You are hereProbabilistic Specification of Metocean Criteria: Generating and Search N-Year Contours for Extreme Response Estimation: The HTCNTR, IFORM, and SURFIT Routines

Probabilistic Specification of Metocean Criteria: Generating and Search N-Year Contours for Extreme Response Estimation: The HTCNTR, IFORM, and SURFIT Routines


Report No. : 
RMS-32
Authors: 
K. Engebretsen
Authors: 
S.R. Winterstein
Published: 
June 1998
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The purpose of this report is to present updated routines to both generate N-year environmental contours, and search them to find critical values [i.e., N-year response levels).

 

Specifically, the three routines are as follows:


HTCNTR: An interactive routine to generate contours, and create Hs,Tp pairs for plotting of contours at a specified return period.


IFORM: A general "inverse-FORM" implementation, which both constructs the N-year contour and searches it for a user-defined response function g. As in standard (forward) applications of FORM (First Order Reliability Methods), the user is required to code their g-function in a Fortran subroutine.


SURFIT: A specificalized version of IFORM. Specifically, this routine also constructs the N-year contour and searches it to find a corresponding N-year response. In contrast to IFORM, the response function g is automatically constructed by response-surface fitting, using grids of wave- and wind-response moments (mean, standard deviation, skewness, and kurtosis) as functions of environmental variables such as the significant wave height H, and peak spectral period Tp, mean wind speed and turbulence intensity.


Earlier versions of these routines have been handed out separately and individually documented; e.g., in RMS Technical Note TN3 (HTCNTR and IFORM) and in RMS Report 28 (SURFIT). This document supercedes these previous documents.


The major change in these routines is their standardization, all now based on a common subroutine (HSTP) which reports Hs-Tp distributions across a range of metocean sites. There are currently 15 such distributions in HSTP, each of which may be used consistently in HTCNTR, IFORM, and SURFIT. Our intent is twofold: (1) to facilitate a unified specification of metocean critera, and their propagation to response analysis; and (2) to promote the establishment of a common database storing such Hs-Tp distributions, as specified in the HSTP routine, for the purposes not only of the three routines supplied here, but also of various response analysis programs generated by other users.


Finally, the new version of SURFIT also includes a new capability: to perform importance sampling to estimate reliability directly. to serve as a check of the FORM-based reliability predictions.