Offshore Substructures Research Articles

Conceptual Design of Offshore Jacket Substructures Using Machine Learning

The offshore wind sector is mature and has led to standardized design methods for offshore substructures. The conceptual design phase is critical for efficiency and cost-effectiveness and forms the basis for further design iterations. As turbine capacity increases, so does the complexity of offshore substructures, making design more challenging. However, the design process still relies on the expertise of the design engineers. These engineers rely heavily on their experience and intuition when designing, which can lead to biases due to limited information. To address this problem, Machine Learning (ML) techniques offer a promising way to improve the accuracy and efficiency of the conceptual design of offshore substructures. The current study is limited to the conceptual design of jacket substructures and was conducted on a self-developed global dataset of real jackets. The ML-based approach proposed in this study is capable of learning from existing data, recognizing intricate relationships between design variables, and potentially providing more accurate estimates for the initial conceptual design of offshore jacket substructures.

A comparative study of laser-arc hybrid welding with arc welding for fabrication of offshore substructures

Laser-arc hybrid welding (LAHW) is an efficient and promising joining method for offshore structures made from metallic alloys. Under a high-power laser output, a keyhole is formed that efficiently melts a metal and provides high penetration depths compared to shallow and wide welds made by conventional arc welding, thus it requires much less time for production. A significant reduction in welding time leads to a substantial reduction of welding consumables and electricity use. Higher gap bridging ability can be achieved and tolerances for plate preparation is reduced when an arc is added to a laser beam. This work explicitly presents and compares different welding methods and their efficiency in application for offshore windmill substructures. According to a preliminary calculation, LAHW may reduce welding time by 10–20 times depending on process parameters and up to 15 times lower consumables usage. It was identified that the highest costs in welding is the filler wire followed by a shielding gas and electricity. This leads to high efficiency of LAHW in term of sustainability and reduction of CO2 emissions in general compared to well-established arc welding. However, the narrow and deep welds made by LAHW may suffer from high hardness in the root area, inhomogeneous filler metal mixing, which may lead to poor microstructure, cracking and porosity. This requires an extensive research work on the process optimization and understanding the underlying physics to provide sound welds which must comply with the international standards.

A comparison study of power performance and extreme load effects of large 10‐MW offshore wind turbines

AbstractThe utilisation of offshore wind turbines has rapidly increased in the last decade, which has resulted in a steady increase in wind turbine sizes. The global average offshore wind turbine size has increased from 1.5 MW to 6 MW in the last two decades. The research community has started to investigate huge 10 to 15 MW offshore wind turbines in recent years, resulting in the study of very innovative floating wind turbines using various substructure technologies. With this backdrop, this paper will investigate and thoroughly compare the power performance of extreme load effects of a large offshore 10 MW turbine installed on the monopile, spar, and semisubmersible substructures. This is performed by using the average conditional exceedance rate (ACER) and Gumbel methods to predict the extreme responses under the operating conditions of 8, 12, and 16 m/s mean wind speed, representing the below‐rated, rated, and above‐rated regions, respectively. The results show that the power performance and extreme loads experienced depends significantly on the operating regions. The mean power generation between the three different types of offshore wind turbines (OWTs) are closely in the whole operating range, which standard deviations differ significantly. Large standard deviations of power generation appear in the spar turbine under the below‐rated condition. Further, it was observed that the spar wind turbine generally experiences larger extreme loads due to larger platform pitch motion. In addition, the ACER method shows a better prediction for the 1, 2 and 5‐year extreme responses than the Gumbel method, which is due to the relatively poor data fitting of the Gumbel method at the upper tail. The study is believed to consolidate and close the knowledge gap in understanding wind turbine responses across the most common offshore substructure technologies and provide a basis for design and deployment of OWTs.

Wind Turbine Technology Trends

The rise in prices of traditional energy sources, the high dependence of many countries on their import, and the associated need for security of supply have led to large investments in new capacity of wind power plants. Although wind power generation is a mature technology and levelized cost of electricity low, there is still room for its improvement. A review of available literature has indicated that wind turbine development in the coming decade will be based on upscaling wind turbines and minor design improvements. These include further improvements in rotor blade aerodynamics, active control of the rotor blade rotation system, and aerodynamic brakes that will lead to increased power generation efficiency. Improvements in system maintenance and early diagnosis of transmission and power-related faults and blade surface damage will reduce wind turbine downtime and increase system reliability and availability. The manufacture of wind turbines with larger dimensions presents problems of transportation and assembly, which are being addressed by manufacturing the blades from segments. Numerical analysis is increasingly being used both in wind turbine efficiency analysis and in stress and vibration analysis. Direct drive is becoming more competitive with traditional power transmission through a gearbox. The trend in offshore wind farms is to increase the size of wind turbines and to place them farther from the coast and in deeper water, which requires new forms of floating foundations. Due to the different work requirements and more difficult conditions of the marine environment, optimization methods for the construction of offshore substructures are currently being developed. There are plans to use 66-kV cables for power transmission from offshore wind farms instead of the current 33-kV cables. Offshore wind farms can play an important role in the transition to a hydrogen economy. In this context, significant capacity is planned for the production of “green” hydrogen by electrolysis from water. First-generation wind turbines are nearing the end of their service life, so strategies are being developed to repower them, extend their life or dismantle and recycle them.

3-D numerical study of offshore tripod wind turbine pile foundation on wave-induced seabed response

Investigation analysis of wave-seabed-structure interaction (WSSI) is a pre-requisite in every marine engineering design and construction due to the huge failure of offshore structures every year, resulting in financial losses. In this study, an integrated 3-D numerical model is established to investigate the wave-induced oscillatory seabed response around the offshore substructure foundation of the tripod support pile. In the integrated numerical model, the Reynolds-Average Navier-Stokes (RANS) equations with k−ε turbulence closure model for the mean fluid flow is applied as the governed equation for the fluid computation, while the Biot consolidation equations are applied as the governing equation for the porous seabed substructure foundation model. The integrated numerical model is validated against physical experimental data from other previous works; to demonstrate that the present numerical model has the capability and capacity of simulating the WSSI around the offshore tripod pile structure foundation, which indicated good results. The results from the wave-induced liquefaction around the tripod support pile show that liquefaction depth at the offshore area and the lateral area of the upstream axillary pile leg tends to be larger than the two axillary pile legs at the lee area.

FLOTANT concept: floater design, integrated modelling & global performance

Abstract This paper presents the final design of the FLOTANT concept developed within the framework of a Cooperation Research Project funded by the European Union’s Horizon 2020 research and innovation programme. The FLOTANT concept is a hybrid concrete-plastic barge-type floating offshore substructure holding a 12MW wind turbine; buoyancy is achieved using plastic material (foam) fitted within the floater substructure. The paper describes the FLOTANT project innovations and presents the principal dimensions of the floating wind turbine system. The floating system has been developed to fulfil strict requirements at two different locations: Gran Canarias (GC) and West of Barra (WoB) sites. Therefore, two different mooring solutions including innovative elastomers to reduce peak loads are presented. The floating system’s intact and damaged static stability is checked along with the hydrostatic and hydrodynamic properties, showing the low-cost barge-type system’s feasibility. To ensure the desired behaviour of the floating system, a relevant subset of design load cases (DLCs) based on international standard requirements have been simulated checking the global performance of floater and moorings. Fully coupled hydro-servo-aero-elastic simulations have been performed to consider the wind and wave loading, the aerodynamic response of the rotor, the hydrodynamic response and structural dynamics of the turbine-floater-mooring assembly, and the control system actions. A summary of the results of the last design loop within the FLOTANT project is presented.

Study of typhoon impacts on the foundation design of offshore wind turbines in Taiwan

Taiwan is hit by an annual average of three to four typhoons with different intensities and routes. To consider the impacts of typhoon events on offshore substructure design, the historical metocean data for storm event-based frequency analysis are generally applied. However, whether a supertyphoon passes directly over an offshore wind farm has a great influence on the results of frequency analysis. Therefore, the American Bureau of Shipping has recently increased the return period of hydrodynamic loads from 50 to 100 years to address the higher variation in the severity of extreme storms in typhoon-prone offshore regions. In this paper, to assess the impacts of the increase in the return period (from 50 to 100 years) on the design of offshore substructures, case studies of different types of substructures (i.e. monopile, three-leg jacket and four-leg jacket) with different water depths were conducted for the offshore wind farms off the coast of Changhua County in central Taiwan. The quantitative differences in foundation design are presented by way of a comparison of substructure weights, overturning moments and pile lengths. The preliminary results of the case studies show that the monopile design is more obviously dominated by earthquakes than by typhoons and the jacket-type foundation is more significantly dominated by typhoons.

Tripod-Supported Offshore Wind Turbines: Modal and Coupled Analysis and a Parametric Study Using X-SEA and FAST

This paper presents theoretical aspects and an extensive numerical study of the coupled analysis of tripod support structures for offshore wind turbines (OWTs) by using X-SEA and FAST v8 programs. In a number of site conditions such as extreme and longer period waves, fast installation, and lighter foundations, tripod structures are more advantageous than monopile and jacket structures. In the implemented dynamic coupled analysis, the sub-structural module in FAST was replaced by the X-SEA offshore substructure analysis component. The time-histories of the reaction forces and the turbine loads were then calculated. The results obtained from X-SEA and from FAST were in good agreement. The pile-soil-structure interaction (PSSI) was included for reliable evaluation of OWT structural systems. The superelement concept was introduced to reduce the computational time. Modal, coupled and uncoupled analyses of the NREL 5MW OWT-tripod support structure including PSSI were carried out and the discussions on the natural frequencies, mode shapes and resulted displacements are presented. Compared to the uncoupled models, the physical interaction between the tower and the support structure in the coupled models resulted in smaller responses. Compared to the fixed support structures, i.e., when PSSI is not included, the piled-support structure has lower natural frequencies and larger responses attributed to its actual flexibility. The models using pile superelements are computationally efficient and give results that are identical to the common finite element models.

Coupled Analysis of Offshore Wind Turbine Jacket Structures with Pile-Soil-Structure Interaction Using FAST v8 and X-SEA

The coupled analysis between a turbine in operating condition and a complex jacket support structure was formulated in this paper for the reliable evaluation of offshore wind turbine structures including pile-soil-structure interactions (PSSIs). Discussions on the theoretical and simulation aspects of the coupled analysis are presented. The dynamic coupled analysis was implemented in X-SEA program and validated with FAST v8 (fatigue, aerodynamics, structures and turbulence) developed by NREL, USA. By replacing the sub-structural module in the FAST with the component of offshore substructure in the X-SEA, the reaction forces and the turbine loads were calculated in each time step and the results from X-SEA were compared with that from FAST. It showed very good agreement with each other. A case study of a NREL 5MW offshore wind turbine on a jacket support structure was performed. Coupled dynamic analyses of offshore wind turbine and support structures with PSSI were carried out. The results showed that in the coupled analysis, the responses of the structure are significantly less than in the uncoupled analysis. The support structure considering PSSI exhibited decreased natural frequencies and more flexible responses compared to the fixed-support structure. The implemented coupled analysis including PSSI was shown to be more accurate and computationally efficient.

Numerical simulation of wave and current interaction with a fixed offshore substructure

Offshore substructures have been developed to support structures against complex offshore environments. The load at offshore substructures is dominated by waves, and deformation of waves caused by interactions with the current is an important phenomena. Wave load simulation of fixed offshore substructures in waves with the presence of uniform current was carried out by numerical wave tank technique using the commercial software, FLUENT. The continuity and Navier–Stokes equations were applied as the governing equations for incompressible fluid motion, and numerical wavemaker was employed to reproduce offshore wave environment. Convergence test against grids number was carried out to investigate grid dependency and optimized conditions for numerical wave generation were derived including investigation of the damping effect against length of the damping domain. Numerical simulation of wave and current interactions with fixed offshore substructure was carried out by computational fluid dynamics, and comparison with other experiments and simulations results was conducted.