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 in Years	June 30, 2017		December 31, 2016
Plants, pipelines and facilities (1)	3-45 (5)	$	35,785.9	$	35,124.6
Underground and other storage facilities (2)	5-40 (6)		3,388.4		3,326.9
Transportation equipment (3)	3-10		172.1		165.8
Marine vessels (4)	15-30		800.5		800.7
Land			266.8		264.6
Construction in progress			4,101.9		3,320.7
Total			44,515.6		43,003.3
Less accumulated depreciation			10,294.9		9,710.8
Property, plant and equipment, net		$	34,220.7	$	33,292.5

(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. (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.

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

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2017		2016		2017		2016
Depreciation expense (1)	$	321.1	$	298.2	$	638.6	$	594.1
Capitalized interest (2)		44.5		46.4		84.1		88.9

(1)Depreciation expense is a component of “Costs and expenses” as presented on our Unaudited Condensed Statements of Consolidated Operations. (2)We capitalize interest costs incurred on funds used to construct property, plant and equipment while the asset is in its construction phase. The capitalized interest is recorded as part of the asset to which it relates and is amortized over the asset’s estimated useful life as a component of depreciation expense. When capitalized interest is recorded, it reduces interest expense from what it would be otherwise.

Azure Acquisition

In April 2017, we closed the acquisition of a midstream energy business from Azure Midstream Partners, LP and its operating subsidiaries (collectively, “Azure”) for $191.4 million in cash. The acquired business assets, which are located primarily in East Texas, include over 730 miles of natural gas gathering pipelines and two natural gas processing facilities with an aggregate processing capacity of 130 million cubic feet per day. The acquired business serves production from the Haynesville Shale and Bossier, Cotton Valley and Travis Peak formations.

The financial results of the acquired business are reflected in our consolidated results from April 30, 2017, which was the effective date of the Azure acquisition. On a historical pro forma consolidated basis, our revenues, costs and expenses, operating income, net income attributable to Enterprise Products Partners L.P., and earnings per unit amounts for the three and six months ended June 30, 2016 and 2017 would not have differed materially from those we actually reported had the Azure acquisition been completed on January 1, 2016 rather than April 30, 2017.

The following table presents the preliminary fair value allocation of assets acquired and liabilities assumed in the Azure acquisition at April 30, 2017. The allocation remains provisional due to ongoing efforts to clarify certain environmental liabilities (estimated at $2.2 million), which are expected to be resolved by December 31, 2017.

Assets acquired in business combination:
Current assets	$	3.1
Property, plant and equipment		194.2
Total assets acquired		197.3
Liabilities assumed in business combination:
Current liabilities		1.4
Long-term liabilities		4.5
Total liabilities assumed		5.9
Cash used for Azure acquisition	$	191.4

The contribution of this newly acquired business to our consolidated revenues and net income was not material for the second quarter of 2017.

Asset Retirement Obligations

We record asset retirement obligations (“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 June 30, 2017 and December 31, 2016 includes $40.3 million and $44.9 million, respectively, of asset retirement costs capitalized as an increase in the associated long-lived asset.

The following table presents information regarding our AROs since January 1, 2017:

ARO liability balance, January 1, 2017	$	$ 85.4
Liabilities incurred		3.1
Liabilities settled		(1.5	)
Revisions in estimated cash flows		(3.2	)
Accretion expense		2.8
ARO liability balance, June 30, 2017	$	86.6