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

text: 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
text: The movements in tangible fixed assets and vessels under construction are reported in the following table:
text: 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).
text: 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

Questions:
question: In which years was the movements in tangible fixed assets and vessels under construction recorded for?
answer: 2018
2019
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
question: What was the aggregate carrying amount of the vessels in 2018?
answer: $4,304,252
scale: thousand
question: In which end year has a higher vessel accumulated depreciation?
answer: 2019
derivation: 907,192 > 595,426
question: What was the change in vessel cost from end 2018 to end 2019?
answer: 414670
derivation: 5,314,348 - 4,899,678 
question: What was the percentage change in vessel net book value end 2018 to end 2019?
answer: 2.39
derivation: (4,407,156 - 4,304,252)/4,304,252 
scale: percent