(4) CONCENTRATION OF CREDIT RISK AND TRADE ACCOUNTS RECEIVABLE

Our primary market areas are located in the United States along the Gulf Coast, in the Southeast, in Brownsville, Texas, along the Mississippi and Ohio Rivers, in the Midwest and along the West Coast. We have a concentration of trade receivable balances due from companies engaged in the trading, distribution and marketing of refined products and crude oil. These concentrations of customers may affect our overall credit risk in that the customers may be similarly affected by changes in economic, regulatory or other factors. Our customers’ historical financial and operating information is analyzed prior to extending credit. We manage our exposure to credit risk through credit analysis, credit approvals, credit limits and monitoring procedures, and for certain transactions we may request letters of credit, prepayments or guarantees. Amounts included in trade accounts receivable that are accounted for as ASC 606 revenue in accordance with ASC 606 approximate $4.7 million at December 31, 2019. We maintain allowances for potentially uncollectible accounts receivable.

Trade accounts receivable, net consists of the following (in thousands):


	December 31,		December 31,
	2019		2018
Trade accounts receivable	$	16,627	$	14,158
Less allowance for doubtful accounts		(127)		(109)
	$	16,500	$	14,049

The following table presents a roll forward of our allowance for doubtful accounts (in thousands):


	Balance at						Balance at
	beginning		Charged to				end of
	of period		expenses		Deductions		period
2019	$	109	$	18	$	—	$	127
2018	$	111	$	—	$	(2)	$	109
2017	$	119	$	—	$	(8)	$	111

The following customers accounted for at least 10% of our consolidated revenue in at least one of the periods presented in the accompanying consolidated statements of operations:


	Year ended		Year ended		Year ended
	December 31,		December 31,		December 31,
	2019		2018		2017
NGL Energy Partners LP	16	%	22	%	26	%
RaceTrac Petroleum Inc.	10	%	11	%	13	%
Castleton Commodities International LLC	9	%	10	%	13	%