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Drilling units (Tables)
 12 Months Ended
Dec. 31, 2018
Property, Plant and Equipment [Abstract]  
Drilling units
The below table shows the gross value and net book value of our drilling units at December 31, 2018 and December 31, 2017.
(In US$ millions)
Cost
 
Accumulated depreciation

 
Net Book Value
Opening balance as at January 1, 2017
$
6,494.1

 
$
(1,153.2
)
 
$
5,340.9

Additions
121.6

 
—

 
121.6

Disposals
(16.7
)
 
—

 
(16.7
)
Depreciation
—

 
(274.9
)
 
(274.9
)
Closing balance as at December 31, 2017
6,599.0

 
(1,428.1
)
 
5,170.9

Additions
115.0

 
—

 
115.0

Disposals
—

 
—

 
—

Depreciation
—

 
(280.3
)
 
(280.3
)
Closing balance as at December 31, 2018
6,714.0

 
(1,708.4
)
 
5,005.6