Page 62 - Marine Structural Design

Part I

Structural Design Principles

Chapter 3 Loads and Dynamic Response for Offshore Structures

3.1 General
One of the key issues in the design of offshore structures is to define the environmental
conditions for the transportation route and installation site, and to determine the environmental
loads acting on the structure for conditions such as transit, installation, operational extreme
and survival. The parameters to be defined in the environmental conditions may be found from
design codes such as API Rp 2T, among of several other codes.
The prediction of extreme values is required for the structural strength evaluation. Various
methods have been proposed for determining the extreme values, (Ochi, 1981, 1990). In this
Chapter, both long- and short-term (surviving storm) wave data approaches are detailed.
The aim of this chapter is to give an overall picture of the environmental conditions and loads
for offshore structural design, and to detail the recent developments in the prediction of
extreme response. A systematic method for structural analysis of offshore structures has been
developed to predict extreme response and fatigue assessment under wave loads.
Vibrations and the associated dynamic effects are also an important factor in structural design
and vibration control. Basics of vibration analysis will be covered in an Appendix of this
Chapter.
The contents related to extreme loads in this Chapter were modified fiom Zhao, Bai and Shin
(2001).

3.2 Environmental Conditions
3.2.1 Environmental Criteria
The collection and selection of the environmental criteria for the design of offshore structures
are the owner's responsibility. Statistical models are essential to adequately describe
environmental conditions. All environmental phenomena of importance should be considered,
such as wind, waves, currents, and tides. In general, environmental conditions as follows need
to be considered in design (API RP 2T, 1997),
wind
waves
currents
Tide