EFFECTS OF EMPIRICAL ORTHOGONAL FUNCTIONS IN SIMPLIFICATION OF STRUCTURAL RESPONSE OF A DEEP-WATER OFFSHORE SYSTEM UNDER CURRENT PROFILE LOADING

摘要

Typical recommended current profiles for marine operations can be found in offshore engineering guidelines and standards. However, for some offshore components (e.g. risers, umbilicals, risers) typical simplified current profiles can easily lead to unrealistic and conservative results. Due to recent developments in current measuring technology, current speed for deep water location can be easily acquired. However, the current speeds are usually recorded for long periods and in many measurement points along the water column. Hence, finding the extreme current profile based on the recorded time-history data is not an easy task since it needs excessive computational efforts. To determine the overall response of an offshore system, various methods have been developed to minimize the required computational efforts in working with big number of irregular current profiles. Mode-based analysis using empirical orthogonal functions is one of these methods. Total number of the utilized modes plays an important role in the numerical complexity of the problem as well as the accuracy of the results. In this study, for a given deep water location, the effects of the reduced number of modes are investigated through response analysis of a simple vertical fixed slender structure under thousands of current profiles. It is found that the reduced-mode profile can produce a good representation of the measured current profile, however it tends to underestimate the structural response.