Impact of Water on the Fatigue Performance of Large-Scale Grouted Connection Tests

Abstract

Grouted connections represent a common joining technique between substructure and foundation piles of offshore oil & gas platforms as well as of offshore wind turbines. Due to cyclic loads arising from wind and wave actions the fatigue performance of the connection has to be considered. In lattice substructures like jackets the grouted connections are located at seabed level being fully submerged during their entire lifetime. Today’s fatigue design regulations are based on investigations neglecting any influence of the surrounding water since they were conducted in dry ambient conditions. So far, only Germanischer Lloyd gives additional recommendations for submerged grouted connections. At the Institute for Steel Construction, Leibniz Universität Hannover, Germany investigations of the joint research project ‘GROWup’ focus on the fatigue performance of axially loaded grouted connections. The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi, funding sign: 0325290) and is the third project in a row dealing with grouted connections. As part of this research project, cyclic loading tests on small-scale and large-scale grouted connections with shear keys are conducted. Small-scale fatigue tests showed a reduced number of endurable load cycles for connections when tested in wet ambient conditions. However, the transferability of these findings to a larger scale was still doubtful due to unknown scale effects. Therefore, the impact of water on the fatigue performance was tested recently at large-scale grouted connections. Previous to the submerged large-scale grouted connection fatigue tests, similar test specimens were exposed to alternating loads at dry ambient conditions. Comparison of both large-scale test results under wet and dry conditions enable to estimate the influence of water on the fatigue performance of grouted connections. Reflection of the small-scale test results gives hints on the scale effect. Test preparation, test results and design recommendations are presented in the paper.

Axially loaded grouted connections in offshore conditions using ordinary portland cement

“…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.…”

“…In dry ambient conditions the crushed material leads to a more homogenous load distribution at the shear keys and thereby a lower stress concentration. In connections with submerged ambient conditions (AC), water can enter the grout annulus by local cracks and bond loss gaps in interfaces [3]. Variable loading causes a pumping effect in cracks and interfaces.…”

Non-linear finite element analysis of grouted connections for offshore monopile wind turbines

“…Initially, to demonstrate the benefit in bending stiffness with increasing grouted lengths, a typical force-displacement curve of selected models is shown in The gap that develops in the steel grout interfaces due to bending is of considerable interest, as it leads to water ingress. This is because it results in reduced friction between steel and grout, which can 291 effectively disrupt the performance of the joint [26]. According to recent studies [27], GCs tested in dry environment have superior performance when compared to GCs in wet conditions.…”

Numerical Analysis of Shear Keys for Offshore Wind Turbine Monopile Grouted Connection with Elastomeric Bearings

“…In the case of the horizontal shear key, it resists an axial load, whereas, in the case of the vertical shear key, it resists the torque of the turbine system. In general, GCs have been designed in accordance with the international design standards (i.e., DNVGL-ST-0126 [9] and DIN EN ISO 19902 [10]); however, GC failure has been reported under ultimate load and fatigue load such as (i) the occurrence of contact destruction between tubular steel pipes and grout material due to the generation of an ultimate or cyclic load that exceeds the lateral stiffness of the friction contact, and (ii) the occurrence of material failure along the grout strut or around the shear keys by shear stresses that exceed the shear capacity of the grout [3]. Therefore, to overcome these problems, the design stage of GCs for monopile offshore structures should be specified in the design of the grout thickness, number of shear keys, shear key size, vertical distance of adjacent shear keys, and grout compressive and tensile strength to prevent material and contact failure of the grout against limit states such as the ultimate limit state (ULS) and fatigue limit state (FLS).…”