Abstract

Recent experience during reeling of pipelines with thick insulation coatings has shown that the combination of this type of coating with small changes of pipeline wall thickness and / or material properties across tie-in welds can result in unacceptable local buckling of the pipeline.

This paper describes the problems experienced during mobilisation for pipeline installation on the Åsgard field. The finite element analysis performed to simulate the behaviour of a pipeline with a thick insulation coating when subjected to plastic bending during reeling is explained. The analysis clearly shows that local buckling of the pipeline can and may occur for unfortunate combinations of pipeline design, coating thickness / stiffness and reeling parameters.

A method has been developed whereby the critical parameters during reeling can be identified, a so-called Reeling Parameter Study. By identifying the relative importance of these parameters, and documenting the effect of remedial measures, it is now possible to prevent reoccurrence of similar problems on future projects.

Introduction

Coflexip Stena Offshore Norge a.s has been awarded a contract by Statoil for the installation of the majority of the infield flowlines for the Åsgard Development, claimed to be the worlds largest subsea development project. The total length of flowlines to be installed is approximately 160 km, ~80 km were installed in the 1998 season, 23 km will be installed this year, and the rest in 2000. The majority of the flowlines, ~140 km, are 10" nominal diameter, the remainder is 12". Two of the 12"flowlines are coated with thin film coatings, the rest are provided with insulation coatings up to 55 mm thick in order to maintain the contents' temperature during operation or shutdown situations.

Most of the flowlines are fabricated from 13%Cr linepipe, a material not used previously for offshore pipelines except on the Gullfaks Satellite Project for the same client in 1997. The qualification of the welding of this material for use with the reeling method is covered by Ref. 1.

The insulated flowlines are prefabricated onshore into 950m lengths at the CSONOR stalk fabrication yard in Orkanger, mid Norway. Immediately prior to offshore installation these long lengths of pipe are welded together. This welding operation takes place with the installation vessel, CSO Apache, moored at the yard. Thick polyurethane field joint coating is applied to the tie-in weld between the stalks and the completed section of pipe reeled onto the vessel. In this way, by repeating the tie-in operation for the appropriate number of stalks, the complete flowline is assembled prior to the transit of the vessel to the offshore location. This method of fabrication of the flowlines ensures that all welding and field joint coating is performed onshore in a well-controlled environment.

During the spooling operation for the 1998 installation season a number of instances of local buckling of the tie-in welds between prefabricated stalks were observed. These occurrences led to the need to cut out and repair the affected welds offshore. The pipes that buckled had all been designed in the traditional manner where the wall thickness required for reeling is identified based on considerations of local buckling and ovalisation of the pipe wall. The occurrence of local buckling of the tie-in joints was therefore a very unwelcome surprise. Full scale testing of the bending of these pipes, to qualify the coating systems, had been performed prior to the reeling operation. No significant buckling of the pipe was observed. Fig. 1 shows a typical local buckle, in this instance from full scale testing performed in January 1999, wher

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