These results reflect a marked sequential increase in activity as four out of five of our operating segments delivered a revenue increase on average of more than 19%.
As announced yesterday, we are raising our EBITDA guidance range to 200 to $225 million for 2021.
For the quarter, we generated adjusted earnings before interest, taxes, depreciation, and amortization or adjusted EBITDA of $60.6 million, exceeding consensus estimates.
During the second quarter, we generated $50.5 million in cash from operating activities and used 12.6 million for maintenance and growth in capital expenditures resulting in free cash flow generation of $37.9 million.
In addition, during the quarter, we retired $30.5 million of our 2024 senior notes through open market repurchases.
In total, our cash position increased by $13.3 million, resulting in a cash balance of $456 million at the end of the second quarter.
Liquidity remains strong with no borrowings against our $500 million revolving credit facility and no loan maturities until November of 2024.
The positive operating results were attributable to a seasonally influenced 14% sequential growth in revenue, complemented by continued operating discipline and incremental efficiency gains.
Sequentially, consolidated adjusted operating results increased by $9.1 million with all of our operating segments generating positive adjusted operating results and EBITDA.
Subsea Robotics or SSR, adjusted operating income improved on nearly 20% higher revenue.
Our SSR quarterly adjusted EBITDA margin of 31% was consistent with recent quarters as pricing remains stable.
The SSR revenue split was 80% from our remotely operated vehicles or ROV business and 20% from our combined tooling and survey businesses, compared to the 78-22 split, respectively, in the immediate prior quarter.
With an increase in days for both drill support and vessel-based services, days on hire were 14,005, as compared to 11,887 during the first quarter.
Our fleet use was 58% in drill support and 42% in vessel-based services versus 64% and 36%, respectively, in the first quarter.
We maintained our fleet count at 250 ROV systems, and our second-quarter fleet utilization was 62%, up significantly from 53% in the first quarter.
Average ROV revenue per day on hire of 8,056 was 2% higher than average ROV revenue per day on hire of $7,874 achieved during the first quarter.
At the end of June, we had ROV contracts on 73 of the 126 floating rigs under contract or 58% market share, which was flat with the quarter ending March 31st, 2021, when we had ROV contracts on 78 of the 105 floating rigs under contract.
Subject to quarterly variances, we continue to expect our drill support market share to generally approximate 60%.
Sequentially, our second-quarter 2021 adjusted operating income line on lower segment revenue.
Adjusted operating income margin decreased to 1% in the second quarter from 4% in the first quarter of 2021 as lower revenue increased the ability to leverage our cost base.
Our manufactured products backlog on June 30th, 2021, was $315 million improving on our first-quarter backlog of $248 million.
Our book-to-bill ratio was 1.3 for the six months ended June 30th, 2021, and was 0.8 for the trailing 12 months.
The sequential decline in adjusted operating income margin from 10% in the first quarter of 2021 to 7% in the second quarter of 2021 and was primarily due to unplanned downtime and related costs associated with the Angola riserless light well intervention project, which was partially offset by higher IMR activities in the Gulf of Mexico.
Integrity management and digital solutions or IMDS, sequential adjusted operating income was higher on a 19% increase in revenue, higher seasonal activity and the start-up of several new multiyear projects contributed to the revenue increase, continuing efficiency improvements, including utilization of field personnel resulted in adjusted operating income margin increasing to 7% in the second quarter of 2021 from 5% in the first quarter of 2021.
Our ADTech second-quarter 2021 adjusted operating income improved from the first quarter of 2021 on a 20% increase in revenue.
Adjusted operating income margin of 18% was better than forecast due to project mix and favorable rate base adjustments.
Adjusted unallocated expenses of $30.3 million was slightly lower sequentially due to lower expense accruals related to incentive-based compensation forfeitures.
We are projecting a decline in our consolidated adjusted operating results on moderately lower revenues with adjusted EBITDA in the range of 50 million to $55 million.
Unallocated expenses are expected to be in the mid-$30 million range due primarily to increased information technology infrastructure costs and normalized accruals for incentive-based compensation.
For ROVs, we expect our 2021 service mix to remain about the same as the 2020 mix of 62% drill support and 38% vessel-based services with higher vessel-based percentages during the seasonally higher second and third quarters.
We estimate overall ROV fleet utilization to be in the mid- to high 50% range, again, with higher seasonal activity during the second and third quarters.
We continue to forecast that our market share for the drill support market will remain around 60% for the foreseeable future.
As of June 30th, 2021, there were approximately 28 Oceaneering ROVs onboard 37 drilling rigs with contract terms expiring by year-end.
During the same period, we expect 33 of our ROEs on 40 floating rigs to begin new contracts.
We forecast that our operating income margins will be in the low to mid-single-digit range for the year the segment book-to-bill ratio will be in the range of 1.1 to 1.5 for the full year.
Our estimated organic capital expenditure total for 2021 remains between 50 and $70 million.
This includes approximately 35 to $40 million of maintenance capital expenditures and 15 to $30 million of capital expenditures.
We forecast our 2021 income tax payments to be in the range of 40 to $45 million.
We continue to expect $28 million in Cares Act tax refunds.
Unallocated expenses are expected to average in the mid-$30 million range per quarter for the second half of 2021.
Our net debt position improved during the second quarter as we repurchased $30.5 million of our 2024 senior notes, and we're able to build our cash balance by $13.3 million.
We had $456 million of cash and cash equivalents at the end of the second quarter.
And as a reminder, we continue to have our $500 million undrawn revolver available to us until November of 2021 and $450 million available until January 2023.
In summary, based on our first-half financial performance and expectations for the second half of 2021, we are raising our adjusted EBITDA guidance to a range of 200 to $225 million for the full year.

Summary:
Sequentially, our second-quarter 2021 adjusted operating income line on lower segment revenue.