First South East European Regional CIGRÉ Conference, Portoroz Deep Water Power Cable Systems? Indeed!

Transcription

1 First South East European Regional CIGRÉ Conference, Portoroz 2016 SEERC 2-06 Deep Water Power Cable Systems? Indeed! M. JEROENSE, O. HANSSON, A. TYRBERG, E. REBILLARD, K. CRONHOLM, J. LINDHE ABB High Voltage Cables Sweden SUMMARY Power cable technology has made significant technological developments during the latest decade and development is still ongoing. Direct current transmission cables are used for long distance high power transmission whereas alternating current cables are used for shorter distances. Direct current transmission voltage and power have seen an outstanding incremental development to high values as 525 kvdc and well above 2 GW, respectively. Alternating current transmission technology has become competitive due to large 3-core cables. The additional requirement the cable technology faces, which is necessary for the Mediterranean area, is increased installation depth. Installation depths more than 1500 meters are a must and values greater than 2000 meters can certainly be needed. Such challenges cannot be met by restricting the development to the cable system only. Indeed the mechanical strength of the complete cable system must be adapted to these depths. But a holistic approach where also the installation technology is taken into account is an absolute must. Cable design and installation at such challenging depths should not be separated. Understanding the technical and weather requirements in an early phase is another valuable aspect of success. The article will elaborate on these different aspects. The CIGRÉ technical brochure TB623 is the results of a 3-year work of an international Working Group that covers all aspects of cable system installation, also at large depths. The TB should be used in the different phases of development, projecting and execution of deep water power cable projects. It will be shown that extruded DC cables and MI are both suitable for deep water installations. KEYWORDS Extruded DC, MI, XLPE, installation, deep water, installation marc.jeroense@se.abb.com

2 INTRODUCTION When planning a submarine power cable link crossing deep water areas, the water depth may give some of the main technical challenges. These challenges can, to a certain limit, be overcome with a combination of cable design and installation methods. This is regardless of the power cable type in question, but every cable type may have advantages or disadvantages towards requirements evolving from the deep water installation. Since most of the future deep sea interconnectors are foreseen as HVDC links, the discussions in this paper will concentrate on DC cables and not on AC cables. The history of submarine DC interconnectors dates back to the 1950 s with the first commercial commission of a submarine Mass Impregnated (MI) cable. During the past few decades, also HVDC cables with extruded insulation have been introduced and are now used in numerous of interconnector projects. When it comes to DC links at greater depths, however, as of today only MI cables have been used. DC projects including cables with extruded insulation have so far been located in relatively shallow waters. There is however no reason for the extruded type of cable, as such, to be disqualified for deep sea installations. The technical challenges around the critical mechanical elements is in fact the same regardless of MI or extruded insulation, e.g. conductor, conductor joints, metallic sheath and armouring. SPECIFIC REQUIREMENTS AT DEEP SEA CABLE SYSTEMS In reality, a high static tensile force in the cable during laying should be regarded as inevitable when talking about cable installation at great depths. From the water depth itself, even without any considerations taken to the installation methods, the additional requirement on a deep sea compared to a shallow installation is the ability to cope with the higher hydrostatic pressure. The cable system must be designed to withstand the surrounding pressure without any devastating deformations of the components such as insulation system and water barriers. Also longitudinal water penetration resistance must be kept at acceptable levels. Taking also the installation methods into account, further mechanical requirements on the cable system can be identified. The obvious one is the ability to withstand side wall or squeeze pressure from the different equipment on the laying vessel. Two types of field joints are necessary at submarine cable installation: Inline joints and Omega or hairpin joints. When it comes to the deep sea inline joints, requirements will be rather the same as for the cable: The ability to cope with tensile forces during laying. Depending on used technique when launching the joint, also some assisting aids such as winches and corresponding fasteners may be necessary. Regarding the Omega joint one major additional requirement will fall on the installation vessel: Two cable ends, connected by the joint, must be held and launched simultaneously. Starting from the cable perspective, there are some basic input values that are needed before any cable design can be drafted. The input includes such things as voltage, ampacity, thermal conditions etc. regardless of deep sea installation or not. In addition to what is needed for a shallow installation, details about possible installation methods must also be known before a deep sea cable design can be established. INSTALLATION The greater part of subsea power cable installation projects are performed by use of laying vessels equipped with horizontal laying system (HLS), see figure 1, comprising different types of laying wheels or chutes. These techniques and methods have their origin from installation of lightweight telecom cables. Installing power cables at deep water leads to high top tension coming from the catenary load. The high top tension in turn leads to high side wall pressure (SWP) if a laying wheel or chute are used. In the case of cable recovery, friction on a fixed chute will increase the recovery force by up to 50 % compared to during normal laying. Therefore fixed chutes have severe limitations when it comes to deep water installation where the tensile forces are large. 2

3 Once again solutions can be inherited from other business areas. The issue with high tension and SWP during installation of flexible risers and pipes for the offshore oil and gas industry has been handled by development of vertical laying system (VLS). See Figure 1. Figure 1. Vertical Laying System (VLS) and Horizontal Laying System (HLS) A typical laying spread of a vessel equipped with VLS is as follows: The cable are guided from the turntable via cable ways up to the top off the VLS tower were a quadrant (Pos 3) equipped with rollers guides the cables towards the tensioners. The quadrant will prevent over bending of the cable. The tensioners on the VLS tower are placed vertically (Pos 4) to take the catenary load. The tensioners has pads designed in order to create friction enough to hold the cable but not damage the cable integrity. The length of the tensioner tracks is also important since it controls the required squeeze force onto the cable. With a VLS the cable does not undergo bending simultaneously as it is in maximum tension, the side wall pressure is thereby kept to a minimum. A laying spread with a VLS tower makes the handling and over boarding of an inline joint challenging but still feasible, it demands that the vessel are equipped with suitable lifting devices. However it has to be considered that the loads are high, this increases the demands on the tools and accessories. Repair of a subsea cable implies an omega or hairpin joint with two cables coming up from the sea to the vessel. These two cable ends, connected by the joint, have to be launched simultaneously. A single VLS tower can then not be used. The joint, including the structure designed to handle the tensile force, can be boarded via chute, if the main part of the catenary load are taken by a load securing device mounted on the vertical section of each cable below the chute. If pulling stockings or a clamping system are used to secure the loads during jointing work and to lower the cable joint to the seafloor one has to take into consideration the high squeezing force the cable will be exposed to. Figure 2. Cable laying vessel mobilized with VLS tower A vessel equipped with VLS tower, see Figure 2, has been used successfully in an installation project of a power cable system for the oil and gas industry in Barents Sea, northern part of Norway. Also cable accessories such as buoyancy modules and wet storage devices were handled with great 3

4 accomplishment. The completed installation project in the Barents Sea showed it feasible to perform subsea cable installations using a VLS tower laying spread also with large, heavy and nonflexible accessories by use of good engineering, skills and equipment. CABLE DESIGN In order to optimize the stress and strain on cable design elements, the cable weight in water should be kept down. A way to accomplish that, is to optimise the choice of materials as well as the layer thicknesses. One obvious example to decrease the cable weight is to use aluminium rather than copper as conductor material. Aluminium has during the last few decades entered the submarine market partly due to the cost effectiveness as such, but in some cases also motivated by the cable weight reduction [1]. One of the critical design elements when it comes to tensile force is the conductor and conductor joints made during manufacturing or installation. During manufacturing the conductor undergoes strain hardening where the yield stress and tensile strength of the conductor is increased compared to the annealed base material. Conductor jointing will be required when installing a long submarine cable. Jointing is performed by welding which will result in a heat affected zone (HAZ) in the vicinity of the weld. In the HAZ the material undergoes some degree of annealing thereby reducing the yield stress and tensile strength. The conductor joint has less strength and is therefore the most critical part of the conductor. The procedures and materials used for conductor joints must be adapted to the higher strain/stress levels encountered during deep water installation. The other main design element involved in the tensile force distribution is the armouring system. The choice of the armouring materials themselves could change when going to deep installation applications. The traditional mild steel, grade 34 with an tensile strength in the range MPa could be replaced by high strength steel, for instance grade 65 with an tensile strength in the range of MPa [2]. Also the geometric of the armouring system, such as lay angels, should be optimised in order to get beneficial division of forces on the different design elements. For a single core cable the strain in the conductor,, will be equal to the strain in the cable,, which is given by: (1) where T is the tension in the cable and EA is the axial stiffness. The axial stiffness of the cable is equal to the sum of the axial stiffness of the different layers. For a double armoured torsional stable DC cable, the axial stiffness can be estimated from:, (2) where E A is the elastic modulus of the armour material, A A the area of the armour layers, the average lay angle of the armour layers, E c is the elastic modulus of the conductor and A C the conductor area. The strain in the conductor results in a tensile force in the conductor and the conductor joint must sustain this load without any damages. The armour layer will dominate the axial stiffness of the cable due to the large area and a high elastic modulus of steel. An increased stiffness is beneficial for reducing the conductor strain, however increasing the area of the armour layer will results in an increased weight of the cable thereby increasing the cable tension. The third design element in the cable system which need close attention is the field joint. This topic will not be further elaborated in this article. 4

5 DESIGN EXAMPLES OF HVDC CABLES, MI VS EXTRUDED CABLES As an example we assume the requirement of a cable link consisting of one pair of DC cables capable of transmitting 1.2 GW in the receiving end. With the condition that the deep sea cables should be separately laid with a burial depth of 0.5 m into sediments with a thermal resistivity of 1.0 Km/W and an ambient temperature of 15 C, an XLPE cable with an 1000 mm 2 Al conductor has ampacity of 1200 A at maximum conductor temperature of 70 C. For an e.g. 500 km link this results in an approximate power in receiving end of 1.2 GW for one pair of cables taking cable losses into account. An MI cable with 1400 mm 2 Al has the corresponding ampacity of 1185 A at conductor temperature of 52 C. Due to lower losses, the transmitted power in the receiving end will be roughly the same for a 500 km link, approximately 1.2 GW. Table 1: Design examples of MI and extruded DC cables Mass impregnated cable Extruded cable Voltage U kv 525 kv Power at receiving end 600 MW/cable 600 MW/cable Conductor material Aluminium Aluminium Conductor cross section 1400 mm mm 2 Diameter complete cable 138 mm 143 mm Mass complete cable 52 kg/m 49 kg/m Cable weight in water 37 kg/m 33 kg/m Axial stiffness* 697 MN 705 MN Current 1185 A 1200 A All values are indicative *From equation (1), two armour layers with 6 mm steel wires and an average lay angle off 14 deg has been assumed. LOADS DURING INSTALLATION In this section the tensile loads and squeeze pressures will be highlighted. Tensile loads The tension in a cable suspended from a cable lay vessel will consist of a static part and a dynamic part varying over time. The static top tension,, will depend on the cable weight in water,, and the water depth,, and can be calculated from the catenary equation: (3) where is the bottom tension at touch down point. The dynamic tension is primarily the result of vessel movements. The induced vessel movements depend on wave height, period and wave direction and are different for different cable lay vessels. Response Amplitude Operators (RAO), established through model testing or analysis in special purpose software, are used to describe the vessel pitch, heave and roll response as a function of the wave conditions. The departure point on the vessel also has a large impact on the dynamic tension. If for instance the cable leaves the vessel over the stern, the pitch response and length of the vessel will be important. To take all these factors into account and to also provide accurate modelling of the wave and current conditions it is common to use special purpose software such as OrcaFlex. 5

6 If the full details of the installation vessel or conditions are not known CIGRÉ technical brochure TB623 [3] provides equations that can be used to calculate a conservative estimate of the expected maximum dynamic tension. The expected installation tension as a function of water depth has been calculated for the MI and XLPE cable design examples presented in Table 1 above. Calculations are performed according to TB623 [3] and based on a significant wave height H s of 2.5 m, a maximum wave height H max of 4.75 m, a wave period of 10 s and with the vertical movement of the sheave,, being 1.5 times larger than the maximum wave height (7.13 m). For this parameter set the dynamic tension contributes with 20 % of the total tension, which corresponds to a dynamic amplification factor of approximately The calculated top tension as a function of water depth for the two cable designs is shown in Figure 3. The conductor strain is an indication of the severity of the tensile load onto the conductor joint. The conductor strain as a function of water depth for the two cable designs is shown in Figure 4. Figure 3: Calculated top tension as a function of water depth for the MI and extruded cable design examples Figure 4: Calculated conductor strain as a function of water depth for the MI and extruded cable design examples The top tension for the two cable designs is quite similar and is approximately 10 % lower for the extruded cable compared to the MI cable which is due to the extruded cable having a lower weight. The axial stiffness of the two cable designs is similar and the strain is approximately 10 % lower for the extruded cable compared to the MI cable for a given water depth. For the cable weights in this example the expected top tension becomes larger than 500 kn at around m water depth, thereby imposing requirements on the cable lay vessel and installation equipment that goes beyond what is normally used in submarine cable installations. The weight of the cables can be reduced to some extent by reducing the armour wire cross section. This will reduce the top tension but it will also result in a lower axial stiffness of the cable and eventually an increased conductor strain for a given water depth. For deep water installation and cable design there is a tradeoff; reducing the tensile load onto the cable components results in increased loads onto the installation vessel and vice versa. Squeeze pressure from tensioners A tensioner system or capstan wheel can be used to hold, feed and recover a cable. In a linear tensioner system grip is achieved by squeezing the cable between two, three or four tracks. The required squeeze force, S, from each track depends on the tensile force in the cable, T, the length of 6

7 the tensioner, L T, the number of tracks, N T, and the lowest friction coefficient,, between the tensioner pad and the outer armour layer and can be calculated from:, (4) The required squeeze force capacity of the cable will therefore depend on both the installation tension and the design of the tensioner system. TESTS FOR DEEP WATER CABLE INSTALLATION Testing is an important part in verifying that a cable system is appropriate for deep water installation. Testing normally starts with component tests to verify the different cable components and this is then followed by development tests and type test on the complete cable system. For deep sea application it is the mechanical loads onto the cable that are of primary interest and most of the mechanical tests on the complete cable system are described in TB623 [3]. This chapter describes some of the most important mechanical tests to be performed to verify a cable design for deep water installation. Tensile test on conductors and conductor joints The most critical part of the conductor is the conductor joint. The joint must have sufficient strength to withstand the stress induced in the conductor due to the cable strain resulting from the tensile force in the cable. As part of the development of the conductor joint technology, both the capacity of the joint and the stress strain relationship of the un-welded conductor need to be understood and investigated through component testing. The requirements on the conductor joint strength will be similar for both extruded and MI cables. Conductor water penetration test. The conductor water penetration test is described in TB490 [4] and shall also be performed for extruded DC cables according to TB623 [3]. The test is performed to simulate water ingress in the conductor during a cable fault at the deepest part of the cable section. The test is not performed for MI cables since the mass-impregnation of the conductor and the papers gives a good longitudinal water tightness of the cable. A development program has been performed to investigate the longitudinal water tightness of extruded cables with large conductor cross sections at water pressures up to at least 200 Bar. It was found that it is possible to achieve longitudinal water tightness for both aluminum and copper conductors by optimizing the water blocking material in combination with good quality in the conductor manufacturing. Both MI and extruded cables can therefore fulfill the requirements with regards to conductor water tightness at large water depths. Squeeze test The crush/squeeze test is described in TB623 [3] and the test replicates the squeeze loads experienced by the cable during installation with a linear tensioner system. Larger installation depth means larger tensile force and that the squeeze force capacity of the cable needs to be increased in order to hold the cable. Figure 5 shows how the load is applied in a 4-track squeeze test and shows a squeeze test performed on a cable. 7

8 Figure 5. Load application in squeeze test and squeeze test performed on cable If the squeeze force is too large this can result in excessive ovalization of the insulation system and/or local indentations into the lead sheath and insulation. The squeeze force capacity of the cable will depend on the cable design but also on the pads used in the tensioner. Squeeze tests on extruded cables as described above have shown that 4-track tensioner systems, in combination with optimized pad geometry and cable design, allow installation at water depths over 2000 m. Tensioners can thus be an alternative to capstan wheels as method of holding the cable during deep water installation of submarine cables. Tensile and Tensile Bending tests The tensile and tensile bending tests are standard tests performed as the mechanical pre-conditioning in most cable type tests. The tests are described in TB623 [3] and replicate the forces that are applied to the cable during installation. The tests should include factory, flexible and rigid joints. The tests will verify the strength of the factory joints, especially the conductor joint. The tensile bending test also reproduces the side wall force experienced on the lay and capstan wheel. If the side wall force is too large the failure modes will be similar as for the squeeze loads; ovalization of the insulation system and/or local indentations in the lead sheath and insulation. If a VLS is used, bending occurs without tension, and the tensile test will reproduce the loads experienced by the suspended cable during installation. For a VLS system the side wall force is instead replaced by the squeeze force from the tensioner system. CONCLUSIONS Deep water installations require a careful design of the cable system taking into account the properties and specific equipment of the laying vessel as well as the weather conditions. Tensile forces, squeeze forces and conductor strains are examples of properties that should be known and designed well. These were calculated in this article for an example Extruded DC and MI cable. Such analysis as well as component test results show that both Extruded DC and MI cables are well suited for laying at large depths. BIBLIOGRAPHY [1] Reference SAPEI Cigré 2012 B1-101 [2] Zink or zinc alloy coated non-alloy steel wire for armouring either power cables or telecommunication cables, Part 2 submarine cables, European Standard, EN :2011, [3] Recommendations for mechanical testing of submarine cables, Cigré Technical Brochure 623, Working Group B1.43, June 2015 [4] Recommendations for Testing of Long AC Submarine Cables with Extruded Insulation for System Voltage above 30 (36) to 500 (550) kv, Cigré Technical Brochure 490, Working Group B1.27, February