The Problem
A crack was identified on a diagonal structural member of a platform jacket structure located in the northern North Sea during a 2015 subsea survey. Connected to this member are two major export risers; 24" oil export and 20" gas export.
Following identification of the crack, the client performed an analysis of the structure to determine the expected deflection of the riser guides if the member were to be completely severed.
The client then requested Jee to analyse the effects of these displacements on the risers. The results of this assessment would determine how soon a repair of the crack was required.
The Solution
We used the finite element analysis tool FS2000 to perform an assessment of the risers under environmental loading. This included the following analysis:
- Evaluate the effect of a 100 year wave on the risers. This was applied in two ways; first using the prescribed displacements provided by the structural analysis, and then including the structural member in the analysis instead.
- Local buckling check to DNV-OS-F101 part D. This used the results of the extreme wave analysis given above.
- Direct wave fatigue analysis using the FS2000 Off-Fat fatigue module.
- Vortex induced vibration (VIV) analysis of the risers and severed member, including a screening and a detailed VIV stress analysis.
The Benefits
Our analysis showed that the severed member did not result in buckling of the risers, and also that the severed member did not result in unacceptable fatigue usage of the riser.
However, the analysis did show the increased risk of VIV the severed member caused. Due to the large mass of the structural member, VIV of the risers could drive vibration of the member, causing a high fatigue usage of the risers.
Based on our recommendations, the client could repair the damage before the member severed. Due to the time sensitive nature of the problem, we were able to give the analysis their full attention, providing results and recommendations to a tight timescale.
