Abstract Description of Åsgard Field

Transcription

1 OTC Smoothbore Flexible Riser for Gas Export Tim Crome, Technip Norge AS, Norway; Eric Binet, Flexi France, Technip Group and Stig Mjøen, Statoil, Stjørdal, Norway Copyright 2007, Offshore Technology Conference This paper was prepared for presentation at the 2007 Offshore Technology Conference held in Houston, Texas, U.S.A., 30 April 3 May This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box , Richardson, TX , U.S.A., fax Abstract This paper describes the development of a novel design of smoothbore riser for dry gas applications. It has been developed for use on the Statoil operated Åsgard field located about 200 kilometres off mid Norway. Shortly after start-up of gas export from, what was then the world s largest semi-sub, Statoil's Åsgard B platform, a serious noise and vibration phenomenon was identified in the gas export system. This system consists of a manifold on the platform topsides connected to parallel flexible risers linked to a subsea Export Riser Base which feeds the gas into the 42" export pipeline to Norway. The noise, essentially a high pitched whistling, and resulting vibration, caused two fatigue failures of small bore piping fittings on the topsides manifold, leading to gas leaks on the platform. There were also concerns related to the subsea piping. The source of the noise, in this dry gas system, was related to vortex shedding of the gas flow past the internal carcass in the flexible risers. Initially two roughbore risers were installed; this was subsequently increased to a total of four including one spare. The increase to three operating export risers had been planned from the start of the project to reflect increased export volumes from Åsgard. However, while it was now possible to manage the gas flow such that the noise and vibration was controlled, this could not be achieved without unacceptable pressure losses along the length of the flexible risers. This problem was related to longer term operation of the Åsgard Transport System as the export volume through it, and hence the required operating pressure, increased with the inclusion of export from new fields, such as Kristin. After comprehensive work with export system monitoring, dynamic simulations, and assessment of alternative export arrangements, Statoil invited Technip to validate the concept of a flexible smoothbore structure for the Åsgard B gas export riser system. This paper describes the background for, and special measures implemented in, the design of the world's first smoothbore flexible riser for gas export. This design is such that noise from the risers is eliminated while, at the same time, a smoother internal bore significantly reduces the pressure losses along the riser, allowing the full potential of the Åsgard Transport System to be achieved. The first smoothbore gas export riser was installed, in late Autumn 2006, as a replacement for one of the existing standard roughbore risers. Hence, the new application of smoothbore riser technology to the novel gas export riser design for Åsgard, gives several benefits; noise and vibration issues are eliminated, pressure losses are reduced and environmental benefits are achieved from the subsequent reduction in need for compression. Description of Åsgard Field The Åsgard field is operated by Statoil and lies on the Halten Bank in the Norwegian Sea, about 200 kilometres off mid-norway. Viewed overall, it is the largest development on the Norwegian continental shelf with the world s largest set of subsea production installations, comprising a total of 55 wells drilled through 17 seabed templates. The Åsgard A oil production ship arrived on the field on 8 February 1999 and became operational on 19 May. Gas production from the semi-submersible Åsgard B platform, see Figure 1, at the time the world s largest structure of this type, began on 1 October The overall Åsgard development comprises: The development of the field itself in the Norwegian Sea. The 42 Åsgard Transport gas pipeline from the field to the Kårstø processing plant north of Stavanger. The Kårstø expansion project. The Europipe II gas trunkline from Kårstø to Dornum on the German coast.

2 2 OTC Total investment in the field is greater than NOK 50 billion with ownership of the field split between the following partners; Statoil (operator) 24,96% Petoro 35,69% Norsk Hydro 9,61% Total 7,68% ExxonMobil 7,24% Eni 14,82% The Åsgard Gas Export System The Åsgard Transport System comprises a 707km long 42 outer diameter pipeline connecting the Åsgard Field to the treatment complex at Kårstø north of Stavanger on the Norwegian mainland. From here the gas is transported further to clients in Europe via Norway s extensive network of gas transport pipelines, totalling more than 7,800km, see Figure 2. To connect the 42 pipeline to the floating production facility in 310m of water, it was necessary to provide an export riser system comprising 4 gas export risers connected to a topsides export manifold and a subsea Export Riser Base ( ERB ). As the maximum size for a conventional roughbore gas export riser for the applicable design conditions was 14 internal diameter, it was necessary to design a gas export system with several risers. The risers are hung off in a Lazy-S configuration, suspended from guide tubes on the southern side of the platform, routed over the Gas Export mid water arch ( MWA ) and connected to the ERB, as shown in Figures 3 and 4. Initially two roughbore risers were installed; this was subsequently increased to a total of four including one spare. The increase to three operating export risers had been planned from the start of the project to reflect increased export volumes from Åsgard. The gas export riser system for Åsgard is unique, no other offshore developments have similar systems with four risers connected in parallel to manifolds both topsides and subsea. The gas export volumes from Åsgard are significant, up to 48 million standard cubic meters per day, equivalent to approximately 620*10 9 standard cubic feet per year. Hence any disruption to the gas export has economical consequences, not only for Statoil and license partners, but also for the Norwegian state. Noise and vibration Shortly after start-up of gas export from the Åsgard B platform, significant noise and vibration was identified in the gas export system. The noise, essentially a high pitched whistling, was audible throughout the process area on the platform. The structural characteristics of some parts of the topsides piping, especially small bore fittings terminated in blind flanges, i.e. those with low mass and a high natural frequency of vibration, resulted in resonance between the acoustic vibrations in the gas and these parts of the piping system. This resulted in vibrations with very high frequency and large stress ranges and the resulting fatigue lives were, in some cases, significantly reduced. Hence, before the magnitude of the problem was understood, the noise and vibration from the risers caused two fatigue failures of small bore fittings on the topsides manifold, leading to gas leaks on the platform. It was also possible, using underwater microphones, or hydrophones, to hear the same noise subsea at the Export riser Base. Extensive analysis of the response of this structure was also performed due to concerns related to the possible fatigue of the subsea piping. Probable cause The most probable source of the noise, in this dry gas system, was related to vortex shedding from the flow of the dry gas over the internal carcass in the flexible risers. Figure 5 illustrates the basic structure of a roughbore flexible riser. For the Åsgard risers there are more than 30,000 gaps between the folded carcass strip that may act as initiation points for vortex shedding. The pressure waves in the gas flow, initiated by the vortices shed by the flow of dry gas past the helical grooves in the riser internal carcass, are probably amplified significantly due to the presence of acoustic resonators in the topsides manifold piping and the subsea ERB. The importance of these two resonators is illustrated by the fact that a two tone whistling can be heard for some flow conditions, with a regular switching between the two tones that occurs at a frequency defined by the time taken for a pressure wave to travel from the topsides end of the riser to the subsea end and back again. Technip have delivered very many risers for gas service around the world and there are very few where noise and vibration have been shown to be a problem. There are no known instances where single simple riser systems, i.e. those where the riser is not connected to topsides and / or subsea piping manifolds, have experienced significant resonance. It should also be noted that the four essentially identical roughbore risers comprising the original Åsgard Gas Export system exhibited different acoustic behaviour. Hence, it has been concluded that it is necessary to consider the entire piping system, including the riser and the piping connected to both ends, when an evaluation of acoustic resonance is to be performed. The integrity of the roughbore risers has also been thoroughly checked and is not threatened by the noise and vibration. Following the discovery of the problems on Åsgard a few other dry gas riser systems were identified where noise could be heard. There were however no other systems where the problem had been serious enough to cause fatigue failures. These instances lead to the establishment of an extensive research program into the problem, including several JIPs. A number of theories have been proposed that link the flow in a

3 OTC roughbore riser to the generation of acoustic vibration, for instance Refs./1/, /2/ and /3/. There is however no simple method to account for the effect of acoustic resonators at the ends of the riser, which are probably essential in the amplification of the initial noise from the riser to a level where it may be heard, or indeed threaten the structural integrity of associated piping systems. No similar instances have been reported with other riser contents, i.e. wet gas or liquid hydrocarbons, probably because only small amounts of liquid are necessary to damp out the acoustic waves. When the problem with noise and vibration of the gas export system was identified on Åsgard B only two export risers had been installed. There were available slots topsides and subsea for two additional risers. To try and control the problem various measures were implemented, these included; rebuilding sections of the topsides piping to eliminate dead ends in the system that could enhance resonance, installation of acoustic blocks into the topsides connections of some of the risers, and, procurement and installation of two additional risers. With these measures implemented Statoil were able to manage the flow through the four risers, by adjusting the amount of gas flow in each riser, such that the required export volumes from the Åsgard field could be achieved without any significant noise and vibration occurring. However, the four identical risers still exhibited differing behaviour, both with respect to the critical level of flow required to generate noise, and the pressure loss through the risers for identical flow conditions. Pressure loss and significance While the pressure loss through the Åsgard export risers was acceptable for the gas produced from that field it was greater than originally planned and resulted in an overall pressure distribution in the Åsgard Transportation System that was unacceptable for the future expansion of the system with the tie-in of other fields. Hence, it was essential for Statoil to implement measures to reduce the pressure loss in the risers and allow the full export capacity of the 42 pipeline system to be exploited. Development of a smoothbore gas export riser When the noise and vibration issues on Åsgard B were identified Statoil and Technip started working on a number of solutions to try to eliminate the possibility of vortex induced excitation of dry gas flows through risers. Statoil were assessing alternative rigid riser concepts in parallel with the Technip work on alternative flexible designs. This included the use of a perforated carcass where consistent shedding is prevented by creating additional flow paths through the riser carcass, and the application of smoothbore riser technology to gas service. Smoothbore flexible pipes have been available for many years but have been restricted to liquid phases such as stabilised oil export or water injection applications. A typical smoothbore structure is shown in Figure 6. A comparison of the build-up of a typical roughbore and traditional smoothbore riser, including a description of the function of the different layers in the riser, is given in Table 1. The smoothbore riser was selected for further development and was proposed as a potential solution to the noise and vibration issues. This design would also result in a significant reduction in pressure loss through the riser, as the internal roughness of the riser is significantly less than the roughness of the original risers with their internal steel carcasses. There were however potential drawbacks with the use of a standard smoothbore riser for hydrocarbon service, the most significant of these was the potential collapse of the internal pressure tube due to an increase of the pressure in the annulus between the internal pressure tube and the anti-collapse sheath. Such an increase in pressure could result from a leak of seawater through the external sheath and the anti-collapse sheath, or from the build-up of liquid inside the inner annulus due to diffusion of water or hydrocarbons through the internal pressure tube. In order to develop a new design of a special smoothbore riser, suitable for the Åsgard field gas export application, a study was initiated in This had the objective of documenting the riser structure and identifying risks and risk reducing measures such that smoothbore technology could be applied to the Åsgard gas export system. This process, and a thorough independent risk review performed by Statoil, resulted in the decision to go ahead with the procurement of a single smoothbore gas export riser. This was delivered in late summer 2006 and installed on the field in November of that year. Capacity tests with the new riser was performed by Statoil in the period 9 th to 18 th December 2006 and demonstrated that the riser system was now capable of accommodating a flow of 48 MSm 3 /day, the maximum capacity required by the platform. Furthermore the pressure loss through the riser was essentially as predicted and significantly less than the pressure loss through the roughbore risers for similar flow rates, ref. Figure 7. Smoothbore flexible riser structure The flexible pipe is an unbonded structure which includes thermoplastic layers to ensure leak proofness of the pipe with respect to both external and internal fluids, and steel layers which are the load bearing elements. Each layer has a specific function and is considered separately. The main functions of each layer for the Åsgard B Gas Export smoothbore riser, illustrated in Figure 8, are summarised in Table 2. The riser is terminated at both ends by end-fittings designed to firmly anchor each element of the riser structure and also to allow access to the internal riser annuli for venting.

4 4 OTC As the new design represented a novel application of smoothbore technology a review of the design features of the smoothbore gas export riser was performed. This resulted in the identification of the following items that required special attention to ensure safe operation of the system: Collapse of the inner pressure tube. There is a risk that this may occur if the pressure in the inner annulus exceeds the pressure in the bore of the riser at any time. Pressure tube size the diameter / thickness combination proposed had not been manufactured before. Small bore tubing in first armour layer the small bore tubing is included to increase evacuation of permeated gas from the subsea end of the inner annulus of the riser. Design of an intermediate anti collapse sheath extruded above the first set of armours. This layer prevents the pipe from hydrostatic collapse in case of external sheath damage. Design of outer armour layers to avoid rupture in the event of failure of the inner pressure tube. Abrasion to external sheath in bellmouth of guide tube on riser. End-fittings as the riser structure has two annuli the end-fittings have two vaults, the lower terminated on top of the upper. In addition, 9 gas vent paths are terminated in the riser topside end-fitting. Overall stiffness of the smoothbore riser and the impact on installation operations. The risks identified for these items and the measures adopted to control the level of risk and maintain it within acceptable levels, are discussed below. Inner Tube Collapse The integrity of the inner tube is critical to the operation of a smoothbore gas export riser. As there is no steel carcass layer within the inner tube to prevent collapse, any differential pressure across the layer acting inwards can only be resisted by the capacity of the tube itself. A differential pressure across the inner tube, acting inwards can only occur if the bore pressure is lower than the pressure in the inner annulus. The design pressure for the riser is 240 bar. Under normal operation the bore pressure is close to this value such that, with the inner annulus free to vent to the atmosphere and therefore at 1 bar, there is no danger of collapse. If the inner annulus is prevented from normal venting, or if there is an obstruction of flow of annulus gasses along the riser, then there may be an increase in pressure in the inner annulus. If this occurs, and there is a depressurisation of the riser due to a planned or unplanned shut down, it is possible for the pressure in the annulus to exceed the pressure in the bore leading to a risk of collapse of the inner tube. Another potential cause for a build up of pressure is condensation of diffused water from the bore of the riser in the inner annulus. It has however been calculated that, under normal operating conditions, the partial pressure of water in the inner annulus is significantly lower than that required to cause condensation. Further, in the event of condensation, under normal conditions the water is expected to evaporate and flow out with the annulus gasses. To minimize the risk of a collapse of the inner tube, the following measures have been implemented: Small bore tubes have been inserted into the inner annulus tensile armour layer to provide redundant flow paths for the annulus gasses. The annulus pressure is continuously monitored via two independent manometers, one linked to the open annulus and one to the small bore tubes Vacuum tests will be performed on the inner annulus prior to planned shut down operations to verify there are no blockages in the inner annulus. A vacuum will be applied through the inner annulus gas vent ports from topside end-fitting during shut down operations to reduce the possibility of a pressure build up in the inner annulus. The suitability of the smoothbore design to the Åsgard Gas Export System is a consequence of the intended service of the riser operating with dried export quality gas. It is not expected that the types of service for the smoothbore riser can easily be extended to other hydrocarbon services, such as production risers. This is because liquids, either liquid hydrocarbons or water, can be expected to accumulate in the riser annulus as a result of diffusion through the pressure tube and condensation. In addition, the noise and vibration issues of gas export system are not expected for roughbore production riser systems. Pressure tube manufacturing The inner tube diameter / thickness combination proposed exceeded previous manufacturing limits in the Flexi France factory in Le Trait, Normandy, France. It was therefore necessary to design and manufacture a new extrusion head to achieve the thickness of 15mm and internal diameter of 348mm. To ensure that the new extrusion equipment was functioning as designed, a trial extrusion was performed on a length of tube sufficient to verify the manufacturing process. This also allowed the flattening of the tube during manufacturing to be tested to ensure that this operation could take place in a controlled manner and that there were no detrimental effects of the flattening, see Figure 9. It should be noted that the thickness of the inner tube had been increased beyond previous manufacturing limits in order to maximize the collapse resistant capacity of this layer. Small Bore Tubing As discussed above, one of the measures implemented to reduce the risk of collapse of the inner tube was the inclusion of small bore tubing within the inner tensile armour layer. Three small bore tubes were introduced to provide redundant flow paths from the subsea end of the riser, where the tubes were perforated to allow the gas to enter them, to gas vent

5 OTC ports in the topsides end fitting. They are wound onto the pipe structure by the armour winding machine together with the armour wires, see Figure 10. This operation required the use of newly developed industrial processes. Flexible riser section prototypes containing tubes had previously been manufactured by hand, tested and qualified as part of research and development projects. End-fitting design As the riser structure has two annuli, the end-fittings have two vaults. The lower vault is terminated on top of the upper vault, see Figure 11. In addition nine gas vent ports are included in the topside end-fitting, three each for venting of the upper end of the outer annulus, the upper end of the inner annulus and the small bore tubing connected to the subsea and of the inner annulus. The end fittings do not however present any new design issues and are in accordance with standard end fitting design rules. They are however significantly longer and heavier than the end-fittings for the original roughbore riser design and needed special care and attention during the riser installation operations. Wear of the external sheath Wear of the external sheath of a flexible riser may result in a hole in the sheath allowing seawater to enter the structure, which can result in corrosion fatigue of the armour layers. The most likely location where wear or abrasion could occur is within the bellmouth on the lower end of the Åsgard B platform guide tubes. To reduce the risk of damage to the external sheath, a protective sheath has been added to the flexible pipe profile for the length of the guide tube, bellmouth, and 5m beyond the bellmouth. This also has the beneficial effect of increasing locally the bending stiffness of the riser and thus reducing curvature in the region of the bellmouth, with a corresponding increase in the predicted service life of the riser at this location. Design of outer armours For a standard design of flexible riser the failure of the inner tube / pressure sheath, although unlikely, would lead to a rapid failure of the outer leak proof layers and hence a burst of the flexible pipe. However, the design of the Åsgard smoothbore riser structure includes specific 55 degree armour wires external to the anti collapse sheath. While the primary function of this armour layer is to protect the anti collapse sheath from damage, this angle of armouring also provides both hoop and axial strength in a flexible pipe structure. In the event of a failure of the inner tube, these outer armour wires have the capacity to support the anti collapse sheath, which will be exposed to the internal pressure in the pipe, and prevent an overall failure of the riser. Although the design life of the flexible pipe can not be guaranteed under these extraordinary design conditions, the use of this design does mean that a failure of the inner tube will not result in an instantaneous burst of the flexible pipe. Hence, an additional barrier towards leakage compared to a conventional roughbore design, is provided. In addition, the inner annulus monitoring, discussed above, allows an immediate detection of sudden pressure variation in the inner annulus. Riser stiffness and installation The bending stiffness of a flexible riser, when depressurised, is essentially dominated by the bending stiffness of the plastic layers in the riser. For a conventional roughbore design there are two plastic layers. The Åsgard smoothbore design incorporates three layers leading to a significant increase in the bending stiffness during installation. The stiffness is furthermore dependant on the ambient temperature during handling operations. The most critical operation is the transfer of the topsides end fitting from the basket on the installation vessel to the top of the Vertical Laying System ( VLS ), used to control the risers configuration during the installation operation. As this operation was performed offshore mid Norway in November, when the ambient temperature was close to freezing, resulting in a further increase in the stiffness of the plastic layers, it was necessary to go to port to perform the transfer of the first end of the riser to the VLS in sheltered waters, with the assistance of a shore based crane. During normal pipelay operations, and transfer of the second end, shown in Figure 12, the increase in bending stiffness is not critical. Conclusions: The first ever smoothbore gas export riser, utilizing the novel design described above, was installed on the Åsgard field in late autumn 2006 as a replacement for one of the existing standard roughbore risers. The riser that it replaced was the one that, due to the generation of noise and vibration at low gas flow velocities, had the lowest export capacity. This operation has proved to be successful, with total gas flows through the Export Riser System of up to 48 MSm3/day, without any significant noise or vibration in the topsides gas export manifold piping or subsea at the Export Riser Base. The new application of smoothbore riser technology to the Åsgard Gas Export System has demonstrated that the new riser design gives several benefits, as follows; Noise and vibration issues are eliminated in dry gas systems. Pressure losses are reduced, leading to reduction in need for compression, which gives an overall benefit to the environment. Furthermore, the overall capacity of the Åsgard Gas Export System has increased.

6 6 OTC Acknowledgments: Statoil Halten area management, especially Idar Grytdal, Subsea Manager. Technip project team and the Flexible Systems Design Supervisor, Johan Kristian Boe, in Oslo, and the Flexi France Product Engineering Division in LeTrait, France. Picture credits : Statoil Figures 2 & 4 otherwise Technip References: 1. Every, M.J et al.: Full-Scale Testing of Flexible Riser System Subjected to Internal Flow-Induced Vibration, OTC Paper 17787, presented at the Offshore Technology Conference, Houston, USA, May 1-4, Løtveit, S.A. et al.: Flow Induced Vibrations in Flexible Pipes, OMAE , presented at The 22 nd International Conference on Offshore Mechanics and Arctic Engineering, Cancun, Mexico, June 8-13, Nørstrup, H. et al.: On Subsonic Flow over Cavities with Aero Acoustic Applications presented at The 13th International Conference on Fluid Flow Technologies, Budapest, Hungary, September 6-9, Figures: 1. Åsgard B Platform 2. North Sea Gas Transport Systems 3. Åsgard Riser Lazy-S Configuration 4. Åsgard Export Riser Base ( ERB ) 5. Roughbore riser construction 6. Typical smoothbore structure 7. Comparison of pressure loss for roughbore and smoothbore risers 8. Åsgard Smoothbore Riser Structure 9. Inner tube on reel at Flexi France 10. Gas vent tubes within armour wire layer 11. Smoothbore riser end fitting on carousel at Flexi France 12. Offshore transfer of second end of riser Figure 1 - Åsgard B Platform Tables: 1. Comparison of Typical Roughbore and Smoothbore structures 2. Main functions of layers in Åsgard smoothbore riser structure

7 OTC Figure 3 - Åsgard Riser Lazy-S Configuration Figure 2 - North Sea Gas Transport Systems Figure 4 - Åsgard Export Riser Base ( ERB )

8 8 OTC Figure 7 - Comparison of pressure loss for roughbore and smoothbore risers Figure 5 - Roughbore riser construction Figure 8 - Åsgard Smoothbore Riser Structure Figure 6 - Typical smoothbore structure

9 OTC Figure 11 - Smoothbore riser end fitting on carousel at Flexi France showing double vault bolting Figure 9 - Inner tube on reel at Flexi France Figure 10 - Gas vent tubes within armour wire layer Figure 12 - Offshore transfer of second end of riser

10 10 OTC Table 1 Comparison of Typical Roughbore and Smoothbore structures Table 2 Main functions of layers in Åsgard smoothbore riser structure Roughbore Standard design for transport of fluid with gas (or pure gas). Has an internal carcass to take external pressure on pressure sheath Gas from diffusion has free access to tensile armour layer. Gas can flow through tensile armour layers to (topside) end-fitting Smoothbore No Gas allowed in Bore (Typically Water Injection). Has an anti-collapse sheath that transfers external pressure loads to the pressure armour. Any gas from diffusion is blocked from tensile armours by anti-collapse sheath. The anti-collapse sheath is protected from external damage by tensile armours. Layer Material Function Pressure tube Pressure vault (Zeta layer) Anti-wear tapes Tensile armours (Inner pair) Tape above armours Anticollapse sheath Tensile armours (Outer pair) External sheath Protective sheath Steel Wires Steel Wires Steel Wires Contains internal fluid Transmit inner pressure to the loading bearing elements Resist radial effect of the internal pressure Support radial loads from external pressure Support radial loads from: Armour wires when the pipe is under tension Caterpillars during installation Protect the steel wires from wear under dynamic loading and reduce the friction between layers Take longitudinal forces from the external tension and end-cap effect from internal pressure Support radial loads from internal pressure Support torsional loads Provide space for flow of diffused gas to topsides end fitting Prevent disorganisation of the armours: During reeling before the overlaying plastic sheath is extruded From torsional loading From the reverse end-cap effect due to external hydrostatic pressure Transmit pressure from the outer annulus to the pressure vault In the event of damage to the external sheath; prevent the pipe from collapsing, and protect the inside layers from the external environment by providing a barrier against seawater ingress Provide physical protection for the anti-collapse sheath Provide secondary protection against bursting Protect the inside layers from the external environment Barrier against seawater ingress Mechanical/abrasive protection of inside layers Mechanical / abrasive protection of inside layers (guide tube section only)