Grouted connections (GCs) consist of two infiltrating steel tubes of different diameters and a grout material bonding the two steel tubes together. Predominantly, the GCs are used to connect the foundation piles with the jacket support structure. Additionally, it can be applied for monopile substructures. A challenging and scientifically still marginally analysed detail of grouted connections is the influence of early age movement (EAM), also known as early age cycling (EAC) on the final load bearing behaviour of the connection. After grouting and until the end of the curing of the material, the offshore structure is partially supported by fluid grout material for several hours. During this time waves and current affect the soft connections. This leads to relative movement during the curing of GCs, which is known as EAM. Previous investigations showed grout material damage due to vertical EAM. These results form the basis of the current offshore guideline limits. According to DNVGL-OS-J101 from 2014, it must be ensured that during the curing process the vertical EAM does not exceed 1 mm. Since 2016 the limit value also applies for the lateral EAM according to DNVGL. However, the influence of lateral EAM on the load bearing behaviour is barely experimentally nor numerically investigated. Thus, the authors are involved in a comprehensive research project (GREAM) addressing this topic. This paper focusses on Finite Element Analysis of different support structures under varying boundary conditions and sea states to analyse the amplitude of lateral EAM of grouted connections.

A grouted connection (GC) is a hybrid connection which joins two telescoped steel tubes by filling the annulus between the steel tubes with grout. GCs are frequently used to enable a force fitted connection between piles and substructure of offshore wind turbines. At latticed substructures this connection is located at mudline level in wet ambient conditions (AC). Nowadays special grout materials are used to achieve not only best mechanical properties but also a good performance during grouting in offshore conditions. To reduce production costs the use of ordinary portland cement (OPC) is investigated as an alternative filling material within this paper. OPC has a much lower tendency to segregate, as there are no aggregates added. This leads to more simplified, stable and cheaper production processes offshore. Further focus is put on the failure mode of OPC filled GCs in submerged condtions. For an appropriate use of OPC offshore a feasible mechanical performance needs to be ensured. Investigating this, small and large-scale laboratory tests were performed at Leibniz Universität Hannover. Using the experimental test setup of previous investigations for special high performance grouts (HPG) [1,2], enables a direct comparison of HPG and OPC. Documenting liquid and solid OPC properties, like slump flow and compressive strength confirms a stable material quality. Small-scale ULS-tests showed significantly lower ULS-capacities and a more brittle failing process compared to HPG. Lagre-scale tests confirmed the observed failure mechanisms of Schaumann and Raba for OPC filled GCs in submerged conditions [3]. Carried out tests showed significant influence of grout material and confirmed influence of grout annulus size on fatigue capacity.

Grouted connections (GC) of offshore structures are susceptible to wave loading and current directly after grouting. Early age movement (EAM) during the curing process can affect the material properties and the interface quality between the grout and the steel structure. Such effects were observed in laboratory tests performed at the Leibniz University Hannover. Generally, EAM can be characterised by high shear rates and identified as a damage mechanism. In this contribution, the authors present a numerical approach that transfers the observed damage mechanism into an ultimate strength analyses of the GCs. For this purpose, EAM simulations and laboratory tests with fluid grout material are combined to determine areas of high shear rates, which mainly affect the grout properties. With an element‐based adjustment approach, the material law of each element is adjusted according to the level of shear rate. The adjusted finite element model is analysed on its maximum axial load resistance.