Casing is one of the most critical well consumables with respect to timing. Total elapsed time for procurement of Oil Country Tubular Goods (OCTGs) varies from 6-18 months. This includes tendering procedures, placing of the purchase order, manufacturing and delivery to the required location.

1 Timing

Normal deliver-time from a mill once an order has been placed is in general, 3-4 months for API materials J55 and K55, 4-5 months for materials N80, L80, P110, and Q125, and six months for C95. Casing with premium connections requires approximately two weeks more than API connections.

In case required delivery times cannot be met by the mills, the following options are available:

·order from a stockist (but selection may be limited and the cost higher);

·borrow from other Operators;

2 Vendor selection

Four criteria have to be met by vendors, i.e.:

• commercially attractive;
• quality management system as per ISO 9001;
• technically acceptable to manufacture OCTGs;
• acceptable delivery performance.

3 Buying description

Reference should be made to API/ISO Specification and Standards. These documents contain specifications for casing, tubing and threads and should be referred to in purchase orders.

API Specification includes suggestions for the minimum contents of a casing 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 such areas as pipe coatings, drift requirements, hydrostatic test pressures and thread protectors.

Materials departments should use standard buying descriptions consisting of the basic pipe and thread requirements in accordance with ISO/API Specifications, together with selected options from those documents.

Operational requirements should also be addressed during preparation of the buying description as described below.

Where handling and storage arrangements permit, API "long" casing (Range 3) should be specified to minimise the number of connections, the running time and, as a consequence, the overall cost of a casing string. Note that API Specification 5CT gives the maximum allowable length of Range 3 casing as 48 ft. In certain cases pipe of this length may cause handling and transport difficulties and transport surcharges may be imposed. Under such circumstances a maximum Range 3 length of 45 ft is often specified in the purchase order.

It may be advantageous to use containers to transport the tubular (custom clearance, handling, etc,) in which case length will be 10.8 m.

In order to eliminate the need for odd bit sizes (e.g. 57/8 in, 83/8 in and 12 in), "alternate drift" casing should be specified whenever possible. Standard drift sizes are given in API Specification 5CT Section 5, while sizes for alternate drift casing are given in. Pipe which is drifted with the larger mandrels should then be marked as such.

At the time of ordering it should be checked that all eventual components of the casing string 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, specially 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, casing hangers, liner hangers, tie-back packers, float equipment and multi-stage cementing 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 (e.g. float equipment) should be encouraged to purchase threaded casing and pup joints for use in their products direct from the mill which supplied the casing. Otherwise, the cost of cutting threads on the finished product is often excessive.

Drift mandrels for casing as set out in API Specification 5CT, are a minimum 12 in (0.3048 m) in length, whereas those for tubing are a minimum 42 in (1.0668 m) in length. It should be noted that this is based on API nomenclature where tubulars having a body outer diameter of less than 41/2 in (0.1143 m) are referred to as tubing and larger sizes are termed 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. Casing drift mandrels should ideally be at least 36 in (0.9144 m) in length to confirm passage for long full-bore tools.

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.