Note 4. Property, Plant and Equipment

The historical costs of our property, plant and equipment and related accumulated depreciation balances were as follows at the dates indicated:

	Estimated Useful Life	December 31,
	in Years	2018		2017
Plants, pipelines and facilities (1)	3-45 (5)	$	42,371.0	$	37,132.2
Underground and other storage facilities (2)	5-40 (6)		3,624.2		3,460.9
Transportation equipment (3)	3-10		187.1		177.1
Marine vessels (4)	15-30		828.6		803.8
Land			359.5		273.1
Construction in progress			3,526.8		4,698.1
Total			50,897.2		46,545.2
Less accumulated depreciation			12,159.6		10,924.8
Property, plant and equipment, net		$	38,737.6	$	35,620.4

(1) Plants, pipelines and facilities include processing plants; NGL, natural gas, crude oil and petrochemical and refined products pipelines; terminal loading and unloading facilities; buildings; office furniture and equipment; laboratory and shop equipment and related assets. We placed a number of growth projects into service since December 31, 2017 including a propane dehydrogenation facility at our Mont Belvieu complex, the first two processing trains at our Orla natural gas processing facility, and a ninth NGL fractionator in Chambers County, Texas at our Mont Belvieu NGL fractionation complex. (2) Underground and other storage facilities include underground product storage caverns; above ground storage tanks; water wells and related assets. (3) Transportation equipment includes tractor-trailer tank trucks and other vehicles and similar assets used in our operations. (4) Marine vessels include tow boats, barges and related equipment used in our marine transportation business. (5) In general, the estimated useful lives of major assets within this category are: processing plants, 20-35 years; pipelines and related equipment, 5-45 years; terminal facilities, 10-35 years; buildings, 20-40 years; office furniture and equipment, 3-20 years; and laboratory and shop equipment, 5-35 years. (6) In general, the estimated useful lives of assets within this category are: underground storage facilities, 5-35 years; storage tanks, 10-40 years; and water wells, 5-35 years.

In March 2018, we acquired the remaining 50% member interest of our Delaware Processing joint venture, which resulted in the consolidation of $200 million of property, plant and equipment. See Note 12 for information regarding this recent acquisition.

In April 2018, we acquired land in the Houston, Texas area for $85.2 million. The consideration paid consisted of $55.2 million in cash with the balance funded by the issuance of 1,223,242 Enterprise common units.

In October 2018, we sold our Red River System and associated crude oil linefill for $134.9 million and recorded a gain of $20.6 million. The Red River System gathers and transports crude oil from North Texas and southern Oklahoma for delivery to local refineries and pipeline interconnects for further transportation to the Cushing hub and Gulf Coast.

The following table summarizes our depreciation expense and capitalized interest amounts for the years indicated:

	For the Year Ended December 31,
	2018		2017		2016
Depreciation expense (1)	$	1,436.2	$	1,296.1	$	1,215.7
Capitalized interest (2)		147.9		192.1		168.2
(1)Depreciation expense is a component of “Costs and expenses” as presented on our Statements of Consolidated Operations. (2)Capitalized interest is a component of “Interest expense” as presented on our Statements of Consolidated Operations.

Asset Retirement Obligations

We record AROs in connection with legal requirements to perform specified retirement activities under contractual arrangements and/or governmental regulations. Our contractual AROs primarily result from right-of-way agreements associated with our pipeline operations and real estate leases associated with our plant sites. In addition, we record AROs in connection with governmental regulations associated with the abandonment or retirement of above-ground brine storage pits and certain marine vessels. We also record AROs in connection with regulatory requirements associated with the renovation or demolition of certain assets containing hazardous substances such as asbestos. We typically fund our AROs using cash flow from operations.

Property, plant and equipment at December 31, 2018 and 2017 includes $72.5 million and $39.9 million, respectively, of asset retirement costs capitalized as an increase in the associated long-lived asset.

The following table presents information regarding our AROs for the years indicated:

	For the Year Ended December 31,
	2018			2017			2016
ARO liability beginning balance	$	86.7		$	85.4		$	58.5
Liabilities incurred		24.4			4.7			4.2
Liabilities settled		(2.5	)		(2.2	)		(5.7	)
Revisions in estimated cash flows		11.5			(6.7	)		24.6
Accretion expense		6.2			5.5			3.8
ARO liability ending balance	$	126.3		$	86.7		$	85.4

The following table presents our forecast of ARO-related accretion expense for the years indicated:

2019				2020				2021				2022				2023
$	8.1			$	8.6			$	9.0			$	9.6			$	10.3