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.

During design stage of high pressure/high temperature pipelines, some conservative parameters are adopted along with sensitivity analyses to assure safe operation in the presence of uncertainties that influence buckle formation, e.g. pipe-soil interaction, as-laid out-of-straightness and initial heat-up. After operation starts and lateral buckles appeared along the line, a survey may provide valuable information regarding confirmation of the design assumptions, evaluation of actual behaviour and the possibility of increase the operating conditions. This work presents the methodology applied to analyse the configuration of the P-53/PRA-1 12″ oil export pipeline in operation using data from a sidescan sonar survey. The aim of such analyses was to gather information for an FE model calibration as well as to obtain preliminary estimates for the bending strains at lateral buckling locations. Special attention was dedicated to smoothing and interpolation of the pipeline coordinates extracted from sonar imagery in order to avoid unrealistic strains estimates.

An account is given of the methods used to evaluate the operating structural performance of a reel laid deepwater oil HP/HT pipeline which had been designed based on the controlled lateral buckling principle. The objective was to develop a finite element (FE) model of the line based on its operating status and to use the model to confirm its present and future structural integrity. The line is surface laid on a fairly undulating soft clay seabed at its deep end and sand at the shallower end. It incorporates three different man-made buckle triggering mechanisms of buoyancy modules, dual sleepers and locally increased lateral curvature along its entire length. The steps involved in the inclusion of the in-situ operating condition of the pipeline, provided through various surveys made of the as-built and operating line and historical records of operating temperatures and pressures and flow rates made at inlet and outlet of the line, into the FE model, is discussed. Several key considerations essential for the successful development and validation of such an operation-based FE model, and for completion of the evaluation task, are highlighted in the paper. Also, a specific challenge encountered as a result of changes in regulatory guidelines on engineering critical assessments, from initial design to current evaluation stage, is discussed. The evaluation has demonstrated that it is feasible to carry out in-situ assessments of laterally buckling subsea lines, and that such assessments can provide not only reliable information regarding current and future structural integrity of the lines, but also invaluable confirmation of initial design data and rationale. This comparison between initial design and the actual operating behavior of the line is not included in this paper but will be described in detail in a future separate paper.

It is imperative to adopt some conservative premises in the engineering calculations undertaken during the design stage of an offshore pipeline susceptible to lateral buckling, in order to achieve a design with adequate levels of robustness and integrity throughout the pipeline’s design life. The conservatism can be attached to many uncertainties such as the pipe-soil interaction — interpreted as-soil friction factors — the seabed stiffness and profile and even the as laid lateral out-ofstraightness. Once in operation, these effects will come into play and the pipeline may behave slightly differently to that anticipated in design, depending on the relative strength of the natural uncertainties compared to the design features such as engineered buckling triggers. The over-riding intention in design is, of course, to enable the pipeline to withstand, with sufficient safety margins, the maximum stresses and strains anticipated to occur by realistic predictions in the design stage. In recent years, many kilometres of deepwater pipelines have been designed and installed along the Brazilian coast using the principle of controlled lateral buckling, in which engineered buckle triggers, such as sleepers and distributed buoyancy sections, are deployed at regular intervals along the pipeline. The purpose of these triggers it to initiate a sufficient number of benign buckles along the pipeline and thereby relax the compressive forces set up as a result of thermal expansion without violating safe limits on stress and strain in the pipelines. In addition to the engineered buckling sites, however, the natural seabed features and associated uncertainties will interact with the pipeline’s behaviour and create additional natural buckle sites. To anticipate these sites and discover their importance at the design stage is recognised as a real challenge, particularly as precise post-installed and in-operation surveys are not normally carried out with the intention of confirming such buckle sites and design assumptions. The work reported in this paper is a detailed comparison between the initial design and observed operational behaviour of an offshore HP/HT pipeline, mainly in terms of the engineered and natural buckles actually formed along the pipeline, the severity of these buckles and some conclusions concerning the effects of initial imperfections and pipe-soil interaction characteristics considered in detailed design. It is hoped that this rare feedback from real operating conditions on installed pipelines, will be of great interest to pipeline designers and lead to more efficient and better understood design processes and encourage Operators to undertake more regular and sophisticated surveys of operating and installed pipelines for the benefit of future projects.

There are many situations during offshore installation of pipelines in which the pipe is held clamped for extended periods on the lay vessel. In the case of installation by reeling this is done at the hang off clamp (HOC) below the workstation of the reel-lay vessel. The increased fatigue damage in the pipe over the duration of the hold period, especially at or just below the HOC, then has to be evaluated and included in the overall fatigue assessment. The behaviour of pipelines during offshore installation under various anticipated sea states have long been routinely predicted using established and mostly purpose-written numerical methods. In these assessments the time histories of vessel movement and wave and current action on the immersed pipe are used as input and the dynamic response of the pipeline is evaluated. The higher local fatigue damage at the girth weld nearest the clamp is then evaluated by using a stress concentration factor (SCF) applied to the predicted stress ranges at that location to take into account the local fixity. In most simulations, however, the SCF is assigned an unnecessarily high value based on full fixity at the clamp which usually results in the predicted fatigue damage being significantly higher than that at the sagbend. The allowable hold period for a given sea state could then be governed by the SCF. In reality, many different factors would be at play at the HOC which reduce the magnitude of an SCF based on full fixity. The more important of these factors are the local frictional slip between pipe outer surface and the pads, the thickness dependent higher compliance of outer thermal insulation coating material that may be on the pipe, and the local flexibilities of the HOC housing assembly on the vessel. This paper provides a discussion and semi-numerical evaluation of the effects of some of these factors on the SCF and consequent assessment of fatigue damage.