Additional information regarding selected balance sheet accounts as of September 30, 2021 and December 31, 2020 is presented below (in thousands):


	September 30, 2021			December 31, 2020
Accounts receivable, net:
Trade	$	112,077		$	109,294
Unbilled revenue	21,523			23,173
Contract assets	22,428			35,870
Other	11,672			3,102
Total accounts receivable	167,700			171,439
Allowance for doubtful accounts	(6,260)			(8,304)
	$	161,440		$	163,135

Allowance for doubtful accounts as a percentage of total accounts receivable	4		%	5		%

As of September 30, 2021, accounts receivable, net in the United States and the United Kingdom represented 77% and 12%, respectively, of the total. No other country or single customer accounted for more than 10% of the Company's total accounts receivable as of September 30, 2021.

For the nine months ended September 30, 2021, the $13.4 million net decrease in contract assets was primarily attributable to $34.6 million transferred to accounts receivable, which was partially offset by $21.1 million in revenue recognized during the period. Deferred revenue (contract liabilities) decreased by $0.9 million in the first nine months of 2021, primarily reflecting the recognition of $6.1 million of revenue that was deferred at the beginning of the period, partially offset by $5.3 million in new customer billings which were not recognized as revenue during the period.

The following provides a summary of activity in the allowance for doubtful accounts for the nine months ended September 30, 2021 and 2020 (in thousands):


	Nine Months Ended September 30,
	2021		2020
Allowance for doubtful accounts – January 1	$	8,304	$	8,745
Provisions	20		2,787
Write-offs	(2,200)		(2,490)
Other	136		1,158
Allowance for doubtful accounts – September 30	$	6,260	$	10,200


	September 30, 2021		December 31, 2020
Inventories, net:
Finished goods and purchased products	$	90,558	$	88,634
Work in process	33,852		27,063
Raw materials	93,779		95,410
Total inventories	218,189		211,107
Allowance for excess or obsolete inventory	(40,111)		(40,731)
	$	178,078	$	170,376

The Company recorded impairment charges totaling $25.2 million in the first quarter of 2020 to reduce the carrying value of inventories to their estimated net realizable value following the March 2020 decline in crude oil prices, which reduced the near-term utility of certain goods within the Offshore/Manufactured Products and Well Site Services segments.

During the third quarter of 2020, the Company recorded an impairment charge of $5.9 million to reduce the carrying value of inventories within the Downhole Technologies segment to their estimated net realizable value based on changes in expectations regarding the near-term utility, customer demand and market pricing of certain goods.

The Company recorded an impairment charge of $2.1 million in the third quarter of 2021 to reduce the carrying value of inventories within the Downhole Technologies segment to their estimated net realizable value based primarily on management's decision to exit a product offering.


			September 30, 2021			December 31, 2020
Property, plant and equipment, net:
Land	$	33,622		$	34,968
Buildings and leasehold improvements							260,772		267,072
Machinery and equipment							240,302		239,986
Completion-related rental equipment							504,890		507,755
Office furniture and equipment							32,936		35,767
Vehicles							75,119		81,607
Construction in progress	8,059			7,207
Total property, plant and equipment	1,155,700			1,174,362
Accumulated depreciation	(815,316)			(790,800)
							$	340,384	$	383,562

For the three months ended September 30, 2021 and 2020, depreciation expense was $14.7 million and $18.0 million, respectively. Depreciation expense was $46.7 million and $56.6 million for the nine months ended September 30, 2021 and 2020, respectively.


	September 30, 2021						December 31, 2020
	Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization		Net Carrying Amount
Other intangible assets:
Customer relationships	$	168,283	$	63,956	$	104,327	$	168,288	$	55,380	$	112,908
Patents/Technology/Know-how	78,692		31,497		47,195		75,920		26,124		49,796
Noncompete agreements	—		—		—		16,044		14,742		1,302
Tradenames and other	53,708		14,385		39,323		53,708		11,965		41,743
	$	300,683	$	109,838	$	190,845	$	313,960	$	108,211	$	205,749

For the three months ended September 30, 2021 and 2020, amortization expense was $4.9 million and $6.2 million, respectively. Amortization expense was $15.4 million and $18.7 million for the nine months ended September 30, 2021 and 2020, respectively.


	September 30, 2021		December 31, 2020
Other noncurrent assets:
Deferred compensation plan	$	24,609	$	22,801
Deferred financing costs	2,886		—
Deferred income taxes	1,281		1,280
Other	5,089		5,646
	$	33,865	$	29,727


	September 30, 2021		December 31, 2020
Accrued liabilities:
Accrued compensation	$	22,072	$	18,463
Accrued taxes, other than income taxes	12,558		7,307
Insurance liabilities	6,684		7,694
Accrued interest	5,185		2,202
Accrued commissions	1,664		1,416
Other	5,268		7,422
	$	53,431	$	44,504