Analysis and design of riser systems for permanently moored FPSOs/FSOs in shallow water are known to be extremely challenging. The complex nature of shallow water hydrodynamics and dynamic responses of the FPSO in extreme environmental conditions, limitations caused by water depth, and the close proximity of risers to each other and other structures, define significant technical and practical challenges for the riser designer. These challenges are typically magnified by conservative estimates of the global response of the FPSO system.
The new riser design codes tend to be vague about global riser analysis requirements and do not clearly define the global mooring analysis outputs, i.e. vessel offset and correlation between low-frequency offset and maximum wave height, to be used in riser detailed analysis. The codes commonly refer to the fully coupled mooring-riser system analysis for more realistic representation of the behavior of the system but do not provide guidance on the application of various offsets, associated directions, and environmental conditions. However, due to the large analysis matrix and level of numerical modeling required for detailed riser analysis, the fully coupled mooring-riser system analysis is not typically used by riser designers in the actual project phase. It is common to perform the riser analysis by assuming the vessel is at an extreme offset and performing dynamic analysis about that location with the maximum wave by using a time history of regular waves, or segments of random wave elevation time histories that contain the maximum wave. While this approach may not cause much difficulty for deepwater riser systems, it has a profound effect on the performance of riser systems in shallow water, leading to challenges in both the design and costs associated with such a system as it exaggerates the dynamic response of the floater, and the loading on the riser system.
In this paper, model test data and numerical time-domain simulations have been utilized to revisit the response of typical shallow-water FPSOs in extreme conditions. The main purpose here is to discuss the natural response of the system and highlight the correlation between the low-frequency and wave-frequency responses.