Fixed Assets and Asset Retirement Obligations

Fixed Assets

Fixed assets consisted of the following:



	December 31,
	2015			2014
Crude oil pipelines and natural gas pipelines and related assets	$	2,501,821		$	466,613
Machinery and equipment	414,100			376,672
Transportation equipment	19,025			18,479
Marine vessels	794,508			731,016
Land, buildings and improvements	41,202			38,037
Office equipment, furniture and fixtures	7,540			6,696
Construction in progress	485,575			222,233
Other	46,455			39,312
Fixed assets, at cost	4,310,226			1,899,058
Less: Accumulated depreciation	(378,247		)	(268,057		)
Net fixed assets	$	3,931,979		$	1,631,001

Depreciation expense was $124.2 million, $73.2 million and $46.3 million for the years ended December 31, 2015, 2014, and 2013, respectively.

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 our 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.

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



December 31, 2013	$	14,332
Liabilities incurred	—
Accretion expense	458
December 31, 2014	14,790
AROs arising from our Enterprise acquisition	158,230
AROs from the consolidation of historical interests in CHOPS and SEKCO	1,988
Accretion expense	4,941
Revisions in timing of expected settlement	9,986
Settlements	(1,273		)
December 31, 2015	$	188,662

Of the ARO balances disclosed above, $9.8 million is included as current in "Accrued liabilities" on our Consolidated Balance Sheet, as of December 31, 2015. The remainder of the ARO liability as of December 31, 2015, as well as the ARO liability as of December 31, 2014, are 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:



2016	$	7,743
2017	$	6,722
2018	$	5,141
2019	$	5,518
2020	$	5,924

Certain of our unconsolidated affiliates have AROs recorded at December 31, 2015 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our Consolidated Financial Statements.