energies Article A Crew Scheduling Model to Incrementally Optimize Workforce Assignments for Offshore Wind Farm Constructions Daniel Rippel 1,2, * , Fatemeh Abasian Foroushani 2 , Michael Lütjen 1 and Michael Freitag 1,2   Citation: Rippel, D.; Foroushani, F.A.; Lütjen, M.; Freitag, M. A Crew Scheduling Model to Incrementally Optimize Workforce Assignments for Offshore Wind Farm Constructions. Energies 2021, 14, 6963. https:// doi.org/10.3390/en14216963 Received: 14 September 2021 Accepted: 19 October 2021 Published: 22 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 BIBA—Bremer Institut für Produktion und Logistik GmbH, University of Bremen, Hochschulring 20, 28359 Bremen, Germany; ltj@biba.uni-bremen.de (M.L.); fre@biba.uni-bremen.de (M.F.) 2 Faculty of Production Engineering, University of Bremen, Badgasteiner Str. 1, 28359 Bremen, Germany; fa_ab@uni-bremen.de * Correspondence: rip@biba.uni-bremen.de; Tel.: +49-(0)421-218-50137 Abstract: In the literature, different authors attribute between 15% to 30% of a wind farm’s costs to logistics during the installation, e.g., for vessels or personnel. Currently, there exist only a few approaches for crew scheduling in the offshore area. However, current approaches only satisfy subsets of the offshore construction area’s specific terms and conditions. This article first presents a literature review to identify different constraints imposed on crew scheduling for offshore installa- tions. Afterward, it presents a new Mixed-Integer Linear Model that satisfies these crew scheduling constraints and couples it with a scheduling approach using a Model Predictive Control scheme to include weather dynamics. The evaluation of this model shows reliable scheduling of persons/teams given weather-dependent operations. Compared to a conventionally assumed full staffing of vessels and the port, the model decreases the required crews by approximately 50%. Moreover, the proposed model shows good runtime behavior, obtaining optimal solutions for realistic scenarios in under an hour. Keywords: offshore installations; crew scheduling; mixed-integer linear programming; model pre- dictive control 1. Introduction Wind energy offers an opportunity to satisfy the world’s need for green and sus- tainable energy. Studies show that the last decade witnessed an exponential increase in produced wind energy [1]. This increase results from more capable turbines [2] and con- tinuously increasing concurrent installation and refurbishing projects [3]. Offshore wind farms can produce higher amounts of energy when compared to their onshore counter- parts, resulting from the ability to install larger turbines and wind farms and higher wind exposure at the open sea [4]. The current political landscape implies that these trends will continue over the following years. In 2020, several countries worldwide reworked their climate policies, potentially resulting in a worldwide reduction of 47% of global emissions by 2050, e.g., banning fossil energy generation or fuel [1]. While offshore wind farms provide large amounts of energy, their installation imposes additional challenges. While higher wind speeds and quickly changing weather condi- tions at the open sea favor energy generation, they render the installation complicated and impede installation operations in more than a hundred meters of height. Moreover, offshore construction sites allow larger farms but render them harder to reach, requiring precise operations planning using weather forecasts and human experience. Besides these planning-related challenges, the installation of offshore wind farms requires highly spe- cialized equipment and certified personnel. For example, installation vessels, so-called jack-up vessels, usually provide the ability to mount themselves onto the sea bed to sta- bilizes themselves for crane operations. In various countries, personnel requires special certification and training for offshore installations in such heights. Several authors in the Energies 2021, 14, 6963. https://doi.org/10.3390/en14216963 https://www.mdpi.com/journal/energies