Riser fatigue insights to benefit offshore projects
Local research into the risers on offshore oil and gas platforms aims to increase the accuracy of fatigue damage estimates to make steel catenary risers (SCRs) more economical to construct.
UWA Centre for Offshore Foundation Systems PhD student Lucile Quéau conducted research into the type of SCR which will be used in the North West Shelf Ichthys project off the northern WA coast.
The risers are pipes that carry hydrocarbons from subsea to sea surface.
Ms Quéau says SCRs are highly stressed, with vessel motions and hydrodynamic loading resulting in two fatigue areas; at the vessel 'hang-off point' where the riser is connected to the floating facility and in the 'touchdown' zone where the riser meets the seabed.
She says gaining accurate fatigue estimates is essential since the risers are often in very deep water (more than 2000m in the Gulf of Mexico), and inaccessible for inspections and repairs.
Currently fatigue estimates are very conservative, making riser construction unnecessarily expensive.
Ms Quéau says more accurate estimates of SCR fatigue damage will reduce their construction cost and give the industry greater confidence in their use.
Her research, "Dimensionless groups governing response of steel catenary risers" was published in Elsevier's Ocean Engineering and is supported by Lloyd's Register Foundation which invests in science, engineering and technology for public benefit, worldwide.
In the article Ms Quéau says design uncertainties arise because of limited understanding of the influence of factors contributing to fatigue like geometry, structure, environmental loading and seabed properties.
Her research aims to provide quantitative guidance on how each parameter affects the induced stresses in SCRs to increase fatigue design accuracy.
Ms Quéau says the evaluation of functional relationships between the input design parameters and fatigue damage is performed by sensitivity studies.
"To conduct pertinent sensitivity studies it is necessary to use the dimensional analysis technique; establishing the complete list of independent design parameters impacting SCR fatigue damage and grouping them, based on their units, into dimensionless groups," she says.
"The published part of this research presently details how the dimensionless groups were established and validated through numerical simulations, laying the groundwork for subsequent sensitivity studies.
"This method facilitates the comparison and interpretation of results but also reduces the number of numerical simulations required.
"The dimensionless groups also assist in the planning of experiments by enabling appropriate scaling of parameters that play a critical role in SCR behaviour, for example centrifuge experiments."