Note 6. 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, 2013		December 31, 2012
Plants, pipelines and facilities (1)		3-45 (6)	$	26,455.2	$	25,382.4
Underground and other storage facilities (2)		5-40 (7)		1,895.8		1,826.3
Platforms and facilities (3)		20-31		664.8		635.2
Transportation equipment (4)		3-10		132.1		136.2
Marine vessels (5)		15-30		713.3		695.0
Land				179.0		167.2
Construction in progress				2,104.7		2,113.1
Total				32,144.9		30,955.4
Less accumulated depreciation				6,578.8		6,109.0
Property, plant and equipment, net			$	25,566.1	$	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 Three Months Ended June 30,				For the Six Months Ended June 30,
	2013		2012		2013		2012
Depreciation expense (1)	$	250.8	$	222.0	$	496.2	$	434.0
Capitalized interest (2)		35.7		29.5		67.3		60.1
(1) Depreciation expense is a component of "Costs and expenses" as presented on our Unaudited Condensed Statements of Consolidated Operations. (2) We capitalize interest cost incurred on funds used to construct property, plant and equipment. 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 January 2013, we sold certain trucking assets for cash proceeds of $29.5 million. As a result of this transaction, net income for the six months ended June 30, 2013 includes a $0.5 million loss from the sale of these assets.

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 six months ended June 30, 2013 includes a $52.5 million gain from the sale of these assets. The Seminole Pipeline remains connected to our Mont Belvieu complex through a newly constructed NGL pipeline that we own.

In April 2013, we sold certain lubrication oil and specialty chemical distribution assets for cash proceeds of $35.3 million. As a result, net income for the three and six months ended June 30, 2013 includes a $6.7 million gain from the sale of these assets.

We received cash proceeds of $14.3 million and $14.9 million from the sale of certain marine transportation assets during the three and six months ended June 30, 2013, respectively. As a result of these transactions, net income for the three and six months ended June 30, 2013 includes a $6.7 million loss from the sale of these assets.

Asset Retirement Obligations

Property, plant and equipment at June 30, 2013 and December 31, 2012 includes $40.9 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 asset retirement obligations ("AROs") during the six months ended June 30, 2013:

ARO liability balance, December 31, 2012	$	105.2
Liabilities incurred		0.1
Liabilities settled		(6.8	)
Revisions in estimated cash flows		2.9
Accretion expense		3.1
ARO liability balance, June 30, 2013	$	104.5

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

Remainder of 2013				2014				2015				2016				2017
$	3.1			$	6.5			$	6.3			$	6.6			$	7.1

Certain of our unconsolidated affiliates have AROs recorded at June 30, 2013 and December 31, 2012 relating to contractual agreements and regulatory requirements. These amounts are immaterial to our consolidated financial statements.