Safeguarding Operational Integrity with Routine Subsea Riser Inspection Using Ultrasonics: A Trendsetter in an Ultra-Deep-Water Field
Authors: Mishra Siddharth , Mishra Divyani , Agrawal Gaurav , Dutta Shaktim , Issabekov Yerlan , Orbelli Gabriele , Miranda Pereira , Gidwani Arjit , Lacaze Chloe , Moreau Olivier Daniel , Drevillon Pierre-Marie , Balou Efficience , Delabrousse-Mayoux Géraud , Cave Adrien , Bledou Manfred 
Published: October 2019

Abstract
Moho Nord is a deep-water exploration and development project launched off the Congolese coast in March 2013. This huge investment is the largest oil & gas project ever undertaken in the Republic of the Congo and with a capacity of 140,000 barrels of oil per day. This project, operated by TOTAL (with CHEVRON and SNPC as partners) taps deposits located in water depths of 650 to 1,200 meters and connects 28 subsea wells to two Floating Production Units (FPU), with an additional 17 wells developed from a Tension Leg Platform (TLP). The project involves numerous operations as drilling, testing and completing the wells. However, these deep-water operations are extremely cost-intensive and environmentally challenging, thereby representing a high-risk/high-reward scenario.
Due to the intrinsic complexities involved in deep-water operations, it demands meticulous operational planning and dictates the use of technology/practices, which maximizes quality information while minimizing rig time; with a minimal impact on the environment. This paper discusses an ongoing success story on Subsea Riser Inspection at Moho Nord, showcasing the unique challenges encountered in the operations. Also, the lessons learnt from the post-operational studies of this campaign can be utilized to optimize the future riser inspection operations globally; to achieve a more engineered approach towards operations.
On Moho Nord TLP, a drilling/production riser is a conduit that provides a temporary/permanent extension of a subsea wellhead to the surface drilling/production facility (where the surface BOP is located); but is extremely critical to preserve the operational integrity of the well. Therefore, robust design followed by routine inspection and monitoring of riser integrity is crucial for the campaign’s success. The literature of oil & gas industry doesn’t report many such cases of successful subsea inspection campaigns, thereby making this a unique campaign for the operator. The campaign involved evaluating the internal and external surfaces of 18.75 inch (0.85 inch thick) Drilling & 10.75 inch (0.45 inch thick) Production risers, for which ultrasonic measurement was utilized. This technique allows scanning the pipe with high radial (up to 1.2 inch) and vertical (0.6 inch) resolutions and the measurement resolutions of 0.002 inch are achieved for both the accurate internal radius and thickness. Minimal operational footprint, well known technology, faster logging speed & quick processing in a short turnaround time (TAT) are some of the additional benefits which have resulted in choosing the ultrasonic measurement as the optimal choice for this campaign.
So far 16 ultrasonic logs have been recorded in the first 8 wells across the production risers, 7 ultrasonic logs across the drilling riser and a time-lapse analysis is being carried out to continuously monitor the internal and external condition of the drilling & production risers thereby ensuring overall operational integrity of the well. This ongoing project has shown exceptional synergy between various stakeholders in the project, which has resulted in a total of &gt; 200 hrs. of operating time with &gt;99% operational efficiency.
This paper documents the lessons learnt, leading to best practices which could be adapted anywhere globally for planning of subsea riser inspection campaigns. Also, the strengths and limitations of the ultrasonic measurement for riser inspection is clearly highlighted.