Design of a 25.5-in Titanium Catenary Riser for the Åsgard B Platform

Abstract

Abstract The Åsgard B semi-submersible production platform is located on the Haltenbanken in the North Sea in 305 m water depth. A free hanging 25.5" (ID) titanium riser has been demonstrated to be a robust engineering solution for a large diameter riser from a semisubmersible platform in this water depth. Pre-engineering covering all main aspects was carried out during the spring of 1999. The purpose of the riser is to accommodate the launching of a multi-diameter pig from Åsgard B and to serve as a gas export riser. The riser is presently a backup solution to the base case of subsea pig launching. Presently, there is no other application of a catenary riser in titanium. So far titanium has only been applied to one top tensioned riser, and components, such as stress joints in a limited number of risers. A limited number of free hanging steel risers have been installed in deep water. Isolation joints have been provided at both ends of the titanium riser. In addition to the riser pipe itself the use of titanium is therefore limited to the taper joint in the pontoon support and parts of the isolation joints. Taper joints in titanium as well as the components in steel are considered proven technology. This accommodates the use of proven designs for all components. In spite of not being critical to performance some technical issues are recognised as areas where further technical development is desirable. The most important aspect is considered to be the qualification of welding and NDT procedures. The most highly stressed welds need to meet strict requirements for initial defects. The requirements are determined through fracture mechanics analysis. It is believed that there is a potential for relaxing these criteria by further screening and grouping of available fatigue data in order to reduce uncertainties inherent in the current data. A base case with a uniform 28 mm titanium wall thickness has been developed, with alternatives allowing for a nonuniform wall thickness. The base case gives simpler purchasing logistics, while the advantages of the alternatives are to reduce the required amount of titanium, to simplify production and to reduce the interface forces applied to the platform. A well suited fabrication site which accommodates a straight riser of approximately 700m is located at Aker Verdal in Mid-Norway. Procedures for assembly of the riser, utilizing two weld stations, have been developed. Load out, tow out and installation methods which are insensitive to varying weather conditions have been developed. Surface towing may be applied for the tow out. To limit weather window restrictions provision has also been made for mid water tow out. The installation procedure has initially the riser suspended between two tugs. One end is transferred to the platform whereafter the rear tug moves over onto a different heading to position the riser in the anchor in a cranked configuration. Contingencies for bad weather have been developed for all phases.