Before choosing a wall thickness it is necessary to choose a pressure rating for the coil. Typically, the high-pressure coil (L1) is rated for the shut-in pressure of the well, and the low-pressure coil (L2) is rated for the maximum working pressure of the downstream equipment. There are many exceptions to this rule and reasons to deviate from it. If designing LI to withstand the well shut-in tubing pressure is too costly, it is common practice to design the coil above the normal operating pressure of the flow line and install a relief valve set at the maximum allowable operating pressure of the coil. If flow is accidentally shut-in by a hydrate plug or other blockage at the choke, L1 could be subjected to total wellhead shut-in pressure unless it is protected by a relief valve.
The wall thickness of the coil can be chosen by using any number of recognized codes and standards. In the United States, the most commonly recognized are American National Standard Institute (ANSI) B31.3 and B31.8, or American Petroleum Institute (API) Specification 12 K. Volume 1 has the tables for ANSI B31.3 and ANSI B31.8. Table 5-1 illustrates the ratings from API Spec 12 K, which uses the calculation procedure from ANSI B31.3, but assumes no corrosion allowance.
After the minimum inside diameter and the required wall thickness, a coil diameter and wall thickness may be selected. Very often, the coil length downstream of the choke (L2) is of a different diameter and wall thickness than the length upstream of the choke (L1).
