While WSD has been used for marine drilling riser analysis and design for many decades, Load and Resistance Factor Design (LRFD), as described in Annex D, can offer the designer a more probabilistic design process over WSD. LRFD technology has been heavily used my multiple industries since the early 1940s and has been incorporated into the design and analysis of marine drilling risers.
(informative)
Riser Management System (QA/QC)
A.1 General
The owner of a riser system is responsible to ensure that the equipment is employed and maintained in conditions that are within the operational envelope of the design throughout the components’ life. This information is typically found in the riser manufacturer’s operation, installation, and maintenance manual.
Section 6 recommends the use of a riser operating manual. The minimal content of this manual is listed in 6.2.
Inspection and maintenance procedures, load ratings, and an accurate log of operating history are a few of the entries listed. A comprehensive RMS assists the owner/user of marine riser systems in properly executing and documenting the recommendations set forth in the OEM manual and the intent of this document. It is the owner’s/user’s responsibility to develop and implement a riser maintenance program.
An RMS creates a roadmap for a systematic execution of this maintenance program.
A.2 Potential Benefits
Some benefits that may be realized through the use of an effective RMS include the following.
— Reduced risk of erroneous operation.
— Reduced downtime.
— Reduce wear and tear by reducing loads on the riser.
— Planned and efficient maintenance cycles.
— Real-time riser stress analysis.
— Fatigue tracking.
— Riser stacking optimization.
— The riser stack-up in the holds or racks may be shuffled to remove joints scheduled for inspection or removal from service. This can save countless hours that can result from unstacking joints to gain access to the “buried” riser that requires removal from service.
— Dynamic monitoring for optimal riser performance.
— Tension, vessel position, and flex joint angles are critical parameters in the operation of riser systems and drilling operations. Dynamic feedback may be used to provide for optimal tension settings, reduced flex joint angles, and favorable vessel position to minimize the wear and tear on the riser system.
A.3 Purpose
The purpose of an RMS is to make and maintain a written and retrievable record of the lifecycle history for the marine riser system components. RMSs may be maintained in the form of computerized software, manual spreadsheets, ledgers, or individual joint logbooks. The advent of modern computers and software serve to automate the collection and storage of data gathered throughout the life of a riser component but is not
73
required to have well-designed and functional program. The complexity of the RMS is determined by needs and goals of the riser system owners.
RMSs can be used to provide documented proof to regulatory agencies for the history, traceability, and adherence to established/required maintenance.
A.4 System Parameters
Section 6 provides an overview of the operating procedures typical of a modern marine drilling riser system.
Guidance and cooperation from the riser OEM’s operating manual should be used to establish an owner/user maintenance program based parameters important to the owner/user. These parameters may be based on applied load, cycle count, or simply by time in service. These system parameters may be used to plan and schedule maintenance intervals and to establish trends that may be used in the determination of the remaining life or condition of the riser component.
Typical information gathered through an RMS includes the following.
— Unique joint identification. This is typically a serial number.
— Joint load rating or condition (serviceable, not serviceable, derated).
— Deployment dates.
— Retrieval dates.
— Number of cycles.
— Sensor data (pressures, temperatures, angles, tensions, stresses, etc.).
— Date of last inspection.
— Description of activity (deployed, tension).
— Date of next inspection.
Additional information may be collected from the drilling unit’s navigation/positioning system, riser tensioner system, subsea stack, or relevant information (directly or indirectly) from other systems. Indirect data may be in the form of calculations. Calculated data that could be useful may include fatigue and usage indicators such as TON-MILES, KIP-FT, WET DAYS, KIP-DAYS, KIP-CYCLES.
The information that is gathered and maintained is determined by the owner/user. It is recommended that the owner/user develop his/her maintenance systems (RMS) in concert with the equipment OEM.
A.5 Data Gathering Methods
Data may be collected through visual monitoring of tensioner settings, manually tallied deployment locations within the string (top, middle, bottom, 500 ft, 6250 ft, etc.), upper and lower flex joint angles, temperature, and pressure probes.
The system may include the use of radio frequency identification (RFID) tags, readers, and databases, pressure and temperature probes, strain gauges, and other automated feedback devices.
Strain gauges and instrumented riser components can be used to provide data that are fed into a riser a management system. Accelerometers may be used to provide cyclic data such as amplitude, direction, and duration (period). By coupling data streams from different components into the RMS, optimal operational parameters may be derived, extending the life of the riser components. Further, anomalous events may be
captured by the system, which may lead to unscheduled activities or inspections. Computer algorithms may be incorporated into the system to generate additional metrics used to determine the riser condition or remaining life. Instrumentation packages may be installed on any riser/vessel component of interest. These may include:
— the telescopic joint;
— the UFJ;
— the lower flex joint;
— the stack riser adapter;
— the stack (temperature and pressure);
— the riser spider;
— the riser gimbal;
— the riser running tool (equipment);
— instrumented riser pup joint;
— riser tensioner(s);
— rig position (DP, GPS, inertial navigation systems);
— driller’s Supervisory and Data Acquisition System (SCADA).
The same data may be collected and recorded manually although the risk of missing data, erroneous entry, etc. may be higher than a robust automated system.
(informative)
Typical Riser Analysis Datasheet
Location
Water Depth/Reference
VESSEL DATA Vessel Name
Vessel Type
Vessel Draft (ft)
Drill Floor to Water Line (ft) Moonpool Dimensions (ft)
DIVERTER
Drill Floor to Diverter Lower Flange (ft) Diverter Housing ID (in.)
Upper Flex Joint, Overall Length (ft)
Crossover, UFJ to Telescopic Joint, Overall Length (ft)
TELESCOPIC JOINT
Collapsed Length (ft) Load Rating (kip)
Outer Barrel Diameter (in.) Inner Barrel Head (in.)
Outer Barrel Wall Thickness (in.) Maximum Stroke (ft)
Outer Barrel Weight, Air (lb) Mud Return Below Drill Floor (ft)
Outer Barrel Weight, Wet (lb) CD1/CD2 (low/high Reynolds Number Dependent)
Outer Barrel Yield Strength (ksi) Mass Coefficient Cm
Drag Diameter (in.) Tensioner Ring Weight (kip)
Inertial Diameter (in.) Tensioner Ring Diameter to Lines (in.)
Length, Tension Ring to Outer Barrel Lower Flange (ft) Inner Barrel Dead Band Safety Margin (ft)
TENSIONER SYSTEM
Number of Tensioners Tension Line Fleet Angle (deg)
Paired or Single Wire Weight @ Tensioner (kip)
Tensioner Line Diameter (in.) Tension Ring Frame: Rot/Non Rot
Tensioner System Design Limits Termination Efficiency
Tension Line Breaking Strength (kip)
RISER JOINT TYPE 1 TYPE 2 TYPE 3
Number of Joints
Buoyant (Yes/No)
Effective Length of Joint (ft)
Coupling Type/Mfr
76
RISER JOINT, continued TYPE 1 TYPE 2 TYPE 3
Coupling Load Rating (kip)
Coupling Yield Strength (ksi)
Coupling SAF
Main Tube OD (in.)
Main Tube Wall Thickness (in.)
Main Tube Yield (ksi)
Main Tube SAF
C/K Line OD/ID (in.)
Booster Line OD/ID (in.)
Hydraulic Line OD/ID (in.)
Riser Joint Weight, Air, Slick (lb)
Riser Joint Weight, Wet, Slick (lb)
Steel Weight Tolerance (%)
Riser Joint Envelope OD (in.)
Buoyancy Type
Foam Density (lb/ft3)
Buoyancy Diameter (in.)
Buoyancy Length (ft/jt)
Buoyancy Weight, Air (lb/jt)
Net Positive Buoyancy (lb/jt)
Buoyancy Weight Tolerance (%)
Buoyancy Loss (Elastic Compression + Tolerances), (%)
Buoyancy Depth Rating (ft)
Drag Diameter (in.)
Inertial Diameter (in.)
CD1/CD2 (hi/lo)
Mass Coefficient, Cm
PUP JOINT/EFFECTIVE LENGTH 5 ft 10 ft 15 ft 25 ft
Main Tube OD (in.)
Main Tube Wall Thickness (in.)
Weight, Air (lb)
Weight, Wet (lb)
FLEX/BALL JOINT + ADAPTER UPPER LOWER INTERMEDIATE
Load Rating (kip)
Inside Diameter (in.)
Rotation Center Above Seafloor (ft)
Rotation Center Below Top Flange (in.)
Rotation Center Above Bottom Flange (in.)
Weight, Air (lb)
Weight, Wet (lb)
Axial Stiffness (kip/in)
Rotation Stiffness (ft-lb/deg)
Max Rotation (deg)
Drag Diameter (in.)
CD1/CD2 (hi/lo Re)
Mass Coefficient, Cm
STACK/WELLHEAD/SS TREE LMRP
LOWER
STACK WELLHEAD
SS TREE
Height (ft)
Inside/Outside Diameter (in.)
Weight, Air (lb)
Weight, Wet (lb)
Drag Diameter (in.)
Hydrodynamic Volume (ft3/ft)
Max Tension (kip)
Max Bending Moment (kip-ft)
Material Yield Strength (psi)
SAF
DRILLING PARAMETERS DRILLING NONDRILLING DISCONNECTED
Drilling Fluid Weights (ppg)
Vessel Offset (% WD)
ENVIRONMENTAL CONDITIONS DRILLING NONDRILLING DISCONNECTED
Design Wave Height (ft)
Wave Period (sec)
Significant Wave Height (ft)
Mean Period (sec)
Peak Period (sec)
Spectrum Type
Current Profile
Water Depth (ft)/knots
Water Depth (ft)/knots
Water Depth (ft)/knots
Water Depth (ft)/knots
Water Depth (ft)/knots
Water Depth (ft)/knots
Max Storm Surge + Tide
VESSEL MOTION RESPONSE (Amplitude/Amplitude)
Surge, Sway Heave (ft/ft) Roll, Pitch (deg/ft)
T (sec)
RAO (ft/ft)
Angle (deg)
T (sec)
RAO (ft/ft)
Angle (deg)
T (sec)
RAO (ft/ft)
Angle (deg)
(informative)
Fatigue
Fatigue damage in drilling risers arises from two primary sources of fatigue: wave-induced fatigue and VIV fatigue.
There are two fundamental approaches to a fatigue analysis. The first approach is based on fatigue tests and S-N (stress range versus number of cycles) curves and can take the form of either deterministic or stochastic (spectral method) calculations. The second approach is based on fracture mechanics principles. For a drilling riser, both approaches require knowledge of the magnitude and probability of occurrence of the expected sea states during either the riser's life or recommended inspection interval. These expected sea states form the
“fatigue weather spectrum” used in the fatigue analysis. The fatigue life of the riser is defined as the total life to riser failure, i.e. the life until the riser fails (“critical failure”).
In the S-N approach, “peak” stress ranges are calculated for each sea state in the fatigue weather spectrum.
These “peak” stress ranges are equal to the product of the dynamic “pipe wall” stresses obtained from the riser analysis and the SAFs calculated for the riser components. The dynamic “pipe wall” stresses are calculated from the dynamic bending moments and the dynamic tension variations. The SAFs are derived by local finite element analysis of a structural component. The SAFs represent the increased stress caused by geometry, three-dimensional effects, and load paths through the structural component.
Fatigue curves specifically for risers have not yet been adopted. However, fatigue curves published for offshore structures have been used to assess riser fatigue [see API 2A; UK DEN (1990); DNV (1984); NPD (1982)]. A difficulty in assessing fatigue for a drilling riser arises from the mobility of the floating drilling vessel.
Over the life of a drilling riser, it is employed at a variety of locations with differing environmental conditions, whereas an offshore production structure occupies a single location throughout its lifetime.
For deterministic and stochastic fatigue analysis methodologies, see the procedures in APl 2A.
It is important to understand that the concern in the fatigue analysis is cyclic stress or stress range, rather than the mean stress itself. Tension-tension, tension-compression, and compression-compression regimes receive equal consideration in the fatigue analysis. If the stress in a structural component remains constant, that component has a fatigue life of infinity and cannot fail because of fatigue.
It is necessary to take care in calculating the stresses and SAFs used in the fatigue analysis. Relatively small changes in the stresses and SAFs can result in large differences in the resulting fatigue life. Since fatigue life is proportional to the stress ranges and SAFs, each raised to the power of the inverse slope of the S-N curve (which ranges from 3 to 5), it can be demonstrated that, for an S-N slope of 5, doubling of either stress range, SAF, or any product of these decreases the fatigue life of a structural component by a factor of 32. For example, if the structural component had a fatigue life of 100 years, doubling of the product of stress range and
SAF would reduce it to 3 years.
In the fracture-mechanics approach, a structure is assumed to have small defects inherent in the parent material and/or weld material. These defects can propagate in the material once a cyclic loading is applied to the zone containing the defect, and the life of the structure is determined by the time these propagating defects take to cause the structure to fail. Once a defect has reached a critical size, brittle fracture can be the controlling failure mechanism. The fracture-mechanics method is based on six parameters:
— defect assessment, based on the size of the initial defect and location in the material;
— propagation parameters, based on material constants and stress ratios;
80
— stress-intensity factor, the influence of geometry on the crack tip as well as the long-term distribution of stress range; this term should not be confused with SAFs;
— fracture criteria, evaluates the mode of fatigue failure by incorporating brittle fracture;
— boundary conditions;
— residual stresses, stresses inherent in the material due to the method of fabrication or welding (see BS 7910).
The S-N approach is a good method to estimate the initial fatigue life of a riser for assumed environmental conditions. The fracture-mechanics method, when coupled with an inspection program, is appropriate for estimating the remaining fatigue life of a riser after use.
Measurement devices such as strain gauges or motion sensors can be used to calculate stress variation and thus help calculate fatigue accumulated during high current and high wave events.
Fatigue calculated through measurement devices can be used to refine an inspection program.
(informative)
Load and Resistance Factor Design
Load and resistance factor design (LRFD) format is based on reliability, in contrast to the standard API format that is based on working stress design (WSD). In the WSD approach, a large factor of safety is imposed on the allowable stress with no differentiation between loadings and how they contribute to the state of stress. In LRFD, load factors are assigned to loading mechanisms in accordance to the degree of uncertainty in the determination of each of the loads. Large factors of safety on the design stresses are eliminated. For example, the self-weight of the riser joints and the drilling fluid density are known to a relatively high degree of accuracy in comparison to the vertical tension and vessel offset, which are known to a slightly lesser degree, and the environmental loads are known to an even lesser degree. LRFD attempts to account for these differences in uncertainty, WSD does not.
Three major components are considered in the development of the LRFD method: uncertainties, risk, and economics. A probabilistic representation of each random variable describes the uncertainties, including unavoidable scatter as well as objective and subjective modeling uncertainties.
1) Uncertainties are measured by the statistical spread in the data.
2) Risk expresses the probability of an unfavorable consequence. The reliability design model invariably defines both loads and strengths as probabilistic random variables. Risk depends on the degree of overlap of the load and strength probability density curves. An important point is that there is no risk free environment.
3) Economics must enter the decision process since there is no zero risk operation. Higher safety margins will move apart and reduce, but not eliminate the load and strength overlap. As risk decreases and initial cost increases, a balance or optimum is reached at which an incremental initial cost is just balanced by an equal decrease in expected consequence cost. The balance point establishes the optimal total cost and the corresponding optimal risk, and hence in principle can be used to derive design criteria, safety margins, etc. A limitation to a direct application of this approach is the limited available data to model the distributions. While the load and resistance factors have been chosen based on reliability considerations, the designer is not faced with carrying out probabilistic calculations. This work will already have been completed in the development of the LRFD code and incorporated in the appropriate design factors. In the LRFD format an effort is made to maintain an engineering understanding of the load and resistance formulations. Formulas are used that were developed for traditional engineering practice rather than relying on a multitude of factors to be read from tables and graphs. All the checking equations are intended to reflect the mean estimate of the members’ ultimate capacity. For the most part, the LRFD formulas are similar to the WSD formulas. An LRFD type format has been adopted by several codes in the United States and other countries. Some of these codes are AISC (American Institute of Steel Construction), ACI (American Concrete Institute), AASHTO (American Association of State Highway and Transportation Officials), DNV GL, BSI (British Standards Institute), CSA (Canadian Standards Association), and API.
82
[1] API Recommended Practice 2A (all parts), Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms
[2] API Recommended Practice 2R, Recommended Practice for Marine Drilling Riser Couplings
[3] APl Recommended Practice 2A-WSD, Planning, Designing, and Constructing Fixed Offshore Platforms—Working Stress Design
[4] API Standard 2RD, Dynamic Risers for Floating Production Systems
[5] APl Recommended Practice 2SK, Design and Analysis of Stationkeeping Systems for Floating Structures
[6] API Bulletin 5C3, Bulletin on Formulas and Calculations for Casing, Tubing, Drill Pipe, and Line Pipe Properties
[7] API Specification 5L, Specification for Line Pipe
[8] API Recommended Practice 9B, Application, Care, and Use of Wire Rope for Oil Field Service
[9] API Recommended Practice 14E, Recommended Practice for Design and Installation of Offshore Products Platform Piping Systems
[10] APl Recommended Practice16J, Comparison of Marine Drilling Riser Analyses [11] API Recommended Practice 16ST, Coiled Tubing Well Control Equipment Systems
[12] API Recommended Practice 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations
[13] ALLEN, D.W., “Vortex-induced Vibration Analysis of the Auger TLP Production and Steel Catenary Export Risers,” OTC 7821, Houston, TX, pp. 169−176, 1995
[14] AMBROSE, B.D., GREALISH, F., and WHOOLEY, K., “Soft Hang-off Method for Drilling Risers in Ultra Deep-water,” OTC Conference Proceedings, 2001
[15] AMJIG, Deep Water Drilling Riser Integrity Management Guidelines, Revision 2, 2H Offshore Engineering Limited, 1999
[16] ASME Boiler and Pressure Vessel Code, Section VIII: Rules for Construction of Pressure Vessels;
Division 3: Alternative Rules for Construction of High Pressure Vessels [17] ASME PTC 25-2001, Pressure Relief Devices
[18] ASME UG 125-136, Section VIII, D1 A PT UG, 2006
[19] ATHERTON, D.P., Nonlinear Control Engineering, Van Nostrand Reinhold, London, England, 1982 [20] Atlantia Offshore Ltd, document No. 99038-01, “Deep Water Drilling Riser VIV Nomograph,” DeepStar
CTR 4502E, 1999
[21] BARSOUM, R.S., “Finite Element Method Applied to the Problem of Stability of a Non-conservative System,” International Journal for Numerical Methods, Vol. 3, 1971
[22] BARSOUM, R.S., and GALLAGHER, R.H., “Finite Element Analysis of Torsional and Torsional-flexural Stability Problems,” International Journal for Numerical Methods, Vol. 2, 1970
[23] BATHE, K.J., Finite Element Procedures, Prentice-Hall, Englewood Cliffs, NJ, 1995
83