Saturday, January 31, 2015

EVALUATION OF SEAL ASSEMBLY AND CEMENT FAILURES. INTERIM SUMMARY OF FINDINGS




EVALUATION OF SEAL ASSEMBLY AND CEMENT FAILURES.  INTERIM SUMMARY OF FINDINGS


On  February 10,  2013,  a  rig  was  conducting  drilling  operations  on  Main  Pass  Block  295 (“MP295”) in the Gulf of Mexico (GOM), when an underground loss of well control event occurred. The loss of well control resulted in gas flow into a shallow sand, below the conductor casing shoe in the well. This loss of well control incident created a risk of broaching of the well to the seafloor, which created significant risks to the crew, the rig, and the environment.
The well design included a dual barrier system in the annulus that should have prevented uncontrolled flow of hydrocarbons. The initial review determined the dual barrier system failed, allowing gas flow to penetrate a shallow formation below the conductor casing shoe. The first possible failure point was the sub mudline casing hanger seal assembly. This system was designed to hold pressure from the well up to the designated pressure and temperature ratings of the seal assembly. The second possible failure point was the cement column within  the conductor casing and surface casing annulus. The cement column should have created a barrier that prevents hydrocarbon flow from the well to the surrounding environment. Subsequent analysis indicated that there were several other potential points for failure which included damaged casing and/or damaged casing threads.
A Quality Control Failure Incident Team (QC-FIT) was assembled to perform the following: conduct a technical evaluation of the equipment involved in this incident to determine if there were global quality assurance/quality control (QA/QC) issues that needed addressing by BSEE and/or industry issues related to the use of the dual barrier systems. The goal of this report is to provide a general assessment of industry equipment and engineering design related to these systems rather than making a definitive determination concerning the loss of well control on MP295.
This technical evaluation includes a review of available data, relevant industry standards, and input from various subject matter experts. A comprehensive recommendations list is outlined at the end of this report. These recommendations are applicable to dual barrier seal systems and equipment used in well construction that serves as barriers.
Summarized key findings and recommendations are:

·         Existing industry practices and BSEE regulations related to pressure testing may not be adequate to evaluate the integrity of either the seal assembly or the cement column. BSEE should consider modifying its regulations to ensure that the integrity of these barriers can be verified after installation.
·         Operators should be required to verify that any pressure containing equipment installed downhole has been designed, tested, and rated for any potential loss of well control condition to which it might be exposed during its service life.
·         A comprehensive analysis of well designs utilizing shallow liners and sub mudline casing
·         hangers  needs  to  be  performed  by either  BSEE  or  the  industry to  ensure  that  best engineering practices are being utilized to minimize the risk of a loss of well control event.



TERMINOLOGY
A liner is a casing string that does not extend to the top of the wellbore (i.e. does not extend to the wellhead). It is suspended from a previous casing string, by way of a  liner  hanger. Therefore, a liner hanger differs from a casing hanger, in that casing hangers are hung off directly from the wellhead and liners are hung off casing strings via a liner hanger. A mudline suspension system is an option for a bottom-supported rig (e.g. jack-up) using a surface BOP, as in the case of this loss of well control event, in order to transfer the weight of the well to the seabed. When using mudline suspension equipment, mudline (or sub mudline, if referring to equipment below the mudline) casing hangers are used to provide suspension points for additional intermediate casing strings that cannot be accommodated by a standard conductor or wellhead housing (as described in API 17D). An annulus seal assembly is a mechanism that provides pressure isolation between each casing hanger and the wellhead housing (also described in API 17D).

In the case of this loss of well control event, the sub mudline casing hanger was hung off of the wellhead, as part of the mudline suspension equipment system. The liner (referred to as a “shallow” liner, because it was hung off at a relatively shallow depth of approximately 703 feet below the mudline) was hung off of the sub mudline casing hanger. The seal assembly in question was at the interface where the shallow liner is hung off of the sub mudline casing hanger (also referred to as the liner top).  The shallow liner is also the surface casing in this case.

See Figure 1 for a schematic of the sub mudline casing hanger seal used on this well.