Cash and Cash Equivalents Concentration of Credit Risk Accounts Receivable Inventories Prepaid Expenses and Other Current Assets Property, Plant and Equipment 42 When long-lived assets are sold or retired, the remaining costs and related accumulated depreciation are removed from the accounts  and any resulting gain or loss is included in income. These capitalized long-lived assets could become impaired if our operating plans  or business environment changes. We review our long-lived assets, including definite-lived intangible assets, for impairment when events or changes in circumstances  indicate that their net book value may not be recovered over their remaining service lives. Indicators of possible impairment may  include significant declines in activity levels in regions where specific assets or groups of assets are located, extended periods of idle  use, declining revenue or cash flow or overall changes in general market conditions. Whenever possible impairment is indicated, we compare the carrying value of the assets to the sum of the estimated undiscounted  future cash flows expected from use, plus salvage value, less the costs of the subsequent disposition of the assets. If impairment is still  indicated, we compare the fair value of the assets to the carrying amount, and recognize an impairment loss for the amount by which  the carrying value exceeds the fair value. We did not record any material impairment charges relating to our long-lived assets held for  use during the years ended December 31, 2013, 2012 or 2011. Property, plant and equipment are carried at cost less accumulated depreciation. Major renewals and improvements are capitalized  while maintenance and repair costs are charged to expense as incurred. They are depreciated using the straight-line method based  on their individual estimated useful lives, except for leasehold improvements, which are depreciated over the remaining lease term, if  shorter. We estimate the useful lives and salvage values of our assets based on historical data as follows: Prepaid expenses and other current assets are comprised primarily of current deferred tax assets, prepaid insurance, value added taxes  and prepaid rents. Inventories consist of manufactured goods, materials and supplies used for sales or services to clients. Inventories are stated at the  lower of cost or estimated net realizable value. Inventory costs are recorded at standard cost which approximates the first-in,  first-out method. Trade accounts receivable are recorded at their invoiced amounts and do not bear interest. We perform ongoing credit evaluations of  our clients and monitor collections and payments in order to maintain a provision for estimated uncollectible accounts based on our  historical collection experience and our current aging of client receivables outstanding, in addition to client’s representations and our  understanding of the economic environment in which our clients operate. Based on our review we establish or adjust allowances for  specific clients and the accounts receivable as a whole, and recognize expense. When an account is determined to be uncollectible,  we charge the receivable to our allowance for doubtful accounts. Our allowance for doubtful accounts totaled $2.9 million and  $3.5 million at December 31, 2013 and 2012, respectively. The net carrying value of accounts receivable is considered to be repre- sentative of its respective fair value. Our financial instruments that potentially subject us to concentrations of credit risk relate primarily to cash and cash equivalents and  trade accounts receivable. All cash and cash equivalents are on deposit at commercial banks or investment firms with significant  financial resources. Our trade receivables are with a variety of domestic, international and national oil and gas companies. We had no  clients who provided more than 10% of our revenue for the years ended December 31, 2013, 2012 and 2011. We consider our credit  risk related to trade accounts receivable to be limited due to the creditworthiness and financial resources of our clients. We evaluate  our estimate of the allowance for doubtful accounts on an on-going basis throughout the year. Cash and cash equivalents include all short-term, highly liquid instruments purchased with an original maturity of three months or less.  These items are carried at cost, which approximates fair value. Buildings and leasehold improvements  3 – 40 years Machinery and equipment  3 – 10 years