Effect of Sacrificial Anodes on Wave Loads on Jacket Structure

Abstract

Wave and current loads on offshore structures are an important factor in the design of offshore structures. These forces are usually evaluated by semi-empirical Morison equation for tubular members wherein the ratio of characteristic dimension to wavelength is less than 0.2. In many cases, offshore structures such as jackets will have appurtenances such as anodes fitted on to them for various purposes, which may not contribute to the overall stiffness. However, these items will contribute to the wave and current loads in the order of magnitude of 10 to 20%. The calculation of hydrodynamic loads on such singular tubular members fitted with appurtenances can be done by taking to account their contribution towards drag and inertia. However, for complex structures, such as jacket structures with numerous members, it becomes practically very difficult and time consuming to do this calculation. In the industry, the general practice is to increase the overall loading due to presence of anodes by around 10 to 20%, based on experience and thumb rules. This paper focuses on the evaluation of wave loads on jackets due to the presence of anodes on jacket legs and braces, and comparing them to that of a jacket without anodes. The evaluation of wave loads is done by both numerical modelling and theoretical evaluation. The numerical model is based on frame analysis using SACS software which has the facility to simulate the wave load on space frame structures. Three different anode-to-jacket weight ratios (total weight of anode to total weight of jacket) are considered. The anodes are modelled as per design requirements and distributed throughout the structure. Recommended hydrodynamic coefficients from codal provisions are used. The overturning moments and base shear are evaluated for design regular waves and current. Results are presented in terms of comparison of base shear and overturning moment to ensure consistency, for three different cases. The recommendations for design engineers within the depth range and region studied can be drawn from this study.