Stell Catenary Riser Systems
- 1 General Sizing
- 2 Materials Selection
- 3 Deepwater Environmental Conditions
- 4 Metocean Data
- 5 Geotechnical Data
- 6 Vessel Motion Characteristics
- 7 Wave Theories
- 8 Steel Catenary Riser Design Analysis
- 9 Strength and Fatigue Analysis
- 10 Construction, Installation, and Hook-Up Considerations
- 11 Pipe-in-Pipe (PIP) System
- 12 Line-End Attachments
- 13 References
In the preliminary stage, the diameter and wall thickness of the riser and pipe must be determined to minimize the cost of the pipes. Factors that influence riser diameter and wall thickness sizing include:
- Operating philosophy: transportation strategy, pigging, corrosion, inspection;
- Well characteristics: pressure, temperature, flow rate, heat loss, slugging, well fluids and associated chemistry;
- Structural limitations: burst, collapse, buckling, postbuckling;
- Installation issues: tensioning capacity of available vessels;
- Construction issues: manufacturability, tolerances, weld procedures,
- Vessel offsets and motions;
- Metocean conditions;
- Deepwater environments.
Factors to be considered in material selection include strength requirements, adequate material toughness for fracture and fatigue performance, weld defect acceptance criteria, and sweet/sour service requirements.
Deepwater Environmental Conditions
For deepwater environmental conditions, four parameters must be analyzed for the design phase of SCRs :
- Hydrodynamic loads;
- Material properties;
- Soil interaction;
- Extreme storm situations.
Metocean DataThe location of a riser may dictate critical design conditions; for example, consider the loop currents in the GoM and the highly directional
- Water depth;
- Tide and surge variations;
- Marine growth.
ROV footage from deepwater operating installations indicates significant and complex riser interaction with the seabed in the SCR touchdown zone (TDZ). Such behavior is largely influenced by the geotechnical properties of the seabed the riser is interacting with. The nonlinear stress/strain behaviors of soil, consolidation and remolding of soil (and associated changes in shear strength), trenching and backfilling, hysteresis, strain rate, and suction effects affect the loads imparted on the riser. It is not possible or desirable to reproduce complex interactions perfectly, but it is important to model those characteristics that have the greatest effects on riser stresses and fatigue lives. Deepwater Soils SCRs are most commonly used as a part of deepwater floating systems. In many deepwater locations (GoM,West Africa, Brazil), soil types found at or close to the seabed are generally underconsolidated, normally consolidated, or lightly overconsolidated clays. Much of the research to date has focused on interaction with such seabeds. However, other soil conditions are also possible. For instance, soils are generally very variable in glacial settings such as in northwestern Europe and Canada, and gravelly and bouldery stiffer clays and sands are often encountered at the seabed.
Vessel Motion Characteristics
The host vessel’s motions are defined by the global performance analysis accounting for wave, wind, and current loads using either time-domain analysis or frequency-domain analysis. The motion data are expressed as time traces of vessel motions or RAOs defined at the center of gravity (CoG) for the floater for predefined loading conditions. The motions at the riser hangoff location will be transferred from the CoG via rigid body assumptions. The riser system is considered to be a cable under current loads with a boundary condition that is defined as the motions at the hang-off location. Vessel RAOs are used throughout the whole design process and it is important for them to be well defined. The following definitions should always be provided with RAOs:
- What the motion phase angle is relative to, whether it is a lag or a lead;
- Vessel coordinate system;
- Location of point for which RAOs are given;
- Direction of wave propagation relative to vessel.
As a general rule, Airy (linear) wave theory is suitable for most applications and is applicable in both regular and random wave analysis. Other heories such as Stokes V have advantages where only regular wave analysis is performed, especially with regard to fluid particle kinematics. This theory resembles those of “real” large-amplitude regular waves.
Steel Catenary Riser Design Analysis
In the pre-FEED phase, an initial design is carried out to define the following: • Riser host layout (for interface with other disciplines); • Riser hang-off system (flexjoint, stress joint, pull-tube, etc.); • Riser hang-off location, spacing, and azimuth angle (hull layout, subsea layout, total risers and interference consideration); • Riser hang-off angle for each riser; • Riser location elevation at hull (hull type, installation, and fatigue consideration); • Global static configuration. A static configuration may be determined based on catenary theory accounting for hang-off angle, water depth, and riser unit weight. The SCR design should meet basic functional requirements such as SCR internal (and/or external) diameters, submerged tension on host vessel, design ressure/temperature, and fluid contents. Due considerations should be given to future tie-back porches to accommodate the variations for a hang-off system, riser diameter, azimuth angles, and the required extreme response and fatigue characteristics.
Strength and Fatigue Analysis
For the preparation of FEED documentation, a preliminary analysis is carried out to confirm: • Extreme response that meets the stress criteria per API 2RD] and extreme rotation for flexjoints; • VIV fatigue life and the required length of strakes (or fairing); • Wave fatigue life; • Interference between risers and with floater hull. In the detailed design phase, installation analyses and special analyses, such as a VIM (Vortex Induced Motion)-induced fatigue analysis, emisubmersible heave VIV(Vortex Induced Vibration) fatigue analysis, and coupled system analysis, are conducted.
Construction, Installation, and Hook-Up Considerations
The effects of construction and installation operations may impose a permanent deformation and residual loads/torques on the riser system while consuming a proportion of the fatigue life. In-service requirements determineweld quality, acceptable levels of mismatch between pipe ends, and out-of-roundness, whereas nondestructive testing (NDT) requirements are determined from fatigue life and fracture analysis assessments.
Risers are to be constructed in accordance with related guides that are consistent with specifications such as ABS Guide for Building and Classing Subsea Pipeline Systems and Riser. The pipe-laying methods and other construction techniques are acceptable provided the riser meets all of the criteria defined in these guides. Guides and specifications are to be prepared to describe alignment of the riser, its design water depth and trenching depth, and other parameters, such as:
- Water depth during normal pipe-laying operations and contingency situations;
- Pipe tension;
- Pipe departure angle;
- Termination activities.
The actual SCR installation philosophy may not be decided at the start of a project. Therefore, the pipeline specifications may have to be drafted with contingencies that consider all installation methods (S-lay, J-lay, and reel lay). In the case of a J-lay, the boring internal diameter is likely to be changed. Therefore, 1 mm must be added to the pipe wall thickness to allow for the change. For smaller diameter SCRs, installation with the reel-lay procedure may be possible. Substantial engineering effort should qualify the SCRs for reeling:
- Full-scale bending trials and testing of centralizers to maintain a constant annulus for a pipe-in-pipe situation;
- Full-scale fatigue testing of the reeling riser welds. It is important to accurately calculate the bending strain in the welds during the reeling operation, to assess crack propagation;
- Careful monitoring of the allowable bending strain.
The term hook-up is another expression for the hang-off system of a riser. To consider a hook-up system, certain parameters have to be determined such as the hang-off location, hang-off spacing, hang-off angle, azimuth angle, and the location of elevation at the hull structure.
Pipe-in-Pipe (PIP) SystemThermal insulation is required for some production risers to avoid problems with hydrate, wax, or paraffin accumulation. The use of external insulation, in some cases, might impair the riser’s dynamic performance by increasing drag and reducing weight in water. However, PIP thermal insulation technology can often be used to satisfy the insulation requirements of lower
The inner and outer pipes of a PIP system, may be connected via bulkheads at regular intervals. Bulkheads limit relative expansion and can separate the annulus into individual compartments, if required. Use of bulkheads can be a good solution for pipelines, but for dynamic SCRs, one must consider the effects of high stress concentrations, local fatigue damage, and a local increase in heat loss. Alternatively, regular spacers (centralizers) can be used, which allow the inner and outer pipes to slide relative to each other while maintaining concentricity. The items listed below are common effects for single-PIP SCRs:
- Residual curvature, which may change along the SCR during installation;
- Residual stresses due to large curvature history;
- Residual axial forces between the two pipes;
- Connection between the inner and outer pipes, including length and play of centralizers;
- Boundary conditions and initial conditions at riser terminations;
- Fatigue life consumed during installation;
- Preloading of inner and outer pipes;
- Axial forces and relative motions during operation, due to thermal expansion and internal pressure;
- Poisson’s ratio effect on axial strains;
- Local stresses in inner and outer pipes due to centralizer contact, including chattering effects;
- Frictional effects between inner and outer pipes;
- Thermal stresses and thermal cycling effects;
- Buckling checks (including helical buckling) due to thermal and general dynamic loading;
- Soil forces on outer pipe;
- Internal and external pressures having different effects on stress in inner and outer pipes;
- Effect of packing material in reversal of lay direction on a reel should be assessed and cross-section distortion minimized; the pipe yields as it is
reeled and it is very soft at the reel contact point;
- Effects of PIP centralizers on pipe geometry during reeling;
- Wear of centralizers;
- Validity of VIV calculations (e.g., with regard to damping);
- Possible effect of any electrical heating on corrosion rates;
- Effect of damage (e.g., due to dropped objects striking outer pipe) on thermal and structural performance.
SCRs can be attached to the floating vessel using a flexjoint or a stress joint.
FlexjointsA flexjoint allows the riser system to rotate with minimum bending moment.
variations can also result in significant changes in flexjoint stiffness. In addition, it should be verified that the flexjoint can withstand any residual torque that may be in the riser following installation or released gradually from the seabed section of the line. Steps may be taken to relieve torque prior to attachment. The bellows protect the elastomeric flex element from the explosive decompression caused by large internal pressure fluctuations in a gas-saturated environment. The cavity between the body/flex element and the bellows is sealed and filled with a water/propylene glycol-based corrosion inhibiting fluid.
Stress joints may be used in place of flexjoints, but they usually impart larger bending loads to the vessel. They are simple, inspectable, solid metalstructures, and particularly able to cope with high pressures and temperatures. titanium. The latter material has the advantage of good resistance to attack from sour and acidic well flows and, of course, gas permeation. Titanium gives lower vessel loads than steel and typically has better fatigue performance than steel.
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