DESIGNING FIBER OPTIC DYNAMIC RISER CABLES FOR OFFSHORE APPLICATIONS Jon Steinar Andreassen (Nexans Norway) Email: <jon-steinar.andreassen@nexans.com> Nexans Norway AS, P.O Box 645 Etterstad, N-65 Oslo, Norway Abstract: The growth in utilizing fiber optics in offshore infrastructure introduces some additional requirements and needs for special engineering, compared to traditional telecommunication subsea deployments. In particular, the dynamic cable section and the associated components relevant for platform installations call for extended engineering, analysis and qualification testing. The reliability requirements are high, due to the cost and risk related to replacement campaigns. This paper describes the various stages of the development and manufacturing of a fiber optic riser cable. 1. INTRODUCTION Up to now, only a relatively small number of telecommunication cable links connecting platforms have been installed. Instead the most common way to deploy optical fibers for the offshore industry is to integrate them in power or control umbilicals. As bandwidth requirements increase, not only for traditional communication needs, but also due to more advanced subsea systems and sensor and monitoring systems, the interest in separate fiber optic cables is growing. For the seabed installed part of such a system, the technology and standards inherited from telecom industry experience is directly applicable. However in case of dynamic cable sections between seabed and the surface, in terms of a floating structure, other issues are introduced compared to standard solutions. 2. DYNAMIC VERSUS STATIC APPLICATION In general, the mechanical performance of traditional submarine fibre optic cables for seabed deployment is determined by dimensioning the axial strength for loads associated by the installation and for allowing a repair situation without degradation or significant reduction in lifetime and reliability. The tensile performance is typically specified by the parameters NPTS (Nominal Permanent Tensile Strength), NOTS (Nominal Operating Tensile Strength), NTTS (Nominal Transient Tensile Strength) in addition to the breaking loads. The above characteristics are defined in international standards [1,2,3,4, as well as methods for testing and verification [5. Except for the NPTS, which reflects allowed residual tension on the cable after installation, the definitions and test methods are focused on high loads and a relatively low number of load cycles. For dynamic cables the load scenario is somewhat different. Throughout its entire operational lifetime, the cable will be exposed to tensile and bend loads which depend on actual weather conditions and sea state. This results in a large number of cyclic loads in terms of tensile and bending, and consequently effort in avoiding long term fatigue is a key issue. The loads to be handled by the cable and associated components making up the complete riser cable system are strongly dependent on where the system is going to be installed. In fact, establishing the full global load scenario is a major part of the Copyright 21 SubOptic Page 1 of 7
conference & convention required engineering. This scenario includes both loads on the cable during installation and throughout the service life. Most of the dynamic riser cables that are installed are custom made umbilicals, made up of power conductors and hydraulic tubes or hoses according to client specifications. Clients, in this case are normally oil companies or subsea equipment providers. It follows that there are few standardized products, and apart from one ISO standard [6 and its equivalent API standard [7 dedicated to cables, it is generally referred to company best practice documents and generic specifications and standards for offshore structures. whether there are existing or planned future installations from same site. In the case of several riser cables from the same topside structure, it will be required that the hydrodynamic behaviour matches in terms of movements for avoiding clashing. The ratio between submerged weight and outer diameter will in this case be a key parameter strongly influencing the design. In figure 2, an example of a fibre optic dynamic cable is presented. The optical fibres are located in longitudinally laser welded stainless steel tubes, similar to what is used for standard submarine cables. The fibre count in this case called for a number of stranded tubes making up the central cable core. The mechanical strength is obtained by 4 contra helical layers of steel wire armour. The outer protection is a high density PE jacket. Figure 1: Artists View on Subsea Infrastructure and Riser Cables from FPSO. The lack of experience with fibre optic dynamic cables and the lack of detailed international standards combined with the variety of parameters influencing the design and strong requirements on long term reliability are major reasons for the need for detailed engineering for every deployment case. 3. DESIGN PHASES FOR DYNAMIC CABLES 3.1. Initial design Based on the fundamental design inputs such as functional requirements, site of installation and initial configuration a preliminary design can be worked out. Another important input is related to Copyright 21 SubOptic Figure 2: Example of fiber optic dynamic riser cable The first step for further evaluation of this initial design is to perform comprehensive structural analysis for determining the fundamental structural global parameters. Important parameters are axial stiffness, bending stiffness and torsional stiffness. Further, it is necessary to determine the safe regions for combinations of tension and curvature based on local stress analysis. Below, some global characteristic performances of the cable shown in figure 2 are presented. Page 2 of 7
,25,225,2,175,15 [% in,125 tra S,1,75,5,25 1 2 3 4 5 6 7 Axial Load [N Figure 3: Response on axial load, straight cable. Free rotating end.,25,225,2,175,15 [% in,125 tra S,1,75,5,25 Strain Rotation 1 2 3 4 5 6 7 Axial Load [N Figure 4: Response on axial load, straight cable. Fixed ends. Strain Torque,5,1,15,2,25,3,35,4,45,5 5 45 4 35 3 25 2 15 1 5 / m e g [d n tio ta o R m [N e u rq T o n c tio e a R The capacity curve in figure 5 represents the load at which the first local stress maximum exceeds yield stress on a spot within a single component or layer. This capacity curve conservatively represents the limits of the cable construction in terms of high loads, and may be considered equivalent to what determines the NOTS and NTTS for submarine cables. The nature of the dynamic application introduces continuos load cycling and variation in local stress. In addition to determine local stress from a yield strength perspective, long term fatigue needs to be addressed. Consequently it is necessary to fully characterise the strength of materials and components in the cable with respect to of high cycle fatigue resistance. The fatigue strength typically is obtained by load cycling specimens at constant stress amplitudes until failure. Results are presented in curves correlating stress amplitudes and number of cycles to failure, and is in general expressed as Calculation of local stress on individual elements in the cable lay up in general involves finite element analysis. The purpose is to determine stress distribution within the cable construction, and evaluate load combinations for which the stress reaches a certain critical level, typically equal to the yield strength for the material in question. Results can conveniently be presented in a so called capacity curve, where allowed tension versus curvature is plotted. Where N is the number of cycles to failure, Δσ is the stress range, and a and m are scale and slope factors determined from experimental studies. 6 N [k d a l L o x ia A 5 4 3 2 1,2,4,6,8 1 1,2 1,4 1,6 1,8 Curvature [1/m Figure 5: Capacity curve Capacity Yield Average, Yield 8% utilization Figure 6: S-N curve, 2.3mm OD laser welded steel tube. (AISI 316) In figure 6, results from high cycle fatigue testing on the laser welded steel tubes used for the cable shown in figure 2 is shown. The data plotted at cycles beyond 2 1 6, are test run outs, that is the cycling was stopped prior to failure. Copyright 21 SubOptic Page 3 of 7
Based on such data, fatigue design curves and possible fatigue limits, as seen in figure 6, can be determined [8. The global structural parameters determine the behaviour and response related to the environmental exposures and loads, and together they form input to the next phase in the development process addressing the load scenario. 3.2. Dynamic analysis After establishing cable design properties and performance, the actual load scenario needs to be determined for verification of applicability. The global loads are in addition to the weight of the cable itself dependant on waves, currents and in case of a floating structure the forced movements of such. Apart from the cable related data, the key inputs are the configuration for the installation, meteorological and oceanographic (METOCEAN) data. For deep water, the riser cable will be installed on a floating structure, in which case data on behaviour and motions at various conditions is needed. Such behaviour is described in the response amplitude operator (RAO) for the floating structure, which typically will be a platform or a FPSO (Floating Production, Storage and Offloading). In addition, the seabed conditions in terms of friction beyond the riser cable touchdown point is an important input. The global configuration analysis will be performed for conditions given by combinations of waves and currents. A typical extreme load case definition is to combine waves with 1 year return period with 1 or 1 year current profile, and vice versa. Waves, current and floater motions normally are assumed collinear, and the analysis are performed for near, far, cross and transverse directions as shown in figure 7. The floater motion effect on configurations is schematically shown in figure 8 for free hanging and lazy wave with buoyancy for near and far cases. In figure 9, the static configurations for a lazy wave riser cable from extreme condition analysis at far and near position, respectively are shown. Far Riser Cable Near Cross Offset Wave Transverse Current Figure 7: Definition of directions for load cases. Bouyancy Modules Near Near Figure 8: Catenary (upper) and lazy wave (lower) configurations for static, near and far load cases. This analysis is performed for a floating platform in the North Sea, and the riser cable is anchored by a clump weight and an 11m long tether at approximately 35m water depth. The static configuration represents the nominal configuration, and wave motions will alter this periodically. One sees that the horizontal movement from far to near is as much as 12m. Far Far Copyright 21 SubOptic Page 4 of 7
Hang off movement, far Original hang off position.7 Hang off movement, near Total curvature (1/m).6.5.4.3.2 static m aximum dyn m in im u m d y n.1 1 2 3 4 5 6 7 Le ngth c oordinate (m ) Figure 9: Far (upper) and Near (lower) configuration resulting from extreme condition analysis for a lazy wave configuration. Along the riser cable, both the tension and curvature will vary. Figure 1 and 11 show the tension and curvature corresponding to the above static configurations as well as Total curvature (1/m).7.6.5.4.3.2.1 maximum dyn static m inim um dy n 1 2 3 4 5 6 7 Le ngth coordina te (m ) Figure 1: Tension and curvature envelope. (Max, Average, Min). Far condition Figure 11: Tension and curvature envelope. (Max, Average, Min). Near condition the minimum and maximum tension and curvature due to dynamics. The combinations of tension and curvature associated with the extreme case loads shown above are to be considered with respect to the capacity curve obtained from the structural analysis. A typical criterion is that the loads should not exceed 8% utilization of yield capacity. Taking floater tilting and abnormal operational situations into consideration, 1% utilization curve can be accepted as criterion. From a fatigue point of view, it is the total set of dynamic loads, rather than the extreme loads that are of interest. From the Metocean data, a wave scatter diagram is made. The wave scatter diagram distributes the number of waves into individual blocks for different wave heights and periods, as well as direction. According to the Palmgren-Miner rule, the total fatigue damage is the accumulated damage from each individual block and can be calculated based on the S-N curves, as seen in figure 6. The total fatigue damage is given by [8 Copyright 21 SubOptic Page 5 of 7
where n i is the number of stress cycles in block i and N i is the number of cycles to failure according to S-N curve in block i. The fatigue life in years is then L is the period covered by the stress cycles in the wave scatter diagram. Required life typically is 25 years, however design life for the riser cable system is multiplied with a safety factor. The SN-curves on various materials connects local stress amplitudes associated with each fatigue block and expected cycles to failure. In general, the areas critical for fatigue damage is at the topside hang-off, touch down point and in case of a lazy wave configuration, close to the buoyancy section. Interference analysis is highly important in order to make sure that no contact between adjacent riser cables takes place when the configuration is altered. In figure 9, the configurations were shown projected to the X-Z plane, however for cross and transverse cases, Y-Z views certainly are as important. This is shown in figure 12 for the configuration resulting from extreme analysis in transverse. Hang off movement, transverse Original hang off position Figure 12: Transverse extreme configuration projected to Y-Z -plane. The interference analysis takes into consideration the similar movements for also the neighboring riser cables. The necessity of matching in terms of hydrodynamic behavior is clear. 3.3. Prototype manufacturing and testing Following the design and analysis phase, the prototype design has to be manufactured and tested. The complete program includes functional and mechanical characterization. The mechanical characterization testing verifies that the cable properties used for analysis and lifetime predictions are correct, as well as verifying proper manufacturing. Fatigue testing is a key issue. Conditions and criteria for a full fatigue test on cable and components are typically defined so that the fatigue damage is equivalent to required operational life. That is equivalent to what is the worst case fatigue damage for operations added the safety factor used for design life for the riser cable. 4. COMPONENTS AND INTERFACES For a dynamic riser cable, there are a number of components that are nearly as important for successful installation as the cable itself. In fact, it is more relevant to consider the riser cable system as a whole. There is a close interaction between the loads acting on the cable and by parameters related to the topside interface, bend stiffener design, design of buoyancy sections, and so on. For the installation, proper pull-in and hang-off arrangements are required. The pull-in head should be flexible, still ensuring acceptable bending diameters for the cable. The dynamic bend stiffener interfacing typically to a tube topside is a component that should be designed in an iterative process in parallel with the dynamic analysis on the cable itself. This component and the topside inclination angle strongly affect the dynamic loads and corresponding local stress in the cable. The inclination angle will in most cases be given for the installation prior to performing analysis. In some cases, as an alternative to installing a bend stiffener, the topside tube ends in a so called bellmouth Copyright 21 SubOptic Page 6 of 7
for limiting bending of the cable. The bellmouth design in such cases is of similar importance. By introducing a buoyancy section, the surface motions and dynamics will not affect the touch down point as will be the case for a free hanging catenary. Of particular importance, adding buoyancy is a method for avoiding compression forces on the cable at touch down region. A buoyancy section is made up by clamping modules onto the cable. The number and size of individual modules has to be determined from the initial configuration and dynamic analysis. In case of multiple riser cables of different designs, this will certainly make the interference analysis even more important. Further, at the touch down point various anchor systems may be used for taking up the tension from the cable and improve stability at touch down. Whenever utilised, anchor systems should be designed in conjunction with the cable. 6. REFERENCES [1 ITU-T G.972, Definition of terms relevant to optical fibre submarine systems [2 ITU-T G.973, Characteristics of repeaterless optical fibre submarine cable systems [3 ITU-T G.974, Characteristics of regenerative optical fibre submarine cable systems [4 ITU-T G.977, Characteristics of optically amplified optical fibre submarine cable systems [5 ITU-T G.976, Test methods applicable to optical fibre submarine cable systems [6 ISO 13628-5, Petroleum and natural gas industries Design and Operation of subsea production systems Part 5: Subsea Umbilicals [7 API Specification 17E, Third Edition, July 23 [8 DNV RP-C23, Fatigue Design of Offshore Steel Structures, April 28 5. CONCLUSION Design of dynamic cables is a complex and comprehensive process. Engineering and development will include thorough characterisation of individual components and materials. This includes: Cross sectional analysis for determining local stress in the cable Determine high cycle fatigue resistance Dynamic analysis for predicting load scenario relevant for the site of installation Compared to traditional riser cables used in the offshore oil and gas industry, a dedicated fibre optic riser cable typically will be small. In case of coexistence, the hydrodynamic properties will be important and ratio between weight and dimension is the vital parameter. Copyright 21 SubOptic Page 7 of 7