Measurement and judgement of API 5B Pin joint of Casing Round Thread

Thread gauge of API Spec 5B Casing Round Thread merely designed and fabricated as per size of short round thread, one size of thread gauge measures same OD but multiple different thread parameters such as LC and SC thread. This issue increases the difficulty of determining the “standoff” of different thread connection. According to the experience of thread inspection, here are two concepts of the special round connection: “Protruded volume” and “Recessed volume”. These two concepts clarify the relationship between “Protruded volume”, “Recessed volume” and “Standoff”.

API round thread casing mainly have long round thread and short round thread. Casings with same outer diameter have different thread sizes according to different steel grade and wall thickness. Standoff, as an important parameter that reflects comprehensively the individual parameters and surface machining quality and has great influence on seal integrity and structural integrity, is essential in API round threaded casing inspection.

For various reasons, API round thread casing gauges are only designed and fabricated with reference to the size of short round threads. It is necessary to use a gauge of one size to measure long and short round thread casing of same OD but different thread size.

Basic conception:

Standoff

API Spec 5B specifies the standoff range for the external thread joints of long and short round thread casing of OD 114.3~ 508.0mm as：

P1±1P

P1：Standoff of working ring gauge established by master gauge (mm)

P：Pitch of round thread, 3.175mm

The measurement schematic diagram is shown in Figure a and b, that is, the axial distance between the small end face of the working ring gauge and the end face of the pipe. When the end face of the pipe extends out of the small end face of the ring gauge, the standoff is negative, otherwise positive but it only applicable when working gauge is in line with the basic dimensions of the short round thread casing (L1, L4, E1)

Protruded volume

API Spec 5B states that the end face of the pipe will extend out the small end of the ring gauge when the long round thread casing is checked with the short round thread ring gauge and the value is：

Pr＝(L1 long - L1 short)-P1

Pr: The protrusion amount of the pipe end extending from small end face of the ring gauge (mm).

L1 long (short): The length of the long(short)round casing end to the hand plane (mm).

The measurement schematic diagram is shown in Figure e. As long as the measured protruded volume is within the range of (L1 long - L1 shot)-P1±1P, the standoff is within range of P1±1P, the measured protruded volume is an indirect measurement of the standoff.

The other case, which is not mentioned in the API Spec 5B, but is an objective one. It is a short round threaded casing with an OD 114.3mm and a nominal mass other than 14.5kg/m (wall thickness other than 5.21mm). When the Pin joint of this short-threaded casing is inspected with a short round ring gauge, the end face of the pipe will also extend out a basic size of the ring gauge end face, as shown in Figure d, with a protruded volume of 15.875mm. As long as the measured protruded volume is within the range of 15.875 - P1±1P, the standoff is within range of P1±1P.

Recessed volume

The recessed volume is also not mentioned in API Spec 5B, but is an objective one. For these 4 special size of short rounds threaded casings with OD of 127.0, 177.8, 219.1,273.1mm and corresponding nominal mass of 17.13 kg/m (WT5.59mm), 25.32 kg/m (WT 5.87mm),35.75 kg/m (WT 6.71mm) and 48.78kg/m (WT 7.09mm), When the Pin joints of these four size are inspected with a short round ring gauge, the pipe end face is also recessed into a basic size of the ring gauge end face, as shown in Figure c, the values were 6.35, 19.05, 9.525 and 19.05mm, respectively. Since the direction of the recessed and protruded is opposite, it can be inferred that as long as the measured recessed volume is within the range of 6.35+P1±1P, 19.05+P1±1P, 9.525+P1±1P, 19.05+P1±1P, the standoff is within range of P1±1P.

Distinguish

It can be seen from the analysis in Figure and above that for the Pin joint of round thread casing, the standoff and the amount of protruded and recessed are three completely different concepts.

The base standoff is 0, and the nominal pipe end and ring gauge small end face are flush; The basic size of the protruded and recessed is not 0. The nominal tube end protruded and recessed is a basic value of the small end face of the ring gauge. The size is the difference between the L1 of two short round casings or difference between long and short round threads.

The actual measured value of standoff may be positive or negative, while the actual measured value of the protruded and recessed is always positive, but the deviation of the protruded and recessed may be positive or negative. You cannot mistake protruded and recessed for standoff. The protruded volume is contrary to the trend of the standoff, while the recessed volume is consistent with the trend of the standoff.

Correlation

Although the concepts of standoff, protruded volume and recessed volume are not equivalent, but there is an inevitable relation between them. The measured of protruded volume or recessed volume is an indirect measurement of the standoff. It can also be used to determine whether the measured protruded volume or recessed volume is qualified or not. The measured values of protruded volume or recessed volume can be converted to standoff. The conversion relation can be summarized as:

Standoff = Nominal value of protruded volume - measured value of protruded volume

Standoff = Measured value of recessed volume - Nominal value of recessed volume

During the practice, if the measurement is standoff, it should be judged according to the tolerance range of standoff. if the measurement is protruded volume or recessed volume, it should be judged according to the tolerance range of protruded volume or recessed volume. It is not necessary to convert all the measured data into standoff. But on no account should the measurement result of protruded volume or recessed volume be written as the measurement result of standoff.