Fixed Assets

Fixed assets consisted of the following:



	June 30, 2017			December 31, 2016
Crude oil pipelines and natural gas pipelines and related assets	$	2,984,884		$	2,901,202
Onshore facilities, machinery, and equipment	762,610			427,658
Transportation equipment	17,857			17,543
Marine vessels	866,584			863,199
Land, buildings and improvements	102,841			55,712
Office equipment, furniture and fixtures	9,681			9,654
Construction in progress	42,882			440,225
Other	55,668			48,203
Fixed assets, at cost	4,843,007			4,763,396
Less: Accumulated depreciation	(629,193		)	(548,532		)
Net fixed assets	$	4,213,814		$	4,214,864

Our depreciation expense for the periods presented was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Depreciation expense	$	50,397	$	48,807	$	100,321	$	88,519

During the period ending June 30, 2017, we sold certain non-core natural gas gathering and platform assets in the Gulf of Mexico that resulted in a gain of $26.7 million.

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	5,581
Change in estimate	729
Divestitures	(7,649		)
Settlements	(12,553		)
Other	240
ARO liability balance, June 30, 2017	$	200,074

Of the ARO balances disclosed above, $20.1 million and $22.4 million is included as current in "Accrued liabilities" on our Unaudited Condensed Consolidated Balance Sheet as of June 30, 2017 and December 31, 2016, respectively. The remainder of the ARO liability as of June 30, 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	$	5,553
	2018	$	9,393
	2019	$	8,627
	2020	$	9,209
	2021	$	9,830

Certain of our unconsolidated affiliates have AROs recorded at June 30, 2017 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our Consolidated Financial Statements.