Evaluation of the Pendulous Installation Method in Ultradeep water

Abstract

The purpose of this thesis was to evaluate the use of the Pendulous Installation Method (PIM) for subsea equipment in water depths down to 4,000 meters. The basic concept of the PIM is to lower the payload in a pendulum trajectory rather than vertically. The idea behind the topic was to study what it would take for the oil & gas industry to start production in deeper water than what is possible today. The installation of necessary equipment was identified as an issue. The scope of the thesis was to describe the challenges of operations in ultradeep water and evaluate the PIM to see if it can be an alternative to conventional installation in 4,000 meters. This was accomplished by a literature study and use of numerical simulations in the SIMO software. A standard risk analysis was also carried out. The PIM was developed by Petrobras in the 2000’s. Their motivation for the development was the prospect of being able to install heavy equipment to ultradeep water without using specialized heavy lift vessels. This necessitated use of fibre rope rather than steel wires. Petrobras did not have access to field proven Fibre Rope Deployment Systems (FRDS), which are necessary to safely use fibre ropes for vertical installation. In the PIM the operation starts with the rope already paid out to full length, solving issues related to handling of the rope. It can thus deploy equipment without special rigging. The SIMO software allows for design of models that can be used for numerical simulation of marine operations. The models used in this thesis included the installation vessel and four different payload types. The equipment was coupled to the vessel by a lifting line. The models were limited to obtaining global results. In this thesis the lift line tension, lowering times and vertical and horizontal motions during the landing were studied. The characteristics of the results compared favourably to the results obtained by Petrobras during their numerical simulations and their full-scale model test. The results obtained in this thesis indicate that the PIM is not very sensitive to water depth. This is due to the use of a fibre rope that is weight neutral in water. The system also has a constant natural period through the lowering, meaning that there is less chance of resonance compared to conventional installation. None of the conditions studied in this thesis led to resonant behaviour, and the total tension in the lifting line never exceeded the minimum breaking load of the line. The lowering time was also significantly lower than the reference used for vertical installation. The landing process is not changed from conventional installation when applying the PIM, but the motions of the payload did only exceed the accept criteria in harsh wave conditions. The PIM is faster than conventional installation, and the highest tension in the simulations occurred after the pendulum trajectory was finished. When measures are implemented the risk in the operation is acceptable. The PIM allows for use of less technologically advanced vessels to lower heavy equipment to the seabed, which can reduce the installation cost. The conclusion of the thesis is that the method can be a good alternative to conventional installation in ultradeep water.