Eigo Miyazaki

Real-Time Riser Fatigue Monitoring Routine: Architecture, Data and Results

Recently, the Modal Decomposition and Reconstruction (MDR) algorithm was developed to accurately estimate fatigue damage in marine risers based on measured acceleration and angular rates at several locations. The greatest benefit for drilling risers can be derived by incorporating the method in an online, fully automated system. In this way, fatigue damage estimates are available to the crew on the rig in real-time for risk quantification and mitigation.To this end, the MDR routine was implemented for online assessment of fatigue damage along the entire riser from acceleration and angular rate measurements at typically 5–10 elevations. This paper discusses the architecture, highlights some measured data and provides results for modes, stress and fatigue damage rate for the Chikyu drilling vessel during two scientific drilling campaigns. These campaigns occurred at the Shimokita site (1180-meter water depth) and the Nankai trough site (1939-meter water depth). To the authors’ knowledge, real-time fatigue monitoring of the entire riser has not been accomplished previously.Robust incorporation of the MDR algorithm into an online computational environment is detailed, including incorporation of top tension and mud weight data from the rig, detection and removal of data errors, and streamlined flow of the data through the computational modules. Subsequently, it is shown by example how the measured accelerations and angular rates are used to determine excited modes, participating modes, stress distribution and fatigue damage along the entire Chikyu drilling riser in an online setting.The technology highlighted advances riser integrity management two steps forward by first using measured data at 5–10 locations and the MDR algorithm to reconstruct stress and fatigue damage along the entire riser, and secondly integrating this approach into a fully automated, real-time computational environment. As a result, drilling engineers are empowered with a tool that provides real-time data on the integrity of the drilling riser, enabling informed decisions to be made in adverse current or wave conditions. Measured data also serves as a benchmark for analytical model calibration activities, reducing conservatism in stress and fatigue in future deployments. Furthermore, cumulative fatigue damage can be tracked in each riser joint, enabling more effective joint rotation and inspection programs.Copyright

Extreme Directional and Planar Profiles for Kuroshio Current Using Inverse FORM and Proper Orthogonal Decomposition

Kuroshio is a major global current that flows near the east coasts of Taiwan and Japan. Kuroshio is a relatively strong current with typical speeds of 3 to 5 knots at the water surface. It is important to properly understand extreme current profiles of these currents for any drilling activity since the response of deepwater risers is known to be sensitive to the shape of the current profile. This paper presents the derivation of extreme two-dimensional (i.e., directional) and planar profiles for Kuroshio currents at a site in Nankai Trough, Japan; water depth is almost 2000 m. About 6000 currents profiles measured over six months in 2010 by JAMSTEC are used. The inverse first-order reliability method (inverse FORM) and proper orthogonal decomposition (POD) technique are employed. While such methodology is well established, its use for this site posed several challenges. Firstly, the first two modes contribute only about 90% of the energy. Therefore, as many as seven modes were included for accuracy. Since an exact solution requiring joint probability distribution for seven variables becomes quite cumbersome, reasonable simplifications were made for efficient calculations. Second, to preserve the directionality in extreme currents, the inverse FORM problem for the two orthogonal components of the current velocity was simultaneously solved, so that extreme profiles for the two planar directions are obtained. Doing so implies solving a four-dimensional inverse FORM problem, even if the full joint distribution of first two modal weights for each direction is used. This four-dimensional problem was reduced to two related two-dimensional problems, wherein the modal vectors in the orthogonal directions are assumed independent; this assumption was found to be valid for this data set. A set containing a limited number of extreme N-year current profiles is derived using the above methodology. It is found that most of the shapes observed in the measured Kuroshio current data are represented in this set of extreme current profiles. The largest riser response obtained from all these current profiles would be the N-year response. A single extreme N-year profile is often sought in analysis, which is also derived from the set of N-year profiles by selecting the profile which maximizes an assumed response function. In summary, this paper presents extreme currents for a site on which little literature exists, and introduces a methodology to derive extreme directional current profiles from measured data.© 2013 ASME

A Method for Estimating Quasi-Static Riser Deformation and Applied Forces From Sparse Riser Inclination Measurements

A method was recently presented for determining quasi-static and dynamic riser angles using measured data typically found in a riser fatigue monitoring system, specifically acceleration and angular rate data. The riser angles were determined at sensor locations. In this paper quasi-static riser displacement, inclination angle, curvature, and stress are estimated along the entire length of the riser, using only the quasi-static inclinations angles at sparse sensor locations. In addition the distribution of applied forces along the entire riser length is also estimated. A rough representation of the current profile can be calculated using the drag coefficients of riser joints.The riser deformation (displacement, inclination, curvature) and applied forces are estimated by solving the matrix equation f = K*x, where f is the vector of forces and moments, K is the stiffness matrix and x is the vector of displacements and inclination angles. In the equation, force and displacement vectors are unknown and the stiffness matrix is determined using Finite Element (FE) modeling. Constraints are applied, setting the inclination angle at the sensor locations to the values derived from measured data. The remaining highly-underdetermined problem cannot be solved in a classical sense, as it admits infinite solutions. To get a solution that is consistent with the physics of riser deformation, smoothness of the solution is enforced as a constraint. The smoothest solution is solved using quadratic programming methods.Following implementation of the method in Matlab®, the procedure was validated using numerical simulations of a riser in applied current. Both connected (to the wellhead) and disconnected cases were simulated. Estimated riser displacements, slopes, curvatures and applied forces are found to match the simulation results closely.The algorithm was then run using measured data from an emergency disconnect event that occurred on the Chikyu drill ship in November, 2012. The riser displacement, inclination and curvature were determined and found to agree well with results determined using another method.The additional capabilities presented herein further expand the utility of a riser monitoring system. Quasi-static and dynamic riser angles are derived from acceleration and angular rate sensors using previously published methods. Using the method developed herein, the quasi-static inclination angles at the sensor locations can be used to determine the displacement, inclination, curvature (stress) and even applied force along the entire riser. These results are particularly useful in strength assessment, model verification, clashing and emergency event reconstruction analyses.Copyright

A Method for Determining Quasi-Static and Dynamic Riser Inclination Using Collocated Accelerometers and Angular Rate Sensors

A method is described for determining quasi-static and dynamic riser angles using measured data typically found in a riser fatigue monitoring system, specifically acceleration and angular rate data. Quasi-static riser inclination and orientation of the inclination plane are determined from the low frequency triaxial accelerations, containing measurement of the gravitational body force. Components of the gravitational body force along the accelerometer axes vary slowly with the riser quasi-static response. The slowly varying Euler angles are determined from the components of gravity along the three axes.Dynamic riser inclination along and transverse to the quasi-static inclination plane are determined by integration of the angular rates, followed by transformation into a coordinate system aligned with the quasi-static inclination plane. The quasi-static and dynamic inclination angles are combined to arrive at the time trace of riser inclination angles.Following implementation of the method in Matlab®, the procedure was validated and the program verified using laboratory test data. A double-gimbaled platform was constructed, on which were mounted a triaxial accelerometer, biaxial angular rate and biaxial inclinometer (reference sensor). A battery of static and dynamic tests was carried out on the platform. Machinists’ levels and angle gauges were used to set the inclination in the various tests. The angles derived from the acceleration and angular rate data were compared to those of the reference inclinometer. Angle estimates were shown to match the reference angles with negligible error.The method was then implemented into the real-time Riser Fatigue Monitoring System (RFMS) aboard the Chikyu drillship. The algorithm was run using data from an emergency disconnect event that occurred in November, 2012. Quasi-static riser inclination angles were quite large due to high currents near the sea surface. Dynamic riser inclination angles proved to be significant due to Vortex Induced Vibration of the lower portion of the riser that immediately followed the disconnect event.It is important to note that the method uses data that is typically already included in real-time riser monitoring systems. Therefore only a software update is required to provide real-time riser angle information. If the method is built into data processing routines for real-time riser monitoring systems, the need for additional instrumentation, such as inclinometers near flex joints, may be circumvented. On the other hand, if inclinometers already exist, the method serves as an independent source of riser angle information at several locations on the riser. The method can also be used to calculate riser and Blow out Preventer (BOP) stack angles from data recorded using stand-alone, battery-powered loggers.Copyright

Drilling Experiences of the deep sea drilling vessel CHIKYU in High Current

Deep sea drilling vessel Chikyu is the first riser equipped scientific drilling vessel in the world. Chikyu started the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) from September 2007. The speed of current encountered with Chikyu has been frequently measured more than 4.0 kt throughout NanTroSEIZE. So far, Chikyu carried out drilling operations. In this report, summaries of experiences and findings associated with high current in drilling operations of Chikyu are described.

Fundamental experiments on motion analysis of deep-water risers for OD21 drilling vessel

The OD21 scientific drilling vessel of JAMSTEC, which is scheduled to be internationally operated from 2006, is planned to equip the 2,500 meters long riser string in the initial stage, and the 4000 meters class in the future. The operations of such deep-water riser strings as more than several thousands meters are still far from enough experiences, and the dynamic motions of the riser suspended from the surface vessel are not well examined. The authors carried out the 1/100 scale model experiments with using the visualization method to analyze the dynamic response of risers, and they obtained basic, but such important characteristics against the actual designs of deep-water risers as the longitudinal dynamic elastic response, the lateral motion induced by the parametric oscillation, and so on.

Manipulator system for deep sea survey

Japan Marine Science and Technology Center (JAMSTEC) has developed several deep sea exploration systems and is operating them. These exploration systems are equipped with manipulators to get samples of rocks and marine life or deploy instruments at the deep sea floor. The manipulator system used underwater has different features from that used for industrial robots on land. In this paper, features and problems of the manipulator system for deep sea survey at JAMSTEC are described.