Patents and other intangible assets

12 Months Ended

Dec. 31, 2014

Goodwill And Intangible Assets Disclosure [Abstract]

4.	Patents and other intangible assets

	December 31,
(U.S. Dollars in thousands)	2014			2013
Cost
Patents	$	35,420		$	34,820
Trademarks—finite lived		596			620
License and other		7,248			7,748
		43,264			43,188
Accumulated amortization
Patents		(33,319	)		(31,739	)
Trademarks—finite lived		(469	)		(454	)
License and other		(2,324	)		(1,949	)
		(36,112	)		(34,142	)
Patents and other intangible assets, net	$	7,152		$	9,046

Amortization expense for intangible assets was $2.3 million, $2.7 million and $2.3 million for the periods ending December 31, 2014, 2013 and 2012, respectively, and is estimated to be approximately $2.2 million, $1.5 million, $1.1 million, $0.5 million, $0.5 million and $1.3 million for the periods ending December 31, 2015, 2016, 2017, 2018, 2019 and 2020 and thereafter, respectively.