The Effect Of Liquid Storage Tanks On The Dynamic Response Of Offshore Platforms

ABSTRACT The sloshing of liquids in storage tanks on fixed offshore structures affects both the natural frequencies and damping. Analytic procedures by which one may account for these effects are presented. Also shown is a method for design of tankage that will result in suppression of dynamic response at the fundamental flexural natural frequencies of the structure. An important aspect is that no new equipment is required, but only optimum configuration of tankage that is already required for storage of water, fuel, mud, or crude oil. INTRODUCTION The ability to measure accurately and predict the dynamic response properties of fixed offshore platforms is an important concern to the industry. In particular, it is important to be able to measure and predict changes in the natural frequencies of the structure, and to measure and predict the damping of the structure at its natural frequencies. It is mown that structural damage will cause small shifts in the natural frequencies of the structure in flexure and torsion. However, detection of the damage by monitoring the changes in frequency will be possible only if all other sources of change are understood, predicted, and accounted for. One important source of change in natural frequency that is not well understood is the dynamic response (sloshing) of liquids in storage tanks1,2. It has been observed that small changes in tank depth can result in substantial changes in response behavior. This paper provides a method by which the effects of liquid storage tanks on the flexural response of a structure may be predicted and accounted for. The modal damping associated with each natural frequency also is important because it controls the dynamic amplification at the flexural and torsional natural frequencies. This amplification must be considered when making fatigue life estimates-for the structure. The damping on jacket structures is very low, typically 1 to 3 percent, and comes from a variety of sources: structural, viscous hydrodynamic, wave radiation, and soils. An additional and not insignificant source, which is generally overlooked, is the damping contributed by the sloshing of liquids stored in onboard tanks. This paper quantitatively assesses the damping contributed by the motions of stored liquids and, furthermore, provides the methods by which tankage design may be optimized to eliminate adverse dynamic properties and maximize the damping available to the structure. The analysis in this paper has been restricted to the two orthogonal fundamental flexural modes of a structure with a rectangular platform. Therefore, it is applicable to most steel structures in the Gulf of Mexico and the North Sea. The fundamental flexural mode was chosen for analysis because it is the most important contributor to dynamic response, and because it is most affected by deck-level storage tanks. It is believed that the analysis may be extended to torsional modes, and higher-order flexural modes as well. Another topic of current concern is the response of a platform to earthquake excitation. The analytic model and methods presented may be extended to estimate the effects of liquid storage tanks on the dynamic response of the structure to earthquakes.