Condition-based maintenance approach allows inspection scope and frequencies to be adjusted, integrity management to be optimized
By Kenneth Bhalla, Stress Engineering Services
The drilling riser on a mobile offshore drilling unit (MODU) features a conduit between the drilling rig and well system that serves a critical role system. Typically comprised of a series of connected 75-ft to 90-ft joints, the riser is the main conduit to drill through. It connects to the top of the BOP, which is latched to the wellhead and casing system on the seafloor.
It is essential to develop a full understanding of riser system usage to ensure that all riser damage and associated risks are captured and quantified accurately based on past loading history and any future loading states. When drilling riser systems fail, uncontrolled release of muds and cuttings into the environment can be catastrophic.
Annual Inspection Rotation
Drilling riser joints are typically inspected at five-year intervals. This is usually performed by rotating 20% onshore every year to be disassembled and inspected. Multiple costly and time-consuming boat trips from the MODU to onshore facilities are required for this procedure, in addition to trucking the riser to the inspection facility once onshore.
Approximately 20 riser joints from each riser system are transported by boat and one riser per truck to the inspection facility each year, making the logistics of performing a drilling riser joint inspection complex and costly. However, since the riser system experiences significant wear, erosion, corrosion, fatigue damage and seal damage during the course of its operations, this process has long been considered an essential method for tracking deterioration over time.
Reliability, Consistency and Accuracy
A laser-based measurement for inspection, together with monitoring of riser systems, has been implemented with a new standard process for collecting critical riser data, which is ABS approved. The aim is to mitigate the costs and time associated with essential MODU drilling riser inspections by empowering drilling contractors to reliably determine the condition of drilling riser joints, consistently predict when vital components will require service and accurately assess remaining component life.
The approach utilizes a lifecycle condition-based monitoring (CBM), maintenance and inspection system that can be deployed on a MODU, enabling resources to be deployed only when necessary. Because drilling riser joints may not be in continual use, it makes sense that maintenance periods are usage- or condition-based instead of calendar-based.
The solution consists of performing a baseline inspection on the riser joints to assess their present state; collecting the environmental and operating data when the rig is on site drilling; and feeding the environmental and operating data into a digital twin. The technology can be used to predict when riser joints may be susceptible to damage.
There is currently no alternative technology that brings together a digital twin of the riser system, onboard riser inspection and load monitoring of the riser to calibrate the digital twin, followed by a determination of actual loading, stresses and fatigue damage over each riser joint, thus accurately assessing remaining life and allowing targeted inspections.
The drilling information, metocean conditions and riser data are collected to determine riser joint fatigue damage. The proprietary system determines stress and fatigue at any location in a riser system/wellhead/conductor casing via measurements from a number of accelerometers and angular rate sensors placed at strategic locations along the riser, together with analytical riser mode-shape information.
Because the only required online inputs are the dynamic riser response, top tension and mud weight, fatigue can be estimated without knowledge of the impinging currents or other forcing events.
Vibration sensors and data-acquisition electronics are housed in the subsea vibration logger (SVDL), which is also ABS approved. The data are then processed using patented technology that integrates a computer algorithm to synthesize stress estimates along the entire riser length using a database of riser dynamic modes.
The estimates are then processed chronologically via rainflow counting to determine fatigue damage accumulated during a drilling riser campaign, thereby providing actionable information to the drilling crew. The data can also be imported into a 3D viewing system for finite element analysis.
The Data Capture Process
Detailed measurements are collected, including vessel, metocean, drilling conditions and either real-time or stored load measurements during the drilling operation on the riser system using SVDL technology to assess fatigue damage. A laser profilometry system is used to collect measurements on the inner diameter of the main bore and auxiliary lines between wells to characterize the state of drilling riser joints.
In addition, other nondestructive examination (NDE) techniques, such as surface NDE, volumetric NDE and wall thickness measurements, are used to assess the health of the drilling riser joint. The SVDL can be installed individually by a ROV on riser joints, wellheads and BOPs in an offline mode. In this mode, the recorded vibration data are retrieved after the measurement campaign and processed offline to estimate stress and fatigue.
A semi-analytical method is used to estimate wellhead fatigue damage directly using the measured BOP stack motion data. Analytical transfer functions are used to directly compute stress time histories and fatigue damage at any location of interest in the drilling riser system. The offline fatigue monitoring system provides the operator with a tool to assess system integrity at any time during a drilling campaign.
The semi-analytical method to compute fatigue from measured vibration provides rapid turnaround of raw data to fatigue consumption, enabling informed decisions to be made in adverse conditions.
Digital Twin Technology
Asset integrity management programs are dependent on a combination of labor-intensive inspection, analysis and measured data. A digital twin is a digital replica of the physical asset – in this case, the drilling riser system and MODU. This digital representation of the system provides the basis of how the asset responds under various operating conditions. The system digital twin integrates a computer model of the physical system with the response data of the system, as well as data analytics, to create a living digital simulation of the system as the asset undergoes various operations.
A digital twin model of the global drilling riser and MODU has been developed and can be used to view the fatigue loading of the actual drilling riser joints. As SVDLs collect data, the data is used to update the digital twin.
The digital twin can also be used for monitoring and diagnostics to optimize asset performance and utilization. The sensor data (SVDL, rig response, etc) can be combined with historical data, human expertise and fleet and simulation learning to optimize performance and improve productivity.
The fatigue damage along the drilling riser is tracked in a digital twin from rig data, such as vessel heading, current profile, sea states, top tension, mud weights, etc. The stresses and, hence, fatigue loading experienced throughout the string are simulated using the data collected from the vessel.
Using SN and fracture mechanics together with rainflow cycle counting, the cumulative fatigue damage on each riser joint is estimated. The specific joints that accrue high levels of fatigue damage are then scheduled for further inspection, based on usage metrics rather than a calendar basis.
Reducing Cost of Logistics
A drilling contractor deploying this program can expect to experience lower inspection costs, as major inspections would be performed according to operating conditions rather than at set time intervals. Because fewer spares would be required due to fewer instances of risers being out of service, the drilling contractor’s riser asset management is expected to improve.
Lower logistics costs would be experienced, with fewer calls for risers being transported by boat. The reduction in logistics can be even more critical in remote operations. Reduced inventory swapping may be able to reduce between-well time and rig nonproductive time.
Classification societies ABS and DNV accommodate this alternative approach by calling for the owner of the drilling riser to maintain a preventive maintenance program to manage riser integrity. Such CBM programs that involve an approved condition-monitoring plan and onboard/onsite inspection can provide tangible benefits. Programs such as CBM and risk-based inspection help adjust inspection scope and frequencies to optimize integrity management based on the actual condition of the riser and its components.
By transferring to a lifecycle CBM, monitoring and inspection system that can be deployed on the MODU, users remove uncertainties surrounding damage of riser joints. The owner is then able to determine whether a riser should be redeployed or replaced. This is the only process that is ABS approved for CBM of drilling riser systems. DC
This article is based on a presentation at the 2019 IADC Advanced Rig Technology Conference & Exhibition, 22-23 October, Amsterdam.