The purpose of the study was to identify and evaluate the variables that detennine the leak resistance limit state functions of API 8-round and buttress [1] connections. This work will be incorporated into Load and Resistance Factor Design (LRFD) equations. Connectors are an integral part any well design program. Therefore, it is vital that they be included in the LRFD design approach. Makeup, tension, internal pressure and dimensional data were among the variables in the evaluation, which was based on finite element analysis, testing and structural mechanics. The leak resistance limit state for round thread connections is defined by contact pressure, stab flank engaged length and coupling yield, while for buttress is defined by contact pressure, stab flank clearance and coupling yield. Leak pressure, as defined by API Bul. 5C3 [2], is a function of makeup and dimensional data independent of thread type, tension, and pipe inside diameter and valid only in the elastic regime. Tension is detrimental in the leak resistance of 8-round connections, but does not compromise buttress leak resistance. Regression analysis was perfonned on structural mechanics results to produce the highest correlation to finite element results to account for end effects on round thread connections. It was detennined by testing that stab flank contact over a minimum length of engagement is not sufficient to prevent 8-round leak, despite sufficient contact pressure level. Teflon impregnated thread compound should be the choice for API buttress. The indications are that optimized makeup should be considered in the leak resistance capacity of API connections. Excessive makeup and/or tension or pressure could result in coupling yield causing leak upon re-pressurization. Further analytical and References and figures at end of paper 653 --------------experimental work would improve the degree of accuracy with which the leak resistance capacity can be detennined.

The paper presents a technique for the analysis of propagating buckles in deep-water pipelines. The development is based on the finite element method and takes into account the large deformation of the pipe, the elastoplastic behavior of the pipe material and the contact between regions of the interior wall of the pipe during buckle propagation. The results of the technique are in excellent agreement with experimental data. The propagation pressure, i.e., the minimum pressure required for buckle propagation, as calculated by the technique, is within 1% of the measured value. Perhaps, more interesting are the results with respect to the state of deformation and stress due to the propagating buckle. The indications are that the deformation in the tail of the propagating buckle is not nearly that ofa collapsed elastoplastic ring in plane strain, as believed in earlier analytical work. Furthermore, the pipe material undergoes considerable unloading and reloading in the course of buckle propagation. INTRODUCTION The phenomenon known as the propagating buckle is of critical significance in the design of pipelines subjected to external pressure, e.g., pipelines for offshore oil production. Its most common occurrence is during pipe-laying operations. The pipe is usually paid off a barge and is subjected to substantial bending, while under external pressure and axial force. The tension necessary for this operation is supplied and controlled by equipment on the barge. In the segment of the pipe near the seafloor, known as the sag bend, where the curvature reaches its maximum, the combination of bending moment, external pressure and axial force may become critical, as a result, for example, of inadequate tension, and local damage may occur. If the external pressure is sufficiently high, this local damage may be transformed into a characteristic mode of deformation which travels along the pipe causing collapse of a long segment. The propagation is terminated when the deformation enters a region where the external pressure is below the minimum level required, or, upon encountering an arrestor, usually a stiffening ring, on the pipe. This mode of deformation is known as the propagating buckle, a term apparently coined by Mesloh et al1 who first observed the phenomenon. The minimum level of external pressure required for buckle propagation is referred to as the propagation pressure and most experimental studies to date have sought to determine its value for the pipes most widely used in practice and develop empirical formulas for practical use, e.g., Mesloh and Sorenson2 and Kyriakides and Babcock3Several analytical studies have been conducted as well. Palmer and Martin4 were first to derive an estimate of the propagation pressure, on the basis of the collapse analysis of an inxtensional ring of rigid-perfectly-plastic material. The deformation of a ring in plane strain, subjected to external pressure, at least at first glance, appears to be almost identical to that of a short segment of the pipe during passage of the propagating buckle.

The purpose of the study was to define the variables that effect the limit states of API t? Round and Buttress connectors This work will be incorporated into Load and Resistance Factor Design (LRFD) equations. Connectors are an integral part of any well design program. Therefore, it is vital that they be included in the LRFD design approach. Variables that determine the structural capacity were identified and evaluated, within the API tolerances, using finite element analysis. The indications are that taper, lead, and friction have no significant effect on the structural capacity of API connections. The effect of internal pressure on the tensile strength of API round thread accounted by API Bul. 5C3 [1] was verified. In addition, internal pressure was shown to have a beneficial effect on the capacity of buttress connection. The API joint strength equation, which in its present form, accounts for makeup only through the engaged thread length. However, the true joint strength was found to also be function of makeup interference.