Atul Ganpatye, Kenneth Bhalla May 17, 2009


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Weak Point Analysis Development Driller 2 Drilling Riser on Horizon Lower Stack 4992 ft Water Depth (Macondo Site)


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Purpose

  • Part 1: Weak Point Analysis – Riser Connected

  • Given that the DD2 riser and stack are installed on the Horizon BOP at Macondo, the aim of this work is to assess riser & well system component strength/integrity in the event of extreme vessel offsets with the riser still connected for the following event(s):

  • - Drive-Off or a Drift-Off Event

  • - Failure of the Emergency Disconnect System

  • Determine potential weak-points and approximate critical vessel offsets

  • Part 2: Response After DD2 LMRP Disconnection

  • This part of the analyses was conducted to estimate the response of the casing after the riser has been disconnected at the DD2 LMRP - in light of the fact that some of the soil may have plasticized and can no longer provide lateral support to the casing and the BOP system may be leaning at some angle.



Important Assumptions (Weakpoint analysis with riser connected)

  • It has been assumed that the soil properties are the same as those assumed for earlier project (PN 119681) at SES. This implicitly assumes that the Deepwater Horizon incident have NOT resulted in any localized changes in the soil properties in the vicinity of the well

  • The wellhead/stack assembly has been assumed to be vertical at the start of the analysis when the vessel is on the well location (zero vessel offset). This precludes the possibility of any plastic (permanent) deformation and subsequent inclination of the casing system following the Deepwater Horizon incident

  • Damage to the wellhead/wellhead connector has not been accounted for.



Additional Assumptions

  • Global ABAQUS model of: riser, tensioners, telescopic joint, flex joints, casing, wellhead, wellhead connector and soils

  • Tensioners are assumed to operate at mid-stroke at 0% offset

    • It should be noted that if the tensioners have larger allowable stroke, the wellhead bending moment and the casing stresses may increase over the mid-stroke case.
  • Planar offset

  • Weakpoint analysis is quasi-static analysis rather than time domain

  • Lateral P-Y Soil Response

  • No dynamic vessel motion or seastate is imposed

  • Linear elastic material models utilized



General Information

  • Development Driller 2 drilling riser configuration in 4992-ft (GOM)

  • Casing program specified per BP

  • Deepwater Horizon BOP Stack on Wellhead

  • Development Driller 2 vessel, tensioners, drilling riser, LMRP and BOP on top of Deepwater Horizon BOP Stack

  • 8.56 ppg mud weight

  • Tolerance factors of fwt =1.02 and fbt = 0.96 assumed

  • 2 of 12 tensioners subject to failure





Riser Configuration Schematic



DD2 Stack Used in the Analysis



Horizon Stack Used in the Analysis



Horizon Stack Used in the Analysis







Soils

  • API RP2A P-Y curves are developed from the soil shear strength, submerged unit weight and 50.

  • Soil data is assumed per:



PART 1 Weak Point Analysis – Riser Connected



Case Matrix







Wellhead Capacity

  • High and Low Pressure Housing Wellhead is Dril-Quip SS-15 System

    • SS-15 is rated for 4 million ft-lb (15-ksi pressure and 1500-kips tension) at 2/3 yield : Working Capacity
    • SS-15 is rated for 6 million ft-lb (15-ksi pressure and 1500-kips tension) at yield : Survival Capacity






















PART 2 Response After DD2 LMRP Disconnection





Results: No Initial Inclination



Results: 1 deg Initial Inclination





Results with Updated Soils Data (doubled soil undrained shear strength and used API 2A-WSD lateral resistance-deflection relationship for short term static load case: per suggestion from Philippe Jeanjean)



Soil Data





Bending Moment Distribution Near Mudline





















Soil Plasticity Concerns



Results: 1 deg Initial Inclination with Vertical – Updated Soil Data



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