$20 million resulting from severance costs of offshore and onshore personnel, (e) approximately $15 million resulting from onshore personnel costs, excluding severance, (f) approximately $10 million resulting from other changes in rig activity and (g) approximately $10 million resulting from reduced costs related to COVID-19 mitigation. These decreases were partially offset by the following increases: (a) approximately $28 million resulting from our allowance for excess materials and supplies due to our identification of certain items that were in excess of our expected future usage based on our current market outlook, (b) approximately $35 million resulting from increased personnel costs, primarily due to unfavorable exchange rates, and (c) approximately $20 million resulting from shipyard and maintenance costs primarily driven by out-of-service activities.
Depreciation and amortization expense decreased for the year ended December 31, 2021, compared to the year ended December 31, 2020, primarily due to approximately $22 million resulting from rigs that were sold and approximately $16 million resulting from assets that had reached the end of their useful lives or had been retired.
General and administrative expense decreased for the year ended December 31, 2021, compared to the year ended December 31, 2020, primarily due to the following: (a) approximately $13 million resulting from reduced personnel costs, including severance, related to our cost savings plan implemented in the year ended December 31, 2020 and (b) approximately $5 million resulting from reduced costs for information systems and technology, partially offset by (c) approximately $6 million resulting from increased strategy and innovation costs.
Loss on impairment or disposal of assets—In the year ended December 31, 2020, we recognized a loss on the impairment of assets, including an aggregate net loss of $556 million associated with assets that we determined were impaired at the time we classified them as held for sale, a loss of $31 million associated with the impairment of our midwater floater asset group and a loss of $10 million associated with the impairment of other assets.
In the year ended December 31, 2021, we recognized an aggregate net loss of $57 million, primarily associated with the sale of a harsh environment floater and related assets. In the year ended December 31, 2020, we recognized an aggregate net loss of $61 million associated with the sale of one ultra-deepwater floater, three harsh environment floaters and three midwater floaters, along with related assets. In the years ended December 31, 2021 and 2020, we recognized an aggregate net loss of $5 million and $23 million, respectively, associated with the disposal of assets unrelated to rig sales.
Other income and expense—Interest expense, net of amounts capitalized, decreased in the year ended December 31, 2021, compared to the year ended December 31, 2020, primarily due to a decrease of $145 million resulting from debt early retired, repaid or restructured, partially offset by an increase of $19 million resulting from debt issued.
In the year ended December 31, 2021, we recognized an aggregate net gain of $51 million associated with the retirement of $323 million aggregate principal amount of the 0.50% Exchangeable Senior Bonds as a result of the 2021 Private Exchange. In the year ended December 31, 2020, we recognized a net gain on restructuring and retirement of debt, primarily due to the following: (a) an aggregate gain of $427 million associated with the restructuring of debt in the private exchange transactions in August 2020 (the “2020 Private Exchange”) and the exchange offers in September 2020 (the “2020 Exchange Offers”), (b) an aggregate gain of $135 million associated with the retirement of $360 million aggregate principal amount of our debt securities in the tender offers in November 2020, (c) an aggregate gain of $36 million associated with the retirement of $147 million aggregate principal amount of our debt securities repurchased in the open market, partially offset by (d) a loss of $65 million associated with the full redemption of the 9.00% senior notes due July 2023 (the “9.00% Senior Notes”).
Other income, net, increased in the year ended December 31, 2021, compared to the year ended December 31, 2020, primarily due to the following: (a) decreased loss of $22 million resulting from impairment of our equity investment in Orion Holdings (Cayman) Limited (“Orion”), (b) increased income of $24 million resulting from a settlement of litigation and other claims, (c) increased income of $23 million related to the non-service components of net periodic benefit income and (d) decreased losses of $7 million resulting from net changes to currency exchange rates, partially offset by (e) decreased income of $25 million related to our investment in Orion and (f) decreased income of $4 million related to our dual-activity patent.
Income tax expense—In the years ended December 31, 2021 and 2020, our effective tax rate was (25.7) percent and (5.1) percent, respectively, based on loss before income tax expense. In the years ended December 31, 2021 and 2020, the aggregate effect of discrete period tax items was a net tax expense of $47 million and benefit of $91 million, respectively. In the year ended December 31, 2021, such discrete items included the effect of tax law changes in Switzerland and jurisdictional ownership changes of certain assets, loss on disposal of assets, expiration and settlements of various uncertain tax positions, gain on retirement of debt, changes to our allowance for excess materials and loss on impairment of an equity investment. In the year ended December 31, 2020, such discrete items included losses on impairment and disposal of assets, gain on restructuring and retirement of debt, revenues recognized for the settlement of disputes, the loss on impairment of an equity investment, the carryback of net operating losses in the U.S., including the release of valuation allowances previously recorded, settlements and expirations of various uncertain tax positions and accruals for withholding taxes. In the years ended December 31, 2021 and 2020, our effective tax rate, excluding discrete items, was (18.5) percent and (23.4) percent, respectively, based on loss before income tax expense. In the year ended December 31, 2021 compared to the year ended December 31, 2020, our effective tax rate increased primarily due changes in the relative blend of income from operations in certain jurisdictions.
Due to our operating activities and organizational structure, our income tax expense does not change proportionally with our income before income taxes. Significant decreases in our income before income taxes typically lead to higher effective tax rates, while significant