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Technip Company Profile |
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 With a workforce of 22,000 people worldwide, TECHNIP ranks among the 5 major players in full-service engineering and construction services in the field of hydrocarbons and petrochemicals.
Technip is one of the most integrated groups providing engineering, technologies and construction services to the oil/gas and petrochemical industry worldwide. With nearly 50 years of experience in the design and construction of large industrial facilities, a wide range of state-of-the-art technologies and operational bases spread over the 5 continents, the Group is able to manage all aspects of major projects at optimised costs, from front end engineering design to turnkey delivery.
With over 30 years of experience, Trelleborg CRP has become a leading designer, manufacturer and supplier of buoyancy, cable and flowline protection and thermal insulation solutions to the Offshore, Marine, Subsea, Telecommunication and Defence industries. Trelleborg CRP offer market leading solutions from its global network of offices and manufacturing facilities. |
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Background |
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 In the Schiehallion FPSO reservoir fluids are transported from the seabed, through a turret in the vessel hull, up to the process plant, via 15 dynamic risers. The risers, which lie on the seabed and rise up to the platform, are subjected to sea currents and waves, continuous loads that can vary quite significantly. Bend stiffeners are present to protect and ease the bending of the riser at the connection to the platform. In the Schiehallion FPSO some of the riser bend stiffeners failed.
 IDAC was approached to carry out an analysis to evaluate the behaviour of the risers under different degree slippages of the stiffeners and to assess what, if any, damage would occur to the riser. The analysis was carried out with a view to sending divers down to carry out remedial work.
The stiffeners consist mainly of two rubber parts an outer bend stiffener (OBS) and an inner bend stiffener (IBS), with a clearance of a few mm between the two. The stiffeners are connected to one another through a bolted (latching) security system, which prevents the IBS from slipping down the riser. It was this latching system that failed in some of the bend stiffeners.
Different designs were proposed and analysed to replace in-situ the old Inner bend stiffeners.
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Analysis Details |
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 15m of riser were modelled, 5 above the bend stiffener and 5 below. The analysis carried out was highly non-linear including non-linear contact, large displacement/strains and non-linear orthotropic materials. The graphic to the left illustrates the finite element model for the first design loop where an IBS clamping system was proposed for the failed units. High order 3D elements were used throughout the model, as highly curved surfaces needed to be accurately represented in contact. Hyper-elastic material models were used to represent the rubber IBS and OBS, and composite materials were used to model the orthotropic response of the riser. Standard surface to surface contact elements were used to ensure load transfer between different components and also to model the friction between them. Bolt pretension was also included in the final phases of the project. |
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The analysis was carried out in several phases as follows:
Firstly a study of the behaviour of the current bend stiffener system was conducted with different levels of slippage of the IBS. In this case the latching system was not modelled. In order to calibrate the model an initial analysis was conducted using ANSYS to calculate the radius of curvature of the riser; this was then verified by the client. To prevent the IBS from further slippage a clamp system was proposed that consisted of a ring located at the end of the IBS and a stopper ring that was clamped to the riser.
Next the latching system was added to the model and a study of the forces at the security system was carried out to assess the reason for the bolts’ failure. |
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The third and fourth phases of the project investigated an IBS replacement. The replacement IBS was designed such that it was made up of two halves, bolted at the top end at the location of the metal insert. The metal insert is depicted in the figure to the left. The figure to the right shows the deflection of both the IBS and OBS, gapping contacts were used to accurately model the opening of the two halves under the bending load.
 The final phases of the project focussed on the behaviour of the fatigue forces at the new clamping system of the two halves of the new IBS, where pretension of the bolts was a crucial step. As a result of this analysis it was verified that the bolts were not seeing any alternating loads, and that the pretension was adequate to cope with the forces exerted on the bolts due to the opening of the two IBS halves.
Design Benefit
The results from Phase 1 of the analysis indicated that carrying out remedial work on the slipped IBS was not urgent and hence time was available to work out a better IBS replacement design and a new bolting strategy and hence provide an efficient repair strategy for the divers. The new strategy was evaluated in phases 3 to 6, which looked at a new IBS, made of two halves and established that the pretension was sufficient to deal |
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Case Study Archive
