Nexans Norway is, together with Marintek, currently developing a software for detailed analysis of complex umbilical cross-section designs. The software development project combines numerical methods with small-scale testing of involved materials, as well as full-scale testing of a wide variety of umbilical designs, essential for calibration and verification purposes. Each umbilical design is modelled and comparisons are made with respect to global behaviour in terms of: • Axial strain versus axial force; • Axial strain versus torsion; • Torsion versus torsion moment for various axial force levels; • Moment versus curvature for different tension levels. The applied theory is based on curved beam and curved axisymmetric thin shell theories. The problem is formulated in terms of finite elements applying the Principle of Virtual Displacements. Each body of the cross-section interacts with the other bodies by contact elements which are formulated by a penalty formulation. The contact elements operate in the local surface coordinate system and include eccentricity, surface stiffness and friction effects. The software is designed to include the following functionality: • Arbitrary geometry modelling including helical elements wound into arbitrary order; • The helical elements may include both tubes and filled bodies; • Elastic, hyper-elastic, and elastic-plastic material models; • Initial strain; • Contact elements, including friction; • Tension, torsion, internal pressure, external pressure, bending and external contact loading (caterpillars, tensioners, etc.). The paper focuses on the motivation behind the development program including a description of the different activities. The theory is described in terms of kinematics, material models and finite element formulation. A test example is further presented comparing predicted behaviour with respect to full-scale test results.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA model for predicting dynamic umbilical stresses due to friction, elastic bending and tension has been developed and compared with strain gauge data from full scale dynamic testing. The results have been used to develop effective stress and fatigue analysis procedures that can be applied for both extreme and fatigue design. These procedures have been applied for both extreme and fatigue design cases to demonstrate the importance of different physical effects. Different applications in terms of tension and geometry may require different procedures for fatigue analysis. Alternative procedures are therefore proposed considering effects from umbilical geometry and water depth. As the oil industry continues to move into deeper waters the challenges related to umbilical design increases. Hence, methods and procedures need to be continuously developed to meet these challenges.
Floating production systems need freely suspended umbilicals, i.e. dynamic umbilicals to remotely control the offset subsea oil wells. Steel tubes have a number of operational advantages that the traditional thermoplastic hoses do not. These advantages are also desirable to have in the dynamic portion of an umbilical system. Super-duplex steel is the most common. and a well-proven material to use in dynamic steel tube umbilicals. A new type coiled tubing has entered the market: zinc-eoated carbon steel. The material has already been used in a number of static umbilical projects, but it has also been desirable to find out whether this material is suited for dynamic service. A dynamic umbilical with this material was therefore designed, analyzed, manufactured and tested. The carbon-steel tube umbilical was designed for service at Shell's Mars field in the Gulf of Mexico so results from the current umbilical analysis are compared with analysis results from the actually installed Mars super-duplex steel tube umbilical although the designs are not identical. A discussion on global dynamic analysis methodology based on the comparison. is also performed. The conclusion from the work is that the zinc-coated carbon steel tubes do have the fatigue properties making it suitable also for use in dynamic umbilicals. The work was originally carried out as part of the DeepStar III session1. Introduction As the trend of oil production moves towards deeper waters and floating production, the need for freely suspended. i.e. dynamic umbilicals has increased. The umbilical is no longer protected by a rigid I-tube from the seafloor to the production vessel. When suspended freely, the umbilical is continuously subjected to the sometimes hostile environment represented by current waves and vessel motions. This naturally increases the demands on the umbilical design, and also creates the need to understand in what way the global configuration affects the umbilical design life. Using steel tubes instead of thermoplastic hoses in the hydraulic lines of a static umbilical has a number of advantages; a quick hydraulic response; no permeability; generally a more impact resistant design and a very high collapse resistance, just to mention a few. Dynamic steel tube umbilicals also have the above listed advantages. but using steel in dynamic applications opens up challenges which must be overcome. The major challenge With. respect to dynamic steel tube umbilicals is fatigue, and details of the global configuration and configuration related products can be decisive on whether the required design life is met or not. Super-duplex steel has up to now been alone as the steel material used in dynamic umbilicals. As a new material has come to the market in the form of coiled. zinc-coated carbon steel, it has also been desirable to find out whether this material can be used in dynamic steel tube umbilicals. This paper therefore gives an overview on the qualification of a dynamic steel tube umbilical using zinc-coated carbon steel. The umbilical, hereafter called the DeepStar dynamic umbilical has been designed as if to see field service at Shell's Mars field in the Gulf of Mexico. The paper therefore also presents comparisons with analysis results from the super-duplex steel tube umbilical that is actually installed at the Mars field.
System Description The Foinaven riser and umbilical system comprises of 10 flexible pipes and 2 umbilical and provides production, test, gas injection, water injection and control for two drilling centre. The size and duty of the risers is as follows:- Drill Centre I 2 × 10" Production 2 × 8" Production/Test 1 × 10" Water Injection 1 × 8" Gas Injection I × Dynamic Umbilical Drill Centre 2 2 × 10" Production 2 × 8" Production/Test I × Dynamic Umbilical The design pressure for the system is 3689 psi (254 bar g). The configuration of the risers is a "pliant wave" (figure I) which is anchored to a gravity base structure by tethers and has buoyancy modules distributed along the lower end. The system is designed such that it can be released from the FPSO in extreme emergency conditions. Design Conditions The Foinaven field is the deepest ever application of a "pliant wave" riser configuration and has to withstand very high currents over the full water column of up to 2 m/s (3.9 knots), When combined with the 100 year design wave of 18 m (sign) this provided very harsh conditions for the design of the pipe (figure 2), the critical areas being the vessel interface, where a bend stiffener is required, and at the riser touchdown point where extreme near and far vessel positions resulted in the need for a hold back anchor to prevent pipe movement and subsequent over bending of the pipe. The large waves in the Atlantic also impose severe fatigue loading on the risers, the most critical risers being the gas and water injection risers, due to the high operating pressure within the system. Next to the environmental conditions the next major influence on the system design was the vessel offset. This is closely linked to the design of the mooring system for the vessel which was far more compliant than originally anticipated in the riser design. When procuring a floating production system the risers influence both the sub sea layout as well as the mooring system design. Care must therefore be taken in addressing this interface. If the interface is defined at the riser touchdown the cost benefit of manufacturing flexible flow line jumpers (especially the flow line to manifold jumpers) at the same time as the riser maybe lost. Also for the umbilical, vessel interface was one of the major challenges. The region that experiences the highest loads both with respect to tension and bending is found at the I-tube exist. Bend stiffener design as well as cross-section design is critical. At touch down two clump weights were needed, One main clump weight taking up virtually all the tension, leading to that the touch down area saw no tension at high bending. the resulting catenary below the tether clamp, however, induced such a high bottom tension in the extreme current and far vessel offset cases that the interface joint between dynamic and static umbilical was anchored to a second, smaller slump weight. Its role is to prevent axial movement of the interface joint thereby preventing potential unwanted configuration changes.
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