ABSTRACT

Conventional fossil fuels are becoming more expensive and cannot cover the rising global energy needs with the remaining reserves in the future. In this context, the relevance of offshore wind energy (OWE) has increased steadily because of high wind resource availability. However, the installation of an offshore wind farm is challenging and cost-intensive because of harsh weather conditions and limited availability of resources. Therefore, a great effort has been devoted to the study of new installation concepts such as feeder concepts. In this paper, a discrete event simulation study is presented to quantify the benefit of using feeder vessels in the installation logistics of offshore wind farms. The results show that the offshore feeder concept outperforms the conventional concept under certain conditions.

INTRODUCTION

Due to CO2-emission and climate change, renewable energies are becoming more important. By using renewable energy, such as wind energy, it is possible to make the energy supply sustainable and climate-friendly. In particular offshore wind energy is of key importance for the success of the energy transition, since for instance in Germany, the theoretically available areas for onshore wind energy installations are typically limited in size and are often already built-up. As a result, the number of offshore wind turbines (OWTs) at sea has increased significantly in recent years. In the first half of 2016, the number of wind turbines has increased to 835 OWTs with a total output of 3,552.22 MW (Deutsche WindGuard 2016). In 2017, approximately 947 wind turbines with a total output of 4,108 megawatts were in operation in the German North Sea and Baltic Sea, and projects under construction or in planning show that the goal of the German federal government will be achieved with the same development of the number of erection sites (BWE Bundesverband Windenergie 2017).

However, the feed-in tariff is going down and the tendering process of offshore wind farms (OWF) in general is realized by auctions. This leads to high market pressure and needs for new saving potentials, whereby harsh weather and sea conditions complicate the planning of the offshore installation process. Due to the existing weather and sea conditions, the construction of the OWE involves logistical challenges. Besides the harsh weather and sea conditions, the installation planning has to deal with limited resource availability. Furthermore, the focus in the next years will be on the installation of high power wind turbines (>= 6MW). The installation of these high power OWTs will essentially take place in deeper water depths (30 – 50 m), and thus in wind farms located in a distance greater than 50 km from the shore (Muhabie, Rigo, Cepeda, and D'Agosto 2018). This leads to significant delays and increased costs in the installation processes. Actually, the erection of OWF is carried out based on a rigid concept, in which the components are transported from production sites to the base port and from the base port to the wind farm location. In the process, different wind turbine components are shipped via waterways from the production sites to the base port. An installation vessel loads the components, sails to the offshore wind farm location and performs the installation. Currently, the share of logistics costs for the erection of OWF is estimated to be between 15% and 20% (Muhabie, Rigo, Cepeda, and D'Agosto 2018) (Dewan, Asgarpour, Jones, and Ouelhadj 2016). This is a result of the great uncertainties at sea, caused especially by unpredictable weather conditions, which make the installation of offshore wind turbines a complex and challenging planning problem. Therefore, a great effort has been devoted to the study of new installation concepts, such as the feeder concepts, which seeks to optimize the use of costly resources like installation vessels and base ports. For this reason, it must be determined which potentials can be raised with such concepts since the technical requirements causes further investment or development costs. For the monetary assessment, economic considerations in the form of investment calculations or cost and performance calculations come to their limits because of the complexity and the restrictions of the processes. A first study of feederbased installation concepts for OWF was presented in (Ait Alla, Oelker, Lewandowski, Freitag, and Thoben 2017). The authors introduced the base port-feeder concept, in which a feeder vessel supplies an installation vessel, called "Jack-Up barge" with nacelles, towers and rotor blades directly in the base port or at shallow waters. In this paper, we extend the work presented in (Ait Alla, Oelker, Lewandowski, Freitag, and Thoben 2017) and introduce another feeder-based concept (offshore feeder concept). For this reason, a simulation study about different installation concepts for offshore wind farms by means of a multi-agent discrete event simulation model is proposed. The aim is to quantify the benefit of an offshore feeder concept. The simulation model considers process times of different operations, weather restrictions and costs. Based on empirical and experimental results, the study performs a comparison analysis. Moreover, the proposed simulation study identifies factors, that affect the offshore feeder-based installation of OWF and investigates their influence on the installation performances.

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