Challenging Design Confronted in a Deepwater/HPHT PiP Tie-Back Flowline System

A challenging problem that pipeline industry has to face in deepwater is the high energy reservoir with high pressure and high temperature. For piping, flowline, and riser, High Pressure (HP) leads to much thicker pipe wall that increases manufacturing and installation cost. High Temperature (HT) has even wider impact on design since the flowline system has to operate over a greater temperature range between non-producing situations such as installation and shut down, and the maximum production flow. Subsea tie-back to the existing floating production facility, generally named as Brown Field Development, has many engineering and financial advantages. It becomes more popular in the Gulf of Mexico (GoM), North Sea, and West African due to the economical benefits. This paper presents some of the design challenges of a deepwater subsea tie-back project, which is composed of an 8″ by 12″ pipe-in-pipe (PiP) flowline loop from three (3) subsea fields to a semi-submersible platform located in the GoM at a water depth of 2,000m (∼6,600ft). Some of key efforts are worth to mention: • Mitigation of thermal expansion and global buckling as facing very soft clay soil; • Transition tie-in of PiP to structure piping - a valiant strength design to meet the deepwater installation loading; • PiP inner pipe lock-in compressive load - effect of flowline (non-bonded) section length variation and locked-in stress; • Tight installation target box for the separately installed structure mudmat and upper module. This paper presents the solutions that Project team has generated to address these design/installation challenges. Lessons learned from the designs and installations are also presented. Advanced analysis tool — FEA are utilized through the entire design stage, from global 3-D flowline modeling to local component strength design.