Design Methodology for Axial Force Response of Pipe-in-Pipe: A Probabilistic Approach

Abstract

As the easy oil is more or less gone, the typical offshore development faces several challenges in the future. These may be related to ultra deep water or difficult operational conditions like high pressure and temperatures. In addition there are often challenges related to flow, for example wax or hydrates during shut-downs or in tail production. Prevention of wax and hydrates is often solved by injection of chemicals or alternatively by some sort of heating, e.g. direct electrical heating. It may also to some degree be solved by superior thermal insulation or a combination of the methods mentioned. A thick insulation coating may give additional challenges with respect to submerged weight. Pipe-in-pipe (PIP) designs, where the flowline is insulated and covered by an outer pipe, solve this challenge and are becoming more and more popular. However, the pipe-in-pipe concepts also provide some specific challenges. DNV has recently been involved in a PIP project with quite challenging operational conditions. The combination of high temperature and high pressure (HTHP) and a corrosive well fluid with a buried pipe-in-pipe without any release of axial force leads to a very conservative design using conventional design approach. This challenge can be solved by applying a stochastic design approach avoiding conservative assumptions on top of each other. A probabilistic analysis targeting an acceptable probability of failure according to DNV-OS-F101 [1] resulted in an optimised design with a balanced selection of input parameters and avoiding ultra-conservative, worst case input combinations.