MEMO CONCERNS. DISTRIBUTION For general information ELECTRONIC FILE CODE AUTHOR(S) DATE. 14F013 9

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1 MEMO CONCERNS PROJECT MEMO SINTEF Energy Research Address: NO-7465 Trondheim, NORWAY Reception: Sem Sælands vei 11 Telephone: Telefax: Direct Electrical Heating System for Preventing Wax and Hydrates in Pipelines DISTRIBUTION For general information Enterprise No.: NO MVA AN NO. CLASSIFICATION REVIEWED BY AN ELECTRONIC FILE CODE AUTHOR(S) DATE HK9572 Harald Kulbotten, Jens Kristian Lervik PROJECT NO. NO. OF PAGES 14F013 9 DIVISION LOCATION LOCAL FAX Electric Power Technology Sem Sælands vei SUMMARY SINTEF Energy Research has been for more than 15 years been working on systems for flow assurance in subsea pipelines. A method to apply direct electrical heating (DEH) is developed. In the heating system the pipe to be heated is an active conductor in a single-phase electric circuit. Feasibility of the system is proven both by scaled tests and on full-scale test installations. The test programme has included pipe dimensions from 8 to 18, and both carbon steel, duplex steel and 13%Cr steel pipes is studied. An important part of the studies has been to provide data for material characteristics of steel qualities, and especially the magnetic permeability. Measuring methods for this purpose have been developed and used on all pipe joints for the projects that are now been built with the DEH system. A computer tool for rating of the heating system is developed and verified through tests. By now the heating method is installed on 16 pipelines in the North Sea. The system is installed on pipeline lengths up to 43 km, and the latest installation was installed this summer on an 18 pipeline. A 30 pipeline at a water depth of approx. 800 m is also prepared using DEH, but then the heating method will be a plug-and-play system for ice melting. The development project for this system is completed this year, in cooperation with cable manufacturer, power equipment supplier, laying contractor and field operator. Further development work will be continued for application of this heating system on longer pipelines and greater water depths.

2 2 TABLE OF CONTENTS Page 1 HYDRATE AND WAX FORMATION PREVENTIVE MEASURES TO HANDLE WAX AND HYDRATE FORMATION WORKING PRINCIPLE OF THE DIRECT ELECTRICAL HEATING SYSTEM (DEHS) DESIGN CRITERIA AND RATING OF THE DEHS KEY VALUES FOR DEH QUALIFICATION WORK MAIN ACTIVITIES FOR A DEH PROJECT REFERENCES... 9

3 3 1 HYDRATE AND WAX FORMATION When transporting untreated well stream in ordinary pipelines, the temperature of oil, gas and produced water will drop rapidly due to cooling from the surrounding seawater. The low temperature results in undesired fluid properties. At high pressures hydrates start to precipitate already at temperatures in the range of o C. Large amounts of hydrate, which is similar to ice crystals, can precipitate on the pipe wall and cause blocking of well stream transport. For some fields wax formation in the flowing crude may also cause operational problems due to increased pressure loss in the pipeline. The viscosity of waxy oil can be of such magnitude, that full shut in wellhead pressure will not be sufficient for getting the cold fluid on stream again after long shut downs. 2 PREVENTIVE MEASURES TO HANDLE WAX AND HYDRATE FORMATION Roughly seen, there are four ways of removing hydrates: mechanical removing, chemical removing, melting with direct heat supply, and melting by reducing the pressure. The three last mentioned methods might also be used preventively, i.e. to avoid hydrates. Additionally the use of improved pipeline insulation may be used as a preventive measure. Application of hot water or heating medium is also possible for short distance pipelines. For removal of wax, the traditional methods include mechanical and chemical methods. Removal of wax by heating requires normally higher temperature than removal of hydrate and hence more power is required. Formation of hydrates is especially important when the oil and gas industry now is moving towards deeper water and longer transport distances. This means colder surroundings for the fluids, and the big depths will also make it difficult to utilize the most common way to remove hydrate plugs depressurization. This is often in practice difficult especially for long transport distances, where the temperature will fall to ambient temperature, and where there are limitations for how low the pressure can fall when having to maintain a flow. Another problem is that the hydrostatic pressure of the liquid in the system will at 2000 m depths of the ocean result in the fact that it is simply impossible to depressurize far enough. Mechanical removing of a hydrate plug will in practice mean the use of so-called coiled tubing, which might be lead to the plug and then simply drill the plug out. Coiled tubing may also be used to supply heat and/or chemicals. Mechanical removing of hydrate masses not plugged, for instance by use of pig has been tried, but experiences indicate that this method might cause packing of the hydrates and a complete plugging.

4 4 The use of chemicals to remove hydrates will in practice mean to use methanol or glycol. These chemicals move the hydrate equilibrium to higher pressure and lower temperature, and thus work in the same way as non-freeze solution in an automobile radiator. The effect of these chemicals is modelled into most available simulation programs for hydrate equilibrium. These chemicals may also be used preventively, as they reduces the hydrate stability area, and give a larger operation area for a real system. The disadvantage with these chemicals is that large amounts are often needed, and during the last ten years intense work has been done to obtain so-called low concentration inhibitors, which may be used to avoid the forming or growth of hydrates. These are found as emulsifying agents dispersing the water so that the particles do not merge to larger accumulations, and as surface-active substances sticking to the crystals surface and preventing their access to new construction materials. An obvious way to remove hydrates is to supply heat to the flow. On a platform or on shore this can be done by injecting hot water or steam into the pipe, but subsea this mostly has to be done by using electrical heating. The direct electrical heating (DEH) system has been developed and qualified for heating of pipelines and is the only system that has been installed on subsea pipelines in the North Sea. Electrical heating of pipelines implies reduced investments of depressurizing systems and recovery plants for chemical residual products. Especially for deep-water fields electrical heating of pipelines is attractive for achieving reliable operation of transport pipelines. 3 WORKING PRINCIPLE OF THE DIRECT ELECTRICAL HEATING SYSTEM (DEHS) The direct electrical heating system is based on the fact that an electric alternating current ( AC ) in a metallic conductor generates heat. The pipe to be heated is an active conductor in a singlephase electric circuit with a single core power cable as the forward conductor, see Figure 1. Topside power supply Electric dynamic riser cable Power cable Thermally insulated pipe Straps Cable splice Connection Static feeder cables Cable splice Power cable ( piggyback cable ) Connection Current transfer zone Anodes on pipe Thermally insulated well stream pipe/pipeline Intermediate anodes on pipe Current transfer zone Anodes on pipe Figure 1: Principle drawing of the direct electrical heating system. The efficiency of the system has a maximum value in a piggybacked installation.

5 5 The cable is located in parallel with and close ( piggyback ) to the heated pipe. The system is supplied via two riser cables from the platform power supply. One of the two single core riser cables is connected to the near end of the pipe, and the other to the forward piggyback cable, which is connected to the far end of the pipe. For safety and reliability reasons, the heating system is electrically connected (i.e. earthed) to surrounding seawater (i.e. an open system ) through several sacrificial anodes. The consequence of applying the open system is that the seawater acts as an electric conductor in parallel to the pipe by the direct electric contact between pipe and seawater at both ends of the heated pipe. The current is divided between pipe and seawater. At the cable connection points the total system current enters the steel pipe. Part of the system current leaves the pipe and is transferred to the water through the anodes in the transfer zone. The length of the transfer zone has been measured directly on full scale test pipelines, and is typically 50 m at 50 Hz. The current that leaves the individual anodes initially has a radial direction. Apart from the transfer zones, the current in the seawater is parallel to the pipe. 4 DESIGN CRITERIA AND RATING OF THE DEHS The heat development and the efficiency of the system depend on the current in the pipe. The basic design parameters of the direct heating concept are as follows: Required steady state pipe temperature and maximum acceptable time for heating of the pipe Thermal properties of the pipe insulation and surroundings, e.g. soil, seawater Geometry, including pipe and cable dimensions and location of cable Electrical and magnetic characteristics of the pipe Resistivity of seawater, seabed and water depth Power frequency Seawater temperature Temperature profile of the pipe at normal production rates for the rating of the supply cable Expected number of heating periods and operational time of the electrical heating system during the lifetime of the pipeline to determine the size of the sacrificial anodes. Complex electromagnetic calculations are necessary to determine the current distribution between pipe and seawater. These calculations are associated with physical laws such as neighbouring effect and skin effect. Thermal calculations are carried out to adapt the heating system to the thermal conditions for the relevant case. The thermal limit of the cables must not be exceeded, and the power supply shall be designed to cover all actual operational situations.

6 6 Calculations are made by applying the computer software Flux2D for solving both thermal and electromagnetic problems. The results are presented as colour shade plots, see Figure 2. Figure 2: Colour shade plot from thermal calculations on a DEH configuration with the piggyback cable located on top of the thermally insulated pipe. Corresponding temperatures and colours are notified in column at left. The generated heat in the pipe is affected by many factors of which material properties (electric resistance and magnetic permeability) and dimensions of the pipe are most important. For the efficiency of the heating system, the distance between pipe surface and parallel cable is of great importance. The reason is that the spacing significantly affects the magnitude of both the pipe current and the current distribution within the pipe. For a constant system current, efficiency has a maximum value in a piggyback installation (where the cable is strapped to the pipe). The verification tests (on both 8 and 18 carbon steel pipelines) have concluded that if the distance between pipe surface and cable is increased from 5 cm to 50 cm, the heat development in the pipe decreases by approx. 30%. That is from approx. 60% at 5 cm distance to approx. 45% at 50 cm, when referred to the total power consumption (included power losses in piggyback cable, seawater and steel pipe.

7 7 4.1 KEY VALUES FOR DEH The rating of the heating system based on the project related design basis data, should be handled as preliminary in the meaning of giving appropriate information about the need for electric power, cable dimensions etc. at an early stage of the project. The design basis data are expected to change throughout the project, and a more exact rating of the heating system is made as a detail engineering study. Experience from design studies performed on direct heating of pipelines indicates that the power need for maintaining a steady state pipe temperature above hydrate formation temperature is estimated to W/m on single 8-12 pipes. This estimate might be acceptable for the conceptual phase of field developments. For a 10 km single pipeline this means that the power requirement is approx. 1-2 MW. The corresponding terminal voltage for such a typical pipeline is estimated to 2-3 kv. However, the heating system will generate reactive power, which might be 3-5 times the generated heat of the pipeline, and must be taken into account when designing the topside power system. If it is required to rate the heating system for melting wax or hydrate plugs, the power requirement is significantly greater than maintaining a steady state temperature. The required time to obtain the acceptable temperature of the fluid will be a dominating parameter. If the heating time is 48 hours, the power requirement might be more than three times the value when rating the system for a steady state temperature (depending on the this temperature level). Reductions of the heating time from 48 to 16 hours will approximately double the power requirement. 5 QUALIFICATION WORK SINTEF Energy Research has made the qualification work on the direct electric heating system funded by Norwegian oil companies. Feasibility of the system is proven both by scaled tests and on full-scale test installations. The test programme has included pipe dimensions from 8 to 18, and both carbon steel, duplex steel and 13%Cr steel pipes is studied. An important part of the studies has been to provide data for material characteristics of steel qualities, and especially the magnetic permeability. A measuring method for this purpose has been developed and used on all pipe joints for the projects that are now been built with the DEH system. More than pipe joints are measured, and these data have been important for obtaining a reliable design of the electric power supply to the system. The development work for application of the heating system will continue for applications on long pipelines ( km) and at great water depths (>1500 m).

8 8 6 MAIN ACTIVITIES FOR A DEH PROJECT SINTEF Energy Research has made preliminary studies on the DEH projects in the North Sea. Simulations are carried out to provide electrical data for the heating system, and hereby also provide important information to enable feasibility analyses for the relevant projects. If decision is taken to apply the heating system, the further work shall be concentrated on following topics: - Relevant companies to be involved for: - handling pre-engineering work for the subsea part of the heating system (including risers), - detail parts lists for deliverables of equipment, costs and time for deliveries, - detail design of topside power supply system, interface topside installation items such as electrical connections at turret, costs and time for deliveries of equipment, - power system analyses for the power supply network onboard the FPU, such as load flow and stability analyses. - Obtain more accurate design data to provide input to sensitivity analyses for the parameters involved in the heating system. - If possible shall magnetic permeability measurements be carried out on relevant pipejoints (available from other projects or from steel work). Depending on margins for the rating of DEH and corrosion aspects it will be necessary to measure a selection of, or all, fabricated pipejoints (after coating) for sorting of pipejoints and location of anodes. SINTEF Energy Research have made similar measurements - Carry out sensitivity analyses to update the computer calculations on the heating system. - Provide information on availability and references from pipe laying companies for the DEH installation procedures. The installation of the DEH system supplied from an FPU has been carried out earlier, and relevant companies for the activities concerning engineering, system analyses, deliverables of components etc. are available in Norway.

9 9 7 REFERENCES The heating method is applied/designed for several pipelines in the North Sea. Project references are: Field/DEH info * Inner diameter (inches) Pipeline length (km) Water depth (m) U-value (W/m 2 K) Cable cross section (mm 2 ) DEH cable rating current/voltage (A/kV) Rated power (MW) / (6 pipelines) (6 pipelines) / / / / / / Year installed / * *-This DEH installation is a backup for hydrate/plug remediation, and the DEH (piggyback) will be installed retrofit. Tests are carried out for verification purposes.