Comparison of Umbilical Installation Analysis Using Two Irregular Wave Spectra
Authors: Felisita Airindy , Gudmestad Ove Tobias , Martinsen Lars Olav 
Published: January 2011

Umbilicals are widely used to provide monitoring and control functions for distant satellite wellheads in subsea developments. Detailed analysis is required to predict the behavior of the umbilical during the installation process. Finite Element Analysis is performed to determine the limiting operational sea-state for the installation operations. This paper presents work carried out with Acergy Norway AS, comparing two irregular wave spectra that are commonly used for installation analysis in the North Sea. The purpose of the study is to select the most suitable method for the installation analysis with respect to different types of installation operations. Selection is carried out based on several limiting criteria, including top tension, compression, minimum bending radius and the tension at the touchdown point. The JONSWAP spectrum and Torsethaugen spectrum are used to model the physical environment of the North Sea. Both of these spectra are especially designed for North Sea environment. The JONSWAP spectrum represents the fetch-limited (or coastal) wind generated seas, meanwhile the Torsethaugen spectrum represents the wave conditions in open ocean areas where the waves are dominated not only by local wind seas but are also exposed to swells (Torsethaugen and Haver, 2004). Two types of installation operations are selected for this work, which are the “buoyancy overboarding” for dynamic umbilical installation analysis and “normal lay” for static umbilical installation analysis. The analysis shows that the Torsethaugen spectrum generates higher tension forces compared to the JONSWAP spectrum. On the other hand, the JONSWAP spectrum generates higher compression and lower bending radius. However, the differences between the results using these two spectra are not large. This is due to the limited wave height and period applicable for installation operations. This work only covers wave height of Hs = 2.5–4.0 m and wave period of Tp = 6–14 s. The selection of these low conditions is based on typical sea states for installation operation (without consideration of survival conditions). Since there is only small variation on the results from the two spectra, both JONSWAP and Torstehaugen spectra are considered suitable for analysis of installation operations. It is further noted that although the Torstehaugen spectra will often provide a more realistic representation of the physical environment, forecast or measured weather data is rarely presented in this form, therefore the JONSWAP or other single peak spectra must be used for decision making offshore. This study has validated that for the range of condition studied, this is an acceptable approach. The conclusion from this study is only applicable for low sea-states and without considering effects from different direction of wind seas and swells components in the Torsethaugen spectrum. Therefore further work is required to fully asses the impact of directionality between the wave components and the impact of higher sea states which are applicable to survival conditions.