Fixed Assets and Mineral Leaseholds

Fixed Assets

Fixed assets consisted of the following:



	June 30, 2018			December 31, 2017
Crude oil pipelines and natural gas pipelines and related assets	$	3,090,475		$	3,028,657
Alkali facilities, machinery, and equipment	506,924			497,601
Onshore facilities, machinery, and equipment	725,624			692,364
Transportation equipment	21,529			21,483
Marine vessels	935,823			918,953
Land, buildings and improvements	230,665			223,186
Office equipment, furniture and fixtures	18,924			18,112
Construction in progress	104,592			151,768
Other	51,597			48,891
Fixed assets, at cost	5,686,153			5,601,015
Less: Accumulated depreciation	(867,465		)	(734,986		)
Net fixed assets	$	4,818,688		$	4,866,029

Mineral Leaseholds

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



	June 30, 2018			December 31, 2017
Mineral leaseholds	$	566,019		$	566,019
Less: Accumulated depletion	(3,704		)	(1,513		)
Mineral leaseholds, net	$	562,315		$	564,506

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



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Depreciation expense	$	70,836	$	50,397	$	139,264	$	100,321
Depletion expense	1,054		—		2,191		—

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	5,496
Change in estimate	513
Settlements	(6,223		)
ARO liability balance, June 30, 2018	$	197,973

Of the ARO balances disclosed above, $9.6 million and $20.9 million is included as current in "Accrued liabilities" on our Unaudited Condensed Consolidated Balance Sheet as of June 30, 2018 and December 31, 2017, respectively. The remainder of the ARO liability as of June 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	$	5,432
	2019	$	9,877
	2020	$	8,710
	2021	$	9,302
	2022	$	9,935

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