ABSTRACT This paper introduces the concept of Water Spider power device, a new floating wave energy system based on linear generator. The device contains horizontal cylinders arranged in a fan pattern, which look like the legs of a water spider. Each of the cylinders is equipped inside with a linear generator. During wave motions, some rotors of the linear generators slide forward and backward, thus generating electricity. The power-generating process reduces loss during energy conversion, and converts wave energy into the mechanical energy of the rotors efficiently. As a case study, an 8-leg conceptual design of the Water Spider device for a nearshore site in East China is presented. The energy potential of the conceptual design was approximately assessed based on observed wave data. The results indicate that, when the apparent power of a linear generator was 10 kW, the energy efficiency of the device was about 65.2% in terms of wave's kinetic energy. The Water Spider device shows advantages because of its simple structure, high efficiency, and easy installation and maintenance. As a small wave energy system, the Water Spider device may provide a promising way for small-scale nearshore wave energy development. INTRODUCTION Normally, a wave energy system converts the energy of wave motion into the high-speed rotation of generator. The energy conversion process can be generally divided into three steps, i.e., collecting wave energy, transmitting energy, and generating electricity by generator (Li et al., 2006). The second step is often achieved by hydraulic transmission or low-head turbines, and sometimes involves a series of procedures, such as energy transmission, voltage stabilization, speed stabilization, and energy storage. The procedures normally make a wave power device complicated, and increase energy loss, thus lowering the energy efficiency of the device. Additionally, large wave power systems, such as Archimedes Wave Swing (Czech et al., 2009) and Wave Dragon (Tedd et al., 2009), may need substantial construction efforts over water.

ABSTRACT The circumferential welds on steel tubular berthing monopiles, namely pile splices, are potential fatigue hot spots. This paper presents the latest progress in linear elastic fracture mechanics (LEFM)-based fatigue safety assessment for welded splices of steel berthing monopiles. Particularly, the fatigue assessment against vessel impacts is studied at the current stage. Practical methods for determining stress intensity factor (SIF) and stress concentration factor (SCF) at the circumferential welds given bending moments are reviewed and summarized. This paper also proposes a new modification factor for Paris law, as well as a Beta distribution for characterizing the hot-spot stress range caused by vessel impacts, which is often doubly bound by operational water levels. The findings of this paper provides practical information for performing LEFM-based fatigue safety assessment of splice welds of steel berthing monopiles. INTRODUCTION Steel marine piles often work against cyclic and harsh operational and environmental loadings. Due to welding flaws or defects (undercuts, cracks, incomplete penetration, and gas pores), circumferential welds on steel piles, also known as splice welds, transversal welds, or girth butt welds, have been recognized as potential fatigue hot spots, given the frequently repeated loadings. Insufficient fatigue strength of welded pile splices may cause severe structural failures (Dailey et al., 1987a, 1987b; Weidler et al., 1987). This paper studies the fatigue reliability of circumferential welds of steel berthing monopoles. Fatigue loadings for a berthing monopile include berthing impacts, wave effects, current oscillations, etc. In many cases, for example well-sheltered harbours, the berthing impact dominates as a major fatigue loading for splice welds. The berthing impact normally leads to substantial bending moments on a monopile. This paper focuses on the cases dominated by berthing impacts. Typically, hot-spot fatigue strength can be assessed with either S-N curves or linear elastic fracture mechanics (LEFM).