Case study

FSPO - Assurance and integrity through strength and fatigue assessments

We aim to ensure the integrity of the vessel for current and future operations.

The challenge

Floating Production, Storage and Offloading (FPSO) systems are valuable and complex assets. To assure our client of the integrity of one such vessel and its ability to withstand its operating conditions, we were asked to perform strength and fatigue assessments.


Our solution

We built a global model of the ship structure, turret and the module frame structure using SESAM (DNV software). Using over 600 different combinations of wave height, frequency and direction to give a complete picture of the vessel’s response, we performed three assessments: 

  • Strength assessment of the ship structure under the impact of extreme waves
  • Spectral fatigue assessment to account for cumulative damage of through life offshore operation, including seasonal changes in wave loading
  • Strength assessment of the mooring system for an extreme 10,000 year wave.

The result

By comparing the predicted stresses on the FPSO hull and internal structures with the classification society rules, we concluded that the vessel was sufficiently strong.

After assessing the potential fatigue damage that the FPSO would accumulate we calculated its fatigue life using a statistical model of wave loading.

Finally, we developed a non-linear model of the mooring system that included the mooring lines, risers and buoyancy. After these additional refinements we applied the extreme loading conditions, and assessed the integrity of the mooring chains.


Frazer-Nash were able to provide assurance to our client that their asset would survive its projected operational life. We not only satisfied their current design needs but were able to save them significant cost by anticipating future requirements that could affect the overall strength of the FPSO.


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Figure 2: Internal Modular structural assessment.
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Figure 2: External FPSO hull strength analysis.

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