When buying tubulars, reference should be made on purchase orders to relevant international standards, such as API Specification 5CT and API Standard 5B on specifications for casing, tubing and threads.
In the case of tubulars for example:
1.API SPEC 5CT, "Specification for casing and tubing". Covers seamless and welded casing and tubing, couplings, pup joints and connectors in all grades. Processes of manufacture, chemical and mechanical property requirements, methods of test and dimensions are included.
2.API STD 5B, "Specification for threading, gauging, and thread inspection for casing, tubing, and line pipe threads". Covers dimensional requirements on threads and thread gauges, stipulations on gauging practice, gauge specifications and certifications, as well as instruments and methods for the inspection of threads of round-thread casing and tubing, buttress thread casing, and extreme-line casing, and drill pipe.
API Specification 5CT also includes suggestions for the minimum contents of a purchase order (with references to the appropriate sections of the Specification) together with a list of optional specifications which may be selected. Optional specifications are available for pipe coatings, drift requirements, hydrostatic test pressures, thread protectors, etc.
Where the lifting/handling facilities permit, longer tubing lengths should be specified to reduce the number of connections and reduce running time. However, consideration should be given to the workover/pulling hoist capabilities.
Standard sizes should be specified whenever possible. Standard drift sizes are given in API Specification 5CT Section 5, while sizes for alternate drifts are given in Section 6. Pipe which is drifted with the larger mandrels should then be marked as per Section 10 of the same document.
Drift mandrels for casing (API Specification 5CT), are a minimum 12 in (0.3048 m) in length, and those for tubing are a minimum 42 in (1.0668 m) in length.
Based on API nomenclature tubulars having a body OD less than 41/2 in (0.1143 m) are called tubing and larger sizes are called casing. In operations however, 51/2 in (0.1397 m) and 7 in (0.1778 m) OD pipe is often used as tubing and pipe having an OD of 41/2 in (0.1143 m) or less may be used as casing in slimhole drilling. Thus, drifting practices should be based on the application of the tubular rather than purely on size.
At the time of ordering it should be checked that all components of the well are dimensionally compatible with the selected pipe and couplings. It may be necessary to specify smaller dimensional tolerances than are given in API Specification 5CT in order to ensure compatibility and sealability, especially for premium connections. For example, API Specification 5CT cites pipe body outside diameter tolerances as +1.0%/-0.5%. Reduction of the pipe body tolerances to +0.75%/-0.5% will eliminate these problems and improves the collapse load bearing capacity.
It should also be ensured that, where necessary, these components are at least as strong as the weaker of the pipe or the connections. Such components include pup joints, cross-overs, hangers, packers, and flow control equipment. A minimum check on these components should include:
- through bore;
- body and coupling OD;
- tensile, collapse and burst rating;
- material specification;
- thread type;
- temperature rating.
Suppliers of accessories should be encouraged to purchase threaded tubulars for use in their products, direct from the mill which supplies the associated tubulars. Otherwise, the cost of cutting threads on the finished product is often excessive.
Note: It should be ensured that the connection thread is concentric with the pipe internal diameter. One way to achieve this is to specify that the machine tool chuck is placed on the internal diameter of the pipe body when cutting the thread.