Mooring line monitoring reduces risk of line failure

The number of floating production systems (FPSs) in operation globally has continued to rise over recent years, reaching around 400 installed facilities in 2014. With E&P activities continuing to move into more isolated locations, this number is expected to grow by a further 50% over the next five years.

Floating installations are subject to a variety of extreme weather conditions, which can have a severe impact on the integrity of mooring systems. Since 2001 there have been almost 30 reported mooring incidents on FPSs, with eight classed as system failures.

Mooring systems on FPSO units are Category 1 safety-critical systems, meaning there are a number of potentially severe human, environmental and economic consequences of a failure. The FPSO vessel Gryphon Alpha in the North Sea provides a stark example of the potential financial impact, where reinstatement costs are expected to reach an estimated $1.8 billion for replacement and upgrades to the mooring system and subsea infrastructure.

FIGURE 1. The INTEGRIpod acoustic data logger is installed on a mooring line. (Source: Pulse Structural Monitoring)

For these reasons, greater attention is being placed on mooring line integrity management (IM) practices as a means to maintain system condition and operational integrity, particularly since FPSO units are increasingly expected to remain on location for longer periods. Historically, these practices have focused mainly on inspection and maintenance, with an emphasis on limiting interruption to production. However, as the demand has risen for more regular and complete information on mooring integrity, monitoring systems have become an increasingly standard feature of IM strategies on FPSs.

Mooring line monitoring

Monitoring systems provide an effective and reliable method to accurately assess mooring line fatigue. Most monitoring systems either use load cells to directly monitor tension or use inclinometers to measure line angle and infer tension using lookup tables that have been established using the catenary equations of the mooring line. These systems provide two main benefits:

• A record of tension history-monitoring can help derive the range of loads imposed on the mooring line together with their frequencies. Long-term averaged tensions can be compared to initial mooring line pre-tensions to indicate any system deterioration; and
• Early warning of line failure-monitoring allows any failure of mooring line components to be identified immediately without awaiting the results of planned inspection activities. This early warning reduces the risk of component breakage turning into a system failure.

One option to monitor mooring line tension is to install a system that uses inclinometers to measure mooring line angle that can be converted into tension using lookup tables. The main advantages of this method are that it requires fewer or possibly no design requirements and constraints for the FPSO mooring system and can be retrofitted to existing FPSO units on station. The monitoring system is designed so that the devices are not in the load path, allowing ease of maintenance. The system components can be replaced without requiring anchor leg tensions to be relieved, and—if required—each mooring line can be retensioned without having to relocate the sensors.

Case study: FPSO unit in South China Sea

The South China Sea is a region where typhoons are frequent, leading to a number of mooring failures in recent years. The mooring system comprises nine mooring lines bundled in three sets of three, 120 degrees from each other around a single-point mooring buoy, making the FPSO vessel naturally weathervaning. Each mooring line is composed of chain and wire segments with clump weights on the upper chain section to reduce offsets and extreme line tension. The mooring system is designed for a 25-year service life and to withstand a 100-year typhoon condition. The FPSO unit is moored in more than 90-m (295-ft) water depth.

The monitoring system provided is a fully diver-installable mooring monitoring system using inclination measurement. The vessel had been on station for five years and required equipment retrofit. Acoustic communication is used to relay data to the control room to avoid the use of cables, which can become trapped and severed during subsea intervention.

FIGURE 2. The acoustic receiver is installed to the underside of the FPSO vessel using a magnetic holder. (Source: Pulse Structural Monitoring)

Equipment

The main components of the system used on the FPSO unit include the INTEGRIpod motion data loggers, acoustic data acquisition units (DAU) and software. Nine acoustic INTEGRIpod data loggers were deployed—one on each mooring line—that communicated to two DAUs installed underneath the FPSO vessel hull (Figure 1). Each data logger communicated with either of the acoustic receivers. The INTEGRIpods are attached to the mooring lines using diver-installable holders. These holders have two design priorities: special coating to resist marine growth and allow simple diver-deployment and removal of logger throughout operational lifetime.

The DAUs are mounted on the underside of the FPSO vessel hull using specially designed magnetic holders (Figure 2). This ensures that communication can be achieved with the INTEGRIpods regardless of FPSO unit orientation and position. The design of the DAU magnetic holder takes into consideration both the ease of diver installation and retrieval and also system durability throughout operational lifetime. Acoustic communication means cables can be avoided along the mooring line, which minimizes risk of system failure. However, cables are used to transfer data from the acoustic receiver to the control room. Magnetic clamps are used to attach the cables along the vessel hull, creating a fully diver-installable and -retrievable system and allowing communication between the subsea equipment and a real-time mooring monitoring software installed in the control room.

Data are transferred to a standard personal computer in the control room running MoorASSURE mooring line monitoring software. Two 200-m (656-ft) cables are run from the turret to the control room: one for power supply and one for data transfer. The software carries out a number of tasks, including the conversion of angle data into tension using a software model of each of the mooring lines, and also presents historical tension and angle data, providing operators with detailed information about the mooring system performance (Figure 3).

FIGURE 3. This MoorASSURE software screenshot shows an example of the angle and tension readings on each mooring line. (Source: Pulse Structural Monitoring)

The software also allows system configuration to specify the regularity of data communication. The data loggers are programmed to measure data for 15 minutes every six hours and communicate these data once a day. Using the software, the operator can alter this regularity to either increase or decrease the number of communications. In this case study, the operator increases the frequency of communication during typhoon events to give a more accurate indication of the condition of the mooring lines and tensions. The software also can use the measured data to generate monthly and yearly reports on the integrity of the mooring system.

System deployment

The monitoring system was installed in the first half of 2013. A diver support vessel was used for the subsea installation of the data loggers, acoustic receivers and subsea cable. Because this is a retrofit application, the vessel and subsea equipment (which had been installed for more than five years) had accumulated marine growth that could have affected the efficiency of the magnetic and mechanical interfacing. This meant that before installation could begin, the mooring lines, underside of the hull and cable routes all had to be cleaned of marine growth using a high-pressure jet hose. Careful consideration was given to the design of the cable installation. A drag force calculation using location-specific current data and the coefficient of friction of the hull was done to determine the magnetic force required. Since installation, the system has witnessed several typhoons. All magnetic interfaces are still attached, and the system continues to deliver accurate data.

Mooring line failures are a very real threat in all offshore oil and gas producing regions. The cost of failure can be substantial. However, the cost of mooring failure is not just measured in financial terms, with reputation, the environment and personnel all potentially at risk. Mooring line monitoring systems provide offshore personnel with reliable data on mooring line integrity, allowing informed decisions to be made.