Fatigue Design Considerations for Deepwater Fixed Platforms
Authors: Hanna S.Y. , Wang W.J. , Nair V.V.D. , Chang N.H. 
Published: October 2008

Fatigue is an important consideration in the design of deepwater platforms. In this study, fatigue design strategies for deepwater fixed platforms are presented. Parameters influencing the fatigue design as well as areas susceptible to fatigue failure are identified. Recommended design solutions and resizing charts to improve the fatigue the are presented. Relevant results from fatigue analyses of typical deepwater platforms are presented. Uncertainties in various parameters contributing to fatigue and research needed to improve the reliability of fatigue predictions are discussed.

INTRODUCTION

As the search for energy moves into progressively deeper water, offshore operators are evaluating new concepts as well as extending the limits of proven platform technology. Several recent industry studies have established the technical feasibility of designing fixed offshore platforms in waters as deep as 1,500 feet. One of the most important design considerations for the deepwater platforms is fatigue. Higher natural periods of these platform cause them to respond dynamically to everyday waves and amp 1ify cyclic stresses of fatigue sensitive tubular joints.

Several technical considerations such as environmenta1 forces due to storm and earthquake ground motions, fabrication and installation aspects govern the design of fixed platforms. This paper focuses on fatigue design considerations. While the fatigue design methodology presented here is applicable to a broad range of platform types, examples are selected from a class of fixed platforms referred to as template type and which are installed by launching off a barge. In this study, the platform is also characterized by a configuration which consists of a number of main legs through which piles are driven to secure the platform to seabed. Additional foundation capacity is provided by a number of skirt pi1es which are framed around the jacket base.

The overall fatigue analysis of the platform can best be accomplished in two stages. A simplified analysis is used in the early stages of the design. A more detailed analysis is used in verification and final resizing stages.

In the first stage, simplified fatigue analysis is performed to determine base shear due to design environmental loading (wind, wave and current) and to establish allowable stresses. Provision of using this simplified approach is presented in the API RP2A (Thirteenth edition) for fixed jackets with periods less than 3 seconds, and based on Gulf of Mexico wave climate. Kinra and Marshall [1] presented some examples of allowable fatigue stresses to be used for certain structures. In this study, the above approach is further extended to platforms in deeper waters and several locations.

The use of simplified fatigue analysis in the early design stage will enable designers to select can sizes, to optimize jacket configuration, and to pinpoint and suppress potential global fatigue problems that might been countered in the final design stage.

Parametric studies on relevant factors reflecting the effect of changing the structural period, leg spacing, conductor locations, deck payload and environmental loading on fatigue strength of tubular joints are included in the present paper.