Engineering critical assessment to BS 7910:2005

As part of a natural gas transmission system expansion strategy a 32” km subsea pipeline was being laid. Defects were found in a number of pipe lengths and in addition mechanical damage to the plate prior to pipe manufacture. Jee Ltd. was requested to carry out an Engineering Critical Assessment (ECA) to BS 7910: 2005 and report on the assessment and its implication on the pipeline and its integrity.

The benefit to the client would be the acceptance of the defects and the subsequent use of the pipe lengths thereby avoiding delay to the pipe lay, cost of manufacturing replacement pipe and the overall impact this would have on the project schedule.

The description of the defects were:

• Pipe - transverse surface breaking cracks located in the weld reinforcement and in the upper region of the internal and external longitudinal double submerged arc welds. Metallurgical analysis determined the defects were as a result of hot cracking during the weld process as a result of copper contamination
• Plate - substantial scratching, metal depressions and pits with the size of defects in the plate ranging from 0.1mm to 2.4mm in depth and up to 2m in length

The scope of work carried out included the following:

• Fracture and plastic collapse assessment of the pipeline during pipe lay
• Assessment of potential for buckling during lay as a result of a combination of pressure, axial force and bending
• The fatigue life of the pipeline during pipe lay and operation
• Determination of the minimum allowable wall thickness
• Determination of allowable defect size
• Determination of allowable grinding dimensions for defect removal

Our assessment provided the client with the following conclusions and recommendations.

• The cracks found within the pipeline were within acceptable limits and neither fracture nor plastic collapse of the pipe was predicted due to the loads on the pipeline during lay
• The minimum wall thickness to prevent buckling of the pipe during lay
• The fatigue of the pipeline during operation was acceptable
• The maximum allowable defect size was determined
• The maximum recommended grinding dimensions to remove cracks was determined

As a result of the work carried out our client was able to use a quantity of the pipe that would have otherwise been rejected.