A New Pipelaying Experience - Piggyback to Dual Lay

ABSTRACT On the British Gas North Morecambe Development Project in 1993, the derrick-lay barge DLB 1601 used a dual lay operation. A 36" Ø pipeline was laid piggybacked onto a 36" Ø pipeline for the shore pull operation and then separated to follow a dual lay for the offshore section. Transformation for piggyback laying mode to the dual /ay mode was performed making use of a new/y developed transition operation. This paper reports the challenges which had to be met to ensure a successful outcome. Results of various engineering studies are first presented. The set-up and observations of the (onshore) prototype testing are then described, Lastly, the special measures taken for the transition operation and certain field observations during the installation phase are briefly discussed. It is beiieved that the information presented in this paper will be of use to other similar projects in the future. INTRODUCTION For the offshore oil and gas industry, thousands of kilometres of submarine pipeline have been constructed all over the world. With the conventional S-lay method, the single-pipe laying technique and the piggyback (or bund[e) pipelaying technique are quite well known. While the former technique involves the laying of a single pipeline, the latter allows eventually the bundle laying of two or more pipelines. In the piggyback pipelaying technique, the small pipelines are generally attached to a comparatively larger pipeline which passes through the tensioners. Two equal-size pipelines have also been laid as a bundle, where both lines were passed through the tensioners. In this technique, different pipelines are fabricated in parallel on the lay vessel. Using saddles and straps, these pipelines are attached to form a bundle near the vessel stern. This thereby allows the pipelines to leave the lay vessel as a bundle and facilitates to lay all the pipelines together on the seabed. In such a case, therefore, touch-down point monitoring of the bundle pipeline is of importance similar to the single-pipe laying technique. Simultaneous laying of two (dual lay) or more (multiple lay) unattached pipelines from the same pipelay vessel are indeed scanty. This pipelay technique is only used in special cases where the pipelines are to be constructed very close to each other, keeping a specified separation. This may be due to any requirement, such as constructional constraints, protection, maintenance, etc. As a result, each pipeline has to leave the lay vessel separately. It necessitates the need for individuallaying ramps having tensioning arrangements for each pipeline. Lay vessel preparation and monitoring of the overall operation associated with such a laying technique therefore need special attention. Over the last two decades, the derrick-lay barge DLB 1601 has performed a number of challenging pipelaying projects. Among these projects, the British Gas North Morecambe Development (NMD) project in 1993 needs a special mention. This is particularly due to the fact that DLB 1601 performed a dual lay operation for the first time, Moreover, an interesting transition operation from piggyback to dual lay was also carried out. This paper describes the engineering and installation challenge.