Hybrid Tubes for Choke and Kill Lines

Abstract

Abstract The main purpose of this paper is to present the technology of hybrid tubes developed to reduce the mass of auxiliary lines of drilling risers used in ultra deep water, by replacing conventional choke and kill lines with lighter tubes. They are composed of a steel core hoop wound with a strip made of carbon fibers impregnated with a polyamide thermoplastic resin. The paper describes the structure of hybrid tubes, the design methodology and the manufacturing process. It presents all the tests carried out to qualify the technology and the conditions of the field test in preparation aboard a drillship offshore Angola. The principle of their integration in riser joints is described. Finally, the architecture of drilling risers including hybrid tubes is commented and their technical benefits are discussed. In conclusion, studies performed at IFP prove that hybrid tubes reduce the weight of auxiliary lines by 50% and that this technology is well suited for ultra deep drilling risers. Introduction In recent years there has been a great increase in the number of wells drilled in the very deep offshore, in water depths initially slightly greater than 2000 m and now in depths approaching 3000 m. Several drilling rigs of a new generation were recently put in service to allow such operations [ref. 1]. Their huge dimensions facilitate the production and the maintenance of the necessary drilling fluids, as well as the storage and the use of a large number of tubulars. Among these, the drilling riser is definitely one of the key components for these complex operations; therefore its design and make up procedures must be studied with a great attention [ref. 2]. Drilling riser problematics The main function of the drilling riser is to extend the borehole throughout the water column in order to guide the drillstring and to return drilling mud and cuttings to the surface. It is composed of tubular elements of 21" outside diameter, joined together by mechanical connectors. It links the drill floor of the floating vessel to the subsea wellhead where the BOP allows the well to be closed in the case of blow out. To ensure the control of the well in such a situation, the riser is equipped with two high pressure auxiliary safety tubes, called kill & choke lines. A third auxiliary tube, called booster line, is used for injecting an additional flow of mud at the bottom of the riser to facilitate the return of the cuttings. Finally, one or several hydraulic lines complete the architecture of the riser. All these tubes are integrated into the 75 ft long riser "joints". Each joint of riser is also equipped with buoyancy modules made of syntactic foam to reduce the wet weight of the system. When studying a drilling riser, the designer has to size the tubes, connectors and buoyancy modules so that the stress levels in the system remain acceptable in all operational conditions [ref. 3]. One of the most important parameters to control is the tension in the riser, particularly at the top end.