2 April 2015
A special report on next generation load instrumentation, measurement and monitoring solutions.
Strainstall is a member of the James Fisher and Sons plc group. The company is a broad-based engineering business, specialising in load measurement and sensor based safety technology. Based is Cowes, Isle of Wight, it has long been associated with the manufacture of standard and bespoke load cells, and has 50 years of experience in assisting industries to operate safely by ensuring that structures, equipment and infrastructure are safe to use.
Through continuous innovation and development, Strainstall has a range of world-class monitoring technologies that continuously monitor physical and performance parameters such as load, stress, temperature, acceleration, pressure and displacement. To date Strainstall has designed, manufactured and commissioned systems that measure force and strain over 1000 structures and applications including the world's largest ship, the world's tallest building, the world's deepest FPSO and the world's largest floating crane, as well as structural health monitoring installations on offshore wind turbine structures.
The use of monitoring systems offshore is well-established. Progressive improvements in technology have resulted in our ability to add monitoring systems to offshore hardware in more locations, in harsher environments and with more reliability. In terms of the instrumentation supporting the sensors within these systems, as an industry we are fortunate to have access to a variety of established and reliable means of interconnection and integration with which to design and build.
The traditional approach to most load monitoring requirements was to place a load cell in-line with the load to provide a direct measurement with a local display. With the introduction of Human Machine Interfaces (HMI), low cost data storage and new sensing technologies there are a multitude of options to provide robust and accurate solutions.
Offshore load monitoring
Load sensing offshore is predominantly concerned with monitoring mooring tensions. These obviously have a large impact on the overall safety of the vessel or platform in question. Typical platform types include Tension Leg Platforms (TLP), FPSO's, Semi-Submersibles, CALM buoys, Jack-Up Rigs and SPARs. The variable nature of met-ocean conditions dictate that monitoring of mooring loads is critical in many regions of the world. This is particularly relevant with the new trend of ultra-deep water floating production platforms.
On many vessels and platforms, the mooring loads alone do not give the operators adequate information to assess the overall safety. More and more the trend is to integrate multiple monitored parameters into a single interface. These parameters will typically include met-ocean conditions, riser tensions, vessel position and motions, hull bending stresses and ballast levels.
On any given vessel there will be multiple monitoring systems and standard communication protocols which now means the transmission of data from one system to another can be done between different suppliers with relative ease. Alarms can be sent from a mooring monitoring system to a centralised Integrated Control and Safety System (ICSS).
This article will take a look at emerging trends in offshore load monitoring, offshore monitoring in general, and the new solutions that are now available to significantly improve safety offshore.
FPSO mooring monitoring case study
As previously explained, monitoring the mooring loads on a floating platform will vary depending on the platform type. There are several parameters to consider when specifying an appropriate monitoring technology and the following will need to be considered:
If we consider a turret-moored FPSO, there are several ways in which mooring tensions can be measured and each has its benefits.
For catenary moorings the tensions can be calculated using the catenary equation. This requires the monitoring of three parameters, namely the chain angle, turret offset and distance to the seabed.
The chain angle is measured using an inclinometer fixed to the chain stopper, the turret offset is measured with a GPS and the depth is typically calculated with draft and tidal data.
With this approach, minimal re-design of existing mooring hardware is required, as the sensors are bolted to the chain stopper so repair or replacement can be easily affected. The angle measurement approach does not achieve accuracy as high as a direct load measurement, but it is usually sufficient, and importantly, can give a better indication of anchor leg failure, should a breakage occur near the seabed. With direct load measurement, a chain failure near the seabed may not trigger any alarms if the weight of chain still imparts a significant force on the chain stopper. There have been several instances of this occurring worldwide, and the failures have only been picked up during annual ROV surveys.
Direct load measurement
Direct load measurement typically provides the most accurate load monitoring solution. There are two general approaches on FPSO turrets and each has its own merits. A load measuring pin is often able to replace an existing pin in a loading assembly so requires minimal re-design to the whole mooring arrangement. This typically gives a direct load output and redundancy can be built in with each load pin having two separate sets of strain gauges and two identical outputs.
The other option that can be applicable where stopper plates are used to hold the chain links is to measure the compressive force applied by the stopper plate to the chain stopper tube. This again is a direct form of measurement and generally has a high level of accuracy. Typically, three or four compressive load cells are fitted into recesses machined into the chain stopper and the stopper plate rests on these. The advantage of this approach is that it enables easy removal of load cells for repair or replacement, although it does require a re-design to accommodate the load cells.
An approach that is becoming more and more popular in terms of mooring load measurement is to use a bolt-on sensor to monitor the strain on a chain stopper, which is directly proportional to the load.
Strainstall's unique strain rings were developed specifically with this application in mind and these sensors have been deployed in numerous offshore mooring applications around the world.
They are most commonly used in dual-articulating chain stoppers that are designed to minimise out-of-plane chain link bending. These chain stoppers, used on both turret-moored and spread-moored FPSOs, typically have a stopper plate at the end of the chain stopper tube, thus subjecting the tube to a relatively uniform tensile load.
The Strainstall strain rings are bolted onto welded fixings on the chain stopper and measure the induced strain in the tubes. The sensors are completely sealed in a 316 stainless steel housing and suitable for use in zone 0 areas as well as subsea. They offer a number of advantages over other monitoring techniques; they are not load bearing and so do not require the stringent materials, manufacturing controls or costs associated with such components. Their bolt-on fixings means that they can be interchanged for repair or replacement as necessary. In addition, their high resolution and dynamic response provides a high accuracy measurement solution.
Subsea monitoring case study
Subsea monitoring of loads, vibrations, current profiles and positions is not a new concept. Most of Strainstall's sensors, including load cells, inclinometers, accelerometers and strain rings are manufactured with subsea housings. However, their use can be limited by the transmission of data back to the platform. Long subsea cable runs are susceptible to damage as often adequate support and protection is not practical.
Therefore, the use of acoustic modems underwater is becoming more prevalent and provides a solution to data transmission over large distances. The range and dexterity of Remotely Operated Vehicles (ROV) also enables maintenance and battery changes to be made for such systems.
Strainstall's Integrated Marine Monitoring System (IMMS) for Total's Moho Nord project utilises such technology in order to monitor the position of subsea buoyancy modules. In this instance a single unit monitors and reports the pressure data to a platform-mounted receiver up to 800 metres away. An ROV-replaceable battery provides a 5 year life expectancy, and multiple buoyancy modules in the field are monitored simultaneously, with data fed to the platform's IMMS. Alarms allow action to be taken in the event of a failure or change in status.
Subsea connectors and cable
Another area of development for subsea monitoring system reliability is the robustness of connectors and cables. Strainstall have worked with sister company RMSpumptools to develop a new type of connector for deployment on the latest generation TLP tendon tension monitoring systems. The load cells in these systems are located around 50m subsea and once installed cannot be replaced during the 30 plus year lifetime of the platform. The use of connectors allows cables to be changed out as required if excessive wear occurs.
The RMSpumptools SeaConnect™ electrical connectors provide a wet mate connector for all subsea monitoring requirements. In common with other high-end connectors, this design provides a double seal between the environment and the critical electrical signals contained within. Copper and cupronickel alloys are used for function-critical components, and coupled with the use of inert low-friction polymor coatings, this is the only connector designed specifically to deal with marine-growth. High-grade stainless steels are used for anti-corrosive and mechanical properties in structural and pressurised casing components.
Often cables are the weakest link in a subsea monitoring system, so their specification and installation are critical to achieving the long-life expected of them. For the most function-critical applications, Strainstall has cable designed and manufactured specifically for the application. In addition, the way in which cable installations are designed is vitally important; time spent at this stage will pay dividends on the project long-term.
As previously discussed, monitoring mooring loads in isolation does not necessarily provide the platform operators with all the information they require to take action to ensure continued safe operation.
Strainstall's recent project for the Moho Nord TLP illustrates how multiple parameters can be effectively monitored and displayed in a manner that allows operators to quickly assess platform safety. For this particular system the following parameters are being monitoring as part of a single Integrated Marine Monitoring System (IMMS):
- Tendon tensions
- Riser tension
- Winch monitoring
- Position and motion
The software can be user-configured to send alarms to other platform software systems such as ICSS and also feeds relevant data into a cloud-storage system to enable remote access to live and archived data.
Throughout the history of offshore monitoring, the display of data from each sensor has moved from being locally displayed to centrally displayed data and with modern networking capabilities data can be displayed in multiple locations on a platform.
Strainstall has moved this technology on one step further to enable users to monitor individual sensors using wireless technologies.
Strainstall have developed a range of low-power zone 1 and 2 wireless transmitters that can be used in conjunction with multiple sensors such as load cells, flow meters etc. Each transmitter has its own unique tag and also internal data storage. A typical transmitter has its own unique tag and also internal data storage. A typical transmitter range is over 200m and with a Strainstall handset any sensor in range can be connected to show the live status. All handsets are interchangeable and certified for hazardous areas. This will enable an operator to move from place to place on a platform and connect with any given sensor in range.
Alternatively, as required, the technology can be provided with smartphone or tablet connectivity. This will enable Apple or Android devices to display live sensor statuses. With the use of appropriate smartphone cases these devices can also be used in hazardous areas.
Download the special Offshore Technology Report.