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Drilling units
6 Months Ended
Jun. 30, 2021
Property, Plant and Equipment [Abstract]  
Drilling units Drilling units
The following table summarizes the movement for the six months ended June 30, 2020:
 (In $ millions)
CostAccumulated depreciationNet book value
As at January 1, 20207,048 (647)6,401 
Additions85 — 85 
Depreciation— (179)(179)
Impairment(1,230)— (1,230)
As at June 30, 20205,903 (826)5,077 
The following table summarizes the movement for the six months ended June 30, 2021:
 (In $ millions)
CostAccumulated depreciationNet book value
As at January 1, 20213,108 (988)2,120 
Additions39 — 39 
Depreciation— (80)(80)
Impairment(152)— (152)
As at June 30, 20212,995 (1,068)1,927 
In March 2020 we recorded a $1,230 million impairment in our fleet, while in June 2021 we recorded a $152 million impairment. Refer to Note 9 – "Impairment of long-lived assets" for further information.