Extreme Wave-Force Calculation Procedure for 20th Edition of API-RP-2A
Authors: Petrauskas Charles , Heideman J.C. , Berek E.P. 
Published: October 2008

The paper decribes the revised guideline design wave force calculation procedure approved for the 20th edition of the API-RP2A WSD and the first edition of the API-RP2A LRFD and contrasts it with the procedures in previous editions of RP2A. It also briefly dicusses the new metocean criteria for the Gulf of Mexico. The revised procedure was developed by the API Task Group on Wave Force Commentary. It embodies research results obtained over the last 25 years. It is a significant revision of the guideline wave force calculation procedures that have been part of the RP2A since the 1976 edition.

INTRODUTION

The paper decribes the revised guideline design wave force calculation procedure approved for the 20th edition of the API-RP2A WSD1 and the first edition of the API-LRFD2 and contrasts it with the procedure in the 19th edition of RP2A3. it also briefly discuesses the new metocean criteria for the Gulf of Mexico. The intent of the paper is to make personnel involved in structure design and assessment aware of the significant changes in the new editions of RP2A and the bases for these changes. The main reference for this paper is the 20th edition of the RP2A, which has a detailed commentary, with an extensive set of reference, that discusses the basis for each element of the revised wave force procedure. The same information is also contained in the first edition of RP2A LRFD. The revised procedure was developed by the API Task Group on Wave Force Commentary. It embodies research results obtained over the last 25 years. It is a significant revision of the guideline wave force calculation procedures that have been part of RP2A since the 1976 edition.

BACKGROUND

The task group was formed at the request of API to write a commentary on the existing RP2A procedure. The first task group meeting was held in December 1987 to discuss objectives and scope, and procedures to archieve them. After reviewing the RP2A procedures the task group concluded that a commentary consistent with the existing guideline disign calculation procedures could not be written. A considerable body of new knowlegde on wave forces had been developed since the first guidelines appeared in the 1976 edition of RP2A. Therefore the task group decided to expand the scope by developing new guidelines and a supporting commentary. Before defining the new guidelines, the task group performed a series of wave force comparison studies to (a) ascertain the accurancy of the computer codes in performing wave force calculations for given environmental conditions, force coeffients, and structure geometry; (b) evaluate differences in Gulf of Mexico (GOM) design wave forces that were being used at that time; (c) and evaluate differences in "best" estimates of GOM 100-yr wave forces. Results of these studies showed significant differences in GOM design force levels (up to a factor of two) but better agreement in the "best" estimates of the 100-yr wave forces. And very importantly, the "best" 100-yr wave forces were considerably higher than the values obtained using RP2A. The study, its conclusions, and recommendations were presented to the API Subcommittee on Fixed Systems Criteria In October 1989. Before accepting the higher forces the task group recommended the following (a) set up an API task group to review uncertainties and biases in structural capacity; (b) have the wave force task group complete its work and expand the scope to develop 100-yr hurricane wave, wind, and current criteria for the GOM incorporating the latest hindcasts; and (c) have industry try the wave force recipe to see if it is consistent with experience in the Gulf of Mexico. The Subcommittee supported the recom