14 - Are Premium Connections Always Better than API's?
Updated: Aug 22, 2018
With more complex wells being drilled nowadays, casing connections, alongside casing steel grades, are considered cornerstones of well integrity.
Casing connections are widely labeled as API and Premium. This last doesn't necessarily command superior characteristics but different features, and is usually associated with a premium price..
API adopts the term "threads and couplings" to refer to casing connectors. In this article we will use the term "casing connections" to refer to the same item.
API Connections - Some History First
Before the development of API standards, casing manufacturers used a sharp V-Shape thread that lacked standardization among industry participants. The V-Shape was fit for simple wells, but as deeper wells were being drilled, the need for a more robust connection became pressing.
This is when API standardized casing connections, and offered specifications for four connections types.
Most lectures and publications list the four connections types in the below specific order, which we believe corresponds to the evolution of the API Connections:
Short Round Threads and Couplings (API CSG)
Long Round Threads and Couplings (API LCSG)
Buttress Threads and Couplings (API BCSG)
Extreme Line Threads (XCSG)
API 8-Round (8-Rd)
The first two types (CSG and LCSG) are known to the industry as:
API 8-Round Short Threaded Connection (STC), and
API 8-Round Long Threaded Connection (LTC) respectively.
The name 8-Round comes from the 8 threads per inch machined on the casing joint and coupling. While both connections have the 8 threads per inch, the LCSG has a longer connection, offering:
greater tensile strength,
higher reliability, and
higher joint efficiency.
However, The API 8-Round connections have two major issues:
prone to joint-pull-out failure caused by thread tapering
Joint efficiency below 100% which reduces the tensile strength of the casing string
Buttress Connections (BTC)
The BCSG (which is widely known to the industry with the acronym BTC) came to solve the issues of the 8-Round connections.
Two major modifications where performed:
thread shape changed from a rounded peak to a squarer peak
extended the pin-box contact length by making the connection longer
These modifications have resulted in:
significantly reducing the joint-pull-out failure risk
Brought up joint-efficiency to 100%.
As with the 8-Round connections, Buttress Connection still need a thread compound to provide the seal (by filling the voids between the thread root and the crest). This sealing mechanism limits their sealing capability with:
gases at high pressure (+500 psi)
solid free and low viscosity fluids
Extreme Line Connections (XLC)
To resolve the sealing limitation of 8-Round and BTC connections, API developed specifications for the XCSG (known also XLC) which brought-in two distinctive features that previous connections didn't have:
First, by being the only API connection with the box machined on the casing joint (integral joint), resulting in eliminating the coupling and cutting the number of threads to half, which reduces the risk of leak.
Secondly, by being the only API connection with a Metal-to-Metal seal, thus improving its gas-tight sealing capability
However, with these features come two constraints:
The Metal-to-Metal seal doesn't eliminate the need for casing dope as the connection requires lubrication for make-up.
The integral joint requires a thicker pipe-wall at the tip of the connection, which results in slightly restricting the casing string ID.
Premium Connections - A Bit Of A Premium Now
API connections offers an affordable variety of casing connections. However; they are far less diverse than proprietary connections (or what is known as Premium Connections).
Premium connections were (and still are) designed and developed by casing manufacturers to address industry needs that API couldn't meet such as:
Enhanced stabbing
Reducing cross threading
Fast make-up
Over torque protection
Dopeless make-up
Improve sealing capability
Maximizing clearance (minimizing OD)
Galling resistance
Tension, compression, torque, burst, and collapse resistance enhancement
Maximize bending capability
"Semi-Premium Connections" is a term used in the industry to identify proprietary connections that offers enhanced features over API Connections, yet compatible with one or more of the 4 API connections types.
Final Words
As stated earlier, Premium doesn't necessarily command superior characteristics since a trade-off is usually necessary to acquire the Premium features (e.g. extra clearance trades-off with joint-efficiency). Even if trade-off is not on specifications, it is usually on price.
It is important to thoroughly design your well and identify all the loads that the casing will endure before deciding on premium connections.
Many exercises can be performed, such as:
break-even analysis to determine the cost and time savings (e.g. Enhanced Stabbing, Cross Threading Reduction and fast make-up features vs normal operations)
Drill Well On Paper (DWOP) conducted with Subject-Matter Experts (SME)
Perform offset wells analysis on wells with API casing and identify any issues
Also, We will be happy to share more details on this post, so please don't hesitate to contact one of our team members.
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