feet) wave in 41m (134.5 feet) water depth (Relative wave height of 0.49), which is not possible to model using Airy and Stokes wave theories. The wave-in-deck analysis is carried out for three different wave heights and the effect of wave height on the wave run-up and loads is analyzed. Introduction Hydrodynamic wave loading on fixed offshore structures has been an issue of concern to the offshore oil and gas industry. A huge wave hitting the offshore platform leads to high wave-in-deck loads that can eventually result in significant platform damage and collapse. Fatalities and damages costing hundreds of millions of dollars can occur. From 2004–2008, five major hurricanes (Ivan, Katrina, Rita, Gustav, Ike) destroyed 180 structures and 1,070 wells in Gulf of Mexico [Kaiser, M.J, 2011]. About 50 Russian crew members were killed after a jack up oil rig capsized and sank in a 6m (19.68feet) wave hitting [News: Reuters, December 2011]. Some of the possible causes of the accidents are:Some areas of the Gulf of Mexico floor have experienced several feet of subsidence, which leads to lower deck height and are more vulnerable to wave loading [Laurendine, T., 2007].The wave crest height according to RP 2A of the American Petroleum Institute (API) is higher than the lower deck elevations of many existing platforms [Bea, R.G., et all,1999]With the occurrence of a tropical storm or hurricane, the wave height exceeds the design height.Old structures are designed to a lower environmental criterion and have lower strength characteristics. So a structural assessment is necessary to determine whether the structure can withstand the peak loads during the huge wave impact. The wave-in-deck loading is very complex and difficult to model with traditional analytical tools. It is also very difficult to assess the loading very accurately in physical model tests carried out in a small scale wave basin, which introduces inaccuracies in the measurements [Grønbech, M. J.et all. 2011]. With the recent advancements in CFD and increased computing power, CFD can be a valuable tool for the assesment of the structures subjected to wave loading.