Nile - Design and Qualification of Reeled Pipe in Pipe for Deepwater

Abstract

Abstract The design requirements for the BP Nile pipe in pipe system required the employment of several high performance pipeline components. Of these components, the insulation system, buckle arrestors and water stops were being installed on a pipeline for the first time. This paper presents the design sequence for these components and a summary of the qualification programme performed. Introduction The Nile field is located approximately 85 miles SE of New Orleans, Louisiana and is located in two blocks: Viosca Knoll (VK) 913 and 914. This field is expected to produce primarily gas condensate. The Nile field will be developed as a sub-sea tieback to the Marlin TLP, located in VK 915 at a water depth of 3245 feet (989 m). The Nile tieback comprises a 7.5 km long 6" × 10" pipe-in-pipe connected to a 6" steel catenary riser. The Overall Heat Transfer Coefficient (OHTC) requirement for the pipe in pipe is 0.21 BTU/°F.ft2.hr (1.2 W/m2K). The design temperature is specified as 95°C. This paper will outline the design process and summarise the qualification programme performed for insulation material, pipeline centralisers, buckle arrestors and reelable water stops. Design: Insulation & Centralisers An insulation requirement of an OHTC of 1.2 W/m2K is particularly challenging, however, when combined with a water depth of approximately 1000 m and the high design temperature, there is a limited selection of suitable insulation materials. The design parameters dictated that a pipe in pipe solution was required and it was on this basis that the project was specified. Insulation Material Selection. There are three pipe in pipe insulation materials which were considered for selection to meet these performance requirements; polyurethane foam (PUF), mineral wool and microporous insulation. Each material has specific advantages in terms of insulation performance, operating temperature range and relative cost. A comparison of the thermal conductivity of each material is presented in Figure 1. In the course of previous CSO development work each of these materials had undergone both simulated installation and thermal evaluation to assess their suitability. Furthermore a detailed heat transfer model had been developed by CSO which allowed the required thickness of each insulation material to be determined. A first assessment of required thickness of the above materials is presented in Table 1. From a first review of the results it was clear that the thickness of mineral wool required is too great for a 10" carrier pipe and a 12" pipe would be required. To use a larger diameter carrier pipe would increase procurement cost, and significantly increase pipe lay top tension. This option was not considered further. Two densities of PUF were considered in the design process. The higher density PUF (85 kg/m3) would require that almost the entire annulus was filled with insulation. This has an impact on the fabrication procedure.