One of the key challenges for offshore pipeline installation are free spans, which are sections not supported by the seabed. Within these sections the stress in the pipeline is increased due to local buckling which is caused by high bending moments at free span shoulders and midspan. Additionally, hydrodynamic loading due to currents and waves, and vortex induced vibrations increase fatigue damage.

There are many solutions to mitigate free spans and their negative effects, like building supports underneath the pipeline, or reducing the span length by burying a section of the pipeline in the seabed at the shoulder. From a deficiency analysis it is concluded that there is a need for a new system concept especially for steep slopes. This new free span mitigation shall be suitable to be performed by Allseas, an offshore construction company, during pipeline installation with the S-lay method, without the need for subcontractors like dredging companies.

Oil and Gas pipelines which are installed onshore do not create free spans as they follow the topography by being bent. This makes route intervention less extensive as it is done offshore.

The standard DNV-ST-F101 which contains requirements, principles and acceptance criteria for submarine pipeline systems, allows for cold field bends for submarine pipelines as long as certain requirements are met. There are many ideas published to bend the submarine pipeline such that it follows the seabed topography like it is the case for onshore pipelines. These approaches have been either patented or presented as case studies but have not been used in projects, so far.

It has been estimated in some of these previous case studies that the bending moments at the shoulders are reduced and stresses in the bend are within an acceptable range. From the presented opportunities and the described need, it is concluded that it is reasonable to develop a feasible new system concept which satisfies the operational and functional requirements defined in this thesis.

From combining different solutions of common subfunctions a number of concepts have been found which have been analysed and narrowed down to one possible most promising design. Using this new tool which is used in combination with an AUXROV it is possible to bend one 12m joint of the size of 32’’ by 18.5°. With the given parameters of this design the free span length and height are narrowed down and the pipeline follows the topography when bent. It is verified that the bending moment at the free span shoulder is indeed reduced as presented in the literature.

The concept design, presented in this thesis, is at an early development stage but can serve as basis for detailed engineering, the next step in concept development. With a tool like this a new opportunity as standard solution for free span mitigation at steep slopes is introduced.