SEGMENT AND GEOGRAPHICAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2022	Jul. 01, 2022	Apr. 01, 2022	Oct. 01, 2021	Jul. 02, 2021	Apr. 02, 2021	Sep. 30, 2022	Oct. 01, 2021
Segment reconciliation
Contract revenue	$ 121,399			$ 98,297			$ 315,882	$ 261,537
Depreciation and amortization	4,405			4,267			13,240	12,678
Interest expense, net	1,435			937			3,216	3,100
Segment profit (loss) before income tax expense	(1,450)			604			(13,611)	(12,884)
Income tax expense (benefit)	(1,526)			(236)			(5,588)	(5,357)
Net income (loss)	76	$ (4,326)	$ (3,773)	840	$ (4,601)	$ (3,766)	(8,023)	(7,527)
Energy
Segment reconciliation
Contract revenue	102,625			81,524			262,186	209,977
Depreciation and amortization	4,166			4,023			12,506	11,934
Interest expense, net							8	5
Segment profit (loss) before income tax expense							(12,525)	(5,607)
Income tax expense (benefit)							(5,143)	(2,331)
Net income (loss)							(7,382)	(3,275)
Engineering and Consulting
Segment reconciliation
Contract revenue	18,774			16,773			53,696	51,560
Depreciation and amortization	239			244			734	744
Segment profit (loss) before income tax expense							8,111	7,507
Income tax expense (benefit)							3,330	3,122
Net income (loss)							4,780	4,386
Operating Segments [Member] | Energy
Segment reconciliation
Interest expense, net	3
Segment profit (loss) before income tax expense	(1,815)			2,138
Income tax expense (benefit)	(1,565)			610
Net income (loss)	(250)			1,529
Operating Segments [Member] | Engineering and Consulting
Segment reconciliation
Segment profit (loss) before income tax expense	3,473			2,673
Income tax expense (benefit)	1,781			1,286
Net income (loss)	1,692			1,387
Corporate, Non-Segment [Member]
Segment reconciliation
Interest expense, net	1,432			937			3,208	3,095
Segment profit (loss) before income tax expense	(3,108)			(4,207)			(9,197)	(14,784)
Income tax expense (benefit)	(1,742)			(2,132)			(3,775)	(6,148)
Net income (loss)	(1,366)			(2,076)			(5,421)	(8,638)
Intersegment Eliminations [Member]
Segment reconciliation
Contract revenue	$ 0			$ 0			$ 0	$ 0