Development of improved solutions for steel catenary riser challenges

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2022
Thesis identifier
  • T16294
Person Identifier (Local)
  • 201751875
Qualification Level
Qualification Name
Department, School or Faculty
  • The current demand for oil and gas resources persists as no established alternative energy source completely meets the world’s high demand for energy. The depleting onshore and shallow-water oil and gas reservoirs are now driving the production of large hydrocarbon reservoirs in deep and ultra-deepwater. Therefore, it is necessary to innovate cost-effective and less risky riser technology for this purpose. The conventional steel catenary risers (SCR) are limited in their application for deep and harsh water environments due to the high stresses and fatigue response at their critical sections. In line with the aim and objective of this research, riser solutions for SCR are developed and investigated in this thesis. These solutions include the branched riser system (BRS), the floating catenary riser system (FCR), the vessel relocation strategy (VRS) and the simulation stage-based pre-trenching technique (SSBPT). Also, the index matching technique (IMT) is developed for the optimisation process of some of the riser solutions. The IMT is a descriptive rather than an inferential technique that applies statistical operations like indexing, intersection, and sorting to obtain optimal solutions to the optimisation problems. The IMT assigns unique index identities to design points in the optimisation design space and tracks the constraint (feasibility) and performance of the objective functions at those design points using those indices. The objective functions are evaluated at the feasible design points and sorted in ascending or descending order depending on whether the problem is minimised or maximised. The vector of the indices of the design points is rearranged to match the order of the sorted objective function vector. The index at the top of the index columns represents the optimum design point of the problem, from which the design variables and objective functions are mapped. The IMT is demonstrated with the SLWR optimisation problem and is further applied to optimise the BSCR, the FCR and the VRS. The BRS concept is developed to address the stress and fatigue challenge of the conventional SCR TDZ. Under limited environmental conditions and vessel motion, a small diameter pipe performs better in strength and fatigue response around the touchdown zone (TDZ) than a larger diameter riser pipe. However, the large bore pipe provides benefits of large fluid throughput from the seabed to the host platform, with lower top-side connections than smaller pipe diameter risers. The BRS concept combines these performances by branching the larger bore riser at an optimum water depth via a connecting structure into two small-bore riser pipes, which extend from the branching dept to the seabed. The BRS configuration variants include the Branched Steel Catenary Riser (BSCR), the Branched Steel Lazy Wave Riser (BSLWR) and the Branched Lazy Wave Hybrid Riser (BLWHR). However, only the BSCR is investigated in this thesis. The BSCR global configuration is developed and analysed to demonstrate its feasibilities. The investigation reveals the stress and fatigue response benefits of the BSCR compared with conventional small bore and large bore SCR. An optimisation methodology was developed to access the optimum BSCR configurations. The FCR concept is developed to address the tieback challenge of the conventional SCR across the congested or environmentally protected seabed. The FCR, with its double “wave bends”, is engineered to extend the riser touch down point (TDP) far beyond the nominal SCR TDP and away from the congested seabed footprint. The riser sections before the nominal SCR TDP are configured to float by installing buoyancy modules. The multiple wave buoyant sections also allow the FCR to decouple its TDZ from the floating platform motion. This can result in a significant reduction in the stress and fatigue damage around the riser TDP. The FCR global configuration is developed and tested for feasibility and was found to provide a better response than the SCR, in addition to its ability to provide a longer span across congested and protected seabed sections. An optimisation method for the FCR is also developed and applied in the selection of optimum FCR configurations. The VRS is the vessel’s planned repositioning within the acceptable limit of the riser design storm responses to help spread and reduce the fatigue damage over a more extended riser seabed section. There is a need to obtain an optimum vessel relocation program that best reduces the SCR TDZ fatigue damage and to know the optimum combination of the number of stations along the relocation axis, the vessel offset limits and the direction of relocation. The constraints on the problem are imposed by the stress utilisation, TDZ compression and top tension. The developed approach includes both symmetric and non-symmetric relocation patterns and can be applied to existing SCRs for life extension purposes. The VRS is developed and demonstrated to show the potential significant reduction in the SCR TDZ fatigue damage compared with SCR with no vessel relocation. The SSBPT is a numerical pre-trenching technique developed to qualify and quantify the trench impact on the SCR TDZ fatigue responses. The method is created using the capabilities of the hysteretic non-linear seabed interaction model, implemented in the OrcaFlex software. The method reveals that the presence of a pre-trench can increase the SCR TDZ fatigue damage. The TDZ damage response depends on several other factors such as the seabed properties, the SCR configurations, the pre-trench depth, the applied wave loads etc. Hence the pre-trench impact on the SCR TDZ fatigue response should be conducted on a case-by-case basis, based on the available design data for the SCR. The SSBPT provide the opportunities to investigate these complex load scenarios. As part of the SCR seabed interaction investigation, the influence of seabed slope on the SCR TDZ response is investigated in this thesis. SCR analyses are usually conducted considering flat seabed, neglecting sloped seabed around the SCR TDZ, which could potentially affect the response of the SCR. The impact of seabed slope on the SCR TDZ strength and fatigue response is investigated using a non-linear (NL) riser soil interaction model. The responses of SCRs on positively and negatively sloped seabed are compared with SCRs on the flat seabed. The investigation reveals that the SCR TDZ responses can be over or under predicted depending on the seabed slope deviation from the flat seabed.
Advisor / supervisor
  • Oterkus, Selda
Resource Type