Identifying how wave loadings will apply to a deck-mounted structure on an offshore vessel is a complex task. A simplified in-phase wave loading approach will apply a factor of safety, but that will undoubtedly lead to an increase in the size, weight and cost of the structure. We developed a new methodology that considered the actual wave phasing; we reduced the stress range throughout the structure to below 50% of that calculated by the simplified in-phase approach, extended the fatigue life by 30% and developed automation tools to complete the analysis in 50% of the predicted time.
OSBIT design, build and deliver high quality and cost-effective offshore systems for some of the world’s largest energy companies. In 2016, as their preferred engineering analysis partner of choice, we were called upon to support one of the most technically challenging and time critical projects ever undertaken in their 10-year history.
Identifying what loads will apply to a deck-mounted structure on an offshore vessel, then determining how those loads will apply to that structure and when they will apply to that structure is a complex and daunting task. All too often, in the absence of good data, Engineers need to adopt a conservative approach to their fatigue assessment. Whilst this conservative approach will apply a welcome factor of safety, in the case of fatigue, it will almost certainly apply an unwelcome increase in the size, weight and cost of the deck-mounted structure too. With a preferred design solution already defined, an increase to the size, weight or the cost of the design were 3 factors that OSBIT and their client (Helix Energy Solutions) were unable to accommodate. With the vessel already alongside in Brazil, time was against us all too, so we had to define a new norm in how we performed the detailed fatigue assessment on the OSBIT-designed structure.
The loads were defined as originating from environmental loads in the North Sea; 3 vessel loading scenarios during operations, transit and survival; 7 structural loading scenarios; and 24 vessel headings. A number of wave periods, sea states and individual wave heights also needed to be considered too. In total, there were 211,680 individual load ‘bins’. A more common, simplified fatigue approach assumes the vessel surge, sway, heave, pitch, roll and yaw response to a given wave will be in phase. This is a conservative assumption to make, as in reality, the vessel response will be out of phase. To reduce the level of conservatism and to reflect the vessel response in a more accurate way, we developed a tool to (i) read in the component stresses per unit of acceleration, (ii) calculate the component stresses due to wave loading, (iii) calculate the principal stress range due to wave loading, (iv) calculate the damage for every load bin and then (v) calculate the accumulated fatigue damage in accordance with the Palmgren-Miner rule. With the tool now considering the actual wave phasing, in some cases, we were able to reduce the calculated stress range throughout the structure to below 50% of that calculated by the simplified in-phase approach. The tool also enabled our Engineers to (a) clearly identify the most fatigue critical regions on a complex structure which was subject to a complex loading regime, (b) illustrate the cumulative fatigue damage as a highly visual contour plot and (c) significantly reduce the analysis computational times. The new analysis approach was heavily scrutinised and widely applauded by all and it now forms the basis for all of our offshore structures fatigue-related work.
- Extended the fatigue life by 30% with no increase in weight.
- Reduced the analysis times by 50%.
- Obtained class approval at the first submission.
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