Fatigue crack growth rates for offshore wind monopile weldments in air and seawater: SLIC inter-laboratory test results

The majority of fatigue crack growth (FCG) data sets available on steels in air and seawater environments are several decades old and may not be appropriate for structural integrity assessment of offshore wind turbine foundations, which are fabricated using contemporary materials and welding technologies. Therefore, the SLIC joint industry project was formed to investigate the fatigue crack initiation and growth behaviour in offshore wind welded steel foundations. The FCG test data from the SLIC inter-laboratory (round robin) test programme have been analysed using a new proposed shape function solution and the results are presented and discussed. The obtained FCG trends in air and seawater environments have been compared with the recommended trends available in standards. The Paris-law constants and ΔKth values obtained from this programme can be used for defect assessment and remaining life prediction of offshore monopile weldments in air and seawater environments. The results from the SLIC project show that for a given value of ΔK the fatigue crack growth rate, da/dN, is on average around 2 times higher in seawater compared to air for the base metal and weldments. This factor of 2 in the seawater environment is almost half of the crack acceleration factor recommended by standards

A novel weight function for RMS stress intensity factor determination in surface cracks

This paper discusses the problem of stress intensity factor determination in surface cracks. In particular, the concept of root mean square stress intensity factors (RMS SIF) is discussed for the general class of semi-elliptical surface cracks. The weight function SIF derivation method is considered problems with the existing techniques are highlighted, and a novel technique for the derivation of the RMS SIF weight functions for surface cracks is presented and results are compared with numerical solutions for a variety of loadings and geometries

Use of a wave energy converter as a motion suppression device for floating wind turbines

Floating offshore wind turbines (FOWTs) are subjected to large amplitude motions that induce greater loads on components and reduce aerodynamic performance. One approach to counteract this has been to use passive damping systems for FOWTs to dissipate the wave-induced energy and therefore reduce the global platform motions. This paper proposes that rather than discard this energy, a wave energy converter (WEC) is utilized on the floating platform to absorb it. A study is carried out on a floating vertical axis wind turbine (VAWT) combined with WEC moving in heave. A range of damping and stiffness coefficients are applied between the FOWT and WEC to establish strategies for two cases: maximum motion reduction and maximum energy extraction. The results and conclusions obtained are presented in terms of modifying the WEC natural frequency, damping and stiffness values

Life cycle optimization for sustainable algal biofuel production using integrated nutrient recycling technology

In this study, a multi-objective optimization of sustainable integration of algal biofuel production using nutrient recycling technology, such as anaerobic digestion and hydrothermal liquefaction, is considered. Gross annual profitability and global warming potential (GWP) are the criteria chosen for the design of the algal biofuel production system. Three scenarios, such as full-scale (baseline), pilot-scale (conservative), and lab-scale (nominal), are chosen based on the expected maturity levels and nutrient demand. The results of the optimization produce Pareto sets of optimal solutions for acknowledging the trade-off between the economic and the environmental criteria of the integrated system. It is found that the anaerobic digestion (AD) technology shows better performance in terms of an environmental perspective, displacing the excessive fertilizer requirements due to its maturity in comparison with the hydrothermal liquefaction (HTL) process. However, HTL is a new, evolving, promising nutrient recycling technology which demonstrates economic preferences compared to the AD process due to its low cost of production

Non-stationary stochastic optimization of an oscillating water column

A non-stationary stochastic optimization methodology is applied to an OWC (oscillating water column) to find the design that maximizes the wave energy extraction. Different temporal cycles are considered to represent the long-term variability of the wave climate at the site in the optimization problem. The results of the non-stationary stochastic optimization problem are compared against those obtained by a stationary stochastic optimization problem. The comparative analysis reveals that the proposed non-stationary optimization provides designs with a better fit to reality. However, the stationarity assumption can be adequate when looking at averaged system response

Risk-based methods for sustainable energy system planning: a review

The value of investments in renewable energy (RE) technologies has increased rapidly over the last decade as a result of political pressures to reduce carbon dioxide emissions and the policy incentives to increase the share of RE in the energy mix. As the number of RE investments increases, so does the need to measure the associated risks throughout planning, constructing and operating these technologies. This paper provides a state-of-the-art literature review of the quantitative and semi-quantitative methods that have been used to model risks and uncertainties in sustainable energy system planning and feasibility studies, including the derivation of optimal energy technology portfolios. The review finds that in quantitative methods, risks are mainly measured by means of the variance or probability density distributions of technical and economical parameters; while semi-quantitative methods such as scenario analysis and multi-criteria decision analysis (MCDA) can also address non-statistical parameters such as socio-economic factors (e.g. macro-economic trends, lack of public acceptance). Finally, untapped issues recognised in recent research approaches are discussed along with suggestions for future research

Design implications towards inspection reduction of large scale structures

Operational management is a key contributor in life cycle costs, especially for large scale assets which are in most times complex in structural hierarchy and with a large nominal service life. Decisions on the operational management may concern the number of inspections or maintenance strategies which may allow full utilization of structural capacity or sacrifice residual life in order to avoid an unscheduled intervention. Design of such assets is often governed by design standards which offer the designer the flexibility to take certain decisions that may affect the CAPEX to OPEX ratio such as that of building a more robust structure which may eliminate the need for costly inspection operations. This paper is investigating this approach, taking the example of offshore wind turbine support structures as the reference case, and examines the relevant provisions of the DNV-Os-J101 Standard with respect to the design implications that such a decision may have to the overall life-cycle cost of the structure. Assessment of the structural properties under different design conditions is evaluated through a combination of detailed cost model and an iterative optimization algorithm. The approach which is followed and documented, can be applicable to other complex structural systems for decision making through evaluation of service life costs. Paper presented at: Complex Systems Engineering and Development Proceedings of the 27th CIRP Design Conference Cranfield University, UK 10th – 12th May 2017

Dynamic modelling of microalgae cultivation process in high rate algal wastewater pond

In this work, a comprehensive dynamic mathematical modelling to simulate the production of microalgae in a high rate algal pond (HRAP) is attempted. A synergetic algal–bacterial system comprising various interrelated biological and chemical system processes is presented. The dynamic behaviour of HRAP system is studied by solving mass balance equations of different components which account light intensity and gas–liquid mass transfer. The model predictions are compared with the previously reported studies in the literature. The influence of kinetic and operating parameters, including the supply of CO2, the maximum growth rate, pond depth and dilution rates, on the pond performance are evaluated. The sensitivity analysis of important process parameters is also discussed in this study. The developed model, as a tool, can be used to assess the factors that affect the pond performance criteria, including algal productivity and the dynamics of nutrient requirements

Stochastic prediction of offshore wind farm LCOE through an integrated cost model

Common deterministic cost of energy models applied in offshore wind energy installations usually disregard the effect of uncertainty of key input variables – associated with OPEX, CAPEX, energy generation and other financial variables – on the calculation of levelized cost of electricity (LCOE). The present study aims at expanding a deterministic cost of energy model to systematically account for stochastic inputs. To this end, Monte Carlo simulations are performed to derive the joint probability distributions of LCOE, allowing for the estimation of probabilities of exceeding set thresholds of LCOE, determining certain confidence intervals. The results of this study stress the importance of appropriate statistical modelling of stochastic variables in order to reduce modelling uncertainties and contribute to a better informed decision making in renewable energy investments

A cluster analysis of investment strategies in the offshore wind energy market

This paper maps different investor strategies in the offshore wind energy market based on data from existing wind farms in the UK. This is realized through the employment of cluster analysis, which classifies offshore wind energy investors - who have purchased equity stakes-in terms of the entry timing, exit timing, purchase timing and stake purchased. We, then, perform a SWOT analysis to identify the major strengths, weaknesses, opportunities and threats encountered by each cluster of stakeholders. Cluster analysis revealed the existence of three distinct investment strategy profiles: i) Late entry investors, ii) Pre-commissioning investors, and iii) Own-build-transfer investors. Corporate and institutional investors tend to be late entry investors, whose strategy is based on buying assets while they are fully operational avoiding construction risks, retaining a risk aversion profile. The exit timing of OEMs and EPCI contractors usually takes place before or right after the commissioning of the wind farm. Finally, major Utilities tend to keep the operating assets on their balance sheet and divest only part of them (mostly minority stakes) during the operating stage; Independent energy companies are found in both 2nd and 3rd cluster; however, exceptions may be observed