. Fixed Assets and Asset Retirement Obligations

Fixed Assets

Fixed assets consisted of the following:



	June 30, 2012			December 31, 2011
Pipelines and related assets	$	205,435		$	167,865
Machinery and equipment	47,639			46,233
Transportation equipment	20,051			21,732
Marine vessels	297,400			262,216
Land, buildings and improvements	13,278			13,140
Office equipment, furniture and fixtures	4,397			3,778
Construction in progress	52,044			14,236
Other	11,934			11,938
Fixed assets, at cost	652,178			541,138
Less: Accumulated depreciation	(139,926		)	(124,213		)
Net fixed assets	$	512,252		$	416,925

Our depreciation expense for the periods presented was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
	2012		2011		2012		2011
Depreciation expense	$	9,077	$	5,804	$	17,827	$	11,640

Asset Retirement Obligations

A reconciliation of our liability for asset retirement obligations is as follows:



December 31, 2011	$	5,900
Liabilities incurred and assumed in the current period	5,995
Accretion expense	400
June 30, 2012	$	12,295

We assumed asset retirement obligations of $5.9 million related to pipelines in connection with our acquisition of GOPL. See Note 2 for information related to our acquisitions.