Buckle propagation under external pressure is a potential hazard during offshore pipeline laying in deep waters. It is normal design practice to install thicker pipe sections which, in case of buckle initiation and consequent propagation, can stop it so avoiding the lost of long pipe sections as well as threats to the installation equipment and dedicated personnel. There is still a series of questions the designer needs to answer when a new trunkline for very deep water applications is conceived: • What are the implications of the actual production technology (U-ing, O-ing and Expansion or Compression e.g. UO, UOE and UOC) on the propagation and arrest capacity of the line pipe, • How formulations for buckle arrestors design can be linked to a safety objective as required in modern submarine pipeline applications. The answers influence any decision on thickness, length, material and spacing of buckle arrestors. This paper gives an overview of buckle propagation and arrest phenomena and proposes a new design equation, applicable for both short and long buckle arrestors, based on available literature information and independent numerical analyses. Partial safety factors are recommended, based on a calibration process performed using structural reliability methods. Calibration aimed at fulfilling the safety objectives defined in DNV Offshore Standards OS-F101 and OS-F201.

Tenaris and Centro Sviluppo Materiali (CSM) launched a Joint Industrial Project aimed at developing heavy wall line pipes. The suitability for very severe applications, involving high service pressures and temperatures, the latter causing large strain fluctuations, in presence of an aggressive sour environment, is analyzed both theoretically and experimentally, including small and full pipe models. The full project program aims at developing a new generation heavy wall product, supported by: a comprehensive laboratory analysis of the material response under severe mechanical loading in aggressive environment; and full scale testing program, including both pipe and girth weld. Both investigations are mainly addressed to basic understanding of impact on design criteria from interaction between severe loading and aggressive environment. Two papers have been already presented on this project, [2] and [3]. The present paper deals with the study, carried out in cooperation with Saipem Energy Services, aimed at setting up a tool for the prediction of ratcheting extent for the pipeline in pressure subjected to axial cyclic, even plastic, straining. In such conditions, ratcheting may develop in the circumferential direction, as a consequence of both material cyclic performance and bi-axial plastic flow. So, detailed characterization of material is required, as well as calibration of plastic performance parameters, particularly in relation to relevant modeling. The final objective of the study is to establish a threshold for the plastic strain development at peak load, beyond which circumferential ratcheting may develop. A numerical model was set up, on-purpose developed and implemented on commercial software, where reverse yielding is modeled by kinematic hardening referring to Von-Mises yield criterion. Use of relevant parameters describing/approximating the actual material response has been made, based on laboratory Multi Plastic Straining Cycling (MPSC) of pipe full thickness samples. Full scale testing of pressurized X65, 10 3/4″ OD × 46 mm WT linepipe has been performed including plastic axial and cyclic straining. A huge measurement campaign allowed to establish the relevant parameters that characterize the response from numerical modeling, facilitating the validation of the set up by comparing the actual ratcheting exhibited by the heavy wall pipe with predictions obtained by the model. Limits of current tools for numerical modeling are also shown, with some degree of dependence on applied straining sequence. Possible paths of numerical modeling improvement are then envisaged.

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In response to the UNCCC held in Paris in 2015 the need to reduce the global warming, due to CO2 release in atmosphere, led to a new business for the capture and storage of CO2 in dedicated deep water reservoir. In this sense the transport of the CO2 at low temperature, necessary to condensate the gas, through offshore pipeline is a commercial and technical valid strategy. One of the issues related to the transport of a condensate gas is the thermal exchange between the transport system, in this case offshore pipelines, and the environment. The gas is usually carried by ships in a liquid phase at very low temperatures, for example −30 °C in case of CO2. The fluid is introduced into the pipeline at the same temperature to not further consume energy for warming up. The design of the offshore pipeline subject to these operating conditions, very cold fluid internally and a water temperature slightly over 0°C at external side, can be affected by the ice formation around the pipe. The ice thickness formation is primarily governed by the external convection coefficient. For the offshore pipelines located in deep waters where the sea currents are negligible, only the natural convection phenomena can occur on the external surface of the pipeline. Considering steady state scenario the heat transfer from the internal fluid to the external environmental is governed by the thermal resistance of each component of the system like fluid, steel, anticorrosion coating, thermal insulation if any and external convection due to the seawater. The low temperatures of both seawater and ice formation, approximately at −2°C, allow to be close to the maximum value of the seawater density: usually this occurs at a slightly colder temperatures depending on salinity and water depth (for the fresh water the maximum is at 4°C). The natural convection is driven by the buoyancy effect due to fluid density variation with temperature: the scenario described above lead to minimizes these effects and consequently the heat transfer due to the natural convection (increasing the thermal resistance). Most of the correlations in literature are related to different temperature ranges, far away from this particular situation: a numerical investigation using computational fluid dynamics technique has been performed. The analysis is executed by means of commercial CFD software FLUENT: the model is based on a two dimensional grid of a pipe submerged in water. In this paper: • The state-of-the-art about the natural convection coefficient estimate for submerged cylinders proposed by different authors through Nusselt number assessment; • A description of the proposed numerical approach is given highlighting the different approaches based on the boundary layer behavior; • A typical application is shown.

The interest of gas companies in the use of high-grade steel pipes, equivalent to X100 and higher, for the construction of long-distance gas pipelines is now a consolidated trend in the world; these pipelines are demanded to operate under extreme operating conditions, such as low temperature, high pressure and high design factor. Under this severe operating scenario the running shear fracture control can not be only assured by the inherent pipe body material self arrestability, and in this case, the use of external mechanical devices, i.e. crack arrestors, becomes mandatory as also envisaged by the draft of the new pipeline ISO standards under discussion. At the same time the knowledge available on the matter mainly dated back to the past, and the existing crack arrestor design guidelines, as those published by PRCI in the 80's, make reference to experimental experience gained on conventional, lower toughness and lower pressure gas linepipe.