This paper summarizes key aspects of work performed to demonstrate the suitability of a combination drilling/production riser concept for the Holstein Deepwater Development Project. Significant tasks included: toughness and fatigue crack growth rate testing of riser component materials; fracture mechanics-based fatigue life estimations; simulation and evaluation of drilling/completion damage to riser components; and evaluation and selection of in-situ riser inspection tools. Finally, the paper discusses a rational in-situ inspection program that could be implemented for the Holstein combo risers.
The Holstein field lies in approximately 4344 ft of water in Green Canyon Blocks 644 and 645, 190 miles south of New Orleans, LA. To develop the field, BP selected a truss spar equipped with dry trees. A top tensioned, dual barrier riser system tied each of 15 wells (original appraisal well plus 14 others) back to its Christmas tree on the spar. BP set the minimum project life as 20 years. Additionally, operational plans called for water injection into the field for pressure maintenance and enhanced oil recovery. As such, the project design included souring of the reservoir during the life of the field.
BP's drilling strategy for Holstein involved using the platform rig to perform drilling and completion tasks through the production riser system on each well and hence the term combo riser. This strategy improved operational efficiency offshore during drilling and precluded the need for a dedicated drilling riser. However, the strategy also required an evaluation program to confirm the technical validity of using the Holstein risers for both drilling and production.
The Holstein Well Systems team designed an evaluation program to confirm the suitability of the combo riser approach that included:
toughness and fatigue crack growth rate testing of candidate riser materials
simulation of drilling/completion damage to riser pipe
evaluation and selection of in-situ inspection tools for detecting damage to the riser components
identification and implementation of in-situ mitigation actions to minimize the likelihood of damage to riser components
estimation of remaining fatigue lives of damaged riser components using fracture mechanics
CTOD and fatigue crack growth rate tests were performed on candidate riser materials. The materials of interest included API 5L Grade X80 seamless line pipe, API 5CT Grade T95 Type I casing and two low alloy steel forging steels, ASTM A707 Grade III modified and ASTM A182 Grade F22 modified. The CTOD tests were performed in accordance with ASTM E1290 at a temperature of 32°F. The test specimen orientation was L-C.
The fatigue crack growth rate testing was performed in accordance with ASTM E-647. The test configuration evaluated the straight line or Paris Law portion of the da/dN versus ?K curve. The lab performed the tests with the machine at constant load amplitude resulting in an increasing ?K test.