Fixed Assets and Mineral Leaseholds

Fixed Assets

Fixed assets consisted of the following:



	September 30, 2018			December 31, 2017
Crude oil pipelines and natural gas pipelines and related assets	$	2,900,469		$	3,028,657
Alkali facilities, machinery, and equipment	515,226			497,601
Onshore facilities, machinery, and equipment	638,726			692,364
Transportation equipment	20,352			21,483
Marine vessels	946,377			918,953
Land, buildings and improvements	220,823			223,186
Office equipment, furniture and fixtures	18,898			18,112
Construction in progress	97,313			151,768
Other	47,800			48,891
Fixed assets, at cost	5,405,984			5,601,015
Less: Accumulated depreciation	(882,833		)	(734,986		)
Net fixed assets	$	4,523,151		$	4,866,029

Mineral Leaseholds

Our Mineral Leaseholds, relating to our acquired Alkali Business, consist of the following:



	September 30, 2018			December 31, 2017
Mineral leaseholds	$	566,019		$	566,019
Less: Accumulated depletion	(4,426		)	(1,513		)
Mineral leaseholds, net	$	561,593		$	564,506

Our depreciation and depletion expense for the periods presented was as follows:



	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Depreciation expense	$	85,282	$	57,117	$	224,546	$	157,438
Depletion expense	722		381		2,913		381

Assets Held for Sale

On August 7, 2018, we granted a third party a time-limited option to acquire certain of our non-core assets in the Powder River Basin in exchange for an option payment of $30 million. As of this date, we reclassified the property, plant, and equipment related to these assets, in the amount of $255.5 million, as current assets held for sale on our Unaudited Consolidated Balance Sheet as of September 30, 2018 (Note 18).

Asset Retirement Obligations

We record AROs in connection with legal requirements to perform specified retirement activities under contractual arrangements and/or governmental regulations.

The following table presents information regarding our AROs since December 31, 2017



ARO liability balance, December 31, 2017	$	198,187
Accretion expense	7,437
Changes in estimate	18,913
Settlements	(9,481		)
ARO liability balance, September 30, 2018	$	215,056

Of the ARO balances disclosed above, $36.0 million and $20.9 million is included as current in "Accrued liabilities" on our Unaudited Condensed Consolidated Balance Sheet as of September 30, 2018 and December 31, 2017, respectively. The remainder of the ARO liability as of September 30, 2018 and December 31, 2017 is included in "Other long-term liabilities" on our Unaudited Condensed Consolidated Balance Sheet.

With respect to our AROs, the following table presents our forecast of accretion expense for the periods indicated:



Remainder of	2018	$	2,459
	2019	$	9,057
	2020	$	8,710
	2021	$	9,302
	2022	$	9,935

Certain of our unconsolidated affiliates have AROs recorded at September 30, 2018 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our Consolidated Financial Statements.