DOT-2014 Flexible Spools Solution at Hybrid Risers Base A. Karnikian, Total and S. Tarbadar, M. Bonnissel, S. Legeay, Technip France Copyright 2014, Deep Offshore Technology International This paper was prepared for presentation at the Deep Offshore Technology International Conference held in Aberdeen, Scotland, 14-16 October 2014. This paper was selected for presentation by the DOT Advisory Board following a review of information contained in an abstract submitted by the author(s). Contents of the paper may not have been reviewed by the Deep Offshore Technology International Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Deep Offshore Technology International Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Deep Offshore Technology International Conference is prohibited. Abstract Rigid spools are commonly used at riser base of hybrid riser systems such as STTRs (Single Top Tensioned Risers) or BHORs (Bundle Hybrid Offset Risers) for the deep water oil fields. The rigid spools design is critical and governed by numerous operating constraints such as dynamic loadings (due to motion of the vertical riser, Vortex Induced Vibration, slugging ), soil properties and soil/spools/structures interaction. Recent surveys on existing brown fields have highlighted unexpected issues on existing rigid spools. The constraints of the brown fields are numerous when the replacement of a rigid spool is envisaged: existing structures developed initially for a rigid solution at riser base giving load limitations, congested field layout, installation constraints due to existing subsea structures and pipelines, and downgraded soil conditions (trenches due to actual rigid spools). To solve these several issues, an innovative flexible spool solution has been developed, combining unbounded flexible pipe structures in a "free hanging" configuration to accommodate the vertical riser motions with mattresses to release the soil downgraded conditions. Based on the experience acquired on these brown field projects and lessons learnt from hybrid risers technology, a specific solution has also been developed for new projects (green fields). A steep wave solution is introduced via a reverse wave configuration with distributed buoyancy over the flexible spools length. Such proposal allows a direct connection between the riser base assembly and the flowline termination, solving the embedment topic and easing accommodation of the hybrid riser motions. This paper deals with the recent issues with the rigid spools design and describes the solutions developed with flexible spool configurations to provide a more reliable riser base spools system for both brown fields and green fields. Introduction and context Since late 90 s the hybrid riser systems (BHORs, STTRs, etc) have been used for the development of some of the deep water oil fields especially in West Africa and Gulf of Mexico. These solutions involve various components forming the pipelines network from the seabed up to the floating unit: Rigid flowlines Flowline End Terminations (FLETs) Rigid spools in-between FLETs and riser base Tensioned rigid riser Top flexible jumpers DOT-2014 1
Top flexible jumpers Tensioned rigid riser Rigid spool FLET Figure 1 Typical arrangement of a Bundle Hybrid Offset Riser (BHOR) The arrangement of the system between the riser base and the FLETs is the key point to ensure the integrity of the rigid spools as well as the connector at each extremity. Figure 2 Typical arrangement of rigid spools linking riser base and FLETs DOT-2014 2
The design of a rigid spool is critical because its in-place behaviour is impacted by a lot of parameters: Rigid riser dynamic displacements due to the combined action of the current, the waves and the floating unit motions, as well as potential amplification due to the Vortex Induced Vibration (VIV) induced by subsea current Rigid riser expansion due to the temperature and pressure FLET displacements on seabed due to rigid flowline expansion and pipe walking VIV on the rigid spool itself due to bottom current Tolerances on elevation and azimuth of the connection at both FLET side and riser base side Interaction between soil and rigid spool Slugging phenomenon inside the rigid spool Stroking of the connectors to make the final connection on FLET and at the riser base Manufacturing tolerances on the 3D rigid spool and accuracy of the metrology Figure 3 Inputs for rigid spool design DOT-2014 3
Taken separately some have more impact on the maximum loads applied on the rigid spool whereas the others have more impact on its fatigue response. However the combination of all these parameters leads to a complex behaviour of the rigid spool which is difficult to accurately model with numerical tools. Figure 4 Finite element model of rigid spools Lessons learnt During the last years several issues have been discovered on rigid spools on different oil fields. The surveys performed by the operators have shown some abnormal configurations of the rigid spools as well as unexpected in-place behaviour. Some rigid spools were found within trenches and partially buried in some places. In addition some oscillations of the rigid spools were clearly observed enabling the formation of the trenches in the seabed. Figure 5-a Buried spool Figure 5-b Spool in trench Further investigations were carried out to explain these unexpected issues. Observation of ROV surveys combined with engineering studies allowed to identify some potential root causes. DOT-2014 4
Dynamic loading The rigid spools could be subject to more dynamic loads than predicted which could contribute to soil erosion, scouring and remoulding: Slugging is a complex phenomenon which is difficult to model More VIV than expected either on the rigid riser or on the rigid spool Quasi-dynamic effects from thermal cycling in the rigid flowline inducing FLET displacements Figure 6 Effect of thermal cycling on rigid spool Pipe-soil interaction For rigid spools lying on seabed, the current way of modelling pipe-soil interaction does not take into account the real soil behaviour: Berm resistance against rigid spool displacement Curve bearing capacity versus depth Soil remoulded condition Soil dynamic loading (including scouring/erosion) Structure-soil interaction Structure-soil interaction is also difficult to predict. Structure displacements cause berm formation which can change the system behaviour but the impact of berm is not currently accounted for because there is no numerical tool able to properly model it. Berm formation Figure 7-a FLET-Survey 2009 Figure 7-b FLET-Survey 2012 DOT-2014 5
Pipe walking This phenomenon shall be carefully assessed during detailed design as it can have a significant impact on the loads induced to the rigid spool at FLET side. However, the assumptions considered for the pipe walking analysis, in particular regarding the frequency of the operating cycles, can be slightly different to what is actually done on site. Figure 8-a Initial FLET position Figure 8-b FLET position after pipe walking VIV on rigid spool Assessment of VIV on 3D rigid spools is difficult because there is no existing theory and no software available today to specifically model this kind of arrangement. In addition it appears that the boundary condition brought by the rigid riser section at the bottom riser assembly could modify the natural frequencies of the rigid spool and thus the characteristics of the oscillations due to VIV. Metrology The metrology performed on site prior to the fabrication of the rigid spool includes some inherent uncertainties. On top of that, the manufacturing tolerances on the rigid spool dimensions, the installation configuration and the stroking of the connectors amplify the uncertainties regarding the as-installed initial shape of the rigid spool and thus the initial constraints applied on it. Figure 9 Lifting of a 3D rigid spool The significant amount of uncertainties linked to the input data and the sensitivity of the design of the rigid spools from the variation of these parameters on their global behaviour initiated the need to propose alternative solutions being less sensitive to these uncertainties. Flexible pipes were naturally some good candidates to accommodate all the constraints applied on the riser base spools for both brown fields and green fields. DOT-2014 6
Brown field solution The replacement of rigid spools by flexible spools has been investigated for an oil field in production. The new designed spools are 8 insulated flexible pipes to be connected between the riser base and either a Multi- Phase Pump or a FLET. Main constraints for the replacement of the spools were the followings: Connection on riser base side is ensured by an existing horizontal connector located between 13m and 17m above the seabed with given load limitations Riser base is subject to displacements in all directions due to rotation of the system around the riser base joint. Significant angular displacements were defined based on accidental conditions (buoyancy tank damaged), extreme vessel offsets (failure of one mooring line) and 100-year return period conditions for waves or current Riser base is subject to vertical displacements due to the thermal expansion of the rigid riser The soil encountered in the vicinity of the riser base is formed by very soft to soft organic clays and contains trenches created by the rigid spools The layout in the vicinity of the riser base is congested Several flexible spool configurations types were compared with different departure angles on riser base side and with or without distributed buoyancy along the flexible spool length to maintain acceptable bending radius and connector s loads. A free hanging configuration has been selected including ancillary equipments needed to accommodate the above constraints. This configuration minimizes the loads at connector interface, reduces the touch down point distance versus the congested field layout and is compatible with the minimum bending radius of the flexible spool due to lateral buckling when the spool is depressurized. In order to have acceptable loads at connector interface and at riser base bends, a single buoyancy module is installed on the end-fitting (flexible spool termination) rear crimping flange via the first collar of the bend restrictor so that it does not affect the configuration behaviour while reducing the loads due to flexible spool / ancillary equipment weights and rigid riser displacement. The buoyancy module has a net buoyancy of about 35kN. Figure 10 Free hanging configuration Brown field DOT-2014 7
The bend restrictor ensures that the minimum allowable bending radius is not violated during extreme conditions. Figure 11-a Near position Figure 11-b Far position To avoid any sinking of the flexible spool into the seabed as experienced by the rigid spools, mattresses are installed underneath the flexible spool on the seabed at touch down point location. They are designed based on soil properties and vertical loads induced by the flexible spool on the seabed. A specific protective outer sheath (extrusion of an additional plastic sheath above the external sheath), which is a simple and cost limited solution, is added to the flexible spool to limit risk of wear due to lateral excursions at touch down point location. Abrasion on mattresses is also limited thanks to their design which includes an external protection layer made of high density polyethylene. Figure 12-a Mattress general arrangement Figure 12-b Mattresses at TDP location The flexible spool free hanging configuration allows the accommodation of the vertical riser displacements. The inherent high structural damping of unbounded flexible pipe structures combined to the free hanging configuration avoid the risk of VIV and slug induced fatigue. Furthermore the fatigue of the flexible spool structure itself due to the rigid riser cyclic motions is very limited. DOT-2014 8
Finally the mattresses protect the soil from erosion, scouring and remoulding effects, avoiding the creation of berm and thus increasing significantly the reliability of the predicted flexible spool behaviour when moving on the seabed. Green field Solution The constraints related to green fields are different from those of a brown field as specific new solutions can be developed to improve the spool behaviour and design a specific riser base bottom assembly arrangement dedicated to a flexible spool solution. Four types of configurations were studied and compared in order to select the most reliable one. These configurations types are as follows: Free hanging: same configuration as on brown field Lazy wave: configuration with distributed buoyancy and contact with seabed Jumper: same configuration as free hanging but with higher connection point on vertical riser side and no direct contact with seabed Steep wave: reverse wave configuration with no direct contact with seabed For each configuration, several evaluation criteria were taken into account: Capability to accommodate vertical riser and subsea structures motions Mitigate field layout modifications Control of the flexible spool bending radius Amplitude of curvature variations Loads on connectors and stiffness of the configuration Need for ancillary equipment s (buoyancy modules, goosenecks, vertebrae, mattresses, etc ) Pipe/soil interaction Installation of the flexible spool Free hanging and jumper configurations require a connector s elevation greater than 15m typically and can accommodate relatively small displacements of vertical riser and subsea structures. However, ancillary equipment s are minimized compared with lazy wave and steep wave configurations onto which buoyancy modules are installed. Nevertheless, those two configurations can accommodate larger motions and smaller connector s height (below 10m). Finally, as opposed to free hanging and lazy wave configurations, steep wave and jumper configurations avoid any interaction with seabed. Figure 13-a Free hanging Figure 13-b Lazy wave DOT-2014 9
Figure 13-c Jumper Figure 13-d Steep wave The best solution consists in a steep wave configuration with bend restrictors and distributed buoyancy over the flexible spool length. This configuration allows being free from seabed interaction and minimizing loads on connectors as well as providing further advantages: The spool is directly connected from the riser base assembly to the subsea structures (FLETs, etc) so that there is no interaction with soil and therefore no risk of spool embedment leading to changes in the configuration characteristics, evolution of the pipe/soil interaction versus time and possible overloading of the subsea connectors. The total net buoyancy is adjusted for each flexible spool structure so that the configuration always keeps its steep shape irrespective of the internal fluid density variations Vertical or horizontal type connectors and goosenecks can be used on both riser base and subsea structure locations, however vertical one is preferred in order to ease installation Length of the flexible spool (typically 50-60m long) is tuned to accommodate riser base displacements, FLET displacements and installation tolerances of the subsea structures as well as to maintain an acceptable minimum bending radius during the entire flexible spool service life. It also minimizes the loads and load variations on both riser base and FLET side which improve their design and service life (size of riser base taper joint for example) Bottom currents are low enough to limit lateral excursions of the flexible spool The inherent high structural damping of unbounded flexible pipe structure combined to the selected shape avoids risk of VIV despite the fact that flexible spools are far less sensitive to this phenomenon than rigid spools Potential slugging induces slight curvature variations along the flexible spool which does not affect its service life contrary to the rigid spool for which it could be a critical issue Distributed buoyancy modules Bend restrictor Gooseneck Figure 14 Steep wave configuration on green field DOT-2014 10
Conclusion The use of flexible spool solution for both brown fields and green fields clearly brings some significant advantages when compared to the rigid spool solution. In particular the flexible spool allows mitigating the majority of the uncertainties impacting the design of the rigid spool, by either physically removing some complex phenomena such as pipe/soil interaction within trenches or taking advantage of the high damping and flexibility of its unbounded structure not to create detrimental fatigue issues or unacceptable extreme loadings. The inherent mechanical properties of the flexible pipe structure enables also to absorb or to better control the manufacturing tolerances (mainly length for flexible spools), the installation tolerances together with the installation constraints (stroking of the connectors), the uncertainties linked to the rigid flowlines and subsea structures response to operating conditions (expansion and pipe walking) as well as the operating conditions themselves (slugging phenomenon). The density variations of the transported fluid have very limited impact on the configuration of the flexible spool thanks to the distributed buoyancy all along its length so that any applications (production, water or gas) can be considered. Besides the accurate behaviour of the flexible spool can be easily predicted using conventional and recognized industrial software as this kind of application is well within the qualification domain of these software. It is deemed that the implementation of such flexible spool system at bottom of hybrid risers significantly increases the reliability of the riser base spools for the intended service life of the riser system. Acknowledgments The authors acknowledge Total and Technip France for allowing them to prepare this paper. DOT-2014 11
Flexible Spools Solution at Hybrid Risers Base Speaker: Alexandre Karnikian, Total Co-authors: Samy Tarbadar, Technip France Mélanie Bonnissel, Technip France Sébastien Legeay, Technip France
Contents Context Lessons learned Brown field solution Green field solution Conclusion
CONTEXT
Context Complex arrangement of 3D rigid spools at hybrid risers base
Context Design and in-place behaviour impacted by a lot of parameters
Context but also: FLET displacements Interaction between soil and rigid spool Tl Tolerances on elevation and azimuth at connections Manufacturing tolerances Metrology
Context Complex behavior Difficult to accurately model with numerical tools
LESSONS LEARNED
Lessons learned Abnormal configurations and unexpected behavior observed Significant oscillations Significant oscillations Rigid spools partially buried Rigid spools within trenches
Lessons learned Dynamic/Quasi-static loading uncertainties Slugging is a complex phenomenon difficult to model More VIV than expected either on the rigid riser or on the rigid spool
Lessons learned Soil interaction uncertainties Impact on friction coefficients Berm resistance against rigid spool displacement
Lessons learned Pipe walking uncertainties Frequency of operating cycles Significant impact on loads induced to the rigid spool and connector
Lessons learned Additional uncertainties Metrology performed on site Manufacturing tolerances Stroking of the connectors As-installed shape Initial constraints on the spool
BROWN FIELD SOLUTION
Brown field solution Main constraints Horizontal connectors Connectors elevation from seabed Soil status in the vicinity of the riser base Congested layout and distance between connections Various flexible spool configurations tested
Brown field solution Free hanging configuration selected
Brown field solution Soil stability at touch down point location
Brown field solution Validation of the flexible spools configuration
Brown field solution Suitable global behaviour of the flexible spool Acceptable loads at connector interfaces Controlled bending radius along the flexible spool No unexpected soil/spool interaction No impact of potential ti slugging Not VIV sensitive Not fatigue sensitive
GREEN FIELD SOLUTION
Green field solution Specific riser base dedicated to flexible spool solution Four types of configurations analyzed Free hanging
Lazy-wave Green field solution
Jumper Green field solution
Steep-wave Green field solution
Green field solution Evaluation criteria Capability to accommodate vertical riser and FLET motions Respect the allowable loads on the connectors Control of bending radius Amplitude of curvature variation Pipe/soil interaction Dynamic analysis
Green field solution
Green field solution Steep-wave configuration selected Minimize the loads on connectors No soil/spool interaction at all Good control of bending radius Limited amplitude of curvature variation Not sensitive to VIV Not sensitive to slugging
Green field solution Steep-wave configuration details
CONCLUSION
Conclusion Flexible solution is a good alternative to increase reliability of the riser base spools and connectors Mitigate the pipe/soil uncertainties More flexibility to absorb positioning tolerances of subsea equipments and motions Very limited it impact of manufacturing tolerances Far less sensitive to slugging and VIV Far less sensitive to fatigue Minimize loading on connectors
Contact information Total alexandre.karnikian@total.com Technip starbadar@technip.comcom mbonnissel@technip.com slegeay@technip.comcom