Intangible assets are recorded at cost, or when acquired as a part of a business combination, at estimated fair value, such as for in-process research and development (“IPR&D”) assets. 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	2021			2020
Cost
Patents	10 years	$	44,561		$	50,326
Developed technology	10 years		43,979			44,334
IPR&D	Indefinite		300			300
Customer relationships	7 years		15,621			15,685
License and other	8 years		18,924			16,941
Trademarks—finite lived	10 years		1,839			1,812
	9 years		125,224			129,398
Accumulated amortization
Patents		$	(41,408	)	$	(46,272	)
Developed technology			(13,409	)		(8,925	)
Customer relationships			(4,520	)		(2,095	)
License and other			(12,528	)		(11,006	)
Trademarks—finite lived			(693	)		(583	)
			(72,558	)		(68,881	)
Intangible assets, net		$	52,666		$	60,517

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 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 deemed finite-lived and are amortized over a period that best reflects the economic benefits provided by these assets.

Amortization expense for intangible assets was $9.4 million, $11.2 million and $7.0 million for the years ended December 31, 2021, December 31, 2020, and 2019, respectively. Future amortization expense for intangible assets is estimated as follows:

(U.S. Dollars, in thousands)	Amortization
2022	$	9,376
2023		8,692
2024		8,254
2025		7,252
2026		6,215
Thereafter		12,577
Total finite-lived intangible assets, net	$	52,366
Indefinite-lived intangible assets, net		300
Intangible assets, net	$	52,666