General and administrative expenses. The decrease in general and administrative expenses for the nine months ended September 30, 2020, as compared to the nine months ended September 30, 2019, was primarily due to reductions in force implemented in February 2019 and May 2020 and a decrease in salaries for all employees effective April 2020.
Pre-petition charges. During the nine months ended September 30, 2020, we incurred legal and financial advisor costs of $9.9 million related to negotiations for the restructuring of our debt prior to filing the Bankruptcy Petitions for relief under Chapter 11 of the Bankruptcy Code.
Depreciation and amortization expense. The decrease in depreciation and amortization expense for the nine months ended September 30, 2020, as compared to the same period in 2019, resulted from a client-related intangible asset that was fully amortized in August 2019.
Interest expense. The increase in interest expense for the nine months ended September 30, 2020, as compared to the same period of 2019, was due to the compounding effect of PIK interest on the Second Lien PIK Notes and interest incurred on the Revolving Credit Facility.
Write-off of debt premium, net. As a result of the filing of the Bankruptcy Petitions, we wrote off $6.1 million of net debt premium associated with our First Lien Notes and Second Lien PIK Notes and $1.6 million of deferred financing costs associated with the Revolving Credit Facility as of September 30, 2020.
Reorganization items. We classified all income, expenses, gains or losses that were incurred or realized subsequent to the 2017 Petition Date and as a result of the 2017 Bankruptcy Proceedings as reorganization items, which primarily consisted of professional fees. See Emergence from 2017 Bankruptcy Proceedings in Note 2 to our unaudited condensed consolidated financial statements.
Income taxes. Historically, we calculated our provision for income taxes during interim reporting periods both by applying the estimated annual effective tax rate for the full fiscal year to pre-tax income or loss and also using the discrete method on the portion of earnings where no such annual estimate could be reasonably calculated. Based on our expectations that for the remainder of the year we cannot reliably estimate the effective tax rate, we employ the discrete method of determining our tax provision based on the pre-tax results for the nine-month period ending September 30, 2020. We will continue to evaluate income tax estimates under the historical method in subsequent quarters and continue to employ the discrete effective tax rate method if warranted.
The relationship between our provision for or benefit from income taxes and our pre-tax book income can vary significantly from period to period considering, among other factors, (a) the overall level of pre-tax book income, (b) changes in the blend of income that is taxed based on gross revenues or at high effective tax rates versus pre-tax book income or at low effective tax rates and (c) our rig operating structures. Consequently, our income tax expense does not necessarily change proportionally with our pre-tax book income. Significant decreases in our pre-tax book income typically result in higher effective tax rates, while significant increases in pre-tax book income can lead to lower effective tax rates, subject to the other factors impacting income tax expense noted above. Additionally, pre-tax book losses typically result in negative effective tax rates due to withholding taxes, local taxation on profitable operations, and deemed profit tax based on revenue even while reporting operational losses. During the nine months ended September 30, 2020 and 2019, our effective tax rates were (3.1)% and (4.7)%, respectively.
Voluntary Reorganization Under Chapter 11
On the Petition Date, the Debtors filed Bankruptcy Petitions for reorganization under Chapter 11 of the Bankruptcy Code in the Bankruptcy Court, and commenced the Cayman Proceedings. On November 1, 2020, the Bankruptcy Court approved the Debtors’ joint administration of their Chapter 11 proceedings under the caption In re Pacific Drilling S.A., et al., Case No. 20-35212 (DRJ).
The Chapter 11 proceedings and Cayman Proceedings were filed in order to effect the Company’s Plan of Reorganization and implement the Restructuring. Copies of the Plan of Reorganization and associated disclosure