Intangible assets are recorded at cost, or when acquired as a part of a business combination, at estimated fair value, less accumulated amortization. These assets are amortized on a straight-line basis over the useful lives of the assets, which the Company believes is materially consistent with the pattern of economic benefit provided by the assets.


		December 31,
(U.S. Dollars, in thousands)	Weighted Average Amortization Period	2022			2021
Cost
Patents	10.0 years	$	40,108		$	44,561
Developed technology	9.8 years		43,699			43,979
IPR&D	Indefinite		300			300
Customer relationships	7.8 years		15,572			15,621
License and other	9.3 years		23,295			18,924
Trademarks—finite lived	10.0 years		1,875			1,839
	9.3 years		124,849			125,224
Accumulated amortization
Patents		$	(37,506	)	$	(41,408	)
Developed technology			(17,830	)		(13,409	)
Customer relationships			(6,938	)		(4,520	)
License and other			(14,386	)		(12,528	)
Trademarks—finite lived			(801	)		(693	)
			(77,461	)		(72,558	)
Intangible assets, net		$	47,388		$	52,666

Acquired IPR&D represents the fair value assigned to acquired research and development assets that have not reached technological feasibility. In a business combination, the fair value assigned to acquired IPR&D is determined by estimating the remaining costs to develop the acquired technology into commercially viable products, estimating the resulting revenues from the projects, and discounting the net cash flows to present value. The revenue and cost projections used to value acquired IPR&D are, as applicable, reduced based on the probability of success of developing the asset. Additionally, estimated revenues consider the relevant market sizes and growth factors, expected trends in technology, and the nature and expected timing of new product introductions by the Company and its competitors. The rates utilized to discount the net cash flows to their present value are commensurate with the stage of development of the project and uncertainties in the economic estimates used in the projections. Any future costs to further develop the IPR&D subsequent to acquisition are recorded to research and development expense as incurred.

IPR&D assets are considered to be indefinite-lived assets until the completion or abandonment of the associated research and development efforts. During the period the assets are considered indefinite-lived, they are not amortized but tested for impairment. Impairment testing is performed at least annually or when a triggering event occurs that could indicate a potential impairment. If and when development is complete, which generally occurs when regulatory approval to market a product is obtained, the associated assets are reclassified to developed technology and are amortized over an assigned useful life that best reflects the economic benefits provided by these assets.

Amortization expense for intangible assets was $9.4 million, $9.4 million, and $11.2 million for the years ended December 31, 2022, December 31, 2021, and 2020, respectively. Future amortization expense for intangible assets is estimated as follows:


(U.S. Dollars, in thousands)	Amortization
2023	$	9,250
2024		8,705
2025		7,692
2026		6,658
2027		6,470
Thereafter		8,313
Total finite-lived intangible assets, net	$	47,088
Indefinite-lived intangible assets, net		300
Intangible assets, net	$	47,388