The Design of Flexible Marine Risers in Deep and Shallow Water

Abstract

ABSTRACT This paper reviews the design methodology of marine riser systems and the analysis performed to substantiate the design corresponding to two field conditions in deep and shallow water. The analytical and numerical tools adopted are capable of handling riser dynamic analysis for cases of survival, operation, interference, installation, and design life verification. The results of computer analyses are presented in this paper in graphical form for the riser system selected in the two cases. Output includes envelopes of riser coordinates, axial force and time histories of forces and wave surface profiles. The obtained results highlight the significance of design parameters for the combined flow of waves and currents on the selection of riser configuration. INTRODUCTION The adoption of flexible pipe for marine risers as an integral part of offshore production systems is no longer viewed as exploratory in nature. Recent installations of key systems world wide have proved the concept to be technically acceptable, economically attractive and often representing a unique solution. Though flexible pipe as a marine product was introduced to the offshore market in the early seventies it was not till 1978 that flexible risers were specified and installed in the Enchova field offshore Brazil [1] as part of a floating production system. Acceptance of flexible marine risers as components of floating production systems or for connection of an export lines to a loading buoy as a viable long term engineering solution was up to recently dependent on the ability of the prospective user or the manufacturer to demonstrate the adequacy of the design configuration chosen. Such evaluation requires certain dynamic analytical models which could only be evaluated through model test or the luxury of full scale validation by actual experience in the field alone with simulated life cycle tests. The availability of such proven tools today along with extensive variety of tests performed so far on samples of the flexible pipe itself and the actual field experience in Enchova [1] Balmoral [2] Jolliet [3] and Green Canyon Block 29 [4] to mention a few, help remove the barriers of acceptance but not necessarily lower the guard in so far as the careful and often lengthy design procedure which such an application calls for. Similar to other engineering technological break throughs, developments often depends on the availability and ease of use of the analytical tools capable of properly simulating the behavior of flexible marine riser in the ocean environment in which it is expected to operate survive, installed and removed if necessary. Life expectancy of 15 - 20 years in not uncommon and life cycle analysis is therefore required in addition to the above mentioned design criteria. The most convincing proof of acceptance of flexible marine risers is best demonstrated if one reviews the outstanding projects currently committed to such engineering solutions in Brazil, Canada, Australia, South East China, North Sea and the Gulf of Mexico.