Collapse challenges and technical solutions
The collapse resistance is a major concern in some environments, and can be the driving technical constraint of well design. This is often seen in deep offshore HP/HT, with Annular Pressure Build up (APB) pressures on the large OD casings, or pre-salt formations with high shear stresses applied by the external environment on the casing strings.
Collapse phenomenon in OCTG is due to external loads applied on the tubular products that have different causes in a well. It can be due to well load cases, such as cementing operations, full evacuation, and APB pressures, or due to floating formations (mainly salt).
Collapse refers to the failure mode of external pressure on a pipe (similarly, burst is the failure mode of internal pressure loads). By extension in Oil & Gas industry, collapse designates more generally any external loads.
Collapse resistance of a tubular depends on mechanical and geometrical parameters, in particular: D/t ratio, ovality, minimum wall thickness, residual stresses and yield strength. Collapse failure will be initiated in the weaker zone of the tubular.
1. DEFINITION OF COLLAPSE
1.1 API Collapse formula (ISO10400)
API 5CT refers to ISO10400 & API5C3 formulas for collapse calculation of API 5CT pipes. There are 4 API formulas defined in these documents, each one applicable on a given domain (D/t ratio and grade ranges). You can refer to chapter 8.4 of ISO/TR 10400:2007 for the detailed equations.
When increasing D/t ratio, the collapse equation that apply will be in this order:
• Yield Strength Collapse
• Plastic Collapse
• Transition Collapse
• Elastic Collapse
A schematic view of these applicable domains is shown below:
For yield strength collapse, the collapse value will depend mostly on the material yield strength. This concerns the low D/t ratios, e.g. tubing sizes or thick pipes.
On the opposite, elastic collapse will be function of pipe geometry mainly, and not yield strength of material. This is typical of very large OD casings.
These equations are known to be conservative.
One can note also that operators usually apply a 1.0 or 1.1 safety factor to the collapse loads for well design calculations.
1.2 Other collapse calculation methods in ISO 10400 Annexes
Annexes of ISO 10400/API 5C3 describe statistical approaches based on Collapse tests
data statistics (Annex G) or production data statistics with Klever-Tamano collapse calculation (Annex H).
These calculations come along with practical limitations:
• Annex G requires to do a very large number of collapse tests from samples in production, for each size and grade combination, per manufacturing plant: this is not very realistic to apply to each single product of a full catalogue, and it is costly.
• Annex H formula is based on empirical collapse database from different OCTG manufacturers (Klever-Tamano equation). Consequently it is not optimized as it depends on the manufacturer process, equipment, and know-how. Also, Annex H would require communicating on process statistics which is sensitive data for suppliers versus competition.
2. SOLUTIONS TO INCREASE THE RESISTANCE
One can sort 3 main ways to increase the collapse resistance of a casing string: increase the material yield strength, increase wall thickness and/or optimize the pipe collapse properties (proprietary High Collapse pipes).
As detailed in the table below, each solution has advantages and drawbacks, and a compromise is to be found, e.g. between clearance and drift constraints, corrosion resistance, weight and cost of the string.
2.1 Increase Yield Strength
Increasing yield strength is not meaningful in certain cases: in the first place it may not be a suitable solution if there is H2S in the well environment. But also, as explained in the previous chapter, for elastic collapse range, the yield strength has no more effect on collapse resistance.
2.2 Thicker wall solution (Integrals and Mixed string)
2.2.1 Suitable connection design
Naturally, increasing the wall thickness will increase the collapse resistance. However, the operator has to consider the clearance and drift constraints of its well design.
Increasing the wall thickness will either increase the OD or reduce the drift of the pipe. To keep the same clearance while increasing the OD, the connection design is usually the most suitable parameter to play with. Typically, a 13 3/8” initial design with threaded and coupled connection (e.g. VAM® TOP) that needs to be switched to a 13 5/8” (88.2# to keep a 12.250 drift) will come with a Special Clearance coupling option (e.g. 13 5/8” VAM® TOP NA) or an integral semi-flush design (e.g. VAM® SLIJ-II). If a thicker wall is still required, pipe could be selected with 14” OD (115# special drift is 12.250”) and for clearance constraint would need to be an integral flush connection (e.g. VAM® BOLT).
This case is presented in the graphs below.
Comparison of collapse performances for different production casing (psi).
2.2.2 Mixed string
For specific cases where only a section of the full string needs High Collapse resistance, like salt formation to be passed through, a mixed string can be the best candidate to avoid a too heavy string (cost and weight purpose) and focus only on the zone where the collapse loads are present.
VAM® MUST is a design developed mainly for this purpose: it is a flush connection threaded on very thick pipes, whose OD is comparable to the related T&C casing string: 7 5/8” 55.3# VAM® MUST could be mixed with 7” VAM® 21 string (both 6.000” drift and similar max OD), or 10 ¾” 109# VAM® MUST with a 9 5/8” VAM® 21 (8.500” drift maintained).
Below a schematic view of the use of mixed string:
Example of a mixed string application with a heavy wall section (10 ¾” VAM®
MUST) part to pass through a salt formation, made to the 9 5/8” T&C casing.
3. HIGH COLLAPSE GRADES
High Collapse pipes consist in optimizing product properties to increase their resistance to external pressure.
The usual influencing parameters that are worked on are shown in the table below.
Table showing API 5CT pipes vs. proprietary High Collapse pipes Specification
The High Collapse pipes are usually fully compliant with API 5CT specifications, in dimensions, manufacturing and control. They are completed by tighter tolerances, additional controls and quality checks and enhanced manufacturing rules from the supplier.
The High Collapse shall meet at least 2 conditions:
• Robust and proven High Collapse formula:
It is based on R&D expertise, collapse tests, and process know-how. This calculation should provide a minimum collapse value guaranteed for the full volume produced.
• Manufacturing control and quality:
This relies on process expertise, state-of-the-art NDT equipment, and specific manufacturing procedure.
The know-how of the supplier is a key factor.
The field record and collapse testing database is also a good indicator of the product reliability. Some operators ask for collapse tests in production. This is a recommended practice when the operator has no record of High Collapse requirement.
3.2 High collapse Model and calculation (V & M proprietary)
Since more than 50 years, Vallourec studied the influential parameters of collapse on the pipes they produce. From this analysis and a large collapse test database, Vallourec developed a proprietary collapse calculation model, from where they can determine the production requirements for a given item to meet a calculated collapse resistance.
Based on this calculation, Vallourec guarantees on the full production of a given high collapse item a safety margin with the claimed collapse rating (which is a minimum “worst case” value).
3.3 Manufacturing procedure and quality controls
Specific production parameters and improved NDT controls are required.
An internal qualification is performed for each Vallourec mill producing High Collapse, consisting in training, rehearsal production, collapse tests on production samples, and audits.
V & M high collapse ratings are guaranteed for all mills through the transfer of know-how, harmonization of best practices, and compliance with V & M model calculation and manufacturing rules.
As an integrated manufacturer, from steel making, green pipe, to heat treatment and threading (finished product), Vallourec can optimize and control all the steps of manufacturing to deliver the best performance.
In average, VM HC pipes will provide +30% collapse resistance guaranteed versus API collapse.
3.4 Collapse testing
Collapse testing procedures are given in API 5C3 document: among other conditions for testing, the minimum length of the sample is a very critical requirement, explicitly defined in API 5C3.
More than 3 000 collapse tests have been performed on V & M high collapse pipes. These tests are done in compliance with API 5C3 collapse test procedure.
Collapse tests are regularly done for R&D purpose but also on production samples. The collapse tests during production are requested by some customers to demonstrate the guaranteed ratings. They are done in-house or at external labs, at a given frequency (1 every 200 pipes for instance) specified by the customer.
Regarding High Collapse grades in OCTG, since there is no API standard for High Collapse manufacturing and values, it is recommended that operators specify the following when looking for High Collapse product:
• The OCTG supplier’s approach and model for High Collapse calculations
• Explanations on manufacturing procedure and NDT controls (the plant shall be equipped and internally qualified to produce with specific rules)
• Collapse tests during production (e.g. 1 every 200 pipes to send to collapse: all the collapse values must be above the claimed rating)
• Qualification of the plant that manufactures High Collapse grades
• High Collapse product catalogue from the manufacturer
This will ensure good quality and reliable performance of the quoted products.
As described in these Tech Files, collapse issues on casing can be managed with different solutions. For the Oil & Gas companies, discussing with premium OCTG supplier to find the most suitable solution will bring added value to their project. Suppliers like Vallourec can advise on a large catalogue of proprietary grades and specific connection designs. Fit-for-purpose solutions could also be developed.