A Critical Review of Transportation Analysis Procedures

ABSTRACT In recent years, with increased emphasis upon deepwater field development, the size and weight of offshore platforms have increased significantly. Consequently, platform transportation has become of great importance, reinforcing the need for a better understanding of the problem. In the past, for relatively small structures, platforms were analyzed for the combined effects of gravity and inertial loads only. The barge was usually assumed to be rigid, implying no structural interaction between the barge and the platform. For the larger and heavier platforms built today, a more sophisticated analysis must be performed. In this analysis the complete platform/barge system is considered. This method accounts for barge flexibility and the resulting platform/barge structural interaction. This paper examines both procedures and highlights the critical aspects and pitfall s of each. Numerical results from a recent study are presented to illustrate the differences in the two procedures. Specifically, several design related questions pertaining to barge flexibility, motion and force phase relationships, tie-down force distribution, platform stresses, fatigue, interpretation of results, and conservatism of results are addressed. 1.0 INTRODUCTION There are two important considerations in the transportation analysis of offshore platforms. From a navigational standpoint, the mission is considered successful if the platform/barge assembly completes the trip from the fabrication yard to the installation site without mishap. This is ensured by enforcing a number of general stability criteria. From a structural standpoint, however, the platform is expected to reach its final installation site without suffering any major structural failure or significant fatigue damage as a result of the transportation phase. Offshore platforms are usually fabricated in a horizontal position, and are then skidded onto a barge for the trip to the installation site. In its seagoing configuration the platform generally rests on skid beams, and is secured to the barge by tie-downs. In this position, the platform is vu1nerable to various structural problems for two main reasons:The platform is designed primarily for its In place upright position.The platform is quite flexible in the horizontal position. These problems are further compounded by the fact that some of the largest platforms designed today are in the range of 70D-1000 feet, while the largest launch barges currently available do not exceed 650 feet in length. This results in the platform overhanging the barge at one or both ends, as well as on the sides. Depending on the geometry of the platform and the barge, overhangs of up to 150 or 200 feet are possible.