On Pipeline Lateral Buckling: Lessons Learned and Current Design Challenges

The subject of lateral buckling design in recent years has by necessity become increasingly more involved as pipeline projects have moved into more difficult environments where there is a need for optimized economic solutions with assured through-life reliability. The authors have had direct design responsibility and specialist involvement with a large number of projects covering a diverse range of environments, single or PIP systems, variable product characteristics and operating conditions, external applied loading type, and geographical installation limitations. These include shallow and deep water, large thin walled and small thick walled diameter pipes, flat to undulating hard to soft seabed, variable cohesive and non-cohesive surficial soil types and various other project considerations which have impacted on the chosen design solution. The purpose of this paper will be to highlight aspects of global buckling design associated with reliable in place systems and conversely those aspects associated with integrity risks to the as-laid operational pipelines. A review of past project challenges along with a commentary as to the state of the art at the time gives an opportunity to evaluate risks and challenges being faced on current projects. Particularly, as it seeks to develop ever more cost effective designs with proven robustness but optimized safety margins for the installation and operation of HT/HP pipelines in marginal fields.

Numerical Study of Hydrodynamic Forces on a Submarine Piggyback Pipeline Under Wave Action

In the paper, a 2D numerical model is established to simulate the hydrodynamic forces on a submarine piggyback pipeline under regular wave action. The two-dimensional Reynolds-averaged Navier-Stokes equations with a κ-ω turbulence model closure are solved by using a three-step Taylor-Galerkin finite element method (FEM). A Computational Lagrangian-Eulerian Advection Remap Volume of Fluid (CLEAR-VOF) method is employed to simulate free surface problems, which is inherently compatible with unstructured meshes and finite element method. The numerical results of in-line force and lift (transverse) force on the piggyback pipeline for e/D = G/D = 0.25 and KC = 25.1 are compared with physical model test results, which are conducted in a marine environmental flume in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, China. It is indicated that the numerical results coincide with the experimental results and that the numerical model can be used to predict the hydrodynamic forces on the piggyback pipeline under wave action. Based on the numerical model, the surface pressure distribution and the motion of vortices around the piggyback pipeline for e/D = G/D = 0.25, KC = 25.1 are investigated, and a characteristic vortex pattern around the piggyback pipeline denoted “anti-phase-synchronized” pattern is recognized.

Irregular Wave Simulation and Fatigue Damage Evaluation of a Flexible Riser Subjected to Bi-Modal Sea States

This paper presents the fatigue evaluation of a flexible riser subjected to bi-modal sea states, where the local wind and swell conditions act simultaneously, and is observed in many offshore regions including Brazil and West Africa. Due to the irregularity of the riser responses, the traditional, regular wave approach for assessing the fatigue damage of a flexible pipe cannot be applied without significant simplifications. A typical deviation would be to treat the combined swell and wind conditions at sea as two sets of separate cases. The regular wave approach can then be applied and the summation of the damage of both cases defined as the final damage of the pipe. As an alternative, this paper presents a more theoretically accurate irregular wave approach. The entire irregular wave simulation was first performed using the commercial software, OrcaFlex™, together with a tensile wire stress model developed in-house. The model implements the pipe bending hysteresis behavior during dynamic excitation, producing corresponding time history stress results, which are used to assess the fatigue damage using a rain-flow counting method. Two case studies are presented, the first being a dynamic simulation performed with two wave trains generated based respectively on the given swell and wind sea spectrums. In the second case study, a single wave train is generated based on the combined spectrum of the swell and wind sea states. Both results are compared with those obtained by the traditional regular wave approach and a preferred analysis method is recommended based on the conservatism and time efficiency.

On the Axial-Flexural-Torsional Coupling of Underwater Slender Cylinders

This paper presents a numerical model for the simulation of the axial-flexural-torsional coupling of undewater cylindrical structures. Cylindrical structures are largely utilized in the marine environment in a wide range of applications as in risers, marine cables, flexible pipes, mooring systems and so on. They may exhibit complex axial-flexural-torsional coupling, which makes the structural analysis highly nonlinear. In addition, the fluid-structure interaction may include flow induced vibrations, frequency lock-in and internal flow effects. The proposed three-dimensional model assumes that the structure aspect ratio is very high, its cross section is circular, the cable is elastic and may experience large displacements and large strains, as long as the elastic regime holds. The steady state load on the cylinder consists of the self-weight and buoyancy, drag and lift forces, in addition to a distributed residual twist along the cylinder. The drag and lift forces are evaluated by Morison type formulation. The governing differential equations are derived from first principles, assuming Newtonian mechanics. Then, they are solved numerically by a finite element formulation based on nonlinear space frame elements. The resulting set of algebraic equations is solved by a minimization technique that uses the Newton-Raphson algorithm. Results show the ability of the model to predict the static configuration of equilibrium of the cylinder and to capture the coupling between axial, flexural and torsional responses of the cylinder.

Aluminum Catenary Production Riser: Design, Testing Results, Ways to Improvement

One of the widely used systems for offshore oil production in water depths up to 500–2500 meters is a steel catenary riser (SCR). Requirements for long-term corrosion resistance of SCR are very stringent, that obliges to manufacture it from expensive steels. Still, the increased water depth leads to increased riser tension, grown pressure, aggravated buckling and oscillation problems. Among alternative materials to manufacture catenary risers, i.e., steel, titanium and aluminum alloys, the aluminum is the best from the “Strength/Weight/Cost” aspects with its high corrosion strength. Design of an aluminum catenary production riser (ACPR) was developed in Russia; and comprehensive tests were performed on mechanical characteristics and corrosion resistance properties of ACPR tubes and their connections. Two possible connections of riser sections were considered, i.e., welded and threaded. Strength analysis of threaded connection was performed by FEM. Mechanical testing included: testing of small samples of pipe material and welded connection cut out of riser section, testing of full-scale specimens of connection prototypes, and measurement of residual stresses. Structural and corrosion tests of samples consist of investigation of standard metallographic characteristics of pipe material and welded connection, and assessment of effects of different types of corrosion in seawater and oil fluid. The results of performed work have led to the conclusion that welded connection is most prospective for ACPR manufacturing. At the same time, the testing revealed certain improvements need to be done in the course of further work on this project.

Fatigue Analysis of Tether Chain in Hybrid Risers

Hybrid risers represent an excellent way to isolate the riser from most of the host vessel motions and thereby limit riser fatigue. A common arrangement features the riser supported by a buoyancy can via a tether chain. The tether chain is a cheap simple way to make the connection while providing flexibility for installation. However, in service the tether is under very high tension, and the chain is not really flexible in the face of small amplitude fatigue loads. The friction effectively “welds” the chain together. Moment and torque input to the system by first order vessel motions and vortex induced vibrations are carried through the chain and induce fatigue loading in the links. Analysis of the chain can be problematic because the determination of the detailed stress in the chain requires a refined FEA model with contact element between the links. From the global sense the analysis may require running hundreds of sea-state realizations in the time domain and the vortex induced vibration (VIV) assessment of thousands of current profiles. In this paper an efficient numerical method is described to rigorously determine fatigue damage at locations throughout the chain.

Hybrid Spool to Meet Deepwater Hybrid Riser Bundle Tower Critical Design Drivers

“Vertical type” spool configuration is commonly adopted as riser base spool for deepwater field development. With increasing water depths and risers subject to harsher environments, the excessive lower riser assembly motions and slugging induced fatigue damage have emerged as the governing design criteria for deepwater riser base spool design. Conventional vertical spool has inherent shortcomings to handle such design conditions. A new “hybrid” concept is therefore proposed, which extends beyond the “traditional” spool concept by integrating both vertical and horizontal spools into a 3D configuration. It inherits the vertical spool’s strength on handling large spatial expansion and utilise the pipe-soil interaction to dampen the potential resonance caused by slug flow. Several prototypes have been examined and the hybrid spool expansion-mechanism is discussed together with an optimisation procedure proposed. An advanced FEA technique using both Abaqus/Flexcom has been employed as part of the hybrid spool development, which consists of rigorous 3D dynamic analysis, bespoke non-linear soil interaction model and utilising the global riser dynamic behaviour. An in-house spool automation tool is developed to optimise the iterative analyses required to obtain a satisfactory hybrid spool configuration. This paper described a successful case study in recent deepwater hybrid riser bundle (multibore hybrid riser) tower development project, where the need to accommodate large lower riser assembly motion and slugging fatigue damage are the two main design drivers. This presentation provides a creative insight into this innovative technology.

Optimized Operations and Integrity Management Through Instrumentation

During drilling and well intervention (DWI) operations today operating limits are normally given as limiting wave height, and sometimes wave periods. The resulting diagrams are often not directly comparable with weather information received on the rig and the final decisions are often based on subjective assessment of wave height and period. The paper will present how BP, on the newly developed Skarv field in the Norwegian Sea, through thorough planning in the engineering phase has implemented a system where operating limits are specified based on directly measurable parameters such as rig heave and upper and lower flexjoint angles. How weather forecasting can be translated to give the rig crew direct forecasting of the limiting vessel or riser responses (e.g. flexjoint angles or heave), will also be presented. It will be shown how this allows for improved operational planning and support from onshore. Over the last years requirements for oil companies to be able to document the structural integrity of their subsea assets, including wells, has increased. On the Norwegian Continental Shelf (NCS) there has been a particular focus on fatigue loading in the wellhead structure, including the upper sections of casing and conductor, due to loads induced by the riser and BOP during DWI operations. There have been cases where the design fatigue life of a wellhead system limits the number of days one can perform operations with a rig on a given well. This in term affects future oil recovery rates as the well fatigue life may not be sufficient to allow for side step drilling or intervention work required to maintain an optimal production from the well. The paper continues to present how BP on the Skarv field, stores and utilizes the measured lower flexjoint response to track and document well integrity. It will be demonstrated how the return on investment of a drilled well can be improved by documenting actual fatigue loading from each operation on a well compared to conservative design calculations. BP has addressed the above issues in a way that is likely to set a new standard for drilling and intervention operations in the North Sea in the future. 4Subsea AS has provided the engineering and instrumentation services that formed the basis for this paper.

Risk Mitigation Through the Application of New Power Umbilical Technology

This paper describes the substantial service life improvements that can be achieved through a new, high technology solution developed for deep water electrical power umbilical and cable applications. The new design represents an enabling technology for power cable projects in the deepest and most dynamic waters, provides a lower risk solution for risers in highly stressed conditions and can give a technically improved solution for the range of electrical power umbilical application. The significant advantages of aluminum alloy cable bundles over traditional copper cable bundles under static and dynamic loading associated with a typical deep water floating installation are presented. A design case study is used to illustrate improvements in structural response and fatigue life associated with the aluminum alloy cable cores against conventional technologies. The paper concludes with an overview of the associated risk reduction through the implementation of the aluminum alloy cables in the form of a failure mode and effect analysis.

Assessment of Different Technologies for Vertical Hydraulic Transport in Deep Sea Mining Applications

Successful mining of deep sea deposits strongly depends on the proper choice of the right equipment. The most probable concept for a deep sea mining system would consist of the three major sub-components: Seafloor Mining Tool, Vertical Transport System and Mining Support Vessel. In this paper, emphasis is placed on the Vertical Transport System. We analyse the pros and cons of the different concepts such as hydraulic transport using centrifugal or positive-displacement slurry pumps, conventional and unconventional airlift systems, vertical offshore mining systems and vortex slurry transportation systems. All these systems are considered for their applicability at different water depths (from the relatively shallow to the relatively deep) for the different types of materials (from the relatively fine to the relatively coarse) and various production rates in terms of the efficiency, reliability and state of the art of technology.