This paper focuses on the overall system of factors which are recognized as being of primary importance in determining hydrodynamic loadings developed by severe storms acting on offshore platforms composed of small diameter members. Current knowledge is reviewed and summarized in the perspective of results from past in-ocean force measurement programs and experience with platforms subjected to intense wave and current loadings.
Published information on the failures and survivals of offshore platforms which have been subjected to intense hydrodynamic loadings are utilized to demonstrate that the industry presently has the ability to successfully design for storm loadings, in spite of the still present large uncertainties.
Current developments and background for our present State-of-Practice (SOP) to describe (analyze) and design for extreme hydrodynamic loadings are summarized in this paper. Current knowledge is reviewed in the perspective of results from past in-ocean force measurement programs and experience with platforms subjected to extreme wave and current loadings.
Five principal categories of factors important in computing hydrodynamic loadings on offshore platforms composed of small diameter members are discussed with respect to their relations with the extreme and nominal loading categories:
Nature of the problem (analysis to replicate vs. design to determine strength).
Oceanographic conditions (methods used to describe wave heights, amplitudes, periods, directional spreading and currents).
Wave and current kinematics (descriptions of water particle velocity-acceleration magnitudes, time history and directional characteristics in free-surface and subsurface regimes, and interactions of waves and currents).
Platform-wave-current interactions (close-spaced and inclined members, fouling, free-surface related effects, element forces and integrated result on platform system).
Force models (models used to translate kinematics to pressures including drag, lift, and inertia as described by models such as those developed by Morison, et al).
This paper is directed toward the platform design engineer. The discussions are developed with the intent of inferring the important effects of platform and oceanographic factors on the resultant forces on offshore platforms, and an evaluation of our present design practice based on past platform performance experience and ocean measurement programs.
In this section we will highlight major factors that influence descriptions of hydrodynamic loadings on offshore platforms. These include objectives of the analyses (design versus research), type of analyses (deterministic versus stochastic), categories of forces (extreme versus nominal), and hydrodynamic regimes (drag, inertia, lift, diffraction).
There has been much confusion and lack of communication between researchers and engineers in the offshore industry because of a failure to recognize the differences between research and design. It will be to our advantage to define the distinction between the objectives of these two processes.
Research struggles with the complex realities of wave and current loadings. The objective is to define the true loadings and develop analytical models which describe the details of these loadings.