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Current installation costs of offshore wind turbines (OWTs) are high and profit margins in the offshore wind energy sector are low, it is thus necessary to develop installation methods that are more efficient and practical. This paper presents a numerical study (based on a global response analysis of marine operations) of a novel procedure for installing the tower and Rotor Nacelle Assemblies (RNAs) on bottom-fixed foundations of OWTs. The installation procedure is based on the inverted pendulum principle. A cargo barge is used to transport the OWT assembly in a horizontal position to the site, and a medium-size Heavy Lift Vessel (HLV) is then employed to lift and up-end the OWT assembly using a special upending frame. The main advantage of this novel procedure is that the need for a huge HLV (in terms of lifting height and capacity) is eliminated. This novel method requires that the cargo barge is in the leeward side of the HLV (which can be positioned with the best heading) during the entire installation. This is to benefit from shielding effects of the HLV on the motions of the cargo barge, so the foundations need to be installed with a specific heading based on wave direction statistics of the site and a typical installation season. Following a systematic approach based on numerical simulations of actual operations, potential critical installation activities, corresponding critical events, and limiting (response) parameters are identified. In addition, operational limits for some of the limiting parameters are established in terms of allowable limits of sea states. Following a preliminary assessment of these operational limits, the duration of the entire operation, the equipment used, and weather- and water depth-sensitivity, this novel procedure is demonstrated to be viable. © Springer Verlag. The final publication is available at https://link.springer.com/article/10.1007%2Fs11804-017-1418-6. This is the authors' accepted and refereed manuscript to the article.

A large earthquake (Mw 7.7) occurred on 16 April 2016 within the source region of the 1906 earthquake in the Ecuador-Colombia subduction zone. The 1906 event has been interpreted as a megathrust earthquake (Mw 8.8) that ruptured the source regions of smaller earthquakes in 1942, 1958, and 1979 in this subduction. Our seismic analysis indicated that the spatial distribution of the 2016 earthquake and its aftershocks correlated with patches of high interplate coupling strength and was similar to those of the 1942 earthquake and its aftershocks, suggesting that the 2016 and 1942 earthquakes ruptured the same asperity. Our analysis of tsunami waveforms of the 1906 event indicated Mw around 8.4 and showed that large slip occurred near the trench off the source regions of the above three historical and the 2016 earthquakes, suggesting that a depth-dependent complex rupture mode exists along this subduction zone.

This paper presents a systematic approach for designing the hydraulic mechanism as a part of the gripper system employed in offshore wind turbine monopile installation. Traditionally, such equipments used in marine operation are designed based on deterministic approach, selecting actuators and power pack by applying a safety margin which is not explicitly derived from a systematic load/load effect analysis, or a reliability based method. The method in this article offers a systematic way of designing the hydraulic power system, actuators and supporting structure to overcome extreme and fatigue loadings during operation. The design starts with a global analysis and modelling of monopile and installation vessel. The forces and motions from global analysis are then employed for designing hydraulic actuators and power system. A dynamic model of hydraulic system is built to analysis dynamic response in hydraulic system. The results from this local dynamic model can be used in power management and system optimization. The proposed method is a step forward to apply reliability-based design on mechanical components in marine applications through a systematic long-term load and load effect analysis. Copyright © 2018 by ASME

Jack-up barges are commonly used for marine operations in the offshore oil and gas, and offshore wind industries. A critical phase within the marine operation activities is the positioning of the jack-up legs onto the seabed. During this process, large impact velocities and forces may arise from the barge’s heave, roll and pitch motions, and structural damage of the legs can occur. This paper numerically investigates the effect of a flopper stopper (FS) on the motion responses of a jack-up barge from the offshore wind industry. The FS is known as a passive roll compensation device. It is suspended from the side of the barge by means of wire ropes and cantilever beams. A simple geometry of an FS is proposed, and the working principle introduced. For the loading condition before the leg-soil impact occurs, global dynamic analyses of the coupled system are conducted. Characteristic values of impact velocities are used to establish the jack-up operational limits in terms of the significant wave height and peak period. By comparing the operational limits for the barge with and without FS, it is found that FS should be placed on the weather side. At beam seas, the current FS can lead to a maximum increase in the operational wave height limit of 35%, whereas for the other wave headings, it may not be beneficial to use FS.Copyright © 2017 by ASME

This paper deals with a general methodology for assessment of the operational limits and the operability of marine operations during the planning phase with emphasis on offshore wind turbine (OWT) installation activities. A systematic approach based on operational procedures and numerical analyses is used to identify critical events and corresponding response parameters. Identifying them is important for taking mitigation actions by modifying the equipment and procedures. In the proposed methodology, the operational limits are established in terms of allowable limits of sea states. In addition, the operational limits of a complete marine operation are determined by taking into account several activities, their durations, continuity, and sequential execution. This methodology is demonstrated in a case study dealing with installation of an offshore wind turbine monopile (MP) and a transition piece (TP). The developed methodology is generic and applicable to any marine operation for which operational limits need to be established and used on-board as a basis for decision-making towards safe execution of operations. This is the authors' accepted and refereed manuscript to the article. Locked until 31 August 2018 due to copyright restrictions.

This paper presents a numerical study for preliminary assessment of the dynamic responses and allowable sea states for the installation of an offshore wind turbine (OWT) tower and rotor nacelle assembly (RNA) based on a novel method. This method is based on the inverted pendulum principle and consists of various sequential activities for which the allowable limits of sea states need to be established. For critical installation activities, numerical analyses methodologies have been applied to model the actual operations. For the parameters limiting the execution of the operations, response statistics are provided. It is found that at least 45 seeds are required to achieve convergence of snap force statistics during the OWT lift-off. The response statistics are used to calculate a characteristic value corresponding to a target probability of non-exceedance. For the lift-off and mating operations, these characteristic values are compared with the allowable limits of the response parameters to establish the allowable limits of sea states. In addition, sensitivity study on key modeling parameters are conducted. Spring coefficients of contact elements and hinged connections, winch speed, and hoist wire stiffness are shown to be important modeling parameters. The results provided in this paper are important for future finite element modeling (FEM) and cost-effective design of the structural components.