Study on hot spot stress distribution of three-planar tubular Y-joints subjected to in-plane bending moment

The three-planar tubular Y-joint (3Y-joint) is the main part of the fatigue assessment of tripod substructures of offshore wind turbines (OWTs). As typical multiplanar tubular joints, 3Y-joints are affected a lot by multiplanar interaction between braces. Moreover, the locations of hot spot stress (HSS) can vary considerably under different load types. Thus, the distributions of stress concentration factor (SCF) and multiplanar interaction factor (MIF) along weld toe curves are necessary to calculate HSS. Considering these requirements, this study focuses on the 3Y-joint considering the wide application of the tripod substructure of OWT. A finite element (FE) analysis method is introduced and validated. Then, a numerical database is established covering common ranges of parameters used in practice. The SCF and MIF of 3Y-joint under in-plane bending moment are analyzed. Distribution formulas are proposed and proved suitable for calculating HSS in engineering design.     

Design and model test of a soft-connected lattice-structured floating solar photovoltaic concept for harsh offshore conditions

Various types of floating solar photovoltaic (FPV) devices have been previously proposed, designed and constructed with applications primarily limited to onshore water bodies or near-shore regions with benign environmental conditions. This paper proposes a novel FPV concept which can survive harsh environmental conditions with extreme wave heights above 10 m. This concept uses standardised lightweight semi-submersible floats made of circular materials as individual modules. The floating modules are soft connected with ropes to form an FPV array. We first present the conceptual design of the floats and the connection systems, including hydrostatic, hydrodynamic, and structural assessments of the floats. To verify the motion response performance, we carried out 1:60 scaled model tests for a 2 by 3 array under regular and irregular wave conditions. From the time series and response spectra, the motion characteristics of the array and the mooring responses are analysed in detail. The proposed concept exhibits excellent performances in terms of modular motions with limited wave overtopping and no contact is observed between adjacent modules under the extreme wave conditions. The findings of this study can serve as a valuable reference to developing reliable and cost-effective FPV technologies for offshore conditions.     

A review of physical flexible ship models used for hydroelastic experiments

Hydroelasticity of ships has been established as a necessary form of investigation for both slender ships and high-speed craft. Experimental investigations have spanned various topics, including symmetric and antisymmetric, harmonic and transient, linear and nonlinear responses. Models have varied in size and the way the structure is modelled, depending of the focus of the investigation. The multitude of interacting physical mechanisms introduce almost-impossible-to-resolve scaling issues, and the eventual compromises depend on the aim of the investigator. This publication provides a comprehensive review of the evolution of these experimental techniques, from the first appearances of the field to the modern state-of-the-art and potential future directions.     

基于2阶HMM的智能车视觉地图定位方法

本文中针对视觉地图匹配问题，将视觉地图匹配问题转化为基于图像序列的最优视觉地图节点匹配问题，并提出基于2阶隐马尔科夫模型（hidden Markov model,HMM）的视觉地图匹配方法。在该模型中，状态变量被定义为高精度视觉地图节点，查询图像被定义为观测数据。在状态转移模型中，引入2阶模型对短时间车辆运动进行匀速运动建模，与传统的1阶HMM相比，可以提高模型的适用性与准确性。提出利用全局图像特征建立查询图像与地图节点之间的匹配关系，并从匹配的汉明距离建立发射概率模型，可有效提高地图匹配的效率。最后，通过前向算法来求解最优匹配的地图节点。为了验证算法的性能，分别在封闭工业园区、开放道路和KITTI公开数据集对算法进行验证。实验结果表明：2阶HMM模型能够有效融合车辆运动信息和图像信息，提高匹配的稳定性和精确度，算法性能明显优于传统的基于单帧匹配和序列匹配算法。     

Dynamic response of a SWATH vessel for installing pre-assembled floating wind turbines

New and efficient installation concepts which can reduce the cost of developing an offshore wind farm are of particular interest. This paper explores a promising concept using the small water-plane area twin-hull vessel (SWATH) to install pre-assembled wind turbines (OWT) onto floating spar foundations. A focus is placed on the hydrodynamic performance of the SWATH and the response analysis of the coupled SWATH-spar system. Firstly, the numerically calculated difference-frequency wave force effect and damping forces of the original SWATH were verified with experimental data. Secondly, the original SWATH was modified to satisfy the criteria of weight-carrying capacity and hydrostatic stability. Thirdly, a multibody numerical model for the SWATH-spar system was developed, in which the hydrodynamic and mechanical couplings between the SWATH and a spar were considered. The SWATH is equipped with a dynamic positioning system to counteract the slow-drift wave force effects. The nonlinear time-domain simulations were carried out for the mating stage when a wind turbine is lifted above the spar foundation. Based on the analysis of statistics of the relative displacement and velocity of the tower bottom and the spar top, the installation concept with SWATH is found to be of decent performance. Finally, recommendations are provided for future research on this concept, which contributes to developing next-generation installation concepts for bottom-fixed and floating wind farms.     

Stress concentration factors in FRP-reinforced tubular DKT joints under axial loads

The stress concentration factors (SCFs) in uniplanar fibre-reinforced polymer (FRP) DKT joints are calculated under five axial loading conditions to determine the maximum SCFs. To this end, 108 finite element models of reinforced DKT joints with different FRPs and geometrical parameters are analysed. Available experimental data and formulas are used to validate the finite element models. The validated finite element models are utilized to investigate the effects of the FRP parameters along with different geometrical parameters on the stress concentration factors in uniplanar DKT joints. The simulations show a reduction of the maximum SCF by around 40% compared to unreinforced DKT joints. The reduction effect increases significantly with increasing the FRP thickness and the number of layers. Despite the notable efficacy of the FRP sheets on the drop of the SCFs in the X-connections, there is not any study or equation on the X-joints with FRP. Therefore, a precise equation is proposed for quantifying the SCFs in X-connections with FRP and is checked against the UK DoE acceptance standard.     

Validation of earthquake analysis methodology of a suction-caisson foundation-structure through model testing

Suction caissons are one of the most widely used foundation solutions for subsea structures and wind farms. Seismic response of subsea structures is however seldom documented properly, often just treated as a foundation capacity issue applying a quasi-static acceleration and not considering the inertial interaction between the structure and the soil. The more relevant tasks to document are the motions of the unit and the response of the externally connected flowlines and equipment/systems on the unit.Based on a case study located in the Shah Deniz field in the Caspian Sea, model centrifuge tests and numerical modelling were carried out to validate the global response of a 4-caisson supported manifold structure subject to seismic motions in soft clay. The centrifuge tests were carried out at 58 g at the centre for geotechnical modelling at UC Davis. To simulate the soil-structure interaction, a series of non-linear springs defined by kinematic hardening models were used in analyses with the ABAQUS software. This development includes the algorithms for determining the required model parameters. A very good agreement between recorded response from the centrifuge test and calculated response from the FE-analyses was achieved.The development and validation of the soil model presented in this paper is an improvement in design methodology for caisson foundations subjected to earthquake loading. The non-linear soil springs are well suited to incorporate in more detailed structural analyses where an accurate representation of the foundation response is required. The paper also briefly describes how the subsequent earthquake design analyses were performed for the Shah Deniz manifold structures making use of the validated soil spring model and the added value it gave to the project.     

浮式平台上部模块非管节点SCF控制方法

应力集中系数（Stress Concentration Factor，SCF）是评估节点疲劳损伤的重要参数。针对浮式平台上部模块非管节点的SCF控制，设计5种加强形式，采用Ansys软件对非管节点进行SCF分析。结果表明：筋板加强可有效改善面外弯矩作用下的应力集中；扣管加强对轴向力、面内弯矩和面外弯矩均具有较好的改善效果。     

Review of the uncertainties associated to hull girder hydroelastic response and wave load predictions

This paper reviews the importance of uncertainties in hull girder loads influenced by flexible fluid structure interactions. The focus is on developments in the field of hydroelastic modelling, simulation and model tests of practical relevance to the prediction hull girder wave load predictions and their validation. It is concluded that whereas hydroelastic methods for use in design development and assessment become increasingly useful, challenges in realizing and modelling uncertainties can be attributed to: (1) the limitations of numerical methods to suitably model nonlinearities; (2) the ambiguity of model tests; and (3) the systematic use of data emerging from computational, model- or full-scale methods. An approach is recommended to assess the uncertainty in the hydroelastic responses to wave loading and an example is provided to demonstrate the application of the procedure.