This paper discusses the importance of dynamic effect on load predictions and structural responses for various types of offshore structures. In addition to wave, wind and current loads, seismic and vibration analyses are emphasized for pile-supported structures. When floating platforms are considered, emphasis is focused on motion and wave load calculations. The calculation procedures and the dynamic effects are illustrated through numerical examples of both bottom-supported and floating platforms.

A sample application of 20-year hindcast wave data is given for predicting wave loads on a column-stabilized unit in the Celebes Sea. Problem areas which deserve research are also identified in the paper.

Introduction

To asses the structural integrity of offshore installation at the design stage, the environmental loads and structural responses must be calculated and evaluated. Methodologies and computational of the wave environment and prediction of motions and loads for an offshore installation in a realistic sea condition. Both the static and dynamic response of an offshore structure can be reasonably predicted at the design stage.

This paper discusses the importance of the dynamic behaviors of various types of offshore structures in design considerations, and identifies the potential problem areas where future research efforts may be warranted. Sample calculations of load predictions for floating platforms and dynamic structural responses of pile-supported and self-elevating platforms performed at the American Bureau of Shipping are utilized to illustrate the computational procedures and the dynamic effects. procedures and the dynamic effects. The representation of environmental wave conditions and its influence on the load calculations and structural responses is also discussed.

Wave Environment

To determine the dynamic behavior of an offshore structure it is important to acquire realistic data on environmental conditions such as wave, wind, current, earthquake, etc., and to properly account for them in the calculations. Although the design loads are dominated by extreme conditions which combine all the maximum probable load components, the examination of dynamic effects can probable load components, the examination of dynamic effects can be performed for an individual load component, such as wave component.

There are three sources for obtaining wave data for design purposes. The most common source is terms of its coverage is purposes. The most common source is terms of its coverage is observed wave data. Such data provides general information but is not readily useful for performing analyses of marine structures due to the lack of spectral details and very often is biased nature. Measured wave spectral data is naturally most suited for use in design and evaluation of marine structures since its describes completely and realistically the distribution of wave energy both in the frequency domain and sometimes in the direction domain.

Unfortunately, reliable wave measurements are costly and time-consuming to obtain and, as a result, are very scare. The other important alternative wave data source is the 20-year hindcast data for more than 2,000 locations in the Northern Hemisphere, which is generated by the Spectral Ocean Wave Model (SOWM) operational at the Fleet Numerical Weather Central of the United States Navy. The hindcast technique, based on theories of ocean wave energy propagation, dissipation and generation using diagnosed wind fields as input, provides directional spectra and wind velocity at six hour intervals over a 20-year period. Such a massive data base not only surpasses the spectral details of measured wave data which are usually expressed as point spectra, but also achieves a better statistical quality because of its uniformity and lack of bias. The applicability of the hindcast data in analyzing dynamic loads and dynamic behaviors of ships and floating marine structures, has recently been examined by comparing a ship's responses with those induced by wave spectra measured by British weather ships at Station India (59 deg N., 19 deg W) in North Atlantic. The good correlation between responses using hindcast and measured data s hows the potential use of hindcast wave data in the dynamic analysis of marine structures, and its adequacy at the early design stage for offshore structures. In this connection, a 20-year data hindcasted for the Calebes Sea region was selected from ABS' wave data bank. To facilitate the computation, the huge amount of hindcast raw data was reduced into a wave special family consisting of seven groups of spectra with varying ranges of significant wave heights. Two of these groups are shown in Fig. 1 as an example, where each wave height group contains eight spectra. The concept of using wave families to represent a seaway has been established for quite a few years in the naval architecture field for performing the so-called long-term prediction of wave loads on performing the so-called long-term prediction of wave loads on ships and marine structures.

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