Fixed Assets, Mineral Leaseholds and Asset Retirement Obligations

Fixed Assets

Fixed assets consisted of the following:



	December 31,
	2017			2016
Crude oil pipelines and natural gas pipelines and related assets	$	3,028,657		$	2,901,202
Alkali facilities, machinery, and equipment	497,601			—
Onshore facilities, machinery, and equipment	692,364			427,658
Transportation equipment	21,483			17,543
Marine vessels	918,953			863,199
Land, buildings and improvements	223,186			55,712
Office equipment, furniture and fixtures	18,112			9,654
Construction in progress	151,768			440,225
Other	48,891			48,203
Fixed assets, at cost	5,601,015			4,763,396
Less: Accumulated depreciation	(734,986		)	(548,532		)
Net fixed assets	$	4,866,029		$	4,214,864

Mineral Leaseholds

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



	December 31, 2017
Mineral leaseholds	566,019
Less: Accumulated depletion	(1,513		)
Mineral leaseholds, net	$	564,506

Depreciation expense was $226.0 million, $194.0 million and $124.2 million for the years ended December 31, 2017, 2016, and 2015, respectively. Depletion expense was $1.5 million for the year ended December 31, 2017, with no similar expense in 2016 and 2015.

During 2017, we sold certain non-core natural gas gathering and platform assets in the Gulf of Mexico included in our offshore pipeline transportation services segment, as well as certain onshore terminal facilities in West Texas included in our onshore facilities and transportation segment. These sales resulted in total gains on asset sales of $40.3 million for the year ended December 31, 2017.

Asset Retirement Obligations

We record AROs in connection with legal requirements to perform specified retirement activities under contractual arrangements and/or governmental regulations. For any AROs acquired, we record AROs based on the fair value measurement assigned during the preliminary purchase price allocation.

A reconciliation of our liability for asset retirement obligations is as follows:



December 31, 2015	$	188,662
AROs arising from the acquisition and consolidation of a previously held equity method investment	20,940
Accretion expense	10,800
Revisions in timing and estimated costs of AROs	(2,254		)
Settlements	(4,422		)
December 31, 2016	213,726
Accretion expense	11,008
Revisions in timing and estimated costs of AROs	7,146
Acquisitions	131
Divestitures	(7,649		)
Settlements	(26,415		)
Other	240
December 31, 2017	$	198,187

At December 31, 2017 and December 31, 2016, $20.9 million and $22.4 million are included as current in "Accrued liabilities" on our Consolidated Balance Sheet, respectively. The remainder of the ARO liability at each period is included in "Other long-term liabilities" on our Consolidated Balance Sheet.

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



2018	$	10,799
2019	$	9,760
2020	$	9,429
2021	$	10,069
2022	$	10,753

Certain of our unconsolidated affiliates have AROs recorded at December 31, 2017 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our Consolidated Financial Statements.