The challenges in the installation of subsea structures are increasing very rapidly in the recent years. Larger and heavier subsea structures are being installed in harsh environments and in deep waters. Sophisticated lifting analysis is needed to establish safe installation criteria. Also, novel installation methods need to be devised to install heavy structures using small vessels.
In this paper, state-of-the-art installation analysis based on DnV-RP-H103 with a new approach to model the complex hydrodynamic loads at splash zone is described. Through recent installation work, this procedure is exemplified and the lessons learnt from the installation of subsea structures and long spools are discussed. A novel method by sub-surface tow, to install heavy subsea structure using a small construction vessel is out lined and the lessons learnt from installation of four structures are discussed.
The project experience showed that DNV-RP-H103 provides an excellent basis for the lifting analysis. In the recent North Sea installation campaign, by applying the new approach, a safe installation was performed at higher sea state. The sub-surface tow method proposed is robust and safe method to install heavy structures using small construction vessels and the lessons learnt from these installation campaigns clearly indicate that this method is easy, safe and robust in harsh environment.