Qynamic Behavior and Design of Offshore Caissons
Authors: Hong S.T. , Brooks J.C. 
Published: October 2008

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 structure. In order to assess the 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 the dynamic behavior of a caisson structure have been conducted and are described in this paper. The analytical simulations check well with the motion measurements in a statistical sense.
A caisson design procedure considering dynamic effects has been 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 has been verified with past caisson operational experience. To illustrate the procedure, a flare pipe support caisson in 185 ft of water has been 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 period 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. In order 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 have been conducted and a procedure for designing a caisson considering dynamic effects has been formulated. Observations from the experimental data and computer simulations of the caisson behavior are described in this paper. Verification of the computer simulation and some useful information in developing and using such simulations as well as practical interpretation of the analytical results are also given. Differences between a static design and a dynamic design are illustrated in an example design of a flare support caisson in 185 feet of water.

MOTION MEASUREMENT
Motion data were taken from a caisson platform offshore Louisiana. General dimensions of the caisson are shown in Figure 1. Motions of the platform were measured by a self-contained, single-channel vibration monitor. The instrument can only take measurements from one direction at a time. Horizontal motions were measured at the heliport, production deck, wellhead deck, and boat landing. The motions were taken in directions both perpendicular to and parallel to the wave crest. Neither a wave staff nor an anemometer was available at the caisson platform and the sea state and wind velocities were obtained by visual estimation. The environmental conditions, maximum platform accelerations and measured platform periods are summarized in Table 1.