Property, Plant and Equipment

Major classes of property, plant, and equipment, which include capital lease assets, consisted of the following (in millions):



	December 31,
(in thousands)	2017			2016
Land	$	—		$	1,344
Buildings	—			4,206
Drilling rigs and related equipment	332,338			294,002
Machinery, equipment and other	1,246			1,571
Capital leases	1,786			1,129
Vehicles	555			405
Software	818			806
Construction in progress	20,706			31,974
Total	$	357,449		$	335,437
Less: Accumulated depreciation	(85,061		)	(62,249		)
Total Property, plant and equipment, net	$	272,388		$	273,188

Repairs and maintenance expense included in operating costs in our statements of operations totaled $14.3 million, $7.7 million and $10.5 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Depreciation expense was $25.8 million, $23.8 million and $21.2 million for the years ended December 31, 2017, 2016 and 2015, respectively.

As of December 31, 2017, property, plant and equipment in our balance sheets included $1.3 million of vehicles under capital lease, which is net of $0.5 million of accumulated amortization. As of December 31, 2016, property, plant and equipment in our balance sheets included $0.8 million of vehicles under capital lease, net of $0.3 million of accumulated amortization.

During 2017, our management committed to a plan to sell our corporate headquarters and rig assembly yard complex in order to relocate to office space and a yard facility more suitable to our needs. As a result, we reclassified an aggregate $4.0 million of land, buildings and equipment from property, plant and equipment to assets held for sale on our balance sheet, after recognizing a $0.5 million asset impairment charge representing the difference between the carrying value and the fair value, less the costs to sell the related property. In the third quarter of 2017, we recorded an additional asset impairment on the property, reducing assets held for sale, of $0.6 million, as a result of water related damage from the heavy rainfall that occurred during Hurricane Harvey in August 2017.

During 2017 and 2016, we recorded an additional $0.8 million and $1.8 million, respectively, loss on disposal associated with the upgrade of the mud systems on our rigs to high pressure status.

During 2015, we began to convert one of our non-walking rigs to pad optimal status, equipped with our 200 Series substructure, omni-directional walking system and 7500psi mud system. As part of this rig conversion, key components of the prior rig were decommissioned and replaced, including the rig's substructure and various mud system components which were no longer compatible with the converted rig. As a result, we recorded a preliminary estimate of the related disposal loss totaling $2.5 million.

During 2015, we recorded an impairment charge of $3.6 million relating to the substructure, mast and various other rig components of our last remaining non-walking rig due to its limited marketability in its current configuration given market conditions.