Safety Assessment of FPSO Turret-Mooring System Using Approximate Reasoning and Evidential Reasoning

Numerous Acts of Parliament and Statutory Instruments which apply to Floating Production, Storage and Offloading (FPSO) developments in the United Kingdom Continental Shelf (UKCS), covering a wide range of issues including health, technical safety, work place safety, lifting operations, environmental protection and pollution prevention and control, are described. A comprehensive study of system safety evaluation of a typical turret-mooring system used on FPSOs is described in this paper. A safety assessment method suggested using approximate reasoning and evidential reasoning approaches is proposed in this study. Subjective safety modelling at the bottom level in a hierarchical framework is carried out using an approximate reasoning approach. The evidential reasoning method is used to combine or aggregate safety estimates at lower levels to produce the safety estimate at the system level. The four main sub-systems (Turret (T), Fluid Transfer System (FTS), Turret Transfer System (TTS) and Interfacing System (IS)) are thoroughly examined in order to perform a subjective safety assessment of the turret-mooring system. Nomenclature PSO: Floating Production, Storage and Offloading. A marine vessel single-point moored to the seabed allowing direct production, storage and offloading of process fluids from subset installations – usually via an internal or external turret system. FSO: Floating Storage and Offloading vessel. It is similar to an FPSO but without production capability. Stores oil products that can be offloaded to pipelines or shuttle tankers. FSU: Floating Storage Unit. It is a generic term for floating installations including FPSOs and FSOs. Single Point Mooring (SPM): Mooring to the seabed from a single location on a Floating Production Unit, usually via catenary’s mooring and involving a number of anchor chains, buoyancy aids and flexible risers. Single location in this context refers to the turret. Turret: A cylindrical single point mooring system geo-stationary with the seabed allowing rotation of the FPSO or FSU vessel in response to wave and wind conditions (weathervaning). Weathervaning: Rotation of the ship about the turret in response to wind, sea and climate conditions. Introduction During the 1990’s there was an increasing move in the North Sea sector to subset and deepwater production with the use of floating production and storage systems. A key innovative technology is the use of Floating Production, Storage and Offloading (FPSO) vessels and other Floating Storage Units (FSUs). They are either purpose built or converted from tankers or bulk carriers. The term FSUs may also encompass simpler systems such as storage and offloading buoys. An integral part of such systems is a turret-mooring system that keeps the vessel on station via single-point mooring and allows the vessel to rotate in response to weather conditions. In 1981 the first FPSO world-wide was installed. This technology has become established relatively quickly since the first FPSO (Petrojarl I) entered UK waters * Corresponding author, email: [email protected]. 2 in 1986 and there has been considerable evolution in design to meet the specific environment conditions of the North Sea. Previously there has been little historical information and guidance available pertaining to maintenance standards, best practice, risk and reliability to Health and Safety Executive (HSE) inspectors as well as to designers and safety analysts. There are currently 14 FPSOs operating in UK waters. It is pertinent to note that most current FPSOs operating in UK waters were installed only in the late 1990s and many of the advances in turret-mooring design have come from Norway with innovative advances in swivel design from both the UK and Norway. The first FPSOs came into service in the Far East and South America in the early 1980s as a cost-effective solution to the exploitation of small and marginal fields to extend production in existing fields or allow early production in new fields. There are particular advantages in using FPSOs in fields with short field lives where the cost of investment in pipelines, structures and field abandonment would be relatively high. These operated in shallow water (less than 150m deep) and in good climatic conditions. The 1990s saw considerable advances in design and application of FPSOs with more than 60 FPSO vessels now operational world-wide, many in severe climates and deep water (up to and exceeding 1000m). An FPSO comprises a vessel with integral process plant, moored to the seabed and attached to risers from subset wells. Two photos of FPSO are shown in Figures 1 and 2. Production fluids flow up the flexible risers to the turret-mooring system where they are transferred to the process plant on the vessel by a swivel or other fluid transfer system. They are then processed and their products stored in tanks on board or exported. The vessel is usually equipped with the facility to offload oil products to subset installations, pipelines, storage buoys or storage vessels. The FPSO is an adaptation of simpler storage vessel technology and the use of mooring buoys for transferring production fluids from wells and subset installations. The first generation of these early vessels was spread-moored with the risers feeding into a simple porch at mid-ship. In most locations spread-mooring is not practicable and a single point mooring is desirable to allow the vessel to weathervane, adjusting to wave and climate conditions. A turret-mooring system is an integral part of most modern FPSO systems, and the method of providing single-point-mooring. This swivels to provide effective weathervaning effect, maintain the system on station and allow fluid transfer from the risers to the process plant onboard regardless of the external environmental situations such as severe weather and wave conditions. Advances in FPSO technology both in design and operations have allowed vessels to operate in increasingly severe environmental conditions, deeper water and to handle higher pressures and more wells. In the initial FPSO designs an external turret with a simple swivel connected to a flexible riser was commonly used. Complex multiple swivel assemblies with turrets internal to the ship are used in more recent design; in some cases, production fluids are handled from multi-wells (30 or more wells). Such designs of turret system are complex and may often include multiple flexible riser connections, pipes for process fluid and well injection, multiple seals, valves, initial separation assemblies, pig launchers and securing bolts. The consequences of failure of this type of design are clearly more severe than for a simple swivel joint. Many FPSOs have a design life of 25 years or more and there are strong cost incentives to the operator to maintain the FPSO on station. The turret-mooring technology is relatively new in the UK and Norwegian sectors although more extensive information exists in Brazil where much of the present deepwater technology was pioneered. Most UK operators now have at least one FPSO in operation and they have gone through somewhat of a learning curve on reliability during initial operation. Experience is more extensive in simple swivel joints in buoys: these have been used in the UK sector for a number of years and formed the initial basis for conventional “Bluewater” turret design as opposed to the “Tentech” central turret designs, which originated later in Norway. The limited number of generic turret system design is advantageous in allowing experience and improvements to be passed on to future developments; but disadvantageous in that problems may become generic across FPSO operations. 3 This paper provides an independent assessment of established and emerging designs of turret and swivel systems for FPSOs and FSUs. This paper should also provide information for evaluation of safety cases, assessing new designs, and raising the level of awareness of safety issues in this rapidly evolving technology. The basis for this paper has been a thorough review of FPSO and FSU turret-mooring systems including consultation with turret and fluid transfer system manufacturers. A system based approach has been used defining generic turret system types, the boundaries between systems, common systems and unique features for individual turret designs. This has produced a comprehensive guidance document of FPSO turret systems, their failure modes and inspection and maintenance practices. General issues such as ship structure and turret location are discussed together with current regulatory and certifying requirements. A subjective safety assessment has been conducted for standard turret designs and design variants. This is to identify critical components, safety concerns and relevance of safeguard strategies such as inspection, maintenance and condition monitoring employed to ensure integrity. FPSOs and FSUs have become established relatively quickly in North Sea operations. There are a number of generic turret design types, yet there is still considerable evolution in design and an increasing number of design and manufacturing companies involved in the market. There are specific challenges. The first one is the accessibility of information, some of which manufacturers may consider proprietary. The second one is that there is a need to identify generic technology and features which are design specific. Finally HSE and offshore operators have limited historical data on reliability, given the speed at which the technology has been exploited. In view of these constraints, a subjective method is suggested in this paper to carry out risk analysis of a turret-mooring system. Regulatory Framework There are numerous Acts of Parliament and Statutory Instruments which apply to FPSO developments in the United Kingdom Continental Shelf (UKCS), covering a wide range of issues including health, technical safety, work place safety, lifting operations, environmental protection and pollution prevention and control. The main item of legislation is the Offshore Installation (Safety Case) Regulations, SI 1992/2885 (HSE 1992). The Safety Case Regulations are goal-setting regulations made under the Health and Safety at Work Act, 1974 (HSE 1974). The key requirement of the Safety Case Regulations governing design is that all hazards with the potential to cause major accidents are identified, their risks evaluated, and measures taken to reduce risks to persons to as low as reasonably practicable (ALARP). The Safety Case Regulations are backed up by the Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations, PFEER, 1995 (S11995/734) (HSE 1995), the Management of Health and Safety at Work Regulations (S11999/3242) (HSE 1999a) and the Offshore Installations and Wells (Design and Construction) Regulations, DCR, 1996 (SI1996/913) (HSE 1996). The main item of legislation dealing with environmental issues is the Offshore Petroleum Production and Pipelines Act (Assessment of Environmental Effects) Regulations 1999 (SI1999/360) (HSE 1999b). The Merchant Shipping Act 1979 (HMSO 1979) and the Merchant Shipping (Prevention of Oil Pollution) Regulations 1996 (SI1996/2154) (HMSO 1996) apply the requirements of MARPOL 73/78 (IMO 2002) on matters of marine pollution with additional UKCS-specific instructions. It is important to note that the Design and Construction Regulations 1996 require design to be based on current good engineering practice, which is appropriately risk-based. Compliance with the existing codes, standards and guidance may not be sufficient to meet the regulatory requirements.