TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA riser fatigue monitoring strategy and implementation on a deepwater Gulf of Mexico Spar top tensioned riser is presented. The paper explains why a fatigue monitoring program is considered necessary to provide the operator with assurance of the riser system performance and integrity in service, the considerations that led to the selection of an appropriate monitoring system and describes in detail the standalone motion logger system adopted.The principles and methods of measurements that permit the monitoring of motions at discrete locations on the riser are presented along with the methods of processing this data such that fatigue damage along the riser can be interpreted.The paper describes how a standalone logger monitoring system has been successfully installed entirely using a ROV, eliminating the need to run the monitoring system during critical path riser installation activities.

Riser VIV response due to ocean current loading is a complex phenomenon governed by both the hydrodynamic and structural properties. In order to obtain better understanding of the global riser VIV response and assist in the improvement of riser VIV design, riser monitoring is being increasingly used. An optimization technique to identify the number of sensors required and the sensor locations for monitoring riser VIV fatigue is presented. The optimization technique has been developed using modal decomposition and linear regression. The paper explains why monitoring at selected locations with limited instrumentation is sufficient to capture global riser response. The principles and methods of using multiple measurement quantities in the optimization technique are also presented along with the adopted methodology, limitations and key conclusions.

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With drilling capability extends to water depths up to 3000m, significantly increased is the risk associated with a failed riser recoil control in the event of an emergency riser disconnect due to loss of vessel station keeping. In deeper waters the tensioner system undertakes higher top tension due to the accumulation of riser length and mud weight. During emergency disconnect, the riser is disconnected between the blow-out preventer (BOP) and the lower marine riser package (LMRP), releasing the base tension and mud pressure. Impact between the top riser system and the diverter housing system should be avoided, and the clearance between the LMRP and BOP should be secured. Efforts have been continuously made in the industry to achieve a more accurate predict of the riser recoil response. The relevance of mud discharge to recoil control has been widely discussed but little quantitative data has been revealed in the literature. In this paper, effect of mud shedding on riser recoil response is discussed. The Herschel-Bulkley rheology model is utilized for mud flow and is considered the latest advance in the drilling industry. Water hammer theories with column separation are modified to account for mud discharge in laminar, transitional and turbulent flow regimes. As a case study herein, recoil response of a drilling riser attached to a dynamically positioned semi-submersible drilling vessel is assessed to present the mud discharge effect on riser anti-recoil control. At emergency events, the riser is disconnected above the BOP, which is located at a water depth of about 2150m for this study. Mud is generally preferred to be freely discharged during an emergency disconnect for ease of anti-recoil control and riser integrity. The density difference between drilling mud in the riser annulus and sea water outside the riser outer casing before disconnect induces a high pressure difference, which drives mud shedding at riser disconnect. Mud flow rate plays an important role in the speed control of the riser uplift. 3D finite element analysis is performed in time domain to simulate riser response before and after disconnect. 2HRECOIL software is integrated into ANSYS user programmable features to better model the riser response and mud discharge.

Steel catenary risers (SCRs) in deepwater environment exhibit complex dynamic dynamic response governed by various factors such as environmental conditions, vessel motions, soil-structure interaction and material degradation. Uncertainties in the SCR design exist in the design data, analysis methodologies, fabrication, and hence a conservative approach has been adopted to overcome these shortfalls. Recent advances in monitoring systems installed on SCRs provide operator the assurance of the integrity of the SCRs in service, verify the SCR design, and enhance basic understanding of the SCR response. This paper outlines a strategy for monitoring the SCRs to characterize the response due to vessel motions, vortex induced vibrations (VIV), and soil-structure interaction. A detailed example of real-time SCR monitoring system with optimized array of motion and strain measurements is presented. The methodology for sensor selection and optimization is based on linear regression analysis. The measured data processing methods include shape matching the response amplitudes with correlating response frequencies. The principles and methods of measured data interpretation to capture global response shape due to wave and vessel motions induced and VIV are presented. The pipe-soil interaction such as soil stiffness, suction, softening and trenching effects characterized using the strain measurements in the touch down zone are presented. In addition, the methods to calibrate the individual vertical, lateral and axial models for pipe-soil interaction are presented.