Session: 02-12-01 Reliability Based Maintenance

Paper Number: 101111

101111 - Combined Fatigue Damages of Bridges From Wind and Wave Actions – Application to Offshore Bridge Structures 

Offshore bridges in offshore environment are subjected to load effects from wind load and wave load. The wind load acts directly on the bridges and the wave load acts on sub-structures such as jackets and the bridge support structure. The combined load effect results in dynamic stresses in the bridge structures which may result in fatigue failures. 

Traditionally, fatigue analyses of the bridge structures are calculated separately for the wind and wave load processes. The total accumulated fatigue damage is calculated by combining the calculated accumulated fatigue damages from these two processes by using analytic expressions. One widely used approach to combine fatigue damage from wind and wave load is the method from DNV-RP-C203 (appendix F). It is demonstrated that the use of this method will lead to conservative estimations of the fatigue damage for certain hot spots.

Applying conservative values for the accumulated fatigue damage will lead to more frequent inspections, as the calculated fatigue damage is the main input to inspection planning. Access of some of the critical inspection points in a bridge structure may require scaffolding and removal of passive fire protection (PFP). In some cases, close-down of the bridge and sub-sequent close-down of the production is required. Therefore, good prediction of the fatigue performance is crucial for safe and economical operation.

To obtain a more appropriate inspection plan that is optimized regarding both integrity and cost, a new method combining wave and wind fatigue damages for offshore bridges in the Norwegian Continental Shelf (NCS) is proposed. The new method consists of taking the separately calculated fatigue damage from wind and wave load for each short-term MetOcean condition and developing the hot spot stress response spectrums for each of the stress processes. From the response spectrum, short-term stress time series for each of the stress processes is simulated. The stress time series for each of the stress processes are then combined and the total stress time series is developed. Finally, stress range counting of the combined stress process is performed, using the Rain Flow Counting algorithm to calculate the combined fatigue damage.

The new method results in significantly longer calculated fatigue lives for certain hot spots than obtained by using the method in DNV-RP-C203.

The paper presents a guidance on how such analysis may be performed and will also show how the fatigue damage from each of the stress processes can be combined to obtain a simplified and more accurate prediction of the total fatigue damage compared to existing combination formula given in DNV-RP-C203. An example will demonstrate how the effect of the new method will influence the inspection plan compared to the conservative method.

Presenting Author: Magnus Knoph DNV AS

Presenting Author Biography: Magnus Leirvik Knoph is a structural engineer in the unit for offshore structures in DNV AS in Norway. He holds a Master of Science in Mechanical engineering from NTNU (2020) in Trondheim, Norway. He specialized in structural mechanics and numerical analysis. He started working in DNV in 2021 and since then he has been involved in several projects related to structural assessment and verification of offshore structures.

Authors:

Magnus Knoph DNV AS
Gudfinnur Sigurdsson DNV AS
Mélanie Devergez DNV AS