Fixed Assets

Fixed assets consisted of the following:



	March 31, 2017			December 31, 2016
Crude oil pipelines and natural gas pipelines and related assets	$	2,848,339		$	2,901,202
Machinery and equipment	467,126			427,658
Transportation equipment	17,049			17,543
Marine vessels	864,103			863,199
Land, buildings and improvements	77,986			55,712
Office equipment, furniture and fixtures	9,642			9,654
Construction in progress	481,595			440,225
Other	48,204			48,203
Fixed assets, at cost	4,814,044			4,763,396
Less: Accumulated depreciation	(591,275		)	(548,532		)
Net fixed assets	$	4,222,769		$	4,214,864

Our depreciation expense for the periods presented was as follows:



	Three Months Ended March 31,
	2017		2016
Depreciation expense	$	49,924	$	39,712

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, 2016:



ARO liability balance, December 31, 2016	$	213,726
Accretion expense	2,808
Change in estimate	(525		)
Settlements	(1,318		)
ARO liability balance, March 31, 2017	$	214,691

Of the ARO balances disclosed above, $26.7 million and $22.4 million is included as current in "Accrued liabilities" on our Unaudited Condensed Consolidated Balance Sheet as of March 31, 2017 and December 31, 2016, respectively. The remainder of the ARO liability as of March 31, 2017 and December 31, 2016 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	2017	$	8,284
	2018	$	9,619
	2019	$	8,841
	2020	$	9,411
	2021	$	10,019

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