Property and equipment, net consisted of the following (in thousands):

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The increase in ships was primarily due to the addition of Seven Seas Splendor. The Company capitalized approximately $341.5 million of costs associated with ship improvements. In 2020, one ship had significant improvements that extended the weighted average useful life of the vessel. Accordingly, we have updated our estimate of both its useful life and residual value based on the new weighted average useful life of its current components. The impact of the change in estimate is accounted for on a prospective basis and is not material. Fully depreciated assets on the vessel were also removed from the ship and ship improvement’s cost and accumulated depreciation. Depreciation expense for the years ended December 31, 2020, 2019 and 2018 was $707.9 million, $627.7 million and $534.9 million, respectively. Repairs and maintenance expenses including Dry-dock expenses were $129.9 million, $199.7 million and $199.5 million for the years ended December 31, 2020, 2019 and 2018, respectively, and were recorded within other cruise operating expense.

Ships under construction include progress payments to the shipyard, planning and design fees and other associated costs. Capitalized interest costs which were primarily associated with the construction or revitalization of ships amounted to $25.2 million, $32.9 million and $30.4 million for the years ended December 31, 2020, 2019 and 2018, respectively.