Patents and other intangible assets

12 Months Ended

Dec. 31, 2015

Goodwill And Intangible Assets Disclosure [Abstract]

4.	Patents and other intangible assets

	December 31,
(U.S. Dollars in thousands)	2015			2014
Cost
Patents	$	35,637		$	35,420
Trademarks—finite lived		439			596
License and other		6,248			7,248
		42,324			43,264
Accumulated amortization
Patents		(32,088	)		(31,198	)
Trademarks—finite lived		(392	)		(469	)
License and other		(4,542	)		(4,445	)
		(37,022	)		(36,112	)
Patents and other intangible assets, net	$	5,302		$	7,152

Amortization expense for intangible assets was $1.7 million, $2.3 million and $2.7 million for the years ended December 31, 2015, 2014 and 2013, respectively. Future amortization expense for intangibles is estimated as follows:

(U.S. Dollars in thousands)	Amortization
2016	$	1,450
2017		1,196
2018		549
2019		551
2020		457
Thereafter		1,099
	$	5,302