Extra High-Pressure High-Temperature (XHPHT) Flowlines: Design Considerations and Challenges

Development of deep water oil reservoirs in the Gulf of Mexico may encounter conditions where the flowline product temperatures approach 177°C (350°F), water depths range to 3000 m (10,000 ft), and tie-back distances up to 40 miles are presently being considered. These high flowline temperatures, water depths and distances, present real challenges to the design of flowlines. The objective of this paper is to present the design considerations and challenges of designing for extra high pressure high temperature (XHPHT) conditions. For such conditions, a pipe-in-pipe (PIP) flowline system with thermal expansion management, and a limit state-based design are viable solutions. This paper is split into three main parts and covers (i) design challenges and how they are overcome, (ii) finite element analysis design methods, and (iii) qualification testing of PIP components. The first section presents the main design issues, and challenges, of designing flowlines for deepwater and high-temperature conditions. The paper discusses aspects of controlling the large axial loads, such as thermal expansion management using buckle initiators and end constraints for flowlines, and presents current methods. The second section describes the use of advanced finite element analysis (FEA) tools for the design and simulation of PIP systems, and presents local and global FEA models, using ABAQUS, to investigate the limit state design of XHPHT flowlines. A 3-D helical response of the inner pipe subjected to high temperature, and the sequential reeling and lateral buckling of flowlines is also discussed. The final section of the paper describes the qualification testing to be undertaken on PIP components to ensure structural integrity and long-term thermal and structural performance. Qualification testing for PIP components for 177°C (350°F) service is discussed, and includes the testing of centralizers, waterstop seals, thermal insulation and loadshares. This paper is based on both theoretical and practical research work.