Deepwater Pipelaying Offshore West Africa : A Comparison Between Rigid Pipe Laying Techniques And Equipments

Abstract

Abstract The West Africa area is a new frontier for deepwater pipelaying technology. Until now, the deepest production comes from the Zafiro field, offshore Equatorial Guinea, where the McDermottETPM Joint Venture laid rigid pipelines in a water depth of approximately 500m in 1997. The paper describes the experience gained from these operations, which were carried out by DP vessel NORLIFT. using the reeling technique. The same year, conventional barges DLB POLARIS and DB I have been active in the same area laying 16 lines, from 4" to 20" in diameter, totalling over 160 kilometers, using the S-lay technique. This experience has been used in the paper to present a comparison between the means, methods and quantities involved with Reel, J-Iay and S-lay techniques, together with their efficiency. The specificities of the West African area are underlined (environmental conditions, reduced availability of industrial resources etc...). It concludes on the advantages and limitations of the conversion of DLB POLARIS to its dynamically positioned mode, which remains a good compromise to lay rigid pipeline in the 2000-3000m water depth range offshore West Africa, once the technical and economical aspects are taken into consideration. 1.0 Introduction The West Africa area is a new frontier for deepwater rigid pipe laying technology. The discoveries made so far and the current exploration prospects are very significant, but their development will only require a limited number of days of dynamically positioned vessels capable of installing rigid pipelines. Until now, the deepest production in this area is the Zafiro field, offshore Equatorial Guinea, where McDermott-ETPM have laid rigid pipelines in approximately 500m waterdepth in 1997. The work was done using the reel lay technique from the MSCL dynamically positioned vessel NORLIFT, but the vessel had to be specially mobilised from North Sea, The vast of existing rigid pipelines in the region were laid using the S-lay technique, from conventional lay barges, of which a few are permanently based in West Africa. In 1997, for example, our barges DLB POLARIS and OBI laid 16 lines, totalling over 160 kilometres, for various customers. The industrial challenge facing the pipelay contractor willing to operate in deepwater West Africa is simple : With which technique and which equipment will it be able to offer the best solution to its customer? The answer is somewhat more complicated and really depends upon the characteristics of the pipe to be laid. To explain the conclusions we have reached, we will first describe the experience gained in 1977. then extrapolate to the deep and ultradeep water depth range. 2.0 Reel lay technique: Zafiro - Phase 2 2.1 Scope of works (see Fig. 1) Within the framework of Zafiro phase 2 development by MOBIL in Equatorial Guinea, and of a contract executed in association with CSO, McDermott-ETPM's role was to build:–2 each 8" rigid carbon steel lines, 6500 meter long from FPSO location in 200 m waterdepth to the Zafiro West "Host" (275 m. W.D.),–one 8" and one 6" rigid carbon steel lines, 8500 meter long each from FPSO location to Topacio "Host" (475 m. W.D.),