Presweeping and Trenching for the Installation of the 40-in. Diameter Zeepipe Gasline on the Dutch Continental Shelf of the North Sea
Authors: Fitzgerald Mel , Smit Jaap , Athmer Joep , Hartsuiker Frits 
Published: October 2008

To avoid over-stressing and freespans of the 40-inch diameter Zeepipe gas line to be laid on the North Sea where nearly 600 sand dunes up to 6 metres in height occurred, a combination of pre-sweeping and. trenching techniques was used. to ensure final on-bottom stability of the pipeline whilst executing the project within the shortest possible time frame. By using the most modern survey positioning and data communication equipment an operational flexibility was achieved. resulting in a minimum volume of soil to be removed.

INTRODUCTION

By means of the Zeepipe system, the longest and largest diameter pipeline system yet laid in the North Sea, gas will be transported from the Norwegian Troll and Sleipner fields to six countries in Western Europe.

The 40 inch diameter pipeline (wall thickness varying between 23.8mn to 24.1 nm) will nm some 1300 kilometres through the offshore sectors of five North sea Countries (see figure 1) from the Sleipler field in Norway to Zeebrugge in Belgium. The total Zeepipe project consists of two phases.

The first phase will be completed by 1993 involves simultaneous laying of nore than 800 kilometres of pipeline between Sleipner and Zeebrugge.

An approx. 200 km section of the North Sea bottom had to be prepared in such a way that the pipe-laying schedule of the lay barge Semac 1 was not jeopardised and post trenching could be executed to achieve the required target line i.e. the bottom of pipeline as specified by the oil company. Installed to this level the pipeline would be in a safe operational condition from a stress point of view, i.e. below 72% of the specified minimum yield strength.

Working back from this target line, and bearing in mind the capabilities of the trenching equipment, the pre-sweeping scope could be established.

The purpose of the pre-sweeping operation was two fold, Le. preparing a smooth enough seabed to lay the pipeline on and to establish such a level that by means of post-lay trenching the required target line could be achieved.

A complication was the large number of vessels working along the route at the same time. This required extremely careful planning on a continuous basis as well as strict reporting procedures.

A contract was awarded to a joint venture comprising of four major Dutch dredging companies, who initially mobilised 5 state of the art trailer dredgers. (Geopotes 15, 13al:ent Zanen, Volvox Delta, Geopotes 14 and. HAM 310). These vessels were able to perform the pre-sweeping activities operating in Europe's busiest shipping lanes, i. e. Eurochannel to Rotterdam and. Deepwater Route 2.

Three of these trailer dredgers were used with dedicated survey vessels, one for each trailer dredger, to carry out the pre-sweeping operations. One main positioning system plus a back-up system were installed on each of the pre-sweep spreads to ensure the required. accuracies at all times. By means of telemetric and. satellite links a constant flow of data was transmitted to and. from the vessels to monitor the ongoing operations and pass on any changes in the scope of work.