Table:
|  | Vessels | Office property and other tangible assets | Total tangible fixed assets | Vessels under construction |
| --- | --- | --- | --- | --- |
| Cost |  |  |  |  |
| As of January 1, 2018 | 4,217,866 | 19,224 | 4,237,090 | 166,655 |
| Additions | 49,036 | 4,678 | 53,714 | 637,046 |
| Transfer from vessels under construction | 642,776 | — | 642,776 | (642,776) |
| Transfer under ‘‘Other non-current assets’’ | — | — | — | (1,650) |
| Fully amortized fixed assets | (10,000) | (192) | (10,192) | — |
| As of December 31, 2018 | 4,899,678 | 23,710 | 4,923,388 | 159,275 |
| Additions | 26,233 | 1,454 | 27,687 | 450,918 |
| Return of capital expenditures | (11,224) | — | (11,224) | — |
| Transfer from vessels under construction | 406,870 | — | 406,870 | (406,870) |
| Fully amortized fixed assets | (7,209) | — | (7,209) | — |
| As of December 31, 2019 | 5,314,348 | 25,164 | 5,339,512 | 203,323 |
| Accumulated depreciation |  |  |  |  |
| As of January 1, 2018 | 460,815 | 3,709 | 464,524 | — |
| Depreciation | 144,611 | 863 | 145,474 | — |
| Fully amortized fixed assets | (10,000) | (192) | (10,192) | — |
| As of December 31, 2018 | 595,426 | 4,380 | 599,806 | — |
| Depreciation | 156,826 | 875 | 157,701 | — |
| Impairment loss on vessels | 162,149 | — | 162,149 | — |
| Fully amortized fixed assets | (7,209) | — | (7,209) | — |
| As of December 31, 2019 | 907,192 | 5,255 | 912,447 | — |
| Net book value |  |  |  |  |
| As of December 31, 2018 | 4,304,252 | 19,330 | 4,323,582 | 159,275 |
| As of December 31, 2019 | 4,407,156 | 19,909 | 4,427,065 | 203,323 |
Paragraphs:
GasLog Ltd. and its Subsidiaries Notes to the consolidated financial statements (Continued) For the years ended December 31, 2017, 2018 and 2019 (All amounts expressed in thousands of U.S. Dollars, except share and per share data)    6. Tangible Fixed Assets and Vessels Under Construction
The movements in tangible fixed assets and vessels under construction are reported in the following table:
Vessels with an aggregate carrying amount of $4,407,156 as of December 31, 2019 (December 31, 2018: $4,304,252) have been pledged as collateral under the terms of the Group’s loan agreements (Note 13).
As of December 31, 2019, a number of increasingly strong negative indicators such as the difference between ship broker estimates of the fair market values and the carrying values of the Group’s Steam vessels, the lack of liquidity in the market for term employment for Steam vessels and reduced expectations for the estimated rates at which such term employment could be secured, together with the continued addition of modern, larger and more fuel efficient LNG carriers to the global fleet, prompted the Group to perform an impairment assessment of its vessels in accordance with the
Question:
Why did the Group perform impairment assessment of its vessels?
Answer: Increasingly strong negative indicators such as the difference between ship broker estimates of the fair market values and the carrying values of the Group’s Steam vessels, the lack of liquidity in the market for term employment for Steam vessels and reduced expectations for the estimated rates at which such term employment could be secured, together with the continued addition of modern, larger and more fuel efficient LNG carriers to the global fleet