Draka Comteq Netherlands / Comparison of Microduct Types for optical cables installation Protected Microducts, Overview and Current Trends W. Griffioen 1, W. Greven 2 1 Draka Comteq Cable Solutions, Amsterdam, Netherlands, 2 Draka Comteq Cable Solutions, Delfzijl, Netherlands, Abstract Microducts have been used many years for optical fiber networks. They appear as tight bundles with protective sheath and as loose bundles blown or prefabricated into protective ducts. Currently thick-walled self protecting microducts showed up. They appear as single microducts, round bundles or strips. All of these products we call protected microducts. They can be used in different situations with specific pros and contras. In this paper an overview is given. Some striking results: in near surface ducts protected ducts may suffer from buckled microducts and in buried conditions some concepts with thick-walled microducts suffer from soil set, leading to strongly reduced cable blowing lengths. 2.1 Loose Bundles in Ducts Keywords: Microduct; protected microduct; thick walled microduct; duct; optical fiber cable; blowing; direct buried. 1. Introduction A protected microduct is defined (IEC) as one or more microducts surrounded by a protective sheath and/or a larger protective duct [1]. Microducts first showed up in the eighties, introduced by BT, as tight bundled microducts surrounded by a protective sheath, used with blown fiber technique [2]. In the nineties loose bundles of microducts, blown into a larger protective duct were introduced by KPN, used with the microduct cable technique [3]. The microducts here are either installed prefab or in the field, allowing the use of existing ducts, empty or even already occupied with a cable [4]. Currently also thick-walled microducts, taking care of their own protection, found their way in the market. They appear as single microducts, round bundles or ribbons. Protected microducts can be used in different situations: direct buried, in underground ducts, in near surface ducts, aerial and sometimes even in water crossings. The different types of protected microducts all have their specific pros and contras. Important factors are ease of installation, branching of individual microducts and, of course, costs. Also important is the behavior in environments with larger temperature variations. Last, but not least, also the soil effect on direct burying plays an important role. Today protected microducts are installed on a larger and larger scale, especially in Europe. It is difficult for a user to find the best solution for his typical situation. In this paper an attempt is made to give a complete overview of protected microducts used and their limitations in some situations. Different specific problems will be treated, supported by theory. Also experiments have been performed, at Draka s test site in Delfzijl, some of them with quite striking results. 2. Overview Protected Microducts In this section different kinds of protected microducts will be listed. The standard microducts are surrounded by a protective sheath or duct. The thick-walled microducts are self-protecting. The pros and contras of the concepts will be given. In another section and in Appendix A tests and calculations are presented. Figure 1. Loose bundles in ducts Loose bundles of microducts are installed in ducts (see Figure 1) in the field by blowing [4,5] or pulling. They can also be installed prefab. There is some free space for the microducts in the duct. This not only enhances blowing in of the microducts, it also improves the impact resistance (microducts can move away) and makes branching of microducts possible (easy access of all microducts) using simple Y-connectors, see Figure 2. Figure 2. Y-Connector for branching ++ Suitable for direct burying, even the best solution. The duct will resist most of the soil set. Most of the little undulations the stiff duct may take are cancelled by the microducts, with their freedom to choose their own path.
Draka Comteq Netherlands / Comparison of Microduct Types for optical cables installation - Not suitable for aerial or near surface ducts. Limited temperature span (20 C). See Appendix A, for a typical free space in the order of 10 mm. + Microducts can be installed later, also in existing empty and even occupied (smaller plastic) ducts [4]. 1 hour temperature relaxation advised. ++ Highest resistance against impact, crush and stones. + Easy to access branching microducts. + Duct and microducts remain straight during unwinding the drum. Outer and inner bend microducts can swap places. - No longitudinal water-tightness (special plugs needed when buildings or closures are entered). - Microduct ends must be fixed during prefab duct laying. If not they may slip inside when the duct stretches during pulling, not coming back when releasing stress, i.e. they may buckle. + Resistance against impact, crush and stones. - Difficult to access branching microducts. Requires relatively expensive boxes. ± Length differences in microducts (on drum) might cause spiralling of the bundle when unwinding, or kinking or flattening of the outer microducts. How serious this is depends on the drum diameter and bundle construction details. + Potential longitudinal water-tightness (no special plugs needed when buildings or closures are entered). - In case of APL shield: Protects as Titanic hull (when microduct damaged), even may generate hydrogen! 2.3 Thick-Walled Microducts 2.2 Tight Bundled with Jacket Figure 4. Thick-walled microducts Figure 3. Tight bundled microducts Tight bundled microducts are surrounded by a protective sheath (see Figure 3). Impact resistance little less than for loose bundles in ducts (even still for DB types) because microducts cannot move away. Branching of microducts needs special boxes because it is difficult to access all microducts (especially middle ones). Tight bundled microducts suffer from length differences between the microducts when unwinding from the drum. On the drum the radial outwards facing microducts make a longer path than the inward ones. Part of this length differenence is frozen in the construction when the bundle is unwound from the drum and laid straight. This may cause flattening of the shortest and buckling of the longest microducts, especially in the outer layer. The length differences may also cause undulations in the unwound bundle (Figure 13) reducing the cable blowing distance. Also undulations may result in kinking of the whole bundle (Figure 14). + Suitable for direct burying. The bundle will resist most of the soil set. forces needed for the microduct (and sheath) couplings (see Appendix A) no suitable connectors exist yet. - Not recommended for old (smaller plastic) ducts, certainly not when occupied with resident cables. Thick-walled microducts (see Figure 4) can be installed without further protection. They are self protecting, but less than for loose bundles in ducts and also less than for tight bundled microducts. Branching of microducts is the most simple for the thick walled microducts. They are easy to access and no separate branching connectors are needed, only the straight microduct connectors. They are less suitable for direct burial. - Less suitable for direct burying. Soil set will cause strong undulations causing decrease in cable blowing length, see tests performed in this paper. ± Microducts can be installed later, also in existing empty (smaller plastic) ducts (although standard microducts suffice here), but less easy than for standard microducts. Use of occupied ducts is not recommended. ++ Easy to access branching microducts. No special Y-couplings needed for branching. + No length differences in microducts on drum.
Draka Comteq Netherlands / Comparison of Microduct Types for optical cables installation 2.4 Strips of Thick-Walled Microducts Figure 5. Strips of thick-walled microducts Strips of thick walled microducts are produced by assembling the microducts on a flat tape of plastic or by surrounding the microducts by a thin jacket. Here the same properties are valid as for single thick-walled microducts. The strips do not offer extra protection (but crossing of microducts is eliminated), neither will the effect of the soil set be reduced (exception is the strip of 2 microducts with thin jacket, part of the family of bundles of thickwalled microducts, see under tests). The individual microducts are just as easy to access. Also the strips can be installed later in ducts (they can be folded to take a minimum of space). - Less suitable for direct burying. Soil set will cause strong undulations causing decrease in cable blowing length, see tests performed in this paper. order of 300 mm (the whole strip in the duct). For the high ± Microducts can be installed later, also in existing empty (smaller plastic) ducts (although standard microducts suffice here), but less easy than for standard microducts. Use of occupied ducts is not recommended. ++ Easy to access branching microducts. No special Y-couplings needed for branching. + No length differences in microducts on drum. 2.5 Folded Strips of Thick-Walled Microducts Figure 6. Folded strips of thick-walled microducts Figure 7. Tool to fold the strips Figure 8. Unwinding and folding strip from drum Folded strips of thick walled microducts are the same as above, but now folded and glued (using pre-applied adhesive with removable covering tape, the latter just removed in Figure 7) when unwinding from the storage drum (where they can still be wound without length differences between the microducts). Because the stiffness of the folded strip is higher and the effective area of the soil above is lower the effect of the soil is much less, as is shown in tests in this paper where the decrease in cable blowing length is acceptable. The individual microducts are still easy to access. The folded strips can also be installed later in ducts (now folded without removing the adhesive covering tape). + Suitable for direct burying. The folded strip will resist most of the soil set.
Draka Comteq Netherlands / Comparison of Microduct Types for optical cables installation ± Microducts can be installed later, also in existing empty (smaller plastic) ducts (although standard microducts suffice here), but less easy than for standard microducts. Use of occupied ducts is not recommended. Note: folding step not done in this case. + Easy to access branching microducts. No special Y-couplings needed for branching. + No length differences in microducts on drum. - Some extra labour (although automatic when using a special tool) for the folding during laying of the strip. 2.6 Bundles of Thick-Walled Microducts ± Less easy to access branching microducts. No special Ycouplings needed for branching. ± Length differences in microducts (on drum) might cause spiralling of the bundle when unwinding (see Figures 13, 15 and 16). How serious this is depends on the drum diameter and bundle construction details. 3. Tests Cable blowing tests have been done for four types of microduct systems: loose bundles in ducts and strips, folded strips and bundles of thick-walled microducts. Tests were also done in direct buried conditions of the ducts. 3.1 Loose Bundles in Ducts Blowing tests were done with a 1.8 mm 4-fiber FttH cable in a bundle of 24 microducts 4/3 mm in a 40/33 mm protective duct. The duct is buried in the ground over 100 m. The microducts were looped, connected by 180 bends with bend radius of 16 cm in a trajectory according to IEC [1]. The cable could be blown in over a length of 1000 m with a pressure of 10 bar, speed at the end 23 m/min. loose tube cable in bundles of 7 microducts 12/9.6 mm in 50/40 mm protective ducts, buried also in the ground over 100 m. The microducts were looped, connected by 180 bends with bend radius of 50 cm in a trajectory according to IEC [1]. The cable could be blown in over a length of 1600 m with a pressure of 7 bar, speed at the end 20 m/min. Figure 9. Bundles of thick-walled microducts Here the thick-walled microducts have been assembled to round bundles, surrounded by a thin jacket that just follows the outer microducts (see Figure 9) or a straight tight jacket. The effect of the soil is small for this relatively stiff bundle. But, problems with length differences occur here again (see Figures 13, 15 and 16). Also the individual microducts are less easy to access (especially the middle one). + Suitable for direct burying. The bundle will resist most of the soil set. - Not recommended for old (smaller plastic) ducts (although standard microducts suffice here), certainly not when occupied with resident cables. 3.2 Strips of Thick-Walled Microducts Blowing tests were done with a 1.8 mm 4-fiber FttH cable in a strip of 7 microducts 5/3 mm. The strip was first laid in the field over 400 m, with one S-curve and one 90 bend. The microducts were looped, connected by 180 bends with bend radius of 16 cm. The cable could be blown in over a length of 500 m with a pressure of 10 bar, speed at the end 23 m/min (usually amply 1000 m is reached, this time a less good quality material was used for the microducts). Then the strip was buried in the ground, a stretch of 50 m small rocks (see Figure 11), a small above ground access section, and the rest sand (see Figure 10). Now the cable blowing length decreased to only 50 m. No difference was seen in behaviour between rocks and sand. loose tube cable in a strip of 4 microducts 14/10 mm (slightly larger inner diameter). This strip was laid parallel to the strip above and connected by 180 bends with bend radius of 50 cm. The cable could be blown in over a length of 1600 m with a pressure of 7 bar, speed at the end 20 m/min. Also this strip was buried then resulting in a cable blowing length of only 630 m at 14 bar blowing pressure.
Draka Comteq Netherlands / Comparison of Microduct Types for optical cables installation Figure 10. Strips before closing trench (sand) Figure 11. Strips before closing trench (rocks) in the previous section. The cable could be blown in over a length of 800 m now with a pressure of 13 bar, speed at the end 20 m/min. Less than for above ground conditions and for loose bundles of microducts in a protective duct (also taking into account the slightly larger inner diameter) but far better than for the non-folded strip. loose tube cable in a folded strip of 6 microducts 14/9.6 mm, also only tested in buried (same as above) conditions. The cable could be blown in over a length of 1566 m with a pressure of 12.8 bar and a speed at the end 7 m/min. Folding the strips helped indeed. Not only the effective area that is exposed to the soil is reduced, also the stiffness of the bundle increased, ranging from 30% to a factor of more than 2 as measured using 3-point bending tests [6]. In theory a factor of 4 is possible, but then the folded strip must be glued slip free. 3.4 Bundles of Thick-Walled Microducts loose tube cable in bundles of 6 and 2 microducts 14/9.6 mm, also only tested in buried (same as above) conditions (in earlier tests with standard microducts from the same manufacturer a blowing length of 1500 m was reached with a pressure of 10 bar and a speed of 20 m/min). The cable could be blown in over a length of 1500 m with a pressure of 11.4 bar and a speed at the end 7 m/min for the bundle of 6 (little different settings as the for the folded strip above, but performing about the same, tested at the same day) and over a length of 780 m (no more length buried) with a pressure of 4 bar and a speed at the end 20 m/min for the bundle of 2. Note that the latter bundle is in fact a strip. However, the (0.75 mm) tight jacket (hard plastic material) made the stiffness of the bundle increase by a factor of 2, as measured using 3-point bend testing [6]. For strips this will probably not work anymore for larger numbers of microducts. The surrounding jacket then has more freedom to buckle in the middle of the bent strip. Figure 12. Residual buckles after digging up 3.3 Folded Strips of Thick-Walled Microducts Blowing tests were done with a 1.8 mm 4-fiber FttH cable in a folded strip of 6 microducts 8/3.5 mm. The strip was only tested in buried (same as above) conditions now, in the same stretch as Figure 13. Unwinding bundle of 6 thick-walled microducts
installation Draka Comteq Netherlands / Comparison of Microduct Types for optical cables In Figures 13 to 16 examples of undulations of different bundles of thick-walled microducts are shown. Some are more serious and some hardly show undulations. The undulating properties depend on the drum diameter and on construction details of the bundles. 3.5 Summary of Tests Tables 1 and 2 show blowing lengths (from Section 3: 2 cable types, same within each Table to compare) converted to blowing pressure of 10 bar, 20 m/min blowing speed and, when needed, to right material or same microduct ID using JETplanner [7,8]. Table 1. Blowing length (m) for 1.8 mm FttH cable in 4/3 mm microduct Figure 14. Kink in bundle of 4 thick-walled microducts Above ground Buried Loose bundle 1000 1000 Strip thick-walled 1000 100 Folded strip thick-walled 1000 600 Table 2. Blowing length (m) for 7.1 mm stranded loose tube cable in 14/9.6 mm microduct Above ground Buried Loose bundle 1900 1900 Strip thick-walled 1900 500 Folded strip thick-walled 1900 1200 Bundle 6 thick-walled 1400 1200 Bundle 2 thick-walled 1500 1200 Figure 15. Folded strip (blue) and bundles (orange) of thick-walled microducts. Note that one of the bundles consists of 2 microducts and is, in fact, a strip. Tables 1 and 2 show that with direct buried thick-walled microducts acceptable cable blowing lengths can be reached when selecting the right construction. However, the longest lengths are still obtained using loose bundles of microducts in a protective duct. Table 3. Comparison of protected microducts LB TB TW ST FS BT Direct burial ++ + - - + + Larger ducts + + + + + + Aerial, near surface - Smaller (plastic) ducts + - - Different materials NA + NA + + + Impact, crush, stones ++ + Branching + - ++ ++ + Length difference + + + + Watertight - + Fixing microducts - + + + + + Figure 16. Bundles of thick-walled microducts (orange ones), one straight and one strongly undulating. Hydrogen risk NA! NA NA NA NA Extra unroll step NA NA NA NA - NA
Draka Comteq Netherlands / Comparison of Microduct Types for optical cables installation In Table 3 a comparison summary is given of the pros and contras of the different microduct designs: loose bundles in ducts (LB), tight bundled with jacket (TB), thick walled microducts (TW), strips of thick walled microducts (ST), folded strips of thick walled microducts (FS) and bundles of thick walled microducts (BT). 4. Conclusions Different microduct constructions were discussed and analyzed, some of them have been tested with cable blowing, above ground and direct buried. Most important conclusions are given below: In near surface ducts large temperature variations may lead to buckling of microducts and therefore reduced cable blowing lengths. Also microducts (and protective ducts) may be pulled out their connecting fittings. It is important to select suitable microduct constructions and connecting fittings. Direct burying of thick-walled microducts, also ribbons, may result in buckling of the microducts due to soil set. This may lead to significant reduction in cable blowing length. Also here it is important to select suitable microduct constructions.