Final report for WP4.3: Enhancement of design methods ... - Upwind

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Figure 3.5: Time series of axial force Fx, shear force Fy and Fz of jacket sensor #1 (from top) for load case 1.1 (wind only) Figure 3.6: Time series of axial force Fx, shear force Fy and Fz of jacket sensor #1 (from top) for load case 1.2 (waves only) 20
4. Advanced modelling approaches There are a number of features unique to multi-member support structures which require enhancement of the current design tool capabilities and techniques. In particular the presence of complex joints needs to be taken into account. The most basic approach to modelling joints in a braced support structure is to include joint cans. Joint can properties differ from basic tube properties in terms of increased wall thicknesses. A preliminary study was performed using the UpWind reference jacket support structure with and without joint cans [54], to determine the influence of joint can modelling on the dynamic simulation of the structure. The mass difference due to joint can modeling was found to be approximately 1.7% with respect to the offshore wind turbine structure as a whole, and the differences concerning the first 18 natural frequencies were under 0.8%. These differences are relatively minor, and therefore it is concluded that the modelling of joint cans is not expected to lead to significant changes in the loads resulting from dynamic simulations. In the remainder of this chapter, an advanced approach to enable a more detailed modelling of joints in multi-member space-frame support structures is analysed, namely the super-element technique. 4.1 Super-element modelling in ADCoS-Offshore As mentioned in Section 2.3, branched support structures for offshore wind turbines (OWT) are basically modelled in ADCoS-Offshore as FE beam elements. These elements are clamped at the nodes, where the centerlines of the members intersect. This leads to a fairly good representation of the single tubular members of the structures, but to a less accurate representation of the joints themselves. Usually, the joint stiffness is overestimated with this approach. This is due to the fact that local joint flexibilities, resulting for example from local chord indentation next to the connection between chord and brace due to a bending moment on the brace, are not taken into account. This is a crucial issue as for numerous OWT structures, fatigue life is a design driver and the greatest calculated fatigue damage is found at the joints of the structures [56]. Furthermore, fatigue life prediction is heavily influenced by local joint flexibilities for comparable structures as described for example in [57]. Thus, further development of OWT design tools towards more detailed joint representation for simulation of turbines with braced support structures is an important development. In the offshore oil and gas industries, simplified beam models of complex support structures are used for load simulation as well. In those simulations, joints may be modeled with beam elements including local joint flexibilities using parametric formulae as developed in [58] for example. A more accurate approach is the so-called super-element or sub-structuring approach. Sub-structuring methods are applicable independent of joint type and dimensions and lead to a stiffness representation of the substructure that is as close to reality as detailed FE simulations with the shell or volume type elements that are used in more detailed models. Furthermore, these methods are implemented in commercially available general purpose FE tools such as ANSYS or MSC.NASTRAN as a standard advanced modeling feature. Sub-structuring methods are described in detail in the corresponding software documentation [59] and [60]. In the course of the UpWind project, a sub-structuring or super-element approach has been implemented in the ADCoS-Offshore code [14] to enhance joint modelling. An analysis using sub-structuring in ADCoS- Offshore is generally performed as described in the following. To facilitate comprehension, different examples are shown: • Detailed joint modeling in a general purpose FE tool like ANSYS or MSC NASTRAN. • Condensation of detailed joint using a reduction procedure such as Guyan reduction [61] or the Craig-Bampton method [62]. • Super-element inclusion in ADCoS-Offshore. • Inclusion of wave loads on the super-element. 21
Page 1 and 2: Project UpWind Contract No.: 019945
Page 3 and 4: Acknowledgement The presented work
Page 5 and 6: Summary The overall aim of the UpWi
Page 7: Table of Contents Acknowledgement..
Page 10 and 11: Task 4.1 Integration of support str
Page 12 and 13: Sequential coupling The sequential
Page 14 and 15: analysis. In the new multibody code
Page 16 and 17: Figure 2.4: 2nd global bending mode
Page 18 and 19: 3.2 Comparison of deterministic loa
Page 22 and 23: • Time domain simulation in ADCoS
Page 24 and 25: • A supplementary load vector for
Page 26 and 27: The following results are found: Fi
Page 28 and 29: Figure 4.5: Output positions in the
Page 30 and 31: It is directly visible that there i
Page 32 and 33: ated eigenfrequency is shifted into
Page 34 and 35: Water levels For the calculation of
Page 36 and 37: guidelines for the inclusion of cli
Page 38 and 39: damaging if the frequency of the ex
Page 40 and 41: uncertain parameters carefully. Thr
Page 42 and 43: Table 5.3 shows annual reliability
Page 44 and 45: Steel substructure for offshore win
Page 46 and 47: Table 5.16 shows the required FDF v
Page 48 and 49: A Fracture Mechanical (FM) modellin
Page 50 and 51: Figure 5.4: Annual reliability indi
Page 52 and 53: Further, the effect of possible ins
Page 54 and 55: Rainflow evaluation The load cases
Page 56 and 57: For DLC 7.2 it has to be considered
Page 58 and 59: Guideline, DLC 1.5). Furthermore, t
Page 60 and 61: Table 5.28: Output locations for da
Page 62 and 63: From these results it can be clearl
Page 64 and 65: Figure 5.11: Normalised DELs with v
Page 66 and 67: Figure 5.14 presents normalized DEL
Page 68 and 69: Figure 5.16: Output locations for e
Page 70 and 71:
UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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