Abstract
This paper presents an experimental characterisation of fatigue at welded connections for the next-generation high-strength low-alloy offshore riser steel, X100Q. An instrumented girth weld is conducted with a parallel programme of physical-thermal simulation (Gleeble) to develop heat affected zone (HAZ) test specimens. X100Q is shown to exhibit superior fatigue performance to the current state of the art offshore riser steel, X80. Significant differences are demonstrated between the parent material and simulated HAZ in terms of hardness, monotonic strength and cyclic plasticity response, which can be related to the observed microstructural transformations: the refined grain and bainitic block size in the fine-grained HAZ are shown to give a harder and stronger response than parent material, whereas the coarsened bainitic lath structure in the intercritical HAZ gives a softer and weaker response. The simulated HAZ materials exhibit superior fatigue performance to the parent material and weld metal. A significant reduction in life is shown for cross-weld specimens, indicating susceptibility to failure due to HAZ softening for matched or over-matched X100Q welds.
Highlights
Flexible marine pipe is currently the most common solution for offshore risers; its viability in deep- and ultra-deepwater is limited due to its high cost and low-pressure capacity
A significant variation in hardness is shown across the girth weld, with a reduction of 17% exhibited in the intercritical heat affected zone (HAZ) for welded X100Q, consistent with findings for other high-strength low-alloy (HSLA) steels
For the welding conditions investigated via physical-thermal simu lation, peak temperature has a greater influence than cooling rate on the microstructure, mechanical and fatigue performance of the HAZ
Summary
Flexible marine pipe is currently the most common solution for offshore risers; its viability in deep- and ultra-deepwater is limited due to its high cost and low-pressure capacity. Due to these limitations, currently, the deepest installed flexible riser is at a depth of 1900 m, with an internal diameter of 190 mm [1]. The microstructure and mechanical performance in the HAZ may be improved to a large extent through close control of the welding process, post weld heat treatment and the addi tion of alloying elements [6]. The use of lean alloy steels and high throughput welding processes are necessary to maximise the effi ciency of offshore welding during pipe laying
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