A Diverless Tie-In System For Large Diameter Pipelines (1995)

ABSTRACT The connection of large trunk lines to subsea export facilities at water depths beyond diver capabilities has been seldom analysed and never carried out. The difficult lies in the fact that tie-in procedures and connection systems available on the market require flexibility of the inboard or outboard hub of the pipeline ends to be connected, which would need very long expansion loops. The solution proposed in this paper does not require any flexibility on the pipeline. This could be of primary importance in the development of future deep water subsea fields, especially in the North Sea, that would benefit from oil or gas transportation in large trunk lines connecting export to receiving facilities. The study has been carried out on a 36 inch pipeline at a maximum water depth of 380 m. Technical and economic sensitivity evaluations have been perform to verify the soundness of this concept for 16÷40 inches pipeline diameters at a maximum water depth of 1000 m. The results obtained are part of a larger R&D project called PINTIES (Pipeline in the NineTIES) sponsored by Norsk Agip, A spool-piece shall be installed to recover distance and angular misalignments between the hub instilled on the export riser base, or on a subsea template, and the hub welded on the pipeline end, The spool-piece is retrievable to perform surface maintenance, lf required. The manufacturer of the spool piece and mechanical connectors was also involved in the design phase of these pieces. INTRODUCTION The trend in the development of offshore hydrocarbon fields is currently moving towards an increase of exploitation schemes maximizing the utilisation of subsea systems (ref. 1). In particular this trend is mainly focusing on deep water applications which require the development of new technologies and new installation criteria. Due to the above a large R&D project, called PINTIES (Pipeline in the NineTIES), has been carried out, sponsored by Norsk Agip, to cope with the new technological requirements for the incoming decade in the North Sea. Within PINTIES project, an activity has been dedicated to the development of new tie-in concepts aimed at providing a thorough examination of current tie-in systems and developing and qualifying new tie-in methods. Through the analysis of the market relevant to future subsea field developments and the assessment of the state of the art for tie-in systems, this activity has allowed the identification of critical field scenarios requiring the development of new tie-in methodologies. These critical scenarios are characterised by deep waters and large diameter pipelines, above 20", for which a lack in tie-in technology was found (ref 2). This paper aims at presenting the new concept developed for the diverless tie-in of large diameter pipelines, The system flexibility, i.e. the possibility to work in different operating scenarios mainly characterised by a range of diameters ranging between 16 and 40 inches and water depths up to 1000 m, is also outlined here below. DESIGN DATA AND FUNCTIONAL REQUIREMENTS The basic design data (ref. 3.), for this new tie-in concept, were identified as representative of those new subsea fields scenarios.