Metocean Criteria/Loads For Use In Assessment Of Existing Offshore Platforms

Abstract

ABSTRACT This paper discusses the basis for the metocean criteria and wave/current deck force calculation procedures that are included in the draft document "API RP2A-WSD, Section 17, Assessment of Existing Platforms". The metocean criteria were developed in 1993 by Work Group 3 on Environmental Loads (WG3) of API Task Group 92-5 "Assessment of Existing Platforms to Demonstrate Fitness for Purpose". The deck force calculation procedures were developed by a sub-group of WG3. Procedures used by WG3 to obtain Gulf of Mexico and West Coast metocean criteria for various exposure categories are described in detail. The recommended procedure for calculating wave/current deck forces is shown to be validated by model-scale wave tank test data. INTRODUCTION The purpose of this paper is to present the basis for the metocean criteria and wave/current deck force calculation procedures that are included in a draft API RP2A document on assessment of existing platforms [1]. The metocean criteria were developed in 1993 by Work Group 3 on Environmental Loads (WG3) of API Task Group 92-5 "Assessment of Existing Platforms to Demonstrate Fitness for Purpose". The deck force calculation procedures were developed by a sub-group of WG3. The metocean criteria consist of the following items:Omni-directiomd wave height vs water depthStorm tide (storm surge plus astronomical tide)Minimum deck heightWave and current directionCurrent speed and profileWave periodWind speed The criteria are specified according to geographical region and water depth. At this time only criteria for the Gulf of Mexico (GOM) and three regions off the West Coast are provided. No metocean criteria are provided for Cook Inlet because ice forces dominate. The criteria are further differentiated according to exposure category and type of assessment analysis. There are six exposure categories depending on possible consequences of failure. There are two environmental impact categories:Significant Environmental Impact andInsignificant Environmental Impact; and three life safety categories:Manned-Not Evacuated,Manned-Evacuated, andUnmanned. Each combination of environmental impact category and life safety category represents an exposure category. For each exposure category metocean criteria are provided for two levels of assessment analysis, design level and ultimate strength. The levels of analyses are in increasing complexity and decreasing conservatism. Design level analysis is like that used in new platform design, including the application of all safety factors, the use of nominal rather than mean yield stress, and the one-third increase in allowable stresses. Ultimate strength analysis excludes all sources of conservatism providing an unbiased estimate of platform capacity. Additional details are in [ 1] and [2]. In both levels of analysis, wave forces for specified metocean criteria must be calculated using the methodology of the 20th ed of RP2A given in [3] and [4] with wave kinematics factors that are provided in [1].