|dc.description.abstract||Subsea umbilicals and power cables are essential components in sea bed infrastructure, and necessary for control and monitoring of remote wells. By acting as a fixed link between a platform and a subsea wellhead, umbilicals make it possible to remotely provide hydraulic control, electrical power and chemical injection. Umbilicals are complex products and associated with various limiting criteria e.g. tension, compression and minimum bending radius. Hence, it is important to predict the behavior of the umbilical during the installation process by detailed analysis work.
This thesis presents a sensitivity study of three different methodologies for installation analysis. The work was carried out in collaboration with Subsea7. Two types of operations have been analyzed, normal lay and lowering of dynamic section. The scope of work was to find the most suitable methodology for installation analysis of the operations investigated. Conclusions were made based on relevant limiting criteria which were top tension, compression, minimum bending radius (MBR), tension in touch down point (TDP) and moonpool clearance. Analysis work was performed in SIMLA.
In the first methodology, installation analysis was performed with regular wave assumption. The methodology was most efficient with respect to preparations necessary in advance of the analysis work. However, regular waves did not provide a realistic representation of the sea behavior and overall results were found too conservative.
The second methodology was based on peak wave height in a three hour storm. The waves were created using a JONSWAP spectra. Characteristic wave trains were selected from multiple long term wave records. Peak wave height was identified and analysis performed using short simulations around the peak. For the responses to be statistically meaningful, peak wave heights were identified for each sea state, nine times, using different seed numbers. The seed number is what provides the randomness in irregular waves. The seed number providing median peak wave height was chosen for the short simulation.
The third methodology was based on peak vessel response in a three hour storm. For each sea state, the seed providing median wave height was chosen, and vessel response peaks identified. Relevant peaks for operations investigated in this work were maximum vessel acceleration as it was associated with maximum top tension and minimum vessel velocity as the peak was associated with minimum TDP tension and MBR.
Results confirmed the anticipated correlation between maximum vessel acceleration and maximum top tension as well as minimum vessel velocity and minimum TDP tension and MBR. Difference between the methodologies was largest where vessel response was largest. For Skandi Acergy, the installation vessel used for these operations, that is for small periods, 6.0 < T < 8.0 seconds, and large periods, T > 12.0 seconds. Given irregular wave assumption, the difference was mainly between methodology based on peak wave height and methodology based on peak vessel response, with methodology based on peak vessel response as the most conservative methodology.
Based on analysis results, it was recommended to perform installation analysis on short simulations at peak vessel response. Governing limiting criteria should be used to evaluate which response peak that is of main interest. The work confirmed a correlation between maximum acceleration and maximum top tension and minimum velocity and minimum TDP and MBR. However, further work should be carried out to confirm that simulations at minimum vessel acceleration provide minimum top tension and simulations at maximum vessel velocity provide maximum TDP tension.