Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Tim Oliver Janele (1) F P Marcus Klose (1) Susanne Landskroener (1)
(1) DNV GL, Hamburg, Germany
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Mr. Janele has recently finished his master's degree in naval architecture and ocean engineering at the technical university of Hamburg. He has been working for DNV GL – Energy, Renewables Certification, as a student assistant for one year. During his master thesis he investigated driving operations of monopiles with pre-fitted flanges.
Pile-driving analyses of monopiles with pre-fitted flanges
Efforts are being made in the offshore wind energy industry to avoid the highly sophisticated grouted connection between the monopile substructure and transition piece by using a steel-to-steel connection with bolted flanges. Therefore, a flange is welded to the monopile, and the pile-driving operation by means of a hydraulic hammer is directly performed on the flange surface. The bolted connection would reduce construction and installation costs, but risks concerning damages at the flange structure may occur. In this investigation a calculation model was developed to predict the damages induced to the flange during driving.
The main objective of the study was to determine the local stresses in the flange area under consideration of the essential pile-driving loads. The load spectrum at the top of the pile was initially estimated with a conventional one-dimensional wave equation analysis. Based on this, a detailed finite element model was developed considering the main components of the pile-driving configuration. In order to capture the dynamic behavior of the system, a nonlinear finite element analysis was performed in the time domain.
Main body of abstract
The pile performance during driving is generally predicted with a driveability analysis, in which the one-dimensional wave equation is solved for a simplified lumped-mass model of the pile and hammer. The soil resistance to driving is applied with springs and dampers considering soil parameters from cone penetration tests. However, this simplified one-dimensional approach is unsuitable for the prediction of local stresses at constructional details like the flange. In order to address this issue, a finite element model was developed that consists of essential hammer components and the detailed pile and flange geometry. The dynamic soil response to driving was considered by a system of vertical springs and dampers, so that the significant soil parameters could be directly adopted from the wave equation analysis. The validation of the finite element model against results from wave equation analyses showed that both approaches capture the same load spectrum for a single hammer strike. This includes the impact force induced by the initial hammer strike as well as the dynamic pile response that results in restrikes with the hammer system. The derived model was used to review a design proposal in detail. The principal risks and uncertainties involved with driving on flanges could be assessed and minimized by optimizing the flange geometry in comprehensive parameter studies. Fatigue damages at critical spots of the flange (like the flange weld) were calculated by multiplying the damage of a single hammer strike with the total number of required hammer blows (obtained from the wave equation analysis).
A reliable method for the detailed simulation of a pile-driving operation was derived in this study. Calculations have shown that driving on flanges is feasible with acceptable damages if certain conditions are met. For example, small deviations in the design of the flange or manufacturing tolerances may have a huge impact on the load distribution at the flange. Thus, it is strongly recommended to perform a detailed analysis of the driving operation in each individual case.
The present study shows how to combine the benefits of the common wave equation analysis (using a simple, but acknowledged soil model) with those of a detailed finite element analysis. The principal characteristics of the pile-driving loads at the flange are presented, and recommendations for the evaluation of ultimate loads and fatigue damages are given.