**Exhibit 10.35**

 
|  | **Fort Worth**

777 Main Street   Suite 2900

Fort Worth TX 76102   Office: 817.862.2000   Toll Free: 866.877.1008

Fax: 817.339.3640 |
|--|--------------------------------------------------------------------------------------------------------------------------------------------|

 

December 19, 2016

 

Karen Thornton

VP Human Resources

Hand Delivered

 

Re: **Updated Retention Bonus Offer**

** **

Dear Karen,

 

On August 5, 2015, you received a letter awarding you a retention bonus in recognition of your contributions and your support of our cost containment initiatives during the oil and gas economic downturn. To this day, the downturn continues; noted as one of the most severe in our industry’s history.

 

Despite our best efforts and industry leading results, we still have a lot of work to do in order to meet our milestones. In recognition of all you have done to keep FTSI moving forward, I would like to offer you a revised bonus and salary replacement schedule. If accepted, this arrangement will replace and cancel your letter dated August 5, 2015.

 

As described below, the bonus and salary replacement will be paid in a manner designed for our future growth and accomplishments.

 

| **Milestone**                                                                    | ** ** | **Bonus Amount Paid**            | ** ** | **Salary Reinstatement**          |
|----------------------------------------------------------------------------------|------------|--------------------------------------------|------------|----------------------------------------|
|                                                                             |       |                                       |       |                                   |
| December 25, 2016                                                           |       | 5% of Salary (as of   August 8, 2015) |       | 50% of August 9,   2015 reduction |
|                                                                             |       | payable on 12/ 30/16   pay date            |       | beginning as of 1/13/2017 pay date     |
|                                                                             |       |                                       |       |                                   |
| Later of July 1,   2017                                                     |       |                                       |       |                                   |
| payable as of 7/14/17   pay                                                      |       | 25% of Salary (as of   8/8/15)             |       | 50% of August 9,   2015 reduction |
| date                                                                             |       |                                       |       |                                   |
|                                                                             |       |                                       |       |                                   |
| or                                                                               |       |                                       |       |                                   |
|                                                                             |       |                                       |       |                                   |
| Achievement of monthly Adjusted EBITDA averaging   $15M for 3 consecutive months |       |                                       |       |                                   |

 

As before, to be eligible for the retention bonus payments, you must be an active full-time employee of the Company on the applicable bonus payment dates. If you are terminated for cause, you will not be eligible for the retention bonus payable thereafter. If you are terminated without cause or the Company completes a sale of a majority of its equity (other than an IPO) or substantially all of its assets, the retention bonus will be paid to you in the next available pay cycle. You must sign FTSI’s Separation and Release Agreement in order to receive this payment.

 

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We have put our minds to work every day, overcoming obstacles and making personal sacrifices. Today, we are well on our way to capturing greater market share, and with our improved efficiencies, and we have set the stage for ongoing growth and profitability. To succeed, I continue to need you to be engaged in the business at hand. So as we enter the home stretch of our journey, continue to be creative and passionate about what we do every day.

 
| Sincerely,                                                  |   |       |
|-------------------------------------------------------------|--------|------------|
|                                                        |   |       |
| /s/ Michael Doss                                            |   |       |
| Michael Doss                                                |   |       |
| Chief Executive   Officer                                   |   |       |
|                                                        |   |       |
|                                                        |   |       |
| I agree to and   accept the terms set forth in this letter. |   |       |
|                                                        |   |       |
| /s/ Karen   Thornton                                        |   | 12/20/2016 |
| Employee   Signature                                        |   | Date       |

 

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