Optimized Design of Pipe-in-Pipe Systems

Abstract

Summary Deepwater subsea developments must address flow-assurance issues, and these increasingly form a more critical part of the design. Pipe-in-pipe (PIP) systems, one of the options available in the designers' toolbox for overcoming these problems, are recognized as thermally efficient, reliable, and proven technology for insulated, subsea transportation of wellbore fluids. Although extremely low U values are achievable, PIP systems come at a cost, with increased weight as a penalty for use in deepwater developments. By applying an "inside-out" optimization process for the design of PIP systems, the top-tension loading on the surface vessel (installation or production) can be reduced significantly while minimizing procurement expenditure on raw materials. Specifically, the design optimization of each component reduces steel volumes as well as the overall outer diameter (OD) of the system. This paper presents the methodology for optimized design of PIP systems and illustrates the potential cost savings in terms of raw materials and installation through a case study for a typical large west African field. Commercial savings related to surface platform hull costs also are presented for a case in which the development employs PIP in catenary risers. Introduction At present, the PIP market is dynamic, with numerous projects requiring PIP solutions and many more examining PIP as a development option. The objective of this paper is to present an optimization design process for PIP systems for deepwater applications, specifically 1000 m or deeper. Rather than employing API standard sizes, this paper focuses on establishing the actual required pipe diameters for the flowline and carrier by performing the thermal and mechanical design in an integrated manner. In this way, the design meets project requirements for production rate and steady-state thermal performance while minimizing the as-installed system cost. Although an important and often driving design consideration in all deepwater developments, cool-down considerations have not been included in the designs generated here. The inside-out design methodology is presented along with the as-installed costs, which have been used as the ultimate comparison condition. The following parameters are investigated, with PIP designs and costs generated for each variable combination.U values of 1.0, 1.5, and 2.0 W/m2K.Flowline lengths of 5, 10, 20, 40, and 60 km.Water depths of 1000, 1500, 2000, 2500, and 3000 m.Two types of insulation material - polyurethane foam (PUF) and microporous (MP) material. In addition to presenting the results for the parametric matrix detailed previously, a project with typical characteristics for a large west African development is discussed, including the cost and top-tension implications on the host platform when employing PIP steel catenary risers (SCRs). With a large number of PIP systems available, it is increasingly difficult to evaluate options rapidly when determining or identifying the most appropriate features on a technical and economic basis. What follows is a brief definition and classification of PIP systems, and two specific criteria can be used to describe any particular one.Insulation type (material-dependent).Structural compliance (configuration-dependent). Associated with each criterion are compatible types of field joints and installation methods. Table 1 is the overall compatibility matrix showing the possible combinations of insulation-material type, field joint, and installation method for the main structural categories because the structural compliance drives the choice of insulation and installation method, with the latter of these heavily influencing fieldjoint selection. PIP systems are more installation-vessel-dependent than conventional pipe. This dependency is more pronounced with:Increasing water depth.Extreme requirements, such as high-pressure/high-temperature (HP/HT) applications.Use for SCRs. Table 2 presents the pros and cons for the three structural classifications. The definitions for structural classification and a description of the insulation-material types are continued in Appendix A. Optimized PIP Design-The Inside-Out Method. The inside-out process for designing focuses on optimizing each layer, from the flowline internal bore outward, so that thickness is minimized. In this way, the cost and weight of the final system are also minimized. This requires the use of non-API-sized pipe for the carrier. The use of non-API size for the flowline is also advocated because it leads to additional cost and weight savings for deep water, particularly for PIP SCRs. For shallow-water applications, the benefits of this approach are limited and may not justify the additional design and procurement complexity. For a deepwater project, significant cost benefits are obtainable. Fig. 1 represents the design methodology for a PIP system with standard API pipe sizes. Fig. 2 represents the design methodology for an optimized PIP system. There is a certain amount of iteration required in this process because the contribution of the carrier pipe's wall thickness to the overall heat-transfer coefficient (OHTC) changes with variations in its diameter and wall thickness. This change in contribution affects the insulation required to achieve the desired U value, which then necessitates recalculation of the carrier diameter and wall thickness. The iteration continues until the optimized combination is achieved. The previously described process is based the calculation of the OHTC (Uo), as provided by the basics of heat-transfer theory and the relative equations. These can be found in Appendix B. It must be stated that this process is based on the simplified 1D heat-transfer rate for convection, conduction, and radiation. Although the OHTC is affected by the fluid and its surroundings' resistance to heat transfer, these are negligible when compared to the insulation and pipes' properties. Furthermore, the process does not take into consideration the Joule-Thomson coefficient, which affects the temperature behavior of the fluid.

Finite element analysis of a sandwich pipe joint

“…The design of a joint usually follows set guidance criteria established wholly by the loadings the field joint would undergo from fabrication and throughout operation (Thielhelm, 1968). For pipe-in-pipe systems, several joint configurations were proposed (Hausner and Dixon, 2004). In sliding pipe-in-pipe systems, field joints require welding of both the inner and outer pipes.…”