Exhibit 23.5



|     |                                                                                                                |                     
|---- | -------------------------------------------------------------------------------------------------------------- | --------------------
|LOGO |                                                                                                                | Collarini Associates
|     | 11111 Richmond Avenue, Suite 126 Houston, Texas 77082 Tel. (832) 251-0160
Fax (832) 251-0157 www.collarini.com

February 15, 2010

Mr. Jerry Kennedy

ATP Oil & Gas
Corporation

4600 Post Oak Place, Suite 200

Houston, Texas 77027-9726

Dear Mr. Kennedy:

In accordance with your request and to enable ATP Oil & Gas Corporation (ATP) to satisfy the requirements of ATPs annual reporting, we
have estimated the proved reserves and future revenue, as of January 1, 2010, to the interest of ATP in and related to certain oil and gas properties, located in the Gulf of Mexico and in the North Sea. The estimate of proved reserves and the
future revenue therefrom conform to all standards and definitions promulgated in Section 210.4-10 of Regulation S-X issued by the Securities and Exchange Commission in November 1988 and amended in December 2008. It is estimated these volumes
represent 98% of ATPs total proved reserves.

As presented in the accompanying detailed projections by reservoir and by reserve
category, we estimate the net reserves and future net income to the ATP Oil & Gas Corporation interest, as of January 1, 2010, in the Gulf of Mexico, to be:



|                 |  |                        |        |  |                        |  |         |              |  |  |                     |  |  |  |           | 
|---------------- |  | ---------------------- | ------ |  | ---------------------- |  | ------- | ------------ |  |  | ------------------- |  |  |  | --------- | 
|                 |  | Net Remaining Reserves |        |  | Future Net Income (M$) | 
|Reserve Category |  | Oil(MBO)               |        |  | Gas(MMCF)              |  |         | Undiscounted |  |  | PresentWorth at 10% | 
|                 |  |                        |        | 
|Proved           |  |                        |        |  |                        |  |         |              |  |  |                     |  |  |  |           | 
|Producing        |  |                        | 6,214  |  |                        |  | 23,528  |              |  |  | 467,087             |  |  |  | 377,333   | 
|Shut-In          |  |                        | 24     |  |                        |  | 11,624  |              |  |  | 40,535              |  |  |  | 35,092    | 
|Behind Pipe      |  |                        | 808    |  |                        |  | 900     |              |  |  | 44,980              |  |  |  | 22,617    | 
|Undeveloped      |  |                        | 44,431 |  |                        |  | 187,873 |              |  |  | 1,976,101           |  |  |  | 1,215,600 | 
|P&A              |  |                        | 0      |  |                        |  | 0       |              |  |  | -116,586            |  |  |  | -50,978   | 
|                 |  |                        |        |  |                        |  |         |              |  |  |                     |  |  |  |           | 
|Total Proved     |  |                        | 51,477 |  |                        |  | 223,926 |              |  |  | 2,412,115           |  |  |  | 1,599,665 | 



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ATP Oil & Gas Corporation

February 15, 2010

Page Two



As presented in the accompanying detailed projections by reservoir and by reserve category, we estimate
the net reserves and future net income to the ATP Oil & Gas Corporation interest, as of January 1, 2010, in the North Sea, to be:



|                 |  |                        |        |  |                        |  |         |              |  |  |                     |  |  |  |         | 
|---------------- |  | ---------------------- | ------ |  | ---------------------- |  | ------- | ------------ |  |  | ------------------- |  |  |  | ------- | 
|                 |  | Net Remaining Reserves |        |  | Future Net Income (M$) | 
|Reserve Category |  | Oil(MBO)               |        |  | Gas(MMCF)              |  |         | Undiscounted |  |  | PresentWorth at 10% | 
|                 |  |                        |        | 
|Proved           |  |                        |        |  |                        |  |         |              |  |  |                     |  |  |  |         | 
|Producing        |  |                        | 4      |  |                        |  | 12,085  |              |  |  | 45,924              |  |  |  | 38,610  | 
|Undeveloped      |  |                        | 25,498 |  |                        |  | 97,497  |              |  |  | 1,018,088           |  |  |  | 338,649 | 
|P&A              |  |                        | 0      |  |                        |  | 0       |              |  |  | -30,156             |  |  |  | -11,189 | 
|                 |  |                        |        |  |                        |  |         |              |  |  |                     |  |  |  |         | 
|Total Proved     |  |                        | 25,501 |  |                        |  | 109,582 |              |  |  | 1,033,856           |  |  |  | 366,071 | 

Oil volumes are generally
expressed in thousands of stock tank barrels (MBO), where one barrel is equivalent to 42 United States gallons. Gas volumes are expressed in millions of standard cubic feet (MMCF) at 60 degrees Fahrenheit and the contract pressure base.

Net sales, as defined in this report, are before deducting production taxes. Net income is after deducting these taxes, and after deducting future
capital costs and operating expenses, but before consideration of federal income taxes. The future net income has also been shown discounted at ten percent to determine its present worth as required by Regulation S-X. This present worth is included
to indicate a standard measure and is not intended to represent the market value of the property. Our estimates of future cash flows include estimates of all costs required to recover reserves.

Reserves in this report were estimated using all applicable engineering and geological data available such as, but not limited to, historic production
volumes, initial flow test information, flowing tubing pressures, shut-in tubing pressures, bottom hole pressures, repeat formation test data, pressure- volume- temperature fluid analysis, geological well logs, sidewall core analysis, and whole core
analysis at the time the report was conducted.

The reserve volumes and their respective classifications and categorizations were estimated by
performance methods, volumetric methods, analogy, or combination of methods. Performance methods generally included decline curve analysis and material balance analysis where representative data was available. Volumetric estimates generally included
a combination of geological and engineering interpretations, while analogy methods included reserve estimates from historical performance of similar wells and reservoirs in the field or nearby fields.

Proved reserve classifications were determined based on the reasonable certainty of recovering the estimated volumes or more. The proved
reserve categorizations were based on the stage of maturity and development of the respective proved reserves.

Based on gross oil equivalent
barrels, approximately 65 percent of ATPs proved reserves are located in the North America, United States, Gulf of Mexico and 35 percent are located in the United Kingdom sector of the North Sea. ATPs proved reserves are approximately 15
percent developed and 85 percent undeveloped in total and in each geographic region.



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ATP Oil & Gas Corporation

February 15, 2010

Page Three



ATPs Gulf of Mexico proved developed reserves are approximately 70 percent proved producing and 30
percent proved non- producing. Approximately 92 percent of the Gulf of Mexicos proved producing reserves were estimated by performance methods while 100 percent of the proved non- producing reserves and 98% of the proved undeveloped reserves
were estimated by volumetric methods. These estimates are based on gross oil equivalent barrels that ATP holds an interest in.

ATPs
North Sea proved developed reserves are 100 percent proved producing. Approximately 73 percent of the North Seas proved producing reserves were estimated by performance and 27 percent were estimated by volumetric methods. Approximately 34
percent of the proved undeveloped reserves in the North Sea were estimated by performance methods, 34 percent were estimated by volumetric methods and 11 percent were estimated by analog methods. These estimates are based on gross oil equivalent
barrels that ATP holds an interest in.

A detail description of methodology used in estimating reserves for each well and reservoir in each
field is tabulated in the respective field section under Reserve Summary included in the detailed report.

Hydrocarbon prices used
in this report are based on SEC price parameters using the average prices received on the first of each month during the 12-month period prior to the ending date of the period covered in this report, determined as an unweighted arithmetic average of
the first-day-of-the-month price for each month within such period. The product prices used to determine future gross revenue for each field were determined by applying benchmark pricing as describe above then adjusted by differentials
only to the extent provided by SEC guidelines. These differentials generally adjust the benchmark prices on a field by field basis to account for product quality, transportation, and marketing. The differentials were provided
to Collarini Associates in detail from ATP. Collarini accepted the differentials as factual data and did not confirm the accuracy of these adjustments. The following table summarizes by major geographic region the average benchmark
prices and the average realized price after the appropriate differentials were applied.



|                  |  |                 |  |           |                    |  |               |  |             |              |  |  |            | 
|----------------- |  | --------------- |  | --------- | ------------------ |  | ------------- |  | ----------- | ------------ |  |  | ---------- | 
|                  |  |                 |  | Price     |                    |  | Avg Benchmark |  |             | Avg Realized | 
|Geographic Region |  | Product         |  | Reference |                    |  | Price         |  |             | Price        | 
|North America     |  |                 |  |           |                    |  |               |  |             |              |  |  |            | 
|United States     |  |                 |  |           |                    |  |               |  |             |              |  |  |            | 
|Gulf of Mexico    |  | Oil/ Condensate |  |           | WTI Cushing        |  |               |  | $61.18/Bbl  |              |  |  | $58.29/Bbl | 
|                  |  | Gas             |  |           | Henry Hub          |  |               |  | $3.87/MMBtu |              |  |  | $4.22/MCF  | 
|United Kingdom    |  |                 |  |           |                    |  |               |  |             |              |  |  |            | 
|North Sea         |  | Oil/ Condensate |  |           | Brent Oil          |  |               |  | $59.73/Bbl  |              |  |  | $56.73/Bbl | 
|                  |  | Gas             |  |           | Heren Index (BNBP) |  |               |  | $4.95/MMBtu |              |  |  | $4.69/MCF  | 



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ATP Oil & Gas Corporation

February 15, 2010

Page Four



Future development costs included in this report were provided to Collarini Associates by ATP and
accepted as factual data. Collarini Associates reviewed, for reasonableness, the estimated future development cost, plug and abandonment cost and their respective timing for each well, reservoir, and field. Collarini did not, however, confirm the
accuracy of these expenditures. These costs are tabulated in the respective field section under Forecast of Expenditures included in the detailed report. These costs are held constant and not escalated. It should be noted, as provided by
ATP, the value of the ATP Innovator floating platform, located at Mississippi Canyon 711, and its associated infrastructure, is $225.676 million as of December 31, 2009. The value is depreciated using a straight-line depreciation method over
its estimated useful life. The depreciated book value is reflected in the appropriate proved category.

Operating costs are based on actual
expenses, as provided by ATP Oil & Gas Corporation. Collarini did not confirm the accuracy of these expenses. These current expenses are held constant through the life of the property. These costs include processing fees where applicable.
Estimated lease fuel-gas usage has been included as a reduction to produced volume in the cash flow forecast.

Collarini Associates utilized
all data, appropriate methods and procedures deemed necessary to conduct and finalize this report to conform to all standards and definitions promulgated in Section 210.4-10 of regulation S  X issued by the Securities and Exchange
Commission in November 1988 and amended in December 2008.

The reserves presented in this report are estimates only and should not be
construed as being exact quantities. They may or may not be recovered, and if recovered, the revenues, costs, and expenses therefrom may be more or less than the estimated amounts. Because of governmental policies, uncertainties of supply and
demand, and international politics, the actual sales rates and the prices actually received for the reserves, as well as the costs of recovery, may vary from those assumptions included in this report. Also, estimates of reserves may increase or
decrease as a result of future operational decisions, mechanical problems, and the price of oil and gas.

All reserve estimates have been
performed in accordance with sound engineering principles and generally accepted industry practice. As in all aspects of oil and gas evaluation, there are uncertainties inherent in the interpretation of engineering data, and all conclusions
represent only informed professional judgments.

The titles to the properties have not been examined by Collarini Associates, nor has the
actual degree or type of interest owned been independently confirmed. The data used in our estimates were obtained from ATP Oil & Gas Corporation and from sources which provide publicly accessible data and are considered accurate.



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ATP Oil & Gas Corporation

February 15, 2010

Page Five



A detailed environmental and mechanical inspection of the properties was not made by Collarini
Associates. A visual inspection of the properties themselves was not considered necessary for the purpose of this report. No assessment of compliance with environmental regulations or future liability for site remediation was made. We are
independent consultants; we do not own any interest in this property and are not employed contingent upon the value of this property. All engineering calculations and basic data used in the analysis are maintained on file in our office and are
available for review.

Very truly yours,

COLLARINI ASSOCIATES

LOGO

Mitch Reece, P.E.

President

MCR/tlk

Collarini Engineering Inc.

Texas Board of
Professional Engineers Registration F-5660