Monitoring concept for grout connections using the FINO3 platform as an example

Kiel University of Applied Sciences - Faculty of Media - Civil Engineering, Institute of Civil Engineering
Prof. Dr.-Ing. Stephan Görtz
Prof. Dr.-Ing. Peter Rozsar
Prof. Dr.-Ing. Kenji Reichling

The grout connection is the joint between the foundation structure and the rising support structure. High-strength mortar (grout) is used to create a force-fit connection between two steel pipes in offshore structures. The conditions for offshore installation are very difficult and the materials used are not unproblematic due to their technical characteristics in conjunction with the processing conditions. The condition of grout connections in offshore structures cannot currently be optimally monitored. The constant changes resulting from the dynamic loads caused by wind and waves are difficult to record.

The aim is to develop a concept for monitoring grout connections. The monitoring concept is based both on robust sensor technology with sufficient corrosion resistance against chlorides and on individually adapted warning levels for the load-bearing capacity assessment.

To ensure sufficient force transmission between the mono-pile and transition piece, the connecting mortar must have sufficient strength, rigidity and durability. Structural damage such as cracking, gravel pockets or high porosity, sometimes caused during production or use, can significantly reduce the strength and stiffness. Experience shows that both shrinkage cracks, which occur shortly after construction and are visible on the surface, and fatigue cracks inside the grout joint can occur.

Monitoring the grout structure is therefore an essential measure to ensure the load-bearing capacity of offshore platforms. However, there are currently no systems available that allow conclusions to be drawn about the residual load-bearing capacity of grout connections.

The Ministry for Energy Transition, Agriculture, Environment, Nature and Digitalisation of the State of Schleswig-Holstein is providing funding from the State Programme Economy 2021-2027.

Activities and goals

The aim of the measurements on the grout joint is to determine strains in the direct and indirect area of the grout joint in order to indirectly draw conclusions about the degree of damage (i.e. cracking). In addition to strain measurements on the mono-pile, the transition piece and the grout mortar, vibration measurements are also to be carried out on the entire structure. In addition, the sensory recording of crack width changes is to be tested and implemented if necessary. Based on the measurement results, conclusions are to be drawn about the force transmission of the grout connection. For this purpose, sensors must be attached primarily around the circumference of the transition piece (above the grout joint) so that conclusions can be drawn about the stress acting on the grout joint. In addition, sensors should be attached to the inside of the mono-pile at various points around the circumference at the height of the groove connection so that the strain increases and thus indirectly the shear force transmission can be inferred.

One concept envisages embedding parts of the measurement technology in a special mortar that has an extremely high resistance to chloride penetration. The measurement technology embedded in this way could be installed in the grout mortar after prior minimally invasive drilling or slotting. This would have the further advantage that the connection between the grout and the sensor could be produced under optimum conditions in the laboratory. Furthermore, the relative displacements between the mono-pile, transition piece and grout mortar should be recorded. Displacement transducers are particularly suitable for this purpose. As these must be protected from corrosive influences, appropriate protection systems must be determined and, if necessary, developed.