A Fatigue Design Method for Deepwater Riser

Abstract

The invention belongs to the design technology of marine oil and gas equipment, and in particular relates to a fatigue design method of a deepwater riser. This method considers the coupling effect of vortex-induced motion of the floating platform and vortex-induced vibration of the riser when calculating the fatigue damage of the deep-water riser. The influence of fatigue damage caused by vortex-induced motion of the floating platform on the fatigue damage of the riser caused by the current is considered, and the influence of the fatigue damage caused by the extreme wave load on the fatigue damage of the riser is considered. The present invention is more in line with the actual fatigue damage accumulation essence of the deep water riser, more in line with the known Miner's linear damage accumulation criterion, more in line with the current environmental load change trend and characteristics, more scientific and reasonable, and the design result is safer and more reliable.

Description

technical field [0001] The invention belongs to the design technology of marine oil and gas equipment, and in particular relates to a fatigue design method of a deepwater riser. Background technique [0002] Marine deepwater riser is one of the important components of modern marine engineering structure system, and it is also one of the weak and vulnerable components. There are generally high-temperature and high-pressure crude oil and natural gas passing through the interior of the deep-water riser, and the external load is subjected to waves and currents. Due to the complexity of the marine environment where the deep-water riser is located, there are many factors that are affected. Fatigue damage is the main failure form of deepwater risers, so the fatigue design of deepwater risers is the key to the design of deepwater risers. [0003] The existing deepwater riser fatigue design method mainly adopts the S-N curve method, which is mainly based on the linear cumulative dam...

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Problems solved by technology

Therefore, it is inaccurate to calculate the riser fatigue damage caused by the vortex-induced motion and ocean current of the floating platform using the method of separate calculation and linear superposition in the prior art

[0021] 2. The fatigue damage caused by the short-term vortex-induced vibration is independently calculated and checked, and the influence of the fatigue damage caused by the short-term vortex-induced vibration on the cumulative fatigue damage of the riser is not considered, which is another shortcoming of the prior art

Although the short-term flow does not occur every year, if the short-term flow occurs at the initial stage of the service of the riser, the fatigue damage caused by it must accumulate with the fatigue damage caused by any subsequent load and affect the safety of the riser

Therefore, it is unreasonable for existing technologies not to consider the cumulative effect of fatigue damage caused by short-term vortex-induced vibration on riser fatigue damage

[0022] 3. The influence of the fatigue damage caused by the vortex-induced motion of the floating platform caused by the short-term flow on the fatigue damage of the deep-water riser is not considered, which is the third shortcoming of the existing technology

Therefore, if the fatigue damage caused by the vortex-induced motion of the floating platform caused by the short-term flow is not considered, the influence of the fatigue damage on the deep-water riser will directly lead to an unsafe fatigue design

Therefore, the existing technology does not consider the riser fatigue damage caused by the vortex induced motion of the floating platform caused by the short-term flow, which is not in line with engineering reality

[0023] 4. Does not consider the influence of fatigue damage caused by extreme wave loads on the fatigue damage of deep water risers, which is the fourth shortcoming of the existing technology

Because, even extreme wave loads have a low probability of occurrence, but can still occur during the 20-year service life of the riser

A 50-year event by definition does not occur every 50 years, so even a riser installed the year after a 50-year wave occurs may have another 50-year event during its 20-year service life Even the wave load that occurs once in 100 years, moreover, the actual service period of the riser will often be extended due to the needs of oilfield development

Therefore, for important structures such as deepwater risers (it is important because once an accident occurs, the economic losses and political impact will be huge) regardless of the impact of fatigue damage caused by extreme wave loads on deepwater riser fatigue damage impact is inaccurate

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