Abstract

Technip-Coflexip has recently developed a technical solution to allow the reeling of a pipe-in-pipe bulkhead. The reeling of bulkheads compared to welding bulkheads into the pipe-inpipe offshore will save a significant amount of offshore time. The design is presented, fabrication procedures are discussed and the results of optimization work by Finite Element analysis utilizing ABAQUS£ are included. Results from fullscale reeling trials of bulkheads on the CSO Apache reelship are also presented, proving the feasibility of this design reelable steel bulkhead design.

Introduction

Steel bulkheads can be included within pipe-in-pipe systems primarily to allow load transfers between the outer and inner pipes during installation and in-service. In addition, the use of pipe-in-pipe in deepwater necessitates an effective repair strategy in the event of a wet or propagating buckle. As bulkheads combine the functions of a buckle arrestor and a waterstop, their installation at regular intervals along the pipeline would allow the replacement of any defective section between bulkheads (Reference 1).

Reel-lay provides clients with a cost effective installation method and high quality installed pipeline. The inclusion of bulkheads during the onshore fabrication procedure of a pipein-pipe system requires a conventional bulkhead design to be further developed in order to achieve a fast assembly and remove the need for half shells. The design, construction and testing of a pipe-in-pipe including an innovative modular bulkhead based on a combination of threaded parts is therefore described. The assessment of the bulkhead behavior during the reeling process and under various loading conditions was carried out by Finite Element Analysis. This theoretical work was validated by a full-scale prototype test performed on the CSO Apache reel-lay vessel and the results are presented hereinafter.

Innovative Design

For the reel lay method individual stalks of pipe-in-pipe up to approximately 5000ft in length are prefabricated on the designated spoolbase prior to vessel arrival. The flowline and carrier pipes are fabricated separately before the flowline systematically inserted into the carrier while spacers and insulation are applied (Reference 2). On arrival of the pipelay vessel the first stalk is spooled onto the reel preceded by a lead string. Each subsequent stalk is connected to the previous by two tie-in welds. First the flowline ends are brought together and welded. Then the carrier pipe of the trailing stalk is slid over the flowline to close the gap for the second weld.

Bulkhead spacing will be dependent on the project requirements. Stalk lengths can be modified to allow the insertion of bulkheads in the pipe-in-pipe assembly during tie in operations. The inclusion of a classical one piece forged mid-line bulkhead in such a fabrication procedure will require the use of half shells. Figure 3 describes a step-by-step assembly of a forged bulkhead at the tie-in station.

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