Dynamic Behavior of a Flexible Line During Installation

ABSTRACT In order to get information about the forces on the connection between a flexible riser and a wet Christmas tree during a guide lineless lay-away operation, a full scale measurement has been performed in Campos Basin. Special transducers, based on LVDT sensors were developed for this purpose and installed on the external surface of the flexible line. Accelerometers and current meters were used for motion and current measurements, respectively. Anumerical simulation of the initial configuration of the flexible line was carried out by using a non-linear finite element program installed on a 386 microcomputer. The experimental and numerical results are presented in this paper and show a reasonable correlation. INTRODUCTION The recent discoveries of two big oil fields in Campus Basin, offshore Brazil, are outstanding in deep water exploration in a world scale. These fields, situated in waters from 200m to 1600m deep, have reservesaround 4 billion barrels of crude oil and 65 billion cubic meters of associated gas. Based upon the analysis of several alternatives, PETROBRAS has decided to proceed with the preliminary exploitationplan of deep water oil fields, based on semi-submersible floating production units and subsea completed satellite wells using wet christmas trees and flexible lines. Flexible lines are classified in static lines and dynamic risers. Lines which lie on the bottom of the ocean are called static lines or just flowlines. Those which connect flowlines to the production platform, namedflexible risers, are primarely flowlines reinforced to withstand the stresses generated by current, wave and platform motions1. The subsea well completion in water depths beyond 300m represents a quite critical operation. To overcome the major difficulty of an underwater couplingbetween the flexible production/control bundle (production, annulus and control lines) and the wet christmas tree, PETROBRAS recently developed the so-called lay-away operation. The primary objective of the lay-away system is to provide a means of running either tree or production base attached to the flowlines in order to prevent the vulnerable pull-in operations. In the first option the tree is run attached to flowlines. In the second option, the production base is run with flowlines and after completing the well, the tree is lowered and locked onto the production base and flowline hub by means of two hydraulic connectors. Bothalternatives have already been successfully tested in Campos Basin A lay-away completion is shown in Figure 1. After arriving on location, the lay vessel positions itself approximately 85m (280 ft) far away from the completion unit. Then, a cable is passed from the completion to the laying vessel and fastened on the flowline hub bridle, which is lowered by paying off the flowlines until a predetermined catenary curve is achieved. The hub is then hooked up to the moon pool and connected onto the production base cradle or tree flowline connector, depending on the option considered. Once locked in position the hydraulic and electrical connectors are tested from the lay vessel. The next step is simultaneously lowering tree or production base and flowlines until landing on wellhead.