Coiled Oilfield Tubing (CT) continually finds new markets, and has experienced continued growth in extended reach wells and high-pressure service. One major improvement is in the quality possible at skelp end welds, when continuously tapered strip1 is employed to join strip of the same thickness.
In this paper, the following are discussed:
Tapered strip and tubing dimensional specifications at all points along the tubing.
Design of CT employing continuously tapered strips.
Effect of tapered wall thickness on theoretical fatigue life.
Effect of skelp-end welds of the same thickness, as compared with those of conventional tapering, on overall string life.
Suggested amount of tubing to be removed if needed during servicing operations.
In order to support high loads and yet maintain high cycle life at skelp-end welds, continuously tapered strip is used in the production of continuously tapered CT strings. It possesses the following advantages over conventionally tapered tubing.
Minimal loss in measured cycle life at gauge-to-gauge skelp-end welds, and thus an overall increase in cycle life along a string.
Reduced ovality at skelp end welds, which has been a problem for tapered welds, such as 0.156 in - 0.175 in.
Ease of manufacture in that no rapid changes are needed to mill set-up during the passage of a taper weld through the electric welder.
Once the model for continuously tapered strip and tubing has been proved valid by measurement, then the following are possible:
Acceptance of strings with continuous tapering for load, theoretical fatigue, and internal volume calculations may be validated.
Determination of the amount of metal that may be removed when an imperfection is detected, within acceptable specification tolerances.
This article covers the results of determining a model for continuously tapered strip, and discusses how the model is used to mathematically determine the above items. Acceptance of continuous tapering has been such that fully 90% of all possible orders for tapered well servicing strings have employed such material.
Manufacture of Continuously Tapered Strip
Strip is manufactured uniquely by rolling a slab of steel from dimensions such as 15 ft. long×40 in. wide×5" thick and weighing 40 tons, to values such as those shown in table 1. Figure 1 shows the continuously tapered strip purchase rolling requirement for the slab mill, illustrating 10% of the length at the two ends are held at constant but differing thicknesses (AB and CD), and a linear taper (BC) accounting for the remaining 80% of the length. With this accomplished, gauge-to-gauge strip bias (skelp-end) welds can be fabricated for an entire string, eliminating the need for step tapered welds.