During a routine inspection of one of their risers, our client noticed that some of their riser guides were oversized, resulting in clearance between the riser and the inside of the guide. The riser had an outer diameter of 10.75 inches. The internal diameter of the guide was 14 inches.
The clearance between the riser and the guide allowed for a degree of riser movement within the guides under environmental loads. We were asked to investigate the potential impact of the oversized guides on the fitness for purpose of the riser.
We undertook an analysis of the riser using the OrcaFlex software. This was capable of performing a dynamic and non-linear analysis of the problem, allowing for a time based analysis which accurately modelled the movement and impact of the riser.
The analysis included an extreme wave analysis, to investigate the integrity of the riser under storm wave conditions, and a wave induced fatigue analysis, in order to understand the effect of the riser movement on the fatigue life of the riser. The fatigue analysis was completed in accordance with DNV-RP-C203 using the OrcaFlex wave summing tool.
An analysis of the effect of the clearance on the vortex induced vibration (VIV) of the system was also assessed, by comparing the magnitude of the clearance with the displacement predicted during VIV under local environmental conditions.
The analysis showed that, whilst peak loads in the riser increased when compared with the nominal (as designed) case, the increase in loads did not represent a threat to the fitness for purpose of the riser, and did not significantly shorten the fatigue life of the riser. The loads on the riser guides were also found to be within the design loads of the guides.
The VIV analysis also showed that the potential for VIV was not changed by the riser clearance, and so the original riser VIV assessment was still valid.
We were able to provide a quick and accurate analysis of the system, due to a wealth of experience in the design and analysis of risers and other subsea structures, and analysis and modelling expertise. This gave the client the confidence to continue operating their system without the need for expensive remedial work.