Dynamic Behavior and Design of Offshore Caissons

Abstract

Free-standing caissons are used for supporting flare pipes and single-well production platforms. However, caissons tend to be flexible and dynamically sensitive, and the static design practice may not be adequate for this type of practice may not be adequate for this type of structure. To assess motion effect on the integrity of the structural system and to quantify the allowable motion for safe operation on board a caisson platform, analytical and experimental studies of platform, analytical and experimental studies of the dynamic behavior of a caisson structure were conducted and are described here. The analytical simulations agree well statistically with The motion measurements. A caisson design procedure considering dynamic effects was developed Design considerations include ultimate strength failure, fatigue failure, excessive motion, and possible damage during installation. A key feature in an effective caisson design is that the upper part of the caisson should be made as small as possible so that wave loading and the caisson period can be minimized The fatigue design procedure was verified with past caisson operational experience. To illustrate past caisson operational experience. To illustrate the procedure, a flare-pipe support caisson in 185 ft of water was designed and analyzed. Introduction Free-standing caissons are used for supporting flare pipes or single-well production platforms. The attractiveness of a caisson structure lies in the potential economy and the short time required for potential economy and the short time required for fabrication and installation. However, a caisson tends to be flexible, and dynamic effects may increase the design requirements from both strength and functional standpoints. To assess the motion effect on the integrity of the structural system and to quantify the allowable motion level for effective operation on board a caisson platform, analytical and experimental studies of the dynamic behavior of a caisson structure were conducted, and a procedure was formulated for designing a caisson considering dynamic effects. Observations from the experimental data and computer simulations of the caisson behavior are described. Verification of the computer simulation and some useful information for developing and using such simulations as well as practical interpretation of the analytical results practical interpretation of the analytical results also are given. Differences between a static design and a dynamic design are illustrated in an example design of a flare-support caisson in 185 ft water. MOTION MEASUREMENT Motion data were taken from a caisson platform offshore Louisiana. General dimensions of the caisson are shown in Fig. 1. SPEJ P. 291