The following table provides a breakdown of TCE rates achieved for the years ended December 31, 2019 and 2018 between spot and fixed earnings and the related revenue days.
During 2019, TCE revenues increased by $8,426, or 2.6%, to $335,133 from $326,707 in 2018. The increase primarily resulted from an increase in average daily rates earned by our fleet and decreased spot market exposure. The total number of revenue days decreased from 7,678 days in 2018 to 7,215 days in 2019. The decrease primarily resulted from three fewer vessels in operation during most of 2019 compared to 2018.
Vessel expenses remained stable at $134,618 in 2019 from $134,956 in 2018. Depreciation expense increased by $1,987 to $52,499 in 2019 from $50,512 in 2018. The increase was due to an increase in amortization of drydock costs and an increase in depreciation expense due to the Overseas Gulf Coast and Overseas Sun Coast, our two new vessels, which entered service at the beginning of the fourth quarter of 2019.
Two reflagged U.S. Flag Product Carriers participate in the U.S. Maritime Security Program, which ensures that privatelyowned, military-useful U.S. Flag vessels are available to the U.S. Department of Defense in the event of war or national emergency.
Each of the vessel-owning companies receives an annual subsidy, subject in each case to annual congressional appropriations, which is intended to offset the increased cost incurred by such vessels from operating under the U.S. Flag. Such subsidy was $5,000 for each vessel in 2019 and $5,000 on one vessel and $4,600 on one vessel in 2018.
Under the terms of the program, we expect to receive up to $5,000 annually for each vessel during 2020, and up to $5,200 for each vessel beginning in 2021. We do not receive a subsidy for any days for which either of the two vessels operate under a time charter to a U.S. government agency.
In June 2019, one of our lightering customers, PES, suffered an explosion and fire at its refinery in the Delaware Bay. The PES refinery complex, which consists of two refineries, has been shut down since the fire. Due to the expected reduction in lightering volumes, we redeployed one of our two lightering ATBs to the U.S. Gulf of Mexico for alternative employment.
In July 2019, PES filed a Chapter 11 bankruptcy petition. At December 31, 2019, we had outstanding receivables from PES of approximately $4,300. The ultimate recovery of these receivables is currently unknown. We established a loss provision of $4,300. We are working diligently to maximize our recovery.
In June 2018, one of our ATBs was berthed to the dock when a third-party ship transiting the channel hit our ATB, causing structural damage to the ATB and damage to the dock. The cost of repairs has been covered by existing insurance policies. We have filed a lawsuit against the third-party ship seeking recovery of our costs of repairs as well as our lost earnings from the ATB being off-hire for 46 repair days.
Table:
 | 2019 |  | 2018 | 
 | Spot Earnings | Fixed Earnings | Spot Earnings | Fixed Earnings
Jones Act Handysize Product Carriers:     |  |  |  | 
Average rate | $25,036 | $57,910 | $31,254 | $60,252
Revenue days | 523 | 4,052 | 1,142 | 3,141
Non-Jones Act Handysize Product Carriers:     |  |  |  | 
Average rate | $30,671 | $13,912 | $25,925 | $12,097
Revenue days | 482 | 417 | 707 | 3
ATBs:     |  |  |  | 
Average rate | $19,117 | $21,861 | $15,333 | $22,207
Revenue days | 255 | 773 | 990 | 998
Lightering:     |  |  |  | 
Average rate | $63,162 | $— | $66,041 | $—
Revenue days | 713 | — | 697 | —


Question: What led to the increase in TCE revenues?
Answer: an increase in average daily rates earned by our fleet and decreased spot market exposure.