Year Ended December 31,

2018

2017

Asset retirement obligations, beginning of period

$

300,446

$

334,438

Liabilities settled

(28,617

)

(72,409

)

Accretion of discount

18,431

17,172

Liabilities incurred and assumed through acquisition

4,286

163

Revisions of estimated liabilities ^{(1) (2)}

15,591

21,082

Asset retirement obligations, end of period

310,137

300,446

Less current portion

24,994

23,613

Long-term

$

285,143

$

276,833

(1)

Revisions in 2018 reflect cost estimate increases as a result of new data on the required scope of work becoming available to us through 2018.  This new data included data realized during the planning phase of the projects, and as the projects proceeded through the execution phase. This new data indicated that the scope was larger and more difficult than the scope used for end of 2017 estimates.  As an example, larger heavy lift vessels would be needed for certain platform removals, and certain wells needed additional well plugging operations to complete the decommissioning per agency requirements.

(2)

Revisions in 2017 were primarily related to increased costs associated with wells at four fields that experienced sustained casing pressure issues.  Wells that experience sustained casing pressure require more days and greater work scope to complete the abandonment project.  Partially offsetting are downward revisions to cost estimates from service providers for plug and abandonment work at certain locations.