What happens when conventional TT fatigue analysis is no longer enough?

Many ageing FPSOs are operating well beyond their original design life. As operators pursue asset life extension and continued production, understanding the true fatigue condition of critical mooring components becomes increasingly important. Traditional tension-tension (TT) fatigue assessment remains a widely used approach for mooring analysis, but what happens when TT fatigue analysis alone no longer captures the full picture? 

For one recent turret mooring system assessment, the challenge was clear: determine the remaining service life of a legacy asset despite significant data gaps and complex connection geometry. The outcome demonstrated why modern fatigue assessments must consider not only tension loading but also out-of-plane bending (OPB) and in-plane bending (IPB) effects.  

The challenge: assessing a legacy FPSO with limited data 

Many offshore assets face a common reality: incomplete historical records and ageing infrastructure. 

In this case, the asset presented several challenges: 

• No hydrodynamic model was available 
• No OrcaFlex model existed 
• No 3D FPSO hull model was available 
• The turret connection geometry was highly complex 
• The operator needed a robust estimate of remaining service life. 

The available vessel information consisted largely of hand-written Offset and Lines drawings dating back to the 1970s. To overcome this limitation, the engineering team rebuilt a detailed 3D FPSO hull model from the available drawings. 

The turret area created additional complexity. Accurate assessment of fatigue damage required detailed modelling of turret-to-mooring line connections, including rotatable hinges. 

Perhaps the greatest technical challenge was the estimation of OPB fatigue damage. Unlike conventional TT fatigue assessment, industry references and historical benchmarks for OPB fatigue are relatively limited, making methodology selection and validation critical.

Why TT-only fatigue analysis is not enough

Why OPB and IPB fatigue matter 

In offshore mooring systems, links are often restricted from bending freely near chain stoppers, H-links and similar connection points. 

When this restriction occurs, OPB and IPB fatigue develops because the connection restricts natural link rotation and introduces additional bending stresses. The resulting fatigue damage typically concentrates in the links closest to the restriction point. 

Why conventional TT analysis misses the full picture 

Traditional TT fatigue analysis focuses solely on cyclic tension loading within the mooring line. 

However, when bending is introduced, the fatigue response becomes significantly more complex. The affected links experience all three: 

• Axial tension forces 
• OPB bending moments 
• IPB bending moments 

These loading mechanisms act simultaneously rather than independently. Ignoring bending effects can therefore underestimate fatigue damage in critical connection regions. 

The assessment becomes even more challenging when the top ends of mooring lines are connected to the turret via hinges, which requires an evaluation for the hinge rotational angle - bending moment relationship. Engineers must evaluate combined fatigue damage generated by the interaction of tension loading, OPB moments and IPB moments rather than treating each effect separately. 

An additional consideration is the hysteretic behaviour of bending moments. As links rotate and slide against one another, capturing this behaviour is essential to producing a realistic fatigue assessment.  

Applying the Acteon-developed methodology for simplification 

To evaluate combined TT, OPB and IPB fatigue, the engineering team adopted a practical methodology developed by Acteon internal research to derive the relationship between relative link angle and bending moment without the significant computational demands associated with detailed finite element modelling. 

Under conventional industry guidance (DNV-OS-E301ver 2024 and BV-NI-DT-R00 E), engineers may need to create numerous detailed chain link finite element models to establish angle-bending relationships. The Acteon methodology implemented a simplified path by deriving these relationships directly from the Orcaflex simulation results. 

Building the fatigue model 

The assessment involved: 

• Reconstructing the FPSO geometry using the available historical offset and line drawings 
• Recreating an OrcaFlex FPSO model with the mooring system 
• Deriving OPB and IPB bending relationships using the above methodology 
• Dynamic simulation for each MDS (Mooring Design State) 
• TT + OPB + IPB fatigue assessment 

Fatigue damage was assessed across multiple locations and stress combinations, resulting in 16 possible TT + OPB + IPB fatigue combinations. This approach provides a more realistic representation of actual operational loading and delivers a significantly more conservative fatigue estimate for critical mooring components. 

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