The wind energy sector has experienced a significant expansion during the past two decades. With the current global appetite for the further expansion of offshore wind farms (OWFs) as one of the main renewable energy resources, a vast number of OWFs are expected to enter the decommissioning stage in the near future which may potentially create serious environmental and economic challenges to different countries. Hence, effective decision-making procedures are required to protect the environment, taxpayers, and local communities against the potential economic and environmental impacts of OWF assets at the end of their lifetime. The main contribution of this study is to develop a new approach for the economic and environmental assessments of OWF decommissioning projects based on a bottom-up model. The approach formulates the costs and emissions based on the available data and experience in the field and tries to provide appropriate assumptions to predict the costs and emissions caused by the different decommissioning activities. To validate and show the applicability of the approach, the economic and environmental assessments of two OWF decommissioning case studies in the UK continental shelf are investigated; the Lincs and Gunfleet Sands OWFs. A cost sensitivity analysis is also performed for different duration and vessel/equipment leasing parameters to identify the most sensitive parameters in the OWF decommissioning projects. The study suggests a set of interesting conclusions on the economic and environmental assessments of OWF decommissioning projects that may be beneficial for policymakers, operators, and local communities in the wind energy sector.