5. TURRETS 49
5.3 Bearing Arrangement
Ability to accommodate any type of ß uid and electric transfer system.
•
Disconnectable versions of the internal turret-mooring system also exists. This design is characterised by a large buoy located beneath the keel of the vessel. When disconnected, the buoy submerges to a pre-determined depth approximately 35-40 metres below the surface where it stabilises whilst still supporting the mooring lines and the risers. As before, this design is for applications in cyclone prone areas.
See Figure 5.3.
Figure 5.3
Submerged Internal Turret Production (STP) Disconnectable Turret Turret system on Pierce Field FPSO, North Sea.
Source – APL.
Finally a more signiÞ cant variation of the internal turret is the STP (Submerged Turret Production). This consists of a buoy located beneath the keel of the vessel, but supporting only the ß exible risers. There are no mooring lines since the vessel is held on station by its thrusters controlled by a dynamic positioning system. When disconnected, the buoy submerges to a pre-determined depth and the vessel may sail away.
5.2 Turret Position
The turret is usually mounted in the forward half of the vessel and the accommodation can
•
be either forward, in front of the turret, or it can be aft (i.e. stern end of the vessel);
If the vessel is passive (i.e. no thrusters) or has a minimum thruster capacity, the turret
•
can be external at the bow in mild environments, and internal within the Þ rst 25% of the vessel length from the bow in severe environments; and
If the vessel is active (i.e. signiÞ cant thruster capacity) the turret is internal and is
•
positioned just forward of midship.
5.3 Bearing Arrangement See Figures 5.4 and 5.5.
Figure 5.4
Espadarte FPSO Turret System (Brazil)
The Petrobras Field in Brazil are developed with many risers (connecting to seabed ß owlines from each well).
Some FPSOs there have up to 75 risers. This large number of risers leads to very large turrets, with many activity decks in the turret. Note – because there is no subsea manifolding of the ß owlines,
this function is performed in the turret decks. A smaller number of lines are then required through the swivel system above.
Source – SBM
Figure 5.5
Turret Design Showing Chain Locking Table and the Bearing System
The mooring lines are pulled into the correct tension by a deck mounted winch, and dropped into the chain stopper receptacle on the chain table at the bottom of the turret. In this internal turret the main bearing system is towards
the bottom (it is a three-roller bearing system). It is below the waterline and is sealed from the seawater. The risers enter the turret bottom via guides and continue to their connection at the turret main deck level.
Source – Azur Offshore Ltd and SBM.
Bearings may be:
Rigid bearings such as ball bearings and roller bearings.
•
And compliant bearings such as bogey wheels and friction pads.
•
Rigid bearings can withstand very large loads around a comparatively small diameter. They can be made to the size required and therefore do not inß uence the size of the turret. The maximum size is of the order of 10m in one solid ring and 17m in segmented form. These bearing systems may be at the main deck level or (more usually) below the waterline. Often, both these types have secondary support systems.
Compliant bearings can also withstand very large loads, but the larger the load the larger the bearing. The reason is that as the load increases, the number of wheels or pads to support it also increases, and these can only be accommodated around larger diameter tracks. Typical track diameters are generally in excess of 20m. These bearings are at the main deck level on the FPSO.
5.4 Mooring Lines and Riser Connections See Figure 5.6.
Figure 5.6 Turret Underside
Source lower turret chain table where mooring lock to FPSO and entry guides for the risers to come up through turret to main deck level.
Source – Azur Offshore Ltd.
The turret ends of the mooring lines always consist of chains since these are easier to handle than steel cables. The chains can be connected either at the bottom or at the top of the turret and are generally handled from the main deck.
These two types of mooring lines connections reß ect the types of turrets:
Connection at the bottom of the turret is used for external turrets and for small diameter 1.
internal turrets equipped with rigid bearings; and
Connection at the top of the turret is used for large diameter internal turrets equipped 2.
with compliant bearings.
If connected at the bottom of the turret, the chains normally terminate at articulated, ratchet action chain stoppers mounted on the chain table. The chains are tensioned by cables which are run through one or more fairleads and up to the deck.
The chains are tensioned by winches which can be mounted either on the turret or on the vessel deck;
additional fairleads are necessary in the latter case. The extra lengths of chains which are needed for installation are stored in chain lockers on board the turret. Alternatively the chains may be cut above the chain stoppers, once tensioned, and the surplus lengths of chains may be stored in lockers aboard the vessel, thereby saving space on the turret.
The risers are generally connected up to rigid piping in a dry environment, inside the turret.
Each riser penetrates the turret bottom through its chain table or ring pontoon and is guided up by individual hawser tubes or funnels. At the entrance point each riser is protected by a bend restrictor which may be inclined from vertical to suit the catenary suspension angle and minimise the strains in the riser.
The risers are generally terminated close to main deck level in an area which is protected, accessible and well ventilated for reason of potential gas leakage. They are connected to rigid piping by standard couplings, but may be welded for further reduction of the risk of leakage upstream of the ESD valves.
5.5 Fluids and Electric Transfers See Figure 5.7.
Figure 5.7
Toroidal Swivel Design and Swivel Stack
The swivel allows the pathway of the risers (Þ xed to the seabed) to connect to the line attached to the FPSO (which can freely rotate – weathervane). Each upward ß ow line goes into the toroidal section. The inner
part (attached to the turret) is separate from the outer part (attached to the vessel). The separate section has high pressure seals. A swivel may have many ß uid paths. Each pass sits on
top of another and the arrangement is known as a swivel stack.
In addition to the ß ow pathways for the ß uids from the reservoir of water injected into the reservoir the swivel must transfer electrical signals and control ß uids and chemicals required at the subsea equipment
level. The swivel components for these are towards the top section. Lastly at the very top there is an inline swivel which is usually reserved for transfer of high pressure gas.
Source – SBM.
Since the turret and the FPSO vessel rotate with respect to each other, there is a requirement for the transfer of ß uids and electric lines between the turret and the FPSO. The technology presently available for ß uid transfer consists of:
The toroidal multi-path ß uid swivel.
•
The toroidal multi-path swivel is a mechanical assembly with bearings and active seals. Only few manufacturers in Europe and the US have experience with this critical technology. In addition, for maintenance purposes, a large gantry is installed above the turret.
The toroidal swivel has a number of advantages:
It offers unlimited rotation in either direction;
•
It is compact, relatively light and can be installed on top of any turret;
•
It can operate in severe environments;
•
It requires little maintenance; and
•
It can accept multi-phase unprocessed ß uids from the subsea production system.
•
Its technical limitations are:
The maximum size of seal diameter, presently of the order of 2.0m, which controls the
•
number of ß ow paths which can be accommodated; consequently manifolding upstream of the swivel may be required for large applications;
The pressure of the ß uids, particularly for gas applications (present maximum of the order
•
of 6,250 psi working pressure).
The size of a toroidal swivel is dependent upon the number and size of the ß ow paths it needs to accommodate.
A recent example is the swivel for the Norne FPSO:
Four toroidal paths, 10-inch at 230 bars.
•
Seal diameter,
•
One in-line gas path, 8-inch at 335 bars,
•
Total stack height 8.9m,
•
Total stack weight 93 tonnes.
•
The transfer of electricity, hydraulic ß uids for power or control and chemicals on-board the turret or subsea systems are accommodated by conventional electro-hydraulic swivels and swivel ring systems.
Examples of multi-path swivel stacks are shown in Figure 5.7.
5.6 Conclusions
It must be remembered that turrets are part of a weathervaning FPSO. Spread-moored FPSOs do not require turrets. Additionally, with the spread-moored FPSO the risers simply drape over the vessel’s sides rather than being grouped through the turret.
Directed Learning: To Þ nd out more about turrets and swivel systems visit the SBM Offshore website. Go to www.sbmoffshore.com Go to MEDIA INFORMATION and the DOWNLOAD CENTRE. Download the brochures on TURRET SOLUTIONS and SWIVEL STACKS.
Review the information.
6. RISERS
6.1 Introduction to Flexible Dynamic Risers See Figure 6.1.
Figure 6.1
Flexible Dynamic Risers
Flexible dynamic risers permit the connection from the seabed to a vessel which can have signiÞ cant surface movements. The continuous risers have a low part which is in tension. Above an arch is formed by laying
it over a rigid support arch or by distributing blocks of foam along its length. The section connecting to the production vessel is in a catenary shape. This catenary section provides the reservoir
of length to accommodate the vessel’s motions (as shown in Figure 6.2).
Source – J E & P Associates.
The production riser system carries well ß uids up from the seabed to the host for production. Some risers will carry ß uids (water and gas for injection into the Þ eld) down from the host to the reservoir. Other risers will carry the dead crude and gas for export to shore.
Early ß oating production vessels deployed steel riser pipes, but these had to be disconnected (after Þ eld shut down) during storm periods. Flexible dynamic risers allow the FPSO to remain fully operational during storm conditions. Such risers provide allowance for the movement of the vessel.
The basis of the ß exible dynamic riser is that it can cope with the FPSO’s movements, even in extreme conditions, by means of its conÞ guration. The riser comes up from the seabed (a “tension” section). It then has an arch conÞ guration – this is either by means of draping it over a mid-water arch or by supporting it with foam blocks distributed along this section. The Þ nal part is the catenary hang section from the arch up to the connection into the FPSO. This catenary portion provides the reserve of length to accommodate the vessel’s movement. See Figure 6.2.
Figure 6.2
Basic Movement Requirements of a Flexible Dynamic Riser between Seabed and FPSO Source - J E & P Associates.
The dynamic ß exible riser concept has been used successfully for several years in various parts of the world in relatively temperate weather conditions and was used by the industry for applications in the most harsh environments, North Sea included, since 1985. See Figure 6.3.
Figure 6.3
FPSO with Mooring Lines and Risers Source – SBM.
6.1.1 Types of Pipe Structures and Main Manufacturers
In the 1980s two types of structure have been on offer, the BONDED rubber based one and the UNBONDED thermoplastic/steel one.
Today only UNBONDED structures are manufactured for ß exible dynamic risers. Bonded structures are only used for short lengths of ß exible pipe jumpers at the top of drilling risers and for ß oating ofß oading hoses, where they can easily be replaced.
A typical unbonded structure with all the various layers and their functions is illustrated in Figure 6.4.
Figure 6.4
Technip Flexible Dynamic Riser Construction and Latest Integrated Production Riser Bundle
The ß exible pipe is a complex fabrication of steel layers and plastic layers. The steel provides the core and the tensile and hoop strength. The plastic provides the ß uid containment requirements. The layers are
not bonded together, but can move with respect to each other.
The latest risers may have insulation and electrical trace heating elements.
Source – TECHNIP Ltd.
Manufacturers have made very large investments to build large and complex factories. The three main manufacturers are:
TECHNIP with factories in France, Brazil, Angola and Malaysia.
1.
WELLSTREAM with factories in the USA and UK.
2.
NTK with one factory in Denmark.
3.
Other factories are planned.
6.1.2 Devices Connected or Attached to Flexible Risers
From the ß oater to the seabed a signiÞ cant number of devices are used. These include:
End Þ ttings
•
Buyoncy modules
•
Mid-water arch
•
Bend stiffeners and bend restrictors
•
Fairing vanes and other guiding tubes and cones
•
PLEM and riser base.
•
6.1.3 New Flexible Risers for Deepwater Developments
The problem of FLOW ASSURANCE does impose a means of maintaining heat in risers, in particular during shut downs.
TECHNIP pioneered the IPB system in 1998 for deepwater projects in Angola (–1,500m). There is extensive thermal insulation, but also an active electrical trace heating wires built into the structure.
Directed Learning: To learn more about ß exible dynamic risers visit the TECHNIP website www.technip.com. Search under SUBSEA. Click Publications. Click Brochures. Look at FLEXIBLE PIPE and FLEXI FRANCE. These give a lot of information on their products – review the information and collect for your Þ le.
6.2 Hybrid Riser Systems
In the early 1990s Mobil proposed a fully welded riser tower concept for 1,400m of water. The same design concept was considered by the ALTO MAR GIRASSOL Group (ETPM, BOUYGUES OFFSHORE, STOLT COMEX) and further designed for the Girassol Field development in 1998. This design was selected against a ß exible riser system with hot water heating.
This new system was locally fabricated and was installed during the summer 2001. It is represented in Figure 6.5.
Figure 6.5
Riser Tower System Details
The riser tower was used in the Girassol Field – Angola. Each of the three riser towers has six 8” pathways for ß ow and two 2” pathways for service ß uids, all surrounded by insulated foam blocks. The towers are some
1.3 km in length. At the bottom they attach to the pile mount with a RotoLock stab-in connection.
At the top there is a buoyancy can which tensions the riser and provides the connection of the ß exible jumper connections to the FPSO riser porch.
Source – Azur Offshore Ltd.
6.3 Steel Catenary Risers for Deepwater Developments
The use of free-hanging steel catenary risers is ideal for benign environments and low vessel motions. A good example of innovation, is the export riser for the Gulf of Mexico Shell Auger TLP. See Figure 6.6.
Figure 6.6
Steel Catenary Risers – Auger TPL (GoM)
With a relatively stable TLP platform, and in some 800m water the steel riser can be draped from the platform to the seabed in a catenary shape.
Source – Azur Offshore Ltd.
For more severe loading conditions, limitations come into play, relating to touch-down dynamics, maximum vessel offset and increased top-end response.
The predominant pipe material considered for the riser is “standard” X 65 grade steel. A small amount of X 80 grade is also used, a light stress-joint (or possibly a short length of titanium pipe) at the top-end ensures that combined stresses are below 50% of yield throughout. Low dynamic response means that a long fatigue life is predicted.
Maximum dynamic rotation over riser end-sections is about ten degrees, and seabed and vessel ends are conÞ gured at a small angle to the vertical, corresponding to a static rotation in the mean position. Uplift on the riser base and vessel weight loading are 100 tonnes and 160 tonnes respectively. Installation would be by controlled depth tow of the entire riser or of several sections joined on-site using mechanical connectors.
The status of SCRs to date is:
The Shell AUGER TLP platform saw the installation of two SCRs 12-inch export risers in 1994. They are in API 5L X 52 pipeline steel, coated with three layers of polyethylene and Vortex Induced Vibration (VIV) suppression strakes in the top 150m. The water depth is 858m, they have a length of 1500m, an inclination angle of 11 degrees +/– 2 degrees from vertical and are connected at pontoon level hang-off points with an elastomeric ß exjoint.
Since the AUGER installation from a TLP, a lot more work has been done, in particular:
Petrobras XVIII semi-sub production platform. Marlim Field. 1998. 10-inch riser in 910m
•
of water. Experiment for mooring.
STRIDE JIP has performed, in 1998 in the UK, test initiatives into several areas for the
•
design of SCR. Tests have been carried out offshore with six-inch and 10-inch risers.
Morpeth Field Mini-TLP GoM. 1998. Two risers, 12-inch and 8-inch for oil and gas export,
•
have been installed in 470m.
SCRs have been installed on the Bonga FPSO operating in Nigeria (see
• Figure 6.7).
Figure 6.7
First Steel Catenary Riser on FPSO (Bonga FPSO in Nigeria)
The Bonga Field in Nigeria is the Þ rst use of steel catenary risers deployed from an FPSO. It is highly instrumented and should provide useful information to the industry about further potential use with FPSOs.
Source – Shell plc.
All the developed conÞ gurations fulÞ l both the Ultimate Limit State (ULS) conditions and fatigue due to Þ rst order wave action and due to vortex induced vibrations. Also, the Fatigue Limit State (FLS) governs the global conÞ guration of the SCR concept.
In order to achieve a conÞ dent design, several design aspects must be studied in detail:
First order wave loading
•
Vortex Induced Vibration
•
Differential effects (from the large volume structure)
•
Riser/soil interaction
•
Fatigue capacity.
•