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Steel Lazy Wave Riser Integrity Review for Excessive Flowline Walking Conditions


 Risers and flowlines are integral parts of deepwater oil and gas field developments. Risers serve as the interface between floating platforms and subsea flowlines and are often subjected to flowline walking (i.e., continuous ratcheting of the riser-flowline transition point (RFTP) towards the floater after each flowline heat-up and cool-down cycle). High temperature and high pressure flowlines have a great propensity to move axially due to thermal cycles, which could be substantial due to riser bottom tensions, as well as other reasons such as axial seabed slopes and thermal transients. The movement will be worse if a curved flowline section close to riser touch down point (TDP) is not stable (i.e., subject to curve pull-out during a flowline cool-down). A riser TDP movement of over 30 m, due to flowline walking, may raise a concern for the riser integrity. Where there is variability and/or uncertainty, riser designs typically use conservative assumptions and would only allow a limited movement at TDP (e.g., within the installation tolerance) due to flowline walking. As a consequence, flowline walking mitigation devices using expensive subsea structures such as huge suction piles are often adopted.
 Steel Lazy Wave Risers offer many advantages over traditional steel catenary risers. As developments are being explored for ultra-deep water, the scalability of SLWRs beyond current water depths makes them increasingly attractive. SLWRs with a direct transition to Steel Flowlines targeting the drill centers provide a simple field architecture solution. However, as HP/HT developments are becoming more common, flowline walking is more prevalent. A stringent allowable riser TDP displacement will result in a costly overall flowline-riser design. Instead of flowline walking mitigation towards SLWR, this paper discusses the idea of allowing a certain amount of flowline walking at the RFTP, so long as the riser integrity is not compromised. Since the Steel Lazy Wave Riser (SLWR) design is complicated and time consuming, general understandings on how much flowline walking (i.e., displacement normalized to water depth) can be accommodated could be very helpful, and may play an important role in the success of the project in terms of quality, cost, and schedule. With this information, steps can be taken in early engineering to ensure the SLWR’s can withstand a predetermined amount of flowline walking.
 In this paper, finite element models of a flowline and riser system are used to investigate the impact of flowline walking on the riser integrity. Influence of flowline walking on key SLWR performance drivers including Strength, Service Life and Interference are presented. Typical riser sizes and configurations, which are selected to cover most likely risers based on recent projects, are considered in the analyses. Parametric studies are performed on key input parameters, such as water depth, internal pressure, and RFTP displacement. Typical FPSO floater motion data is used in the analyses; various cases with different riser design margins are investigated.