Robotics revenues decreased $13.1 million, or 37%, in the first quarter 2021 compared to the first quarter 2020. The decrease in revenues year over year was due to a reduction in vessel days as well as a reduction in ROV utilization compared to the first quarter 2020. This decrease was offset in part by increased trenching activity year over year. Chartered vessel utilization increased slightly to 90% during the first quarter 2021 compared to 89% during the first quarter 2020; however, total vessel days during the first quarter 2021 decreased to 165 compared to 405 during the first quarter 2020. Vessel days during the first quarter 2020 included 272 spot vessel days primarily attributable to the seabed clearance project in the North Sea and utilization on the Ross Candies in the Gulf of Mexico compared to three spot vessel days during the first quarter 2021. ROV, trencher and ROVDrill utilization was 24% in the first quarter 2021 compared to 34% in the first quarter 2020, and vessel trenching days in the first quarter 2021 increased to 72 days compared to 42 days in the first quarter 2020. Robotics results from operations declined $0.1 million in the first quarter 2021 compared to the first quarter 2020 due to lower revenues, offset in part by lower operating costs, year over year.
Production Facilities
During the first quarter 2021, Production Facilities revenues increased $1.4 million, or 10%, compared to the previous quarter and $0.9 million, or 6%, compared to the first quarter 2020 primarily due to higher oil and gas production revenues in the first quarter 2021.
Selling, General and Administrative and Other
Selling, General and Administrative
Selling, general and administrative expenses were $15.2 million, or 9.3% of revenue, in the first quarter 2021 compared to $12.8 million, or 8.0% of revenue, in the fourth quarter 2020. The higher expenses during the first quarter were primarily due to the timing of employee benefit related costs as well as certain credits related to employee benefits recognized during the prior quarter.
Other Income and Expenses
Other income, net was $1.6 million in the first quarter 2021 compared to $8.4 million in the fourth quarter 2020. Other income, net includes unrealized foreign currency translation gains related to the British pound, which strengthened 1% during the first quarter 2021 compared to 6% during the fourth quarter 2020.
Cash Flows
Operating cash flows were $39.9 million in the first quarter 2021 compared to $40.2 million in the fourth quarter 2020 and $(17.2) million in the first quarter 2020. The increase in operating cash flows year over year was due to higher earnings, lower regulatory certification costs for our vessels and systems and improvements in working capital during the first quarter 2021.
Capital expenditures totaled $1.3 million in the first quarter 2021 compared to $1.1 million in the fourth quarter 2020 and $12.4 million in the first quarter 2020. Capital expenditures in the first quarter 2020 included capital spending related to the completion of the Q7000, which was placed into service during the first quarter 2020. Regulatory certification costs for our vessels and systems, which are included in operating cash flows, were $1.8 million in the first quarter 2021 compared to $0.8 million in the fourth quarter 2020 and $17.8 million in the first quarter 2020. Regulatory certification costs during the first quarter 2020 included dry dock costs on the Q4000, the Q5000 and the Seawell as well as certification costs for several intervention systems.
Free cash flow was $38.5 million in the first quarter 2021 compared to $39.1 million in the fourth quarter 2020 and $(29.6) million in the first quarter 2020. The increase in free cash flow year over year was due to higher operating cash flows and lower capital expenditures. (Free cash flow is a non-GAAP measure. See reconciliation below.)