In order to meet the growing demand for oil production in even deeper waters, new technologies have been developed, making the exploration of such fields possible. A system used as an alternative for the control of extreme operating conditions (high temperatures and pressures) of such exploration fields is the pipe-in-pipe system. This kind of system is extensively used in offshore applications in which exceptional thermal insulation capability is required, preventing hydrate/wax formation and maintaining the production temperature up to the arrival facilities. However, extreme operating conditions can cause the system to experience thermomechanical buckling, which can lead to a structural failure of the system. In order to control these thermomechanical loads and ensure that pipeline is within a safe operating margin, the potential buckling formation locations need to be assessed and may need to be mitigated. The key point in the thermomechanical design of offshore pipelines is to define whether the buckling phenomena should be controlled or not. The optimal solution may often involve addressing natural imperfections whilst establishing the required engineered mitigation measures. Design assumptions such as the pipeline as-laid lateral Out-Of-Straightness (OOS) and the pipe-soil interaction parameters are common input data uncertainties existent during design of HP/HT offshore pipeline systems. A robust design should be impervious to variations in the values of these design parameters, considering values that lie within reasonable and feasible limits, such that the project can proceed with reasonable certainty and within sensible cost limits. In this scenario full of uncertainties, reliability analysis has been implemented in the thermomechanical design of offshore pipeline systems. The purpose of reliability analysis is to reduce the design conservatism by quantifying the probability of failure associated with the pipeline system. This paper presents a novel and viable proposal for conducting probabilistic analysis associated with lateral buckling of a full length pipe-in-pipe system, through the application of detailed finite element analysis. The information contained in the paper can be used as guidance for future reliability evaluations of offshore pipeline systems.

Raven is the third stage of the West Nile Delta development (following Taurus / Libra and Giza / Fayoum) from two BP-operated offshore concession blocks, North Alexandria and West Mediterranean Deepwater. The Raven project included the design of various rigid pipelines, of which one specifically is the subject of this paper. The 16" RSM to RP in-field flowline is approximately 4.8 km long, connecting a manifold (RSM) to a PLEM (RP) through a route that crosses a prominent geological feature identified as the Rosetta Channel, a submerged canyon that extends for about 30 km. The Rosetta Channel is about 2.5 km wide at the location of the 16" flowline route crossing, with steep slopes going down for approx. 40m (in height) on the RSM side, and then climbing up approx. 150m (in height) towards the RP side. Although it is typically preferred to avoid very rough geophysical features, this is not always possible or practicable and it is not uncommon to come across challenging seabed features that demand complex engineering solutions in order to minimise risks and associated costs. This paper addresses the numerous technical challenges involved in the design of the 16" flowline that crosses the Rosetta Channel. Following close collaboration between all involved stakeholders, a robust, reliable and cost-effective solution was achieved after a detailed engineering process, where the final design required a unique combination of mitigations including seabed excavation, pre-lay rock carpets, post-lay rock berms, cable jetting, curve bollards and sleepers.

The Electrically Heat Traced Flowline (EHTF) is characterised by a combination of high performance dry annular thermal insulation of Pipe-in-Pipe (PiP) with a restricted electrical heating capability provided by helically wound copper wires laid between the inner pipe and the insulation in the annulus. The main advantage of EHTF are: future tie-back integration, unlock marginal reserves, access to HPHT pipeline, extend field life and maximise economic recovery and reduction in chemical and energy usage operational flexibility in controlling the flowline temperature and preventing the formation of wax and hydrates in shutdown conditions. Fibre optic cables are deployed in the EHTF system to measure the temperature of the flowline. This paper presents the development of a detailed finite element model to predict the mechanical behaviour of the helically wound cabling during reeling operations. The wires and cables were represented explicitly in the model as initially straight and then wound helically around the inner pipe with specified pre-tension. The EHTF PiP system was then cyclically deformed against a former to simulate the reeling process. A fibre optic cable (FOC) containing a local imperfection due to denting was included in the model to assess the impact of reeling process on the continued acceptability of accidentally dented FOC. The effects of friction between the cabling and the inner pipe and insulation surfaces, the pre-tensioned helical winding process and helix pitch, and the restraint provided by the thermal insulation layer and centralizers, were all investigated. Physical tests were conducted to establish the cyclic material properties of the electrical wires and results from these tests were used to calibrate the FE model. This paper details Subsea 7's technical expertise in modelling the highly complex behaviour of the EHTF cabling system as it experiences multiple bending cycles due to reeling. The paper highlights some important key results describing the behaviour of the wires and consequent predictions of integrity which have since been verified through full scale physical tests. The FE modelling also contributed to the insight gained regarding the overall behaviour of the system.