Fixed Assets

Fixed assets consisted of the following:



	June 30, 2016			December 31, 2015
Crude oil pipelines and natural gas pipelines and related assets	$	2,657,642		$	2,501,821
Machinery and equipment	418,013			414,100
Transportation equipment	18,991			19,025
Marine vessels	821,895			794,508
Land, buildings and improvements	49,431			41,202
Office equipment, furniture and fixtures	9,347			7,540
Construction in progress	566,804			485,575
Other	46,915			46,455
Fixed assets, at cost	4,589,038			4,310,226
Less: Accumulated depreciation	(463,244		)	(378,247		)
Net fixed assets	$	4,125,794		$	3,931,979

Our depreciation expense for the periods presented was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
Depreciation expense	$	48,807	$	22,512	$	88,519	$	44,549

Asset Retirement Obligations

We record AROs in connection with legal requirements to perform specified retirement activities under contractual arrangements and/or governmental regulations. As a result of the Enterprise acquisition of the offshore pipeline and services business of Enterprise Products Partners, L.P. on July 24, 2015, we recorded AROs based on the fair value measurement assigned during the preliminary purchase price allocation.

The following table presents information regarding our AROs since December 31, 2015:



ARO liability balance, December 31, 2015	$	188,662
AROs arising from the purchase of the remaining interest in Deepwater Gateway	10,470
AROs from the consolidation of historical interest in Deepwater Gateway	10,470
Accretion expense	5,079
Change in estimate	4,590
Settlements	(2,071		)
ARO liability balance, June 30, 2016	$	217,200

Of the ARO balances disclosed above, $5.5 million and 9.8 million is included as current in "Accrued liabilities" on our Unaudited Condensed Consolidated Balance Sheet as of June 30, 2016 and December 31, 2015, respectively. The remainder of the ARO liability as of June 30, 2016 and December 31, 2015 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	2016	$	5,473
	2017	$	9,900
	2018	$	8,204
	2019	$	8,731
	2020	$	9,293

Certain of our unconsolidated affiliates have AROs recorded at June 30, 2016 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our Consolidated Financial Statements.