Reliability Analysis of Blowout Preventer Systems: A comparative study of electro-hydraulic vs. all-electric BOP technology
A blowout preventer (BOP) is a large valve used to seal, control and monitor oil and gas wells. It serves as an important barrier against blowouts. Excessive downtime on the BOP is a problem for drilling companies worldwide, which causes increased costs and delays for everyone involved in a drilling project. The background for this thesis is Odfjell Drillings experience with downtime on the BOP during drilling operations on board their mobile offshore drilling units.
The downtime and associated cost due to failure on the BOP increases with the water depth of a drilling project because the time it takes to recover and re-install the BOP stack increases. In a deepwater operation the unproductive downtime from a problem that requires the BOP stack to be recovered to the surface may be 1-2 weeks. The magnitude of the resulting daily loss, both for the owner and the client involved, illustrates how important reliability of the BOP is.
Deepwater drilling operations may also experience new challenges compared to operations in more shallow depths. Examples include increased loads on the riser system, higher pressure and temperature in the well and energy loss in subsea accumulators. Today, drilling companies worldwide have a strong focus on reducing BOP downtime. Improved technology and new solutions for subsea BOPs are therefore believed to be a necessity for future deepwater drilling.
This thesis is a case study of the electro-hydraulic BOP on board Deepsea Stavanger, a drilling unit owned and managed by Odfjell Drilling. The first focus is to analyse BOP failures that have led to downtime on this rig and to relate them to the technical mode of operation of the BOP.
The company Electrical Subsea & Drilling AS (ESD) is working on developing an all-electrically operated BOP. They claim that their new technology can provide many benefits versus the electro-hydraulic BOP systems, both with respect to environmental and operational safety, as well as cost reduction for drilling- and oil companies. Additionally, they claim that their BOP concept is more reliable and less prone to excessive downtime. The second focus is therefore to establish a thorough system description of this concept, to analyse potential failure modes and to compare them with the failures experienced on board Deepsea Stavanger.
The overall goal for this thesis is to compare the conventional electro-hydraulic BOP system with the all-electric BOP concept developed by ESD, with respect to reliability. The purpose of such a comparison is to see if any of the recurring failures Odfjell Drilling experiences on board Deepsea Stavanger are less likely to occur if the BOP is all-electrically operated.
To compare the two BOP concepts, a reliability analysis is performed on each system. The reliability analyses are performed in four steps:
1. Functional analysis
3. Reliability block diagram analysis
4. Fault tree analysis
Reliability data is gathered from experience data on board Deepsea Stavanger, engineering judgment input from a workshop performed with Odfjell Drilling and ESD, previous reliability studies and comparative components in OREDA.
The results from the reliability analyses yield that the all-electric BOP concept is more reliable and less prone to failures than existing electro-hydraulic BOP systems. However, this is a result based on a single case study with numerous assumptions involved. There are also other factors, in addition to reliability, that are important to consider when assessing a BOP system.
There are many arguments in favour of the all-electric BOP concept. An electric system contains fewer and more reliable components than an electro-hydraulic one, making the all-electric concept simpler than existing BOP systems. The lack of a shuttle valve and the use of subsea batteries instead of accumulators are the most obvious advantages with the all-electric concept. In addition, the concept is weight saving, has a greater amount of redundancy in the control system, offers better and more precise monitoring and is less polluting. Still, there is considerable uncertainty associated with the new technology, both with respect to human impacts, maintenance, repair hours and profitability and there are more issues to be examined before a certain conclusion can be drawn regarding which system contributes the least to BOP downtime.
For new technology to be developed and implemented there must exist some market drivers. The fact is that today there are no market drivers for an all-electric BOP system. A promise of high reliability is not enough to create success. An all-electric BOP concept can solve many of the challenges the drilling industry is facing in the years to come, but time will show whether or not the concept proves to be both technically and financially profitable.
Place, publisher, year, edition, pages
Institutt for marin teknikk , 2014. , 155 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-26702Local ID: ntnudaim:11044OAI: oai:DiVA.org:ntnu-26702DiVA: diva2:750224
Utne, Ingrid Bouwer, ProfessorStautland, TarjeiRød, Magne