By Edward Naranjo, Emerson Process Management, Rosemount
Over the last 30 years, the use of FPSOs (Floating, Production, Storage, and Offloading) and other Floating Storage Units (FSUs) has emerged as a key technology to produce oil and natural gas from subsea fields. These vessels can be deployed quickly, and advances in FPSO technology have allowed them to operate in increasingly severe environments and deeper water, and to handle higher pressure and more wells. Not surprisingly, the share of offshore production installations comprised by ships is growing, accounting for 30 percent of the UK Continental Shelf production, in one instance [UK Health and Safety Executive (HSE), 2014].
Despite the increasing number of floating production installations, controlling hydrocarbon releases and reducing the risk of fires and explosions remain key concerns for FPSOs. As higher temperature, higher pressure reserves are harnessed, exploration and production equipment is subjected to greater stress. According to the UK Health and Safety Executive, FPSOs and FSUs have a higher rate of hydrocarbon releases than fixed installations [HSE, 2014].
Most FPSO risk assessments identify the turret system as one area of potentially highest risk [International Association of Oil and Gas Producers (OGP), 2006; HSE, 2014]. Turret systems are the structure from which production fluids from the flexible risers are transferred to the process plant on the vessel by a swivel or other fluid transfer system. Turrets maintain the vessel on station through single point mooring and allow rotation of the vessel to adopt the optimum orientation in response to weather and current conditions. In most cases the vessel can freely rotate through 360o. An FPSO turret system comprises three main systems: the turret; a fluid transfer system (FTS), a multi-path swivel to transfer the production fluids to the process plant on the vessel; and an intermediate manifold known as the turret transfer system (TTS) that links the turret and the fluid transfer system. All parts of a turret system are shown in Figure 1 below.
As with other offshore systems, FPSO turret systems can be subject to degradation in service. Corrosion, abrasion or fracture, and changes in material properties can lead to loss of containment of hydrocarbons. Likewise, hydrocarbon releases can also result from poor maintenance practices, insufficient operational controls, or damage from dropped objects. Regardless of failure mode, ignition of released hydrocarbon can cause or contribute substantially to a major accident.
According to a report commissioned by HSE [Wall et al., 2002], the consequences of a turret explosion could be the following:
According to the same report, the frequency of hydrocarbon gas releases within the turret system is approximately 2 x 10-2 per year and that of turret explosion is 2 x 10-4 per year. The gas release frequency is similar to that of reciprocating compressors (7.1 x 10-2 per year) and centrifugal compressors (1.1 x 10-2 per year) used in offshore and onshore installations [OGP, 2010], and other process equipment used for the handling of fluid.
Because control measures may fail, it is essential for turret systems to have such measures in place as flame and gas detection and fire deluge arrangements. Ultrasonic gas leak and point combustible gas detectors can be installed to monitor the turntable manifold and the fluid transfer system, which has high pressure dynamic seals. In offshore production in the UK, many of the reported incidents between 1995 and 2000 have been associated with the turret transfer system in internal turret designs. The combination of ultrasonic gas leak detection with point gas detectors is particularly effective, since leak detectors respond to the source of the release, while gas detectors help assess hazard severity. Other areas to monitor are the swivel access structure and the gas export swivel.
In some instances, it might be necessary to monitor path of travel to protect worker accommodations, many of which are close to the turret system in internal turret designs, as well as the process plant. In such cases open path detectors could be beneficial. Similarly, flame detectors are required to monitor the main turret, the multi-path swivel stack of the fluid transfer system, and the turntable manifold of the turret transfer system. In the fluid transfer system, it is not uncommon to site one or more flame detectors on each story of the swivel access structure.
Floating structures for production, storage, and offloading have been used safely and reliably over many years. Turret technology has played a key role in mining fields, as turrets have become larger and more complex to handle increasing levels of production and the particular process conditions of individual wells. Nonetheless, with higher throughput and operation in increasingly severe environments, addressing the potential of hydrocarbon gas releases becomes an important element in accident mitigation.
One tool to reduce the risks of escaping gas or process fluids is flame and combustible gas detection. Ultrasonic and combustible gas detectors may arrest the escalation of an incident, while flame detectors can offer early warning of jet fires. Since no one detection technology is 100% effective, the use of a combination of ultrasonic gas leak detection, gas detection, and flame detection increases detection efficiency and offers the most effective means to reduce the consequences of hydrocarbon releases. More information on gas and flame detection solutions can be found here.
How have you set up your gas and flame detection systems to help ensure the best possible safety coverage for your application?
References
HSE. 2014. Offshore Oil & Gas Sector Strategy 2014 to 2017. London, UK: HSE Books. http://www.hse.gov.uk/offshore/offshore-strategic-context.pdf.
International Association of Oil and Gas Producers (OGP). 2006. Guideline for Managing Marine Risks Associated with FPSOs, Report No. 377. London, UK: OGP. http://www.ogp.org.uk/pubs/377.pdf
International Association of Oil and Gas Producers (OGP). 2010. Risk Assessment Data Directory, Report Vol 434-1. London, UK: OGP. http://www.ogp.org.uk/pubs/434-01.pdf
Wall, M., Pugh, H.R., Reay, A., and Krol, J. 2002. Failure Modes, Reliability and Integrity of Floating Storage Unit (FPSO, FSU) Turret and Swivel Systems, Offshore Technology Report 2001/073. Abingdon, UK: HSE Books. http://www.hse.gov.uk/research/otohtm/2001/oto01073.htm