We were commissioned by our client to assess the static and cyclic strength of over 50 subsea Xmas Tree (XT) flowloops. We needed to define the thermal and mechanical loads and determine the resistance of the flowloops to plastic collapse, hydrogen induced stress cracking (HISC) and fatigue failure. The work on low cycle fatigue delivered the potential to extend the fatigue life of the flowloops by up to 70% and by deploying our automation tools, we were able to cut the analysis times by 24 weeks and reduce the project costs by over £100,000.
Subsea Xmas Trees (XTs) are located on subsea wellheads and they control the flow of well intervention fluids into the well and production fluids out of the well. The XTs house a multitude of key components, all of which are critical to the function of the entire subsea production system. The various components of a subsea XT are typically located around the main valve block on so-called ‘wing blocks’. The components on the various wing blocks are connected together by a number of ‘flowloops’. The flowloops are either fabricated to drawing before the XT is assembled, or they are fabricated to fit after the XT is assembled. When the flowloops are fabricated before the XT is assembled, there is a potential tolerance stack up effect which may induce stresses when the flowloop is installed on the XT. When in operation, the well intervention or production fluids will induce further pressure and temperature loads onto the flowloops and the cycling and fluctuation of these pressure and temperature loads can ultimately lead to low cycle fatigue failures.
Due to their critical nature, their expected life duration and the inaccessibility of their location, subsea XTs must be designed in accordance with stringent design codes to ensure they will meet their short, medium and long term structural and functional needs. Finite Element Analysis (FEA) is used to assess the performance of the subsea XTs against these design code allowables and functional needs; it’s a complex and a time-consuming task to perform all of the required analyses in the right way and it’s one that should only be undertaken by a competent engineer who is familiar with the applicable codes, the analysis software and the entire functionality of the XT. With their in-house analysis team already committed to other things, our client asked us to intervene.
In the first instance, we had to decide what needed to be done. In essence, there were 4 things: (1) a global assessment in line with ASME VIII Div. 3 to determine the plastic collapse; (2) a local capacity assessment in line with ASME VIII Div. 3 to perform a tri-axial strain check; (3) a Hydrogen Induced Stress Cracking (HISC) analysis in accordance with DNV-RP-F112 on the components made from Duplex stainless steel; and (4) a low-cycle fatigue assessment in accordance with DNV-RP-C203, applying a Neuber Correction from NORSOK – N006. Then, given the sheer number of flowloops involved, we had to define how the analyses should be done. To drive accuracy, consistency and efficiency, we (a) generated a global model of each flowloop to identify the worst case combination of frame deflections and tolerances; (b) defined an elastic-plastic material model to calculate the global and local capacities to ASME VIII Div. 3; (c) used Pseudo-elastic stress ranges, in combination with S-N curves from DNV-RP-C203 and NORSOK – N006, to determine the cumulative fatigue damage for multiple load cases; (d) where standard Stress Concentration Factors (SCFs) were too conservative, we created detailed sub-models of the areas of interest and weld geometries, with cut boundaries being mapped onto the sub model from the global model, to determine the cyclic strain range; and (e) deployed a number of our Automation Tools to automate the analysis and reporting processes.
- Extended the fatigue life of the XT flowloops by 70%.
- Reduced the analysis lead time by 24 weeks (50%).
- Reduced the project cost by over £100,000.