A Tool for Modelling Installation Analysis of a Deepwater Suction Pile

A suction pile is a thin-walled steel cylinder used as mooring anchors and foundations for subsea equipment. The normal operation for the installation of the pile is using a floating crane vessel that carries it on deck to target location and then lowers it to the seabed, using the crane. The pile penetrates the soil in two stages, firstly by self-weight with a vent cap opened on the top. Once it comes to rest under its own weight, the second stage in which the top cap is sealed and the pressure inside is slowly lowered by a Remotely Operated Vehicle (ROV).

Due to the large size and weight of the piles, it brings critical scenarios during the installation process. Numerical analyses to assess the loads on the structures and vessel crane and to assess the coupled motions between payload and vessel are important to enable the methodology and used equipment for the proper installation of the structure.

This paper describes a direct time domain modelling procedure using OrcaFlex software and numerical findings of the pile installation for the following operation phases: overboarding, lowering through splash zone and finally landing into the target box on the seabed. The modelling strategy focus on the proper calculation of the hydrodynamic forces and environmental factors. The model includes the vessel, the crane wire, the slings, and the pile system and is subjected to the vessel motions, the coupling between structure and vessel and the pile flooding. Moreover, the statistical evaluation of the dynamic responses from different wave realizations is used to estimate the extreme responses of various sea states. Analysing the responses and following all the criteria from DNV standards, for a secure installation, it is possible to provide the allowable sea state for each operation.

It is presented the most updated methodology we have for the installation analysis and shown an in-house tool we developed for the modelling of the cases to be analysed. This software allows the best solution for the suction pile installation analysis as it may be faster, standardized, and a practical way to get results when already in operation.

Furthermore, the influence of coupled dynamics of the vessel and the lifted pile, shielding effects, fin plates, crane payout modelling, and line torsion under hydrodynamic responses and the allowable sea states are analysed.