Deepwater Pipelay: Extending The Limits

ABSTRACT The Stena Apache laid pipe in a record 900 metres water depth in 1994 following vessel modifications and remote systems development. The potential limits for further advances into deeper waters are explored. It is concluded there is scope for further enhancement such that potential deeper water developments can be economically brought on stream. INTRODUCTION In recent years, the offshore industry has shows a determination to develop prospects in deeper water. This has resulted in the frontiers of deepwater pipelay being pushed to new limits. Pipelay is typically performed using J-Lay or S-Lay methods from laybarges, or by reel-lay as with the Stena Apache reelship. The Stena Apache uses what is essentially a J-Lay method over an adjustable ramp which is fed by pipe from the reel. The reel is loaded with pipe at a welding spoolbase located at a convenient onshore location. The Stena Apache has undertaken many pipelay works, primarily in the North Sea. Work outside the North Sea has taken the vessel to the Bass Strait in Australia, the Gulf of Mexico, the Malaspina and Georgia Straits off Vancouver, Canada and the Campos Basin in Brazil. In mid 1993, Petroleo Bràs sileiro S.A. (Petrobras) awarded a contract to Stena Offshore Ltd. for the installation of multiple flowlines for the Marlim Phase I Development Project, located in the Campos Basin, Brazil. Stena Offshore had previously performed other work for Petrobràs in the Campos Basin area. In the most recent works, completed in early 1992, the Stena Apache laid 10 inch pipe in a maximum water depth of 705 metres. The progression of pipelay depth for the Stena Apache has increased rapidly in the last five years, from average water depths of about 140 metres in the Northern North Sea, to the recently completed works off Brazil to 900 metres, see Fig. 1. The industry is currently looking to lay pipe in even deeper waters, for example to 1800 metres in the Gulf of Mexico and to 3000 metres in the Arabian Sea1. The means by which the Stena Apache was able to successfully extend the depth lay record to 900 metres is discussed. The depth and pipe size limits for the Stena Apache, with the proposed modifications scheduled for the winter of 1994/1995, is investigated, and possible further enhancements which could further extend the depth and weight handling capacities, are reviewed. CAPABILITIES OF THE STENA APACHE REELSHIP The Stena Apache is a specialised pipelay vessel, unique in its technique for the handling of pipe by the use of a large, vertically mounted main reel, see Fig. 2. The reel measures 25.0 metres in diameter with a hub diameter of 16.5 metres and has the capacity to carry 1815 tonnes of pipe. Pre-fabricated pipe stalks are spooled on to this reel from an onshore spoolbase, thus minimizing the field duration required for pipelay operations. This allows the utilisation of shorter weather windows and decreased operating time in the field.