Note 8. 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			December 31,
	Useful Life in Years			2013		2012
Plants, pipelines and facilities (1)		3-45	(6)	$	27,540.4	$	25,382.4
Underground and other storage facilities (2)		5-40	(7)		2,101.8		1,826.3
Platforms and facilities (3)		20-31			659.6		635.2
Transportation equipment (4)		3-10			138.9		136.2
Marine vessels (5)		15-30			744.8		695.0
Land					176.6		167.2
Construction in progress					2,655.5		2,113.1
Total					34,017.6		30,955.4
Less accumulated depreciation					7,071.0		6,109.0
Property, plant and equipment, net				$	26,946.6	$	24,846.4

(1)	Plants and pipelines include processing plants; NGL, natural gas, crude oil and petrochemical and refined products pipelines; terminal loading and unloading facilities; office furniture and equipment; buildings; 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)	Platforms and facilities include offshore platforms and related facilities and other associated assets located in the Gulf of Mexico.
(4)	Transportation equipment includes tractor-trailer tank trucks and other vehicles and similar assets used in our operations.
(5)	Marine vessels include tow boats, barges and related equipment used in our marine transportation business.
(6)	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; office furniture and equipment, 3-20 years; buildings, 20-40 years; and laboratory and shop equipment, 5-35 years.
(7)	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 Year Ended December 31,
	2013		2012		2011
Depreciation expense (1)	$	1,012.4	$	900.5	$	776.6
Capitalized interest (2)		133.0		116.8		106.7

(1)

Depreciation expense is a component of "Costs and expenses" as presented on our 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.

In December 2011, we sold our equity interests in Crystal Holding L.L.C. ("Crystal"), which owns natural gas storage facilities and associated pipelines located in Mississippi for $547.8 million in cash (net of working capital adjustments). We recorded a $129.1 million gain on the sale of Crystal. The net carrying value of our investment in Crystal was approximately $411.9 million, of which $356.2 million was the total net carrying value of Crystal's property, plant and equipment. We determined that the financial results of Crystal did not meet the criteria to be classified as discontinued operations. We enter into commercial contracts and have operational arrangements with the Petal and Hattiesburg natural gas storage facilities, which are adjacent to and currently share operating assets with our Petal, Mississippi NGL storage facility.

In March 2013, we sold the Stratton Ridge-to-Mont Belvieu segment of the Seminole Pipeline, along with a related storage cavern, for cash proceeds of $86.9 million. As a result, net income for the year ended December 31, 2013 includes a $52.5 million gain attributable to the sale of these assets. The Seminole Pipeline remains connected to our Mont Belvieu complex through a newly constructed NGL pipeline that we own.

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 (i) above-ground brine storage pits, (ii) offshore Gulf of Mexico platform and pipeline assets and (iii) 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 ARO obligations using cash flow from operations.

Property, plant and equipment at December 31, 2013 and 2012 includes $37.4 million and $40.3 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 periods indicated:

	For the Year Ended December 31,
	2013			2012			2011
Balance at beginning of period	$	105.2		$	112.0		$	97.1
Liabilities incurred		1.7			1.7			0.7
Liabilities settled		(14.2	)		(27.8	)		(7.3	)
Revisions in estimated cash flows		(8.6	)		13.7			15.0
Accretion expense		6.1			5.6			6.5
Balance at end of period	$	90.2		$	105.2		$	112.0

The following table presents our forecast of accretion expense for the periods indicated:

2014				2015				2016				2017				2018
	$	6.1			$	6.5			$	6.9			$	7.4			$	8.0

Certain of our unconsolidated affiliates have AROs recorded at December 31, 2013 and 2012 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our consolidated financial statements.