Learn how to model the dynamic behavior and fatigue life of subsea risers using different types, methods, steps, challenges, and best practices.

In my experience, modeling riser fatigue should only start in the project. Just as we perform history matching for reservoirs on a relatively routine basis, when in operation, we should be utilizing the actual internal and environmental conditions the riser has been through (including inspection results) to understand how much 'life' we have taken from the riser since installation. This has proved quite helpful when operational issues came up and the team needed some 'flexibility' to operate 'at or beyond design' for short periods of time.

How do you model the dynamic behavior and fatigue life of subsea risers?

Subsea risers are the pipes that connect the offshore platforms or floating production systems to the subsea wells or equipment. They are subjected to various environmental and operational loads, such as waves, currents, vessel motions, pressure, temperature, and internal flow. These loads can cause dynamic responses and fatigue damage in the riser structure, which can affect its performance, reliability, and safety. Therefore, it is important to model the dynamic behavior and fatigue life of subsea risers accurately and efficiently.

1 Riser types and configurations

There are different types of subsea risers, such as top-tensioned risers, steel catenary risers, flexible risers, and hybrid risers. Each type has its own advantages and disadvantages, depending on the water depth, platform type, and operational requirements. The riser configuration also depends on the layout of the subsea field, the number and location of the wells, and the connection points. The riser configuration can be vertical, lazy wave, steep wave, or arch, among others. The riser type and configuration affect the dynamic behavior and fatigue life of the riser, as they determine the riser geometry, stiffness, damping, and boundary conditions.

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Ali Maboodi

Executive Deputy at Qeshm Oil Investment Company (QOIC)
(edited)
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In my opinion, the rigid risers and flexible risers should be categorized under two different major types. It means that configuration of each type, should be studied in different way.

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2 Riser analysis methods

There are different methods to analyze the dynamic behavior and fatigue life of subsea risers, such as analytical methods, numerical methods, and experimental methods. Analytical methods are based on simplified assumptions and equations that can provide quick and approximate solutions for some basic cases. Numerical methods are based on discretizing the riser into finite elements or segments and solving the governing equations using computational software. Numerical methods can capture more complex and realistic scenarios, but they require more computational resources and validation. Experimental methods are based on testing physical models or prototypes of the riser in laboratory or field conditions. Experimental methods can provide valuable data and insights, but they are costly and time-consuming.

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3 Riser modeling steps

Modeling the dynamic behavior and fatigue life of subsea risers using numerical methods involves defining the riser properties (material, diameter, thickness, weight, and coating), geometry (length, curvature, and inclination), boundary conditions (top and bottom connections, supports, and clamps), environmental loads (wave spectra, current profiles, and wind speeds), operational loads (internal pressure, temperature, and flow rate), and analysis parameters (time step, output frequency, and convergence criteria). The dynamic analysis should be run using a suitable software such as OrcaFlex, Ansys or Abaqus to extract the dynamic responses (displacements, stresses, strains and forces). The fatigue analysis should be run using a suitable software or method such as rainflow counting, S-N curves or fracture mechanics to extract the fatigue results (fatigue damage, fatigue life and hot spots).

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Ali Maboodi

Executive Deputy at Qeshm Oil Investment Company (QOIC)
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It should be highlighted that the type and specification of anchor flanges have significant effects on the both fatigue and dynamic analysis.

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4 Riser modeling challenges

Modeling the dynamic behavior and fatigue life of subsea risers can be challenging due to their nonlinear and coupled nature, which can cause complex phenomena such as vortex-induced vibrations, slugging, and bending hysteresis. Additionally, the variability and randomness of environmental and operational loads can significantly affect the riser responses and fatigue damage. The accuracy and availability of input data, such as material properties, load coefficients, and fatigue parameters, can also have a large influence on the modeling results and reliability. Moreover, the computational efficiency and robustness of software and methods should be taken into consideration to ensure the feasibility and quality of the modeling process and outcomes.

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5 Riser modeling best practices

To effectively model the dynamic behavior and fatigue life of subsea risers and overcome the associated challenges and uncertainties, it is recommended to use appropriate riser types and configurations that suit the project objectives. Validated software and methods that are suitable for the analysis should be employed, as well as realistic and representative input data that reflect the actual or expected conditions and scenarios of the riser operation and environment. Additionally, sensitivity and uncertainty analysis should be conducted to evaluate the effects of different input parameters and assumptions on the modeling results and reliability. Lastly, verification and validation techniques should be used to check the consistency and accuracy of the modeling results against analytical solutions, experimental data, or other sources of reference.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Adam L. Ballard, PhD

Capturing and Utilizing Knowledge through Digitalization
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In my experience, modeling riser fatigue should only start in the project. Just as we perform history matching for reservoirs on a relatively routine basis, when in operation, we should be utilizing the actual internal and environmental conditions the riser has been through (including inspection results) to understand how much 'life' we have taken from the riser since installation. This has proved quite helpful when operational issues came up and the team needed some 'flexibility' to operate 'at or beyond design' for short periods of time.

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Andrew Crawford

Integrity Manager at Buffalo Inspection
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We used Von Mises analysis years ago and always assumed that there were indications that were missed during the initial construction inspection. We used industry available POD models of the time and assumed a missed defect of 6mm in length by 3mm in depth as the worst case. This then allowed us to optimize inspection frequencies that would pre-empt a failure of such a defect and maintain integrity.

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How do you model the dynamic behavior and fatigue life of subsea risers?

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1 Riser types and configurations

2 Riser analysis methods

3 Riser modeling steps

4 Riser modeling challenges

5 Riser modeling best practices

6 Here’s what else to consider

Subsea Engineering

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