Dsigning risers for deepwater (>500m) and. An Innovative Flexible Riser Solution for Large Diameter, Ultra-deepwater Asian Fields.

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1 Technology An Innovative Flexible Riser Solution for Large Diameter, Ultra-deepwater Asian Fields Future developments of large gas reservoirs in the Asia Pacific region are ever increasing the need for deepwater, large diameter risers. These deepwater applications represent a challenge for larger production and export services utilising conventional rigid and flexible lines. This paper looks at the progress in flexible pipe technology and adaptations of riser configurations to address the challenges faced by large diameter deepwater risers. In particular, it focuses on innovative flexible riser configuration concepts drawing upon several years of research and development. These configurations aim at circumventing the challenges that are associated with free hanging catenary configuration such as high tension loads at the top. The configurations in question have been investigated within the context of a generic Asia Pacific development using computer simulations in combination with a review of progress in flexible pipe technology development to highlight the key technical challenges and solutions. The key findings of this paper will provide insight to the industry towards pushing the envelope of deepwater large diameter risers for future developments in the Asia Pacific region. Dsigning risers for deepwater (>500m) and ultra-deepwater (>1000m) brings additional challenges over conventional riser design due to the extreme loads which vary in nature as the riser descends through the water column. Add to this the larger diameter pipes often utilised in the Asia-Pacific region for gas development and the inherent reduction in strength and increase in mass tend to compound the already challenging conditions. A riser system can most of the time be designed for a given field conditions, however it is key for the operator that the interface load topsides are not too onerous for the type of platform selected. The riser system also needs to meet the fatigue requirements of the field service life up to 40 years. That is why innovative riser configurations aim at taking advantage of developments in flexible pipe technology to increase the range and performance of deepwater large diameter risers. This paper will first present the specific challenges related to a large diameter deepwater risers continued by a section detailing configurations that would allow the use of such risers without impacting too much on the platform design. The following section will present specific results extracted from the studies perform on this subject. Deepwater, Large Diameter, Flexible Pipe Technology Unbonded Flexible pipe Unbonded flexible pipes have been used in the offshore oil and gas industry for more than 30 years. An unbonded flexible pipe is basically a multilayered structure of helically wound metallic strips and tapes, and extruded thermoplastics. Refer to Figure 6 for a cutaway drawing of a flexible pipe structure. Steel layers provide the mechanical resistance performances of the pipe; that is, resistance to hoop stress, JAN-MAR

2 tension and hydrostatic pressure. The plastic layers act as pressure barriers against internal and external fluids. For high-pressure applications, an additional layer of helical reinforcement over the pressure armour, or a second set of tensile armour layers, may be applied. For deepwater designs, additional reinforcements are applied over the tensile armour layers to avoid buckling of the armour wires under the compression load caused by the external pressure. Since its introduction in the oil and gas market flexible pipe, innovation has allowed the product to be used in deeper waters and for more complex operations (larger diameter; increased design pressures). The following three areas have been taken as examples each being key for the subject of this paper i.e.: a large diameter deepwater development. In this study, the focus will be on rough bore pipe used for gas production risers. To improve the resistance to collapse of flexible pipes two parameters can be modified the improvement of the internal carcass and/or the pressure vault. Both these layers playing a key role in determining the collapse pressure of a rough bore flexible pipe. The design collapse mode for rough bore pipe is when the hydrostatic pressure is applied on the pressure sheath. In this case the pressure vault (Item 1) acts as a support for the internal carcass (Item 2) which restrains the carcass deformation. Therefore for a given carcass size and material, the higher the stiffness of the pressure vault, the higher the collapse resistance (refer to the Figure 3). Collapse Developments Collapse resistance is the capability of a flexible pipe to resist the hydrostatic pressure during its entire service life including the installation/precommissioning phase. As demonstrated experimentally, flexible pipes can collapse in two different modes, the first one is called ovalisation and is typical of smooth bore pipes. The second one is heart collapse mode and is typical of carcass collapse of a rough bore when the hydrostatic pressure applies on the pressure sheath. Figure 3 provides an indication of collapse pressures for a standard and an infinitely stiff pressure vault. It shows clearly that for a large diameter in deepwater, the careful selection of both carcass and pressure vault material and size is key as an increased quantity of steel will lead to an increase weight and top tension generating issues elsewhere in the system. 38 JAN-MAR 2012 Visit our websites at

3 Lateral Buckling Testing / Development If submerged and left unpressurized a pipeline will experience a compressive force known as Reverse End- Cap that could for a flexible pipe lead to a birdcaging of the tensile armour wires. Birdcaging is a well understood failure mechanism that can be designed out using high strength tapes. However once high strength tapes have been applied on the tensile armour wires, the main degree of freedom for the armour wire is to move sideways i.e. within the volume defined by the armour wire annulus as show in Figure 4). Under compression with cyclic bending the tensile armour wire kinematic is a combination of axial and transverse motions. If these wires are not resistant enough to take the transverse loadings they can buckle laterally, this is called lateral buckling of the armour wires. This phenomenon was first observed 10 years ago Ref [5] and following extensive testing the main mechanism and driving parameters of the armour wires lateral buckling phenomenon are understood. The performances of these tests whether inside hyperbaric chambers or offshore during DIP test campaigns have validate designs down to 3000m (see Table 1). With this paper in mind, the key design criteria to prevent lateral buckling are the material selection, use of large wires and the effect of armouring wire angle. Armour wire developments Drawing from the previous paragraph, one of the important layer of flexible pipe design in deepwater is the tensile armour layer. This layer supports the weight of the flexible pipe (which needs to resist the hydrostatic pressures using a heavy carcass and pressure vault), as well as resisting the lateral buckling phenomenon in the touch down region. In order to limit the top tension it has been demonstrated through several projects including Baobab (Ref [6]), Kikeh (Ref [3]) and recent Brazilian projects that flexible risers can be split in sections two or more to allow optimisation of each section. The top section will always be in tension and will resist the highest tensions and will be submitted to a severe fatigue regime. The middle section will see less severe loads but a higher hydrostatic pressure, and the third section will see the lowest tension but will be submitted to high hydrostatic pressure and a larger curvature variation at the TDP. The middle and in some instances the top section can benefit from the use of carbon fibres as described in Ref [9] to reduce the overall weight of the riser. These sections will also be designed to resist their lower hydrostatic pressures enabling optimization in the material quantities used and further reduction in the topside tension. The bottom section will be optimised for hydrostatic collapse and lateral buckling resistance drawing from work performed within the last 10 years Ref [5] and [8]. One way to address the failure modes associated with deepwater risers is to investigate the overall system and try and provide more favourable conditions for the flexible pipe structure. The following section investigates a riser system concept which intends to alleviate the onerous combination of forces acting upon the flexible pipe structure. The FSFR Concept The Free Standing Flexible Riser is a configuration that emerged from the ideas that with a large riser in deepwater, the top tensions will reach levels that will make the design of the host platform (weight) as well as JAN-MAR

4 has to withstand during the field service life. The FSFR flexible riser is in a configuration which has no cyclic curvature at depth, hence eliminating the tensile armour lateral buckling phenomenon. Riser Base - The riser base acts as the anchor point for the riser on the seabed. For deepwater fields the base would be a suction anchor design to hold the riser in place against the local geotechnical data. Ancillaries - Several bend stiffeners are required for the installation phase and in service conditions of this configuration. Bending stiffeners creating a smooth transition between a fixed connection and the pipe are key to the FSFR configuration and it is expected up to four would be required. Bottom Jumper This jumper connects the seabed infrastructure to the bottom of the riser. This jumper can be a rigid or flexible pipe. the riser (fatigue), more difficult and maybe un-economical (unfeasible) for some projects. The FSFR configuration can be broken down into the following components: Jumper - The jumper connects the host platform to the submerged buoy. The jumper structure is designed specifically to be resistant to fatigue loads as it will have to accommodate the platform motions during the field service life. Submerged Buoy - The submerged buoy supports the riser system during the service life of the field. The buoy due to its floating nature and flat top surface also acts as an installation aid when the riser and jumpers are connected to it. Riser - The riser itself connects the submerged buoy to the seabed and act as the main conduit for the transported fluids. The riser can be made of one or several sections connected to each other during the installation. The design of these sections takes into account the internal and external loads that the riser FSFR versus Traditional Deepwater Risers For the development of a deepwater field it is expected that the following configurations would be studied. 1. Flexible Free Hanging Catenary 2. Flexible Mini Lazy Wave 3. Steel Catenary Riser Configuration 1 and 3 are the simplest ones as no additional structures are required together with limited ancillary equipment. However both of them will generate large top tension that could have a negative impact on the fatigue performance of these configurations. Note that configuration 3 as well as configuration1 in some instance can also be affected by minimum bending radius and fatigue limitations at the Touch Down Point. Configuration 2 through the use of buoyancy modules reduces significantly the touch down point fatigue issues affecting either 1 or 3. The top tensions are also reduced however the top tension reduction is a direct function of the amount of buoyancy modules installed. This could become uneconomic for large risers. Benefits of FSFR over Other Hybrid Risers (FSHR) Another configuration available that uses the advantages of both rigid and flexible risers is the FSHR (Free Standing Hybrid Riser). It is similar to the FSFR in that it uses a flexible jumper to connect the plat- 40 JAN-MAR 2012 Visit our websites at

5 form to the supported rigid riser location where flexibility and fatigue resistances are key. The rigid riser located in milder environmental conditions can then connect the flexible jumper to the seabed and beyond to the wellhead. Hybrid riser also allows uncoupling of the riser installation schedule from FPSO schedule. Other hybrid riser configuration requires larger installation vessel able to handle both the rigid and flexible lines which may not be available or economic for every development. For example the flat buoy associated with the FSHR can be fabricated on land and towed to site therefore not requiring a heavy lift vessel. Additional benefits with regards to the installation of the flat buoy configurations are presented in Ref [4]. Deepwater Riser Configuration Dynamic Performance Comparison To quantitatively compare the dynamic and fatigue performance benefits of the FSFR riser over traditional FHC and MLW risers, a dynamic analysis case study has been performed. The case study was performed with a generic set of design inputs based on common and expected deepwater Asia- Pacific conditions. These were used to design flexible pipes and run a dynamic analysis for each of the configurations using the three-dimensional finite element analysis software Orcaflex. Case Study Design Input The design conditions utilised in the case study were formed by performing a review of field conditions in the Asia-Pacific region and selecting commonly proposed production platforms and their characteristics. The water depth was chosen to encompass the majority of foreseeable developments in the region and the metocean criteria selected to represent common currents and wave heights associated with typhoons / cyclones for 100 Year return period conditions. For the purposes of the study, the number of input criteria and consequently dynamic analysis load cases were limited to those which commonly exhibit the worst riser loads, curvatures, and hang-off angles. The design parameters used are summarised in Table 2. Flexible Pipe Structure Design The flexible pipe structures for the Asia-Pacific Case study consist of standard unbonded flexible pipe construction components designed to industry standards Ref [1] and Ref [2]. The FHC and MLW riser configurations are two part risers with structures optimised for the top and bottom sections taking into account the changes in tension and internal / external pressure as you descend through the water column Ref [3]. The two part construction also allows for packing onto standard reels which are better suited to the majority of ISV s in the region. The structure designed for the top section of the FHC and MLW is the same structure used for the jumper section of the FSFR, optimised for fatigue resistance. The vertical riser section of the FSFR is specifically designed for the conditions seen by the FSFR. The top structure for the FHC and MLW, and the riser structure for the FSFR consist of the following layers and properties: JAN-MAR

6 The intermediate sheath between the sets of armour wires creates a second annulus and an additional barrier to corrosive gases diffused from the production stream. This allows the use of higher strength sweet steels in the outer annulus leading to a stronger lighter flexible pipe. A benefit of the FSFR riser configuration is that the riser section can be kept in tension at all times and the nominal amount of tension can be adjusted by modifying the size and consequently the uplift of the buoyancy can. This can be used to the advantage of the flexible pipe structure design in that the tension near the base of the riser can be selected such that the reverse end cap effect and lateral buckling phenomena are offset. This means that the armour wire dimensions and angles can be optimised for the tensile loads without being driven by the deepwater buckling effects. The bottom structure of the FHC and MLW, optimised for the higher internal and external pressures consists of the following layers and properties: Dynamic Analysis Method The dynamic analysis was carried out using time domain simulations in the three-dimensional finite element analysis software Orcaflex. The risers are modelled as slender cylindrical beam elements with the mechanical and hydrodynamic properties with the configurations based on working knowledge of past projects and scaled / adjusted for the specific field conditions. The production platforms motions are modelled by means of response amplitude operators derived from diffraction analysis. During any standard dynamic analysis a large number of load cases are performed using various combinations of input parameters to cover the broad spectrum of conditions a riser configuration is subject to. For the purpose of this study the number of cases has been restricted and combined to give rise to the most onerous expected conditions. Table A in the appendices presents some of the most common parameters considered in a dynamic analyses and how they have been considered for this specific case study. A load case matrix detailing how the parameters from Table A in the appendices are combined is presented at the end of the paper This shows that this case study only considers 36 different load cases where as a full dynamic analyses can range from hundreds to tens of thousands of individual load cases 42 JAN-MAR 2012 Visit our websites at

7 to be assessed when considering the various combinations of all the parameters. Note however that the cases studied give a good general overview of the riser behaviour and hence provide a basis for comparing riser configurations. Configuration Summary The final dimensions of the risers are presented in the Table B in the appendices. Note that the dimensions are based on the risers attached to the semisubmersible, to account for the extra distance between the Semi and FPSO hang-offs, 45m is added to the FHC Riser top section, MLW riser top section and the FSFR Jumper. The distributed buoyancy modules attached to the MLW provides 24Te of uplift across the 60m of buoyancy section. The MLW configuration for the FPSO and the MLW and FHC configuration for the semi converged upon during the dynamic analysis displayed very similar scale dimensions to previously designed risers from projects and studies. Early simulations performed showed that the freehanging catenary configuration was not suitable for use with the FPSO. This was due to large vertical displacements and accelerations at the FPSO s riser hangoff from the vessels pitching motion, this carried directly to the FHC risers touch down point causing a buckling type phenomena. This lead to both unstable simulations and unsuitable bend radii s in the riser, this is similar to what is described in Ref [6]. Increasing the horizontal distance between the hang-off point and the TDP, tightening the riser, did not alleviate this issue while keeping the riser within suitable top and bottom end tensions. This highlights that for deepwater developments where the motions at the riser hang-off are particularly onerous, the simple free-hanging catenary riser may not be a feasible solution. Dynamic Analysis Results The critical results of the dynamic analysis are presented in Table C of the appendices. As mentioned one of the major benefits of the FSFR over the FHC and MLW is the reduction in topside loads on the host platform. This has been quantified by the dynamic analysis which shows a 465Te reduction over the MLW and a 557Te reduction over the FHC for the semisubmersible platform. The angle that the riser makes from the nominal hang-off angle is also an important factor affecting the loads seen by the host platform. This combined with the tension determines the shear force and bending moments seen by the hang-off equipment. Below is a plot showing the Tension vs. Angle of each riser connection with the semisubmersible. Each case for each riser has three points plotted, max tension and associated angle, max angle and associated tension, and the combined tension and angle which is associated with the pseudocurvature. The pseudo curvature is a calculated value which often forms the most onerous combination of tension and angle. With all three of these points plotted for each case, they give what can be considered as the design envelope for the bend stiffener, the bend stiffener geometry then determines the subsequent shear force and bending moment transferred to the topside hang-off equipment. The envelopes above were used to generate shear forces and bending moments with an identical bend stiffener for each configuration, this clearly shows that tension alone does not provide the full picture as to the loads the platform connection will see. In saying this however the bending moments and shear forces are taken locally by any hang-off structure whereas the tensions may have a much larger effect on the platform in general. JAN-MAR

8 In this particular study it is clear by comparison of the results that the FPSO is more onerous to the riser configurations than the semi-submersible. This was particularly so for the FHC which proved a challenging configuration for the FPSO under these conditions. It should be noted however that this is not always the case for FHC configurations connected to FPSO s as there are many examples of this worldwide in particular in Brazilian waters where this arrangement is very common. One example of a FHC connected to a FPSO locally is at the Kikeh field in Malaysia (Ref [3]). As is observed by the FSFR Riser connection at the buoy, the maximum tension and angle between the Semi platform and the FPSO are very similar. This occurs because of the decoupling of the riser from the platforms motions which leads to a far greater consistency of expected behaviour and potentially a wider range of feasibility for this configuration. Deepwater Riser Configuration Fatigue Performance Comparison As the loads on the risers increase due to deeper water, the fatigue performance becomes a more critical factor in determining the service life and acceptability of the flexible pipe design. The fatigue critical locations are generally those that exhibit high tensions and / or high curvatures. One of the more onerous areas in terms of riser fatigue is near the topside hangoff, particularly in the region of the bend stiffener. At the bend stiffener the mean and alternating stresses, and hence the fatigue performance, are governed by both the tensions and curvatures. To get a basic idea of the fatigue performance benefits of the FSFR riser over traditional FHC and MLW risers, the tensions and curvatures can be compared. The tensions and curvatures are extracted from simulations performed using the same model as used to assess the dynamic performance of the risers, to simplify the comparison we will only assess the risers connected to the semi-submersible. Selected fatigue waves will be used in the simulations. Fatigue Waves The simulations will be performed with wave types similar to those used to assess the fatigue performance of risers in previous deepwater projects. The waves selected are presented in the table below and are to be run using a regular dean stream wave type, the associated directions are the same as described in the dynamic analysis section above. The total number of fatigue cases considered takes into account each of the wave heights combined with each of the periods and directions, in total 36 simulations were performed. The full fatigue load case matrix can be seen in the appendices. Fatigue Analysis Methodology The fatigue performance comparison will be performed in the same manner as the dynamic analysis using time domain simulations in the three-dimensional finite element analysis software Orcaflex. The riser configurations will be those described above and have the same flexible pipe designs. Preliminary bend stiffeners will be added to the FHC, MLW and FSFR Jumper at the top hang-off and for the FSFR jumper and riser terminations at the buoy. The FPSO will not be included as the semisubmersible platform was found during the dynamic analysis to be suitable for the FSFR, MLW and the FHC hence will provide a good direct comparison between the three. Riser fatigue is generally a long term effect based on repetitive low return period environments so as well as the waves being reduced in magnitude, the current and vessel offsets are also reduced. To take this into account the current and semi-submersible offset specified for the dynamic analysis have been reduced by 50% in order to bring it closer to the one year return period environments seen in typical deepwater Asia Pacific fields. To give a more conservative result the risers will consider full marine growth for all simulations, the risers will be considered full of water. From the simulations performed the time history of the tension and angles at will be extracted at the 44 JAN-MAR 2012 Visit our websites at

9 semisubmersible hang-off location of the FHC and MLW risers and the FSFR jumper. As the riser section of the FSFRs connection to the buoy is also considered a critical hang-off location where a bend stiffener is present, the time history of the tension and angles will also be extracted here. Fatigue Analysis Results The results of the fatigue analysis gave a clear indication towards the improved fatigue behaviour of the FSFR over the MLW and FHC. As expected, they also highlighted the benefit with regards to fatigue of the MLW over the FHC for the studied conditions. Figure 9 displays the curvature vs. tension envelopes of the different risers at the semisubmersible hangoff. This was generated by plotting the min and max tensions from each case combined with the min and max curvatures. This method generates slightly more conservative envelopes than using the tensions and curvatures from associated time steps however it still provides a good indication of the comparative fatigue behaviour without reducing the loads down to the local stress levels. It can be seen from the size and location of the envelopes that the FSFR offers improved fatigue performance compared to both the MLW and FHC and that the MLW offers improved fatigue performance over the FHC. The limitation with the chart above is that the tension / curvature range for each individual case is not clearly represented. This becomes more apparent when plotting the curvature variation vs. the tension variation which has been done in Figure 10. When taking into consideration the envelopes presented in Figure 9 with the curvature and tension ranges presented in Figure 10 it becomes even clearer that the fatigue dynamic behaviour is improved for the FSFR configuration. An interesting phenomena present in the values of the FSFR riser section plotted is the two distinct trends, one being with nearly no tension variation and the other with linearly increasing tension and curvature variations. These two specific trends were found to be clearly related to the wave period used, the former linked to the 4 second period and the latter the 8 second period. The buoy was found have higher motion for the greater period due to the greater water particle velocity, this in turn leads to the higher tension ranges observed. Using the airy wave theory shows that for a 4 second period there is virtually no water particle motion at the level of the buoy in comparison to the 8 second period. This is significant in that in a standard riser fatigue analysis, the lower period waves usually occur for a greater number of cycles over the field life and can in turn lead to greater fatigue damage, ensuring the buoy is placed at the correct depth would serve to prolong a riser s service life. Conclusion The research and development efforts that have JAN-MAR

10 made by the industry will enable oil and gas companies in the Asia pacific region to monetize discoveries even when they are located far from land and in deep water. As the paper demonstrated there is a range of possible flexible riser configurations that can be used for such developments depending on the size / service life of the field and the type of host platform. This paper highlights the FSFR configuration is well suited for these developments as it combines a reliable design using qualified flexible pipes with a limited installation footprint. Acknowledgements The authors would also like to thank all the personnel at Technip s flexible pipe R&D centre in Le Trait, France working on flexible pipe research as well as all the Technip TCS Paris R&D team working on the FSFR and flat buoy systems especially Romain Vivet and Rene Maloberti. References [1] API Specification 17J: Specification for unbonded flexible pipe, and ISO [2006]: Unbonded flexible pipe systems for subsea and marine applications, second edition. [2] API RP 17B: Recommended practice for flexible pipe, fourth edition, 2008 and standard ISO : petroleum and natural gas industries design and operation of subsea production systems part 11: Flexible pipe systems for subsea and marine applications, second edition, 2007 [3] Berton, H., Hanonge, D., Anson, K. Challenges and Solutions for Deepwater Flexible Risers in the Asian Regions, Technip, Presented at the 5th PetroMin Deepwater and Subsea Technology Conference Kuala Lumpur [4] Vivet, R., Technip, Minguez, M., Seal Engineering (a Technip Company), Berhault, C., Principia Jacquin, E., Hydrocean Flat buoy concept for free standing riser application: An improvement of the in place hydrodynamic behaviour, OMAE presented at the 30th International Conference on Ocean, Offshore and Arctic Engineering. [5] Bectarte, F., Coutarel, A., Instability of tensile armour layers of flexible pipes under external pressure Technip, OMAE presented at the 23rd International Conference on Offshore Mechanics and Arctic Engineering. [6] Decoret, L., Mullot, D., Paterson, J., Taylor, T., Innovative optimisation of large-id Sour-Service Flexible Riser for the Baobab project in 1000m Water Depth, West of Africa OTC presented at the 2007 Offshore Technology Conference [7] Averbuch D., IFP, Paumier, L., Felix-Henry, A., Technip: Flexible pipe curved collapse resistance calculation, OMAE Presented at the 28th International Conference on Ocean, Offshore and Arctic Engineering, 2009 [8] Secher, P., Bectarte F., Felix-Henry, A., Lateral buckling of armor wires in flexible pipes: reaching 3000m water depth OMAE presented at the 30th International Conference on Ocean, Offshore and Arctic Engineering [9] Do, A.T., Beaudoin, O., Technip, Odru, P., Grosjean, F. IFP, New Developments in High Strength Composite Materials for Lightweight Offshore Flexible Risers, Third International Conference on Composite Materials For Offshore Operations. Houston, TX, October 31 - November 2, JAN-MAR 2012 Visit our websites at

11 Appendices Notes: a) This relates to the connection at the platform for the FSFR Jumper, MLW and FHC. This is the connection at the buoy for the FSFR Riser. b) Maximum angle during dynamics from the nominal hang-off (built in) angle. c) This relates to the connection at the buoy for the FSFR Jumper, MLW and FHC. This is the connection at the buoy for the FSFR Riser. d) Maximum angle during dynamics from the nominal hang-off (built in) angle, this is not applicable for the FHC or the MLW second ends as these are on the seabed. e) As the risers in the dynamic analysis were modelled with pinned connections at either end, this does not take into account bending in this region. PP JAN-MAR

12 This publication thanks Philip Ward, Henri Morand and Antoine Felix-Henry of Technip for providing this paper for publication. 48 JAN-MAR 2012 Visit our websites at