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 the structures' natural frequencies and damping. Analytic procedures by which one may account for these effects are presented. A procedures by which one may account for these effects are presented. A method for designing tankage that suppresses the dynamic response at the fundamental flexural natural frequency of the structure is proposed. Supporting data from full-scale field studies are presented. Introduction The ability to measure accurately and to predict the dynamic response properties of fixed offshore platforms is an important concern of the industry. In platforms is an important concern of the industry. In particular, it is important to be able to measure and particular, it is important to be able to measure and predict changes in the natural frequencies of a predict changes in the natural frequencies of a structure and to measure and predict the damping of that structure at its natural frequencies.Structural damage is known to cause small shifts in the natural frequencies of a structure in flexure and torsion. However, detection of the damage by monitoring the changes in frequency is possible only if all other sources of change are understood, predicted, and accounted for. One little-understood predicted, and accounted for. One little-understood source of change in natural frequency is the dynamic response (sloshing) of liquids in storage tanks.Small changes in tank depth have been shown to result in substantial changes in response behavior. This paper provides a method by which the effects of liquid storage tanks on the measured flexural natural frequencies 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 estimating the fatigue life of a structure.The damping on jacket structures is very low, typically 1 to 3%, and comes from various sources-structural, viscous hydrodynamic, wave radiation, and soils. An additional, not insignificant source generally overlooked is the damping contributed by the sloshing of liquids stored in on-board tanks.This paper quantitatively assesses the damping contributed by the motions of stored liquids and also provides the methods by which tanks may be provides the methods by which tanks may be designed to suppress adverse dynamic properties and to increase the damping available to the structure.Our analysis emphasizes the two first-order, end-on and broadside flexural modes of a structure with a rectangular platform. It is, therefore, applicable to most steel structures in the Gulf of Mexico and the North Sea. The first-order flexural modes are of principal concern because they are the most principal concern because they are the most important contributors to dynamic response and because they are most affected by deck-level storage tanks. This analysis may be extended to torsional and also to higher-order flexural modes. An example is presented in the Appendix. presented in the Appendix. 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. JPT P. 1231