1.4. Approach and methodology
Due to an extensive amount of simulations and large Finite Element (FE) models within this study, an efficient modelling approach will be adopted to approach this problem. This chapter describes the basis of this approach. Based on this, a methodology is presented in order to meet the objectives and answer the overall research question.
1.4.1. Modelling approach
In order to quantify the amount of EAC movement, the relative movement between the outer shell of the jacket leg and the inner shell of the foundation pile needs to be determined. This can be done by modelling the full jacket geometry to match the actual behaviour at sea. This includes a full geometric representation of the foundation piles including the pile-leg interface with a friction connection as visualised in Figure 1.11. The relative EAC movements can then be monitored by measuring the deflection between the outer shell of the jacket leg and the inner shell of the foun-dation pile. In order to accurately model the behaviour of the friction connection, the pile and leg need to be modelled using solid elements. The jacket can be modelled using beam elements. This model, as displayed in Figure 1.11, will be referred to as the full model. The only simplification be-tween the full model and reality is the linearization of the soil-pile interaction to an apparent fixity (AF) approach. The components in this model will be further explained in chapter 2 and chapter 3.
Figure 1.11: Full model. Figure 1.12: Global model (left) and detailed model (right).
Investigating this problem using a full model as displayed in Figure 1.11 has several downsides.
First of all, the computational time is significant due to the complexity of the model. With roughly 70000 elements and 175000 nodes, simulations are slow. Furthermore, due to the frictional interface between the pile and leg, an extensive amount of contact iterations needs to be performed per time step. Since the basis of this research will rely on performing a large amount of analysis on the pile-leg model, it is beneficial to reduce this simulation time.
Therefore, most simulations will be performed using two models as visualised in Figure 1.12. First of all, interface forces will be determined using a global jacket model constructed with beam ele-ments. After, a detailed study can be performed on a detailed pile-leg model constructed with solid elements. Since the number of detailed pile-leg models decreases from 4 to 1 when comparing Fig-ure 1.11 with FigFig-ure 1.12, computational time is reduced with almost 75%. Furthermore, one could
1.4. Approach and methodology 12 modify the detailed model while keeping the interface conditions from the global unchanged. Fig-ure 1.13 summarized this approach. In order to assess the uncertainty imposed by using two mod-els, a validation will be performed using the full model. This will be further discussed in section 2.4.
Figure 1.13: Modelling approach.
Model assumptions
Throughout this study, the assumption is made to not incorporate any grout stiffness into the model simulations. This assumption is based on the guideline of EAC as stated inDNVGL-ST-0126[17]. In-corporating grout stiffness is in reality difficult as its strength development is highly dependent on time and temperature in the first 24 hours. As discussed in Appendix A, the stiffness of grout is neg-ligible for the first hours of curing. Only for the last hours of curing, whenever the right temperature is reached, grout stiffness can become significant in the reduction of EAC movement. However, in-cluding this would be advised after performing small scale tests on the material. Otherwise, due to a lack of reliable data, this would impose a large uncertainty in the simulation. This would therefore be a recommendation for any further research.
Three main assumptions that will also be used throughout this study are:
1. Dividing the total system into a global model and detailed model (discussed in this subsection and section 2.4).
2. Linearizing the soil-pile interaction by an apparent fixity method (discussed in subsection 2.2.5.
3. Simulating for a an equivalent largest design wave to replace irregular wave conditions (dis-cussed in section 2.3 and subsection 3.2.3).
The uncertainty imposed by these assumptions will be discussed in the recommendations in sec-tion 5.2.
1.4. Approach and methodology 13 1.4.2. Research methodology
Figure 1.14 visualises the methodology of this research which will be used to meet the objectives as discussed in section 1.3. As can be observed, the total study will be divided into three phases. Each phase consists of a number of simulations indicated by the orange colour in the flowchart. The output of these simulations, which are movements, are indicated by the green colour. The input for these simulations, which are forces, are indicated by a blue colour. The methodology per phase will be discussed separately.
Phase 1
The main objective of phase 1 is to set up a global jacket model which can be used to determine the response for various load cases. Interface forces will be extracted from this model and used as an input for the reference model in phase 2. Phase 1 starts with the determination of the reference site as this partly results in the key geometry of the jacket structure. A global model will be made and simulated for a number of load cases. From this model, interface forces can be extracted to be used for phase 2.
Phase 2
The main objective of phase 2 is to set up a detailed reference model in order to quantify the EAC movements for the previous defined load cases. Furthermore, the objective is to gain insight into the behaviour of EAC movement for various loading conditions. Phase 2 starts with a detailed analysis of the pile-leg interface and a FE model set-up. Simultations will be run for the pile-leg model using the interface forces as determined in phase 1. The EAC results following from these simulations will be analysed. The load cases yielding the governing EAC movements will be used for the analysis of phase 3.
Phase 3
The main objective of phase 3 is to analyze the effect of a number of key stopper parameters on the behaviour of EAC movement. Phase 3 starts by investigating three configurations for the governing load cases as determined in phase 2. Each configuration will be analysed by performing an area and friction sensitivity study. Furthermore, each configuration will be tested for varying wave directions.
1.4. Approach and methodology 14
Figure 1.14: Research methodology.