Offshore Floating Production
Appendix A
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Floating production units – history, design, installation and operation
Part (i) – FPSO history
A Overview
A.1 There have been three main phases in the development of the FPSO concept:- 1. The 1970s to early 1990s saw a 20-year period of slow growth, characterised by small, low budget projects, in mild environment areas. Virtually all projects were based on converted tankers, and floating production technology was not widely accepted (especially for harsh environments) by the oil majors and large contractors. Key technologies such as turret moorings, production swivels and flexible risers were developed and refined;
- 2. In the 1990s there was a period of transition in which the FPSO technology matured and several parallel developments led to widespread acceptance. The volume of oil production handled by FPSOs quadrupled during the 1990s. Low oil prices and new FPSO leasing contractors facilitated the acceptance in the North Sea area. This familiarised the oil majors with the technology and allowed them to consider FPSOs for future major projects elsewhere. A mix of newbuild vessels and converted tankers were used but sizes and production capacities remained relatively modest;
- 3. 2000 onwards, FPSOs became the key enabler for development of large oilfields in deep water offshore West Africa and Brazil. Many of these projects required large, purpose built FPSOs with heavy topsides, and were often led by the oil majors. The FPSO is accepted as a ‘standard’ technology.
B Early platforms for offshore oil production
A.3 Offshore oil and gas production began in the shallow waters of the Gulf of Mexico and was initially based on extending existing civil/harbour engineering technology toPage 212
C The drive to deeper water
A.6 These projects were pushing the practical boundaries of fixed offshore structure technology which was not economically feasible for the development of oil and gas fields in waters reaching more than 1,000m depth. Floating systems are much better candidates because the weight and cost increase gradually with water depth, while that of fixed structures increases more or less exponentially. The growth in floating production, storage and offloading systems (of various types, functions and features) began. A.7 Around the turn of the millennium, floating platform technology received a further boost when deepwater drilling units capable of operations in water depths of 3,000m or more began to appear (before then, 500 metres was considered ‘deep’). Oil and gas reserves were soon found in these deep waters and for these cases FPSOs or other floating production concepts became necessary. A.8 Some examples of the water depth progression of drilling technology and FPSOs are given by Agip’s Firenze (1998) in 830m in the Adriatic, Shell’s BC-10 (2009) in 1,780m offshore Brazil using steel catenary risers, and Shell’s Stones FPSO (2016) in 2,900m in the US Gulf of Mexico.D FPSO use on marginal fields – milder environments
A.9 At the same time as moves towards deeper water were taking place in the 1980s, smaller, marginal oil fields were being developed in remote regions where there was little infrastructure and subsea oil export pipelines could not be economically justified. For such projects, often in relatively shallow water, the tanker based FPSO was a convenient solution, as the oil could be stored onboard and periodically exported by tanker rather than by pipeline. Floating production units were attractive for marginal fields with short production durations because their ability to be moved elsewhere allowed (in theory at least) their cost to be spread over more than just the initial short life of the marginal field. These projects allowed the FPSO technology to be developed and gain acceptance during the 1980s. A.10Page 213
E FPSO application to harsh environments – North Sea
A.13 In the North Sea, early reluctance to use FPSOs was primarily due to technical concerns related to the effect of the harsh environment on the vessel motions and moorings. A few monohull floating storage and offloading units (FSOs) had been used in the UK North Sea since 1981 (Shell’s Fulmar floating storage unit – a converted tanker). The breakaway of this unit from its rigid single point mooring in 1989 did not help the case for deployment of FPSOs. A.14 However, by the early 1990s the declining size of field discoveries on the UK sector and a resulting need to use more economic development techniques coincided with increasing confidence in the key FPSO technologies. Advances in mooring and offloading systems and in fluid swivel technology were key factors in the acceptance and development of modern FPSOs. A.15 There are three candidates for the title of the first FPSO to work in the North Sea:- 1. The Petrojarl I was a turret moored FPSO ordered in Japan on speculation by a Norwegian ship owner. The vessel, although small, was sophisticated and purpose designed. She arrived in Norway in 1986, where she was initially engaged in well testing at the Oseberg field from 1986–1988. This was followed by several short-term test contracts until Amerada Hess hired the vessel for its first full field development contract at the UK Angus field from 1991–1993. This was followed by multiple later deployments (Petrojarl I is still operating in Brazil as of 2021);
- 2. BP’s dynamically positioned Seillean produced and stored oil in the North Sea shortly before the Angus field commenced production. This unique vessel was BP’s idea for sequential production from small fields and was also a sophisticated, purpose-designed newbuild. Unfortunately, the concept used the soon to be outdated rigid riser technology and dynamic positioning instead of mooring.
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- 3. Another key North Sea milestone was the successful use in 1993 of an FPSO by Kerr McGee for full field development of the Gryphon field. This FPSO was based on a fast-track topsides addition to a purpose designed but speculatively built hull. The field was still producing in 2021.
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F
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Technological barriers and breakthroughs for acceptance of FPSOs
A.18 Until the late 1990s, the FPSO concept often met with resistance from engineers and firms who preferred to use fixed platform technology. This resistance was strongest in hydrocarbon basins where the weather was harsh, the water depth not too great and where the regulatory regime was ‘sophisticated’; areas such as the North Sea (UK and Norway).
A.19 The primary concerns were the reliability/feasibility of station-keeping (i.e. mooring) and fluid transfer, especially in harsh environments. A related concern at the time was that the motions would be too much for efficient separation of oil and gas in the topsides process plant (in the end this was not a major problem).
A.20 The move away from the early single point mooring systems towards turret moorings was important. While both permitted weathervaning, the former were (rightly) perceived to be susceptible to single point failures (e.g. Garoupa, Fulmar). In contrast, the ‘earth fixed’ part of the turret mooring is connected to multiple mooring legs, offering some redundancy. The development of high-pressure swivels associated with these turret moorings permitted the well fluids to be brought onboard a weathervaning monohull.
A.21 Before the advent of flexible risers, fluid transfer from the seabed to the surface was another concern for FPSOs. While semi-submersible FPUs could handle rigid production risers, the greater motions of a monohull meant that this was a much greater challenge for an FPSO, especially in rough weather locations.
A.22 Therefore, a key enabler of the FPSO concept was the emergence in the early 1980s of flexible riser technology which allowed floating vessels to be safely connected to subsea wells despite wave induced motions and excursions on the mooring systems. These riser pipes are composite structures of steel and synthetic materials which can safely contain hydrocarbons under high pressure.
A.23 Brazil was where much of the pioneering work with subsea production systems and flexible risers took place. In 1979, the first flexible production risers were deployed on the Penrod 72 semi-submersible FPU at Enchova Leste. In the North Sea, the first use of flexible risers was for water injection duty on the 1983 tieback of the Duncan field to the Transworld 58 semi-submersible FPU on the Argyll field (the production risers were rigid). This was followed in 1986 by a full suite of flexible production risers on the Balmoral semi-submersible FPU.
A.24 Brazil has continued to pioneer various aspects of floating production and subsea systems (e.g. first use of a steel catenary riser on the P18 semi-submersible in 1998).
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Part (ii) – Different types of mobile production units
A Introduction
A.25 Mobile production units (MOPU) are moveable platforms designed for the handling of streams from hydrocarbon production wells and the onboard separation of the well fluids into oil, water and gas. In addition, they may be required to inject water for reservoir support or compress gas for ‘lifting’ the production from the well. A.26 The output of these production units is stabilised oil and gas (although some now export liquefied refrigerated gas). The produced oil is exported by visiting tankerPage 216
B Characteristics of different hull types
(i) Jackups and bottom fixed platforms
A.32 A jackup rig is a self-elevating bottom-fixed platform with three or four tall legs. During operation, the legs rest on the seabed and the hull with mission equipment is jacked up on the legs until it is well above the sea surface and the reach of the waves. When the legs are not deployed downwards, jackups float on a hull which is normally triangular in shape (three-legged jackups) or rectangular (four-legged designs). A.33 These hulls allow jackups to be towed from location to location or transported on heavy lift ships. However, careful weather routing is normally required for long ocean transits. Because of their great height, jackup legs can suffer rapid and catastrophic fatigue damage if the rig experiences extreme motions when transported with the legs elevated upwards. A.34 Jackups developed as the most popular type of mobile drilling unit because, being bottom supported, they provided a very stable working platform. Some have been adapted for use as production or accommodation platforms. In production service, the stability of the jackup platform allows surface wellheads to be used, rather than the more expensive and difficult to access subsea wells associated with FPSOs or semi-submersible production units. Such jackup units are normally operated in conjunction with an export pipeline, or a nearby floating storage tanker or subsea oil tank. A.35Page 217
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(ii) Semi-submersibles
A.42 For many decades, the offshore industry has used the semi-submersible hull form, with its deeply submerged lower hulls (pontoons) and relatively slender columns or legs, to provide a stable drilling platform in water too deep for jackups, especially in exposed locations. This hull concept is relatively transparent to wave action, permitting operations to continue in conditions where conventional monohull designs would experience extreme motions. The semi-submersible form has also been used to provide stable and comfortable offshore accommodation units, floating production units (FPU), and offshore construction and crane vessels. A.43 In some of the later developed oil and gas basins the offshore production units were semi-submersibles, such as at the Argyll Field in the UK (first oil in 1975) and offshore Brazil. This was possible because the low motion characteristics of the type permitted the use of rigid risers (adapted from drilling duty) before flexible risers were developed. A.44 Several early semi-submersible production units converted from drilling rigs were leased by the oil companies from their contractor owners. However, most modern semi-submersible production platforms are purpose designed and built and owned by the oil companies. A.45 Most semi-submersibles are built to the rules of a classification society and the IMO MODU Code (the equivalent of SOLAS). Tragic accidents involving semi-submersible units (for example, the accommodation unit Alexander Kielland in 1980 and the drill rig Ocean Ranger in 1982) caused significant loss of life and led to the development of special rules for the stability, structural integrity and ballasting systems of these vessels. A.46 In addition, such units are generally considered to be offshore installations by coastal states and so are subject to special legislation governing the hydrocarbon activity, which is usually more onerous than standard maritime regulations. Application of such coastal state requirements can cause problems in project execution if they are not clearly defined and diligently followed up. A.47 In production duty the benefits of the semi-submersible are a reasonably large deck area and the ability, if required, to permit workover of the subsea wells via an onboard rig because of the low motion characteristics. A.48 All semi-submersible production units make use of multi-leg spread moorings. Unlike a monohull, a semi-submersible has no need to weathervane to face extremePage 219
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(iii) Spar
A.53 The spar hull is a long, slender vertical hull, usually of circular cross section. This hull shape offers excellent heave, pitch and roll motion characteristics. The vertical motions are so small that some applications have used rigid risers with surface Christmas trees. A.54 Surface Christmas trees are particularly advantageous in oil fields where the wells may require frequent access for workover. With subsea wells in deep water, workover would require a hired-in semi-submersible or drillship, which is very expensive. A.55 The very deep draught means the topsides must be installed in deep water offshore after upending the hull (the complete platform construction cannot be completed at one site). A.56 The concept is not tolerant of weight growth in design or in service (due to its small waterplane area). The limited deck area forces a vertical arrangement of the topsides which may mean that accommodation etc. is located over or near to wells and production (which can be a safety concern). A.57 This low tolerance for weight changes means that no useful oil storage is normally possible – so a pipeline or FSO is required to export produced oil.(iv) Tension leg platforms (TLPs)
A.58 The tension leg platform is a buoyant hull, looking somewhat like a semi-submersible, tethered to the seabed by vertical tendons. Usually, the hull has horizontal pontoonsPage 221
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(v)
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Floating production, storage and offloading vessels (FPSOs)
A.64 FPSOs are the most common type of mobile production unit. When originally pioneered in the 1970s they were based on conversions of trading oil tankers. This is still popular, although newbuild hulls have also become common. The tanker hull offers a very big carrying capacity in proportion to its own weight and this permits large oil storage volumes as well as supporting heavy topsides. Most tanker conversions make use of 2 million barrel capacity VLCC hulls, although some projects in certain parts of the world have utilised smaller vessels of Aframax size (about 700,000 barrels) or even less.
A.65 The design is generally tolerant of weight alterations during design or service and the large deck area permits arrangement to suit conflicting safety and process demands. The motions do not permit the use of surface Christmas trees and so FPSOs are all associated with subsea wells or nearby wellhead platforms. Motions again mean that the use of an onboard rig for performing workover operations on subsea wells is generally not a practical option.
A.66 The marine aspects of tanker conversions are generally centred around life extension and enhancement of the hull structure and coatings to achieve a long life offshore without docking, increasing the accommodation capacity, together with the modifications required to interface with the topsides processing equipment and the mooring system.
A.67 In sea areas with relatively modest wave conditions and/or a predominant direction for the environmental forces (such as West Africa), FPSOs may be moored by means of spread moorings. In such cases, the subsea risers may simply be hung off the side of the vessel. In such cases there is relatively little complexity and cost associated with the shipyard scope for the mooring and riser systems.
A.68 In harsher environments such as the North Sea, vessel motions and mooring loads may be reduced by allowing the FPSO to weathervane into the weather. This requires a turret bearing system and a high-pressure swivel system which transfers fluids from the earth-fixed risers below the FPSO to the process systems onboard which are fixed to the FPSO and so rotate around the mooring. These turret and swivel systems can become large and complex, with some weighing more than 10,000 tonnes and costing considerably more than the FPSO hull. There are few qualified suppliers of such special systems. Fabrication and integration of the turret mooring system is therefore a major element of such projects, and the project contracting strategy must account for this.
A.69 In some parts of the world, disconnectable turret moorings are employed, in order to allow the FPSO to avoid rare but extreme events such as hurricanes, typhoons or icebergs. These FPSOs maintain a self-propulsion capability and the ability to navigate as a conventional ship. For most other FPSOs there is no propulsion equipment, or it is used only on the delivery voyage to the field.
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(vi) FPSO buoy shaped (Sevan type)
A.70 This is an FPSO, simply of a different shape to conventional ships or barges. The hull is circular in plan view, meaning that the area presented to the wind and waves is the same in any direction. This means that a weathervaning mooring is not required (such FPSOs are always spread moored), which means that the expensivePage 223
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C Liquefied gas platforms
(i) LNG regasification units (FSRUs)2
A.74 FSRUs are not, strictly speaking, production units, as they are not connected to hydrocarbon reservoirs and simply receive product (LNG) from visiting tankers and export it in the form of gas. However, they are typically owned by shipping companies or offshore contractors and chartered into energy projects which differ significantly from the standard cargo transportation business the basis LNG ships were originally developed for. They are discussed briefly in this Appendix for that reason. A.75 LNG carriers have been converted or built from new to serve as LNG importation terminals, capable of regasifying methane cargo prior to sending onshore into a client onshore gas pipeline network. The ship receives LNG from visiting LNG carriers and serves as a floating storage and regasification unit (FSRU). The regasification equipment is modest in terms of size and complexity, effectively heating the cold LNG with warm seawater so that it vaporises. A.76 While the donor LNG carrier will be derived from a well-established design, many other FSRU design aspects will be bespoke for the intended place of operation and the client’s requirements. The FSRU will also be part of a much larger project, which introduces design interfaces and areas where the non-performance of one aspect of the project may affect another. A.77 Unlike a standard LNG carrier, the FSRU must have a permanent mooring system to maintain its own position. This may be a fixed jetty or a weathervaning type turret mooring system, through which is connected the gas export pipeline. While manyPage 225
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(ii) Floating LPG offtake
A.80 Floating units capable of storing liquefied petroleum gases (LPG) such as butane and propane and offloading the product to visiting LPG tankers are an important stepping stone towards the modern FLNG. They embody certain key features such as refrigeration offshore, side-by-side berthing and offtake operations, and cryogenic cargo transfer at sea. A.81Page 226
(iii) Floating LPG production
A.83 The first floating LPG production vessel was the Sanha LPG, which has been in production since 2005 offshore Angola. A.84 This unit has cryogenic LPG storage of 135,000m3 in the steel hull. The hull and storage technologies employed were those of standard LPG trading tankers. The production capacity is 5,940m3/day of butane and propane.(iv) Floating LNG production vessels
A.85 Unlike FSRUs which are designed to receive already processed liquid LNG, floating LNG production vessels (FLNG or LNG FPSO) are designed to receive gas (sometimes combined with other well fluids) and export LNG. The core of all FLNG units (and onshore LNG trains) is therefore a liquefaction plant capable of lowering the
Table A1 FLNG units in operation as of 2021