Fixed Assets and Asset Retirement Obligations

Fixed Assets

Fixed assets consisted of the following:



	December 31,
	2016			2015
Crude oil pipelines and natural gas pipelines and related assets	$	2,901,202		$	2,501,821
Machinery and equipment	427,658			414,100
Transportation equipment	17,543			19,025
Marine vessels	863,199			794,508
Land, buildings and improvements	55,712			41,202
Office equipment, furniture and fixtures	9,654			7,540
Construction in progress	440,225			485,575
Other	48,203			46,455
Fixed assets, at cost	4,763,396			4,310,226
Less: Accumulated depreciation	(548,532		)	(378,247		)
Net fixed assets	$	4,214,864		$	3,931,979

Depreciation expense was $194.0 million, $124.2 million and $73.2 million for the years ended December 31, 2016, 2015, and 2014, 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, 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
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

At December 31, 2016 and December 31, 2015, $22.4 million and $9.8 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:



2017	$	11,117
2018	$	9,408
2019	$	8,619
2020	$	9,176
2021	$	9,772

Certain of our unconsolidated affiliates have AROs recorded at December 31, 2016 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our Consolidated Financial Statements.