OTC 19899

Qualification of the Grouped SLOR Riser System Daniel Karunakaran, Dan Lee and John Mair, Subsea 7

Copyright 2009, Offshore Technology Conference This paper was prepared for presentation at the 2009 Offshore Technology Conference held in Houston, Texas, USA, 4–7 May 2009. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract The growing trend of deep and ultra deepwater developments necessitates the use of risers that will give good stress response and fatigue performance, and be able to optimise field architecture to accommodate complex and congested seabed layouts. In order to achieve this, Subsea 7 and 2H Offshore have developed the Grouped SLOR, a hybrid riser solution which captures the above stringent riser requirements and maintains maximum operability in deepwater developments at water depths greater than 700m.

The Grouped SLOR consists of individual free standing risers, SLORTM and/or CORTM grouped together by a buoyant guide frame tethered down at either ends to suction piles. Connection between the host vessel and the SLORTM or CORTM is provided by a flexible jumper from a gooseneck located at the top of the riser assembly.

The paper describes the technical developments, key features of the riser system, and the qualification programmes that have been performed to validate the robustness of this concept and design, namely:

• Finite element analysis assuming constant hydrodynamic properties of the air cans; • Computational Fluid Dynamics analysis to validate the assumed hydrodynamic coefficients of the air can arrays; • Model test in tow tank for air can arrangements with varying currents and varying current incidence directions to

validate the hydrodynamic coefficients of the air can array • Model test on the complete Grouped SLOR assembly, including air cans, guide frame, risers and tethers to investigate

the flow phenomena around the air can assembly; • Full installation assessment.

Introduction Grouped SLOR is an “open bundle” riser solution developed specifically to optimise the riser/vessel interface, production vessel approaches and seabed layout. It uses a buoyant truss frame to guide the freestanding risers, constraining all risers to move collectively, and thus eliminating the risk of clashing.

The Grouped SLOR has great potential for large deepwater developments, which typically have a complex and congested seabed layout immediately adjacent to the production vessel. This is due to the large number of risers and umbilicals often required to meet production, injection and export requirements, and the spatial constraints imposed by mooring lines and vessel offsets. This poses significant constraints on the riser design to achieve an acceptable riser arrangement whilst ensuring that clashing and interference are avoided. In addition, the fatigue requirements, stringent insulation and gas lift requirements (met by the use of a concentric riser system) greatly favour the use of Grouped SLOR.

In order to qualify the application of Grouped SLOR for deepwater environments, a series of qualification projects had been performed to validate the robustness of the concept and design, and they are listed as follows:

• Finite element analysis based on assumed constant hydrodynamic properties of the air cans; • Computational Fluid Dynamics analysis to validate the assumed hydrodynamic coefficients of the air can arrays; • Model test in tow tank for air can arrangements with varying currents and incidence directions to validate the

hydrodynamic coefficients of the air can arrays; • Model test on the complete Grouped SLOR assembly, including air cans, guide frame, risers and tethers to investigate

the flow phenomena around the air can assembly; • Full installation assessment.

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The complete Grouped SLOR model test was carried out by MARIN (Maritime Research Institute Netherlands). The tests were carried out for 5 current directions and 10 current velocities of up to 2m/s (at Mean Sea Level) and the test results obtained should be applicable for deeper water depth by the use of stiffness and truncation factors. The results indicate that the riser and air can assembly has stable VIV response at high currents without any indication of large amplitude motion or galloping.

All the above qualification projects indicate that the Grouped SLOR riser concept and design is robust and well suited for deep to ultra deepwater harsh environments.

Grouped SLOR Concept SLOR and COR are trademarks of 2H Offshore Engineering Ltd (2H). Subsea 7 is in collaboration with 2H to develop and launch the Grouped SLOR so that they can offer it as part of a SURF (subsea umbilical, risers and flow lines) solution for deepwater field developments.

2H has had tremendous success with its standalone hybrid riser design, the Single Line Offset Riser (SLOR). Operators have successfully employed the SLOR, as well as its pipe-in-pipe variant, the Concentric Offset Riser (COR) in deepwater developments offshore Africa, and recently offshore Brazil and the Gulf of Mexico. The SLOR/COR offers an attractive solution due to its excellent fatigue performance and ability for pre-installation, thus taking it off the field development critical path.

Recently, new field developments with larger riser numbers and existing developments with the need for additional tiebacks have been identified, and these requirements pose significant problems for SLOR/COR systems due to clashing issues. The problem is further compounded by use of turret moored FPSO, which further decreases the available riser porch spacing as shown in Figure 1.

Figure 1 Turret Moored FPSO with SLOR™

In order to meet the riser requirements of large offshore developments, Subsea 7 and 2H have jointly developed the Grouped SLOR concept. This is a variant of the SLOR and COR design which incorporates a guide frame connecting between 2 or more risers constraining them to move collectively and ensuring positive separation as depicted in Figure 2.

Figure 2 Grouped SLOR (6/4 Risers Arrangement)

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SLOR and COR Design The SLOR arrangement consists of a rigid steel riser pipe extending from the mudline to an air can situated 50 m - 200 m below the mean sea level depending on environmental loading conditions. The air can provides up thrust which applies tension to the riser pipe and generates an over pull at the mudline of between 50 Te and 150 Te depending on water depth. The gooseneck and flexible off-take can be located either above or below the air can depending primarily on installation strategies. The air can can be connected to the riser either by chain or a flex element. Alternatively, if the off-take is above the air can, the riser pipe can be routed through the centre of the air can and terminated at an upper bulkhead.

The SLOR is typically situated around 200 m away from the vessel or turret depending on water depth. Connection between the two is achieved using a flexible jumper via a steel gooseneck assembly connected to the top of the riser pipe. The flexible jumper is then connected to the host vessel through an I-tube or J-tube assembly with a bend stiffener.

At the base, the SLOR is connected to a foundation pile (suction, driven, gravity or drilled) and terminated with an off-take assembly that facilitates connection to the flowline with a rigid spool. Connection to the foundation pile is achieved via a roto-latch (articulation joint) or a flex element.

The COR design follows a similar design to the SLOR except that the design incorporates a pipe-in-pipe configuration. The outer annulus is used for gas lift purposes, and as such the COR is usually used for production risers. The upper assembly is modified to incorporate a smaller gooseneck with access to the annulus so that gas can enter the COR via a flexible jumper attached to the vessel and inject into the production flow at the base of the riser through a gas lift crossover forging. COR is extremely useful if the development requires a gas lift riser adjacent to the production riser. COR can also be used to meet stringent thermal requirements, having a high thermal mass, excellent insulation capability and achieving long cool down durations.

The SLOR and COR arrangements have already been successfully utilised on projects in West Africa with further applications planned in the coming years. Grouped SLOR Design The Grouped SLOR consists of a number of risers in close proximity which eliminates clashing issues, whilst maintaining a practical distance between each riser to facilitate installation, inspection and maintenance, including removal and reinstallation if necessary. The individual SLOR/COR is connected into the guide frame assembly via the receptacle.

Grouped SLOR Arrangement

The individual SLOR/COR design is similar to the standard freestanding arrangement, with the riser pipe running through the central bore of the air can. The main modification is the elongated large diameter upper stem between the top of the air can and the gooseneck connector, similar to that used on a SPAR riser. This stem interfaces with the guide frame via a bearing assembly. The riser pipe is terminated at the top of the stem transferring the air can upthrust to the riser string resulting in tension on the riser.

The air cans are typically 5 m to 6 m in diameter, with a centre-to-centre spacing of 2 diameters when connected with the frame, giving a 1 diameter air gap between them. The length of each air can depends on the water depth and required over pull; for a mid-depth development with a 150 Te SLOR base tension, the air can length varies from 10 m to 20 m depending on riser purpose and diameter.

To aid installation, the gooseneck is designed to be removed and attached after the SLOR and guide frame have been installed. This allows the flexible jumper to pass over the top of the guide frame, optimising the allowable space and increasing the overall stability of the system.

Riser Guide Frame The guide frame depicted in Figure 3, is the component that differentiates the Grouped SLOR from the standalone SLOR/COR design. It is fabricated from steel tubular in a truss arrangement to minimise its weight in water and maximise its stiffness. It is installable using a light duty vessel or simply towed to site. For the 4-SLOR group configuration shown in Figure 2, the frame is approximately 45 m long with a central bolted flange connection, such that the frame can be handled in 2 sections, if required.

Connection to the seabed is by spiral strand steel tethers connected to either end of the frame. These are splayed out below the frame to provide stiffness to resist rotation during operation. The tethers are connected to the mudline using suction piles via a length of chain connected to the tethers and terminated at the top of the piles as shown in Figure 2. The chain aids installation as it allows the height of the frame to be easily adjusted during installation to account for inaccuracies from pile installation.

Although the frame tubular provide buoyancy when installed, additional buoyancy tanks are attached to the top of the guide frame at either end of each half as illustrated in Figure 3. The buoyancy tanks, which are sealed air tanks or syntactic foam, are configured such that typically 50 Te of over pull are maintained at the base of each tether at all times.

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The risers are guided within the frame using a receptacle with a swinging gate which is bolted on to the front of the frame as depicted in Figure 3. This includes a central opening which allows two arms to swing open to accommodate the riser guide stem, these arms are then closed using an ROV and locked using a pin.

The riser stem slides within the receptacle relative to the frame to account for vessel movement, temperate and pressure effects, and also different stages of the installation process. The inner surface of each receptacle is equipped with an ultra-high molecular weight polyethylene (UHMWPE) bearing pad to resist wear over the duration of the field life. Since the receptacles are bolted to the frame, they can be easily replaced if required without disturbing the adjacent risers.

Figure 3 Grouped SLOR Guide Frame Assembly and receptacle Qualification Programmes Finite element (FE) analysis was performed to confirm the stability and arrangement of Grouped SLOR configuration. Computational fluid dynamics (CFD) was carried out to confirm that the hydrodynamic coefficients employed for the FE analysis were appropriate. Model test campaign was subsequently carried out to confirm the CFD results and the hydrodynamic response of the air can arrangements under extreme current loading conditions. Further model test on the complete Grouped SLOR assembly, including air cans, guide frame, risers and tethers was conducted to investigate the flow phenomena around the air can assembly. Finite Element Analysis In-place FE analysis on the Grouped SLOR shows that the system is more stable, with lower displacements than a standalone SLOR or COR, which has already been proven to be excellent in fatigue and operational performance, see Dale et al. (2007).

The maximum displacement of the guide frame under normal operating condition is typically less than 2% water depth for deepwater environment (800 m) and increases to 6% water depth for ultra deepwater environment (2000 m). This reduction is due to the difference in geometry configuration from 800 m to 2000 m and will not be an issue as the guide frame is typically 200 m away from the vessel.

The greatest bearing load on the guide frame is less than 10 Te for a water depth of 2000 m, which is well within the bearing capacity of the UHMWPE wear pads. For a lower water depth of 800 m, the maximum bearing load is around 6 Te.

The guide frame rotation for a deepwater development of 2000 m under normal operating condition is less than 1.5 degree. Considering the most unstable installation configuration, whereby only one SLOR is installed, the rotation increases to around 25 degree, however this will not affect the stability of the frame. Lower frame rotation is expected for a shallower water depth development.

The Grouped SLOR response depends greatly on the arrangement of the SLOR/COR within the frame and their service conditions. It is therefore recommended that pairs of riser with similar requirements are installed opposite one another. It will be preferable to put the two production risers in the middle of the frame and the two injection risers on the outside or vice versa such that the frame loads are relatively balanced. The relative stiffness of the risers along with the weight of their flexible jumpers ensures that the frame remains balanced and stable at all times. Computational Fluid Dynamics The hydrodynamic stability and proximity of the riser air cans to one another within the group are critical in ensuring the system remains stable. In light of this, CFD analysis was conducted to ensure the hydrodynamic coefficients used in finite element analysis are appropriate.

The CFD analysis results confirmed that the maximum drag coefficient (CD) experienced by the air cans is less than 2.0, assuming an infinitely long air can. In reality, the air cans are typically a maximum of 6 diameters in length, and therefore the end effects reduce this maximum value (as confirmed during tank testing detailed below). Snapshots of the CFD analysis are given in Figure 4, along with the drag parameters obtained.

GUIDE FRAME

CENTRAL FLANGED CONECTION

TETHER

BUOYANCY TANK

SLOR STEM RECEPTACLE

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The flow interaction and vortex-induced loading measured from CFD analysis concludes that the air can spacing of one diameter gives the best response in terms of vortex induced vibration (VIV), wake effects and galloping. Due to the simplistic design and relative low-cost of the guide frame, the air can spacing can easily be increased for developments where high extreme currents are observed.

Figure 4 CFD Velocity Vectors Around Air cans Model Test of Air Cans In order to confirm the hydrodynamic response of the air can arrangement, scaled tow tests were conducted at MARIN’s test center in the Netherlands. The tow tests shown in Figure 5 were conducted for a range of current velocities typical of developments in the Gulf of Mexico or offshore Africa with air can spacings ranging from 0.5 diameters to 2 diameters representing full scale spacing of 3 m to 12 m respectively.

Tow test results indicate that as a result of air can end effects and the variability of the air can lengths, the maximum CD likely to be experienced is approximately 1 to 1.5 with an air can spacing of 1 diameter (D). This is less than the CD value of 2.0 used for the finite element analysis. Finite element analysis confirmed that in order to generate clashing between the air cans, the required drag coefficient for a spacing of 1D is 16.0.

It can therefore be concluded that the SLORs within the group remain stable under extreme current activity with no possibility of clashing.

Figure 5 Grouped SLOR Tow Model Test

CFD Analysis of Air Cans, Mean Transient Drag Coefficients4 Air Cans in Series, 1.0D Separation, 0.1m/s Current, Reynolds Number 0.55x106

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40 50 60 70 80 90Angle (°)

Dra

g C

oeffi

cien

t, C

d (-)

Air Can 1 - Inline Air Can 2 - Inline Air Can 3 - Inline Air Can 4 - InlineAir Can 1 - Cross Flow Air Can 2 - Cross Flow Air Can 3 - Cross Flow Air Can 4 - Cross Flow

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Grouped SLOR Complete Model Test Model test was performed on the complete Grouped SLOR assembly, including air cans, guide frame, risers and tethers for the investigation of the flow phenomena around the air can assembly. Model scaling of 1:73 was adopted. The tests were carried out for 5 current directions and 10 current velocities of up to 2 m/s (at MSL). Further details can be found in Karunakaran et al. (2008).

Test Set Up

The model test was carried out by MARIN using their Depressurised Towing Tank as shown in Figure 6, which was 240 m long by 18 m wide and 8 m deep. The tank has an overhead carriage with an accurate speed control between 0 m/s and 8 m/s. The maximum effective tow length was 200 m.

Figure 6 MARIN’s Test Facility with Carriage

The riser configuration for the model test was based on a Grouped SLOR system proposed by Subsea 7 for a West Africa field development in a water depth of 800 m, similar to the 4 riser configuration seen in Figure 2. The two CORs were located at the middle of the guide frame with one SLOR located at each end.

The air can outer diameters were fixed at 5.5 m with the length varied between 17 m and 25 m to give a riser base tension of 150 Te to accommodate riser sizes ranging between 11.5” and 15”. Buoyancy modules were attached to the guide frame to ensure the tether tension of 50 Te was achieved.

A scale of 1: 73 was adopted, accounting for the four air cans, guide frame complete with buoyancy modules, and riser and tether system which were truncated at 520 m water depth.

(a) (b)

+ve Surge

+ve

Sway

Figure 7 (a) Rotating Base Plate for Grouped SLOR (b) Test set up in Tow Tank

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The Grouped SLOR model was mounted on a rotating base plate (Figure 7a) to allow for current incidence variations. It was hung from the carriage on 4 steel wires (Figure 7b) at around 7 m water depth and was towed by the carriage. The flexible jumper was not considered in the test, depicting the scenario where the Grouped SLOR was waiting for the arrival of the production vessel, before jumper connection between individual riser and the vessel’s I or J-tube.

The risers and tethers were modelled by means of steel wires with correctly scaled weight in air and underwater weight. Lead weight and foam beats were used for adjusting the weight distribution. The bending stiffness of the risers was not modelled, as they were considered to be small as compared with the stiffness of the Grouped SLOR assembly.

Sensors

The six degrees of motion of the guide frame of the Grouped SLOR assembly were measured by means of a “Krypton” optical tracking system. The Krypton system measures the motions of three infrared LED light sources with an absolute accuracy better than 0.1 mm (at model scale). Based on the measured motions of these three light sources, the Krypton system calculates the surge, sway, heave, roll, pitch and yaw. The measuring frequency was 100 Hz.

The light sources were mounted on a triangular plate, known as “target”. This light weight target was mounted on two small diameter rods through the waterline on top of the guide frame of the Grouped SLOR as illustrated in Figure 8. Tensions in the risers (or wires for the model) were measured by means of small ring type strain gauges at three locations, namely:

• Under the air can of the (aft) water injection SLOR; • Under the air can of the aft Production and Gas Lift COR; • Under the bridle of the aft tether.

An underwater camera to capture the model behaviour during the tests, was mounted on a streamlined strut at approximately 2 m offset from the model. It was positioned approximately at the same water depth as the air cans.

Figure 8 Grouped SLOR Assembly with Krypton Optical Tracking System

Test Matrix The base case model was tested for five flow directions, namely 0, 10, 30, 45 and 90 degree with current velocity varying from 0.17 m/s to 1.71 m/s (constant over the water column). Two further configurations, Series 9 and Series 10 as described in Table 1, were considered to determine the optimise locations of the combined SLORs and CORs for the Grouped SLOR assembly. Details of the test matrix are given in Table 1.

Configuration Flow Direction (degree) Current Velocity (m/s) Remarks 2 CORs in Middle 0, 10, 30, 45, 90 Base Case 2 SLORs in Middle 0 Series 9 2 SLORs in Middle With 2 x Tether Tension 0

0.17, 0.34, 0.51, 0.68, 0.85, 1.02, 1.19, 1.36, 1.54, 1.71, 1.88 (Series 9 only) Series 10

Table 1 Test Matrix

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Truncation and Calibration The stiffness of the Grouped SLOR assembly in surge direction was determined. The water depths considered were 555 m and 800 m. The former water depth was chosen for confirming the model test truncated depth of 520m while the latter water depth was typical of West Africa field development.

The horizontal stiffness of the full scale Grouped SLOR model was determined by pulling it to an offset with a thin wire connected to the guide frame. The offset was measured with the Krypton motion tracking system and the pulling force was measured with a small ring type strain gauge in the pulling wire.

The stiffness results comparison indicated that the model test stiffness was similar to that obtained from the Flexcom model for a similar depth. This was subsequently used to determine the stiffness and truncation factors for the natural period at different water depths.

The surge natural period of the truncated model was measured to be 75 s from the decay test. This gives a corresponding value of 96 s for a water depth of 800 m by applying the stiffness and truncation factors. This compares very well with the natural period of the computed Grouped SLOR assembly in surge direction, which were 77 s and 102 s for water depths of 555 m and 800 m respectively.

The above results show that the model natural periods compared very well with those determined using the finite element programme. This adds confidence to the model test and its set up.

Test Results The Grouped SLOR model was tested at different current velocities, ranging from 0.17 m/s to 1.71 m/s by towing at a constant speed (over the water column), ensuring that 20 to 40 vortex induced motion (VIM) test cycles can be obtained. The flow angles of 0, 10, 30, 45 and 90 degrees were tested as illustrated in Figure 9a for 0 degree and Figure 9b for 90 degree flow angles.

(a) (b)

Current Current

Figure 9 Grouped SLOR Tested for (a) 0 Degree and (b) 90 Degree Flow Angles

Yaw and Surge Responses The frame yaw motion is the dominant mode of response for in-line conditions (i.e. 0 and 10 degree flow angles). For cross flow conditions (90 degree flow angle), surge is the dominant mode.

A clear onset of the frame yaw response can be observed in Figure 10 at a reduced velocity of approximately Ur = 7 (Ur = UT/D, where U is the current velocity, T is the natural period and D is the air can diameter) corresponding to a model current velocity of 0.5 m/s (or 0.4 m/s for water depth of 790 m). At lower current velocities, the yaw response is very small or negligible. The largest response is observed for the 0 degree and 10 degree flow angles, which may be due to the influence of the wake field from the downstream riser.

The onset of the frame surge response for the 90 degree current flow, occurred at a reduced velocity of approximately Ur = 5 as indicated in Figure 10. The response increases with increasing reduced velocity up to a maximum of 1.4 m at Ur = 10 and then decreases for higher reduced velocities of up to Ur = 25. Noticeable pitch response of the individual air cans relative to the guide frame was observed. The air cans articulated in roll and pitch motions around the receptacle on the guide frame.

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Yaw

Surge

Figure 10 Frame Yaw Motion and Surge Motion with Respect to Reduced Velocity and Flow Angle

Air Can Response It was observed for current velocities between 0.5 m/s and 1.0 m/s, the response seems to be dominated by the vortex shedding from the air cans. For higher current velocities (1.0 m/s to 1.7 m/s), it was possible that other types of instabilities caused by the downstream wake may contribute. However, there were no indication of high amplitude motions or galloping observed. A reduced velocity, Ur = 5 can be considered as a good indication on the onset of the VIV as shown in Figure 11.

Figure 11 Air Can Motion with Respect to Reduced Velocity and Flow Angle

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Optimium SLOR and COR Arrangement Two alternative configurations as mentioned in Table 1 were tested to determine the optimum SLOR and COR arrangement, namely Series 9 with the COR at the ends, and Series 10 with the COR at the ends and tether tension increased from 51 Te to 108 Te.

Locating the COR at the ends of the guide frame has an adverse effect on the yaw response of the Grouped SLOR assembly. The maximum yaw response of around 3.5 degree is observed at a low current velocity and at reduced velocity Ur above 10. The increase in tether tension (Series 10) did not help to reduce the yaw response.

The above results suggest, placing the two CORs at the middle of the guide frame and the SLOR at each end of the guide frame is the optimium Grouped SLOR arrangement for combined SLOR and COR.

Individual SLOR Comparison The motions at the receptacle for the Grouped SLOR model were assumed to be representative of the individual SLOR air can as discussed in MARIN (2008). These motions subsequently normalised to the diameter of the air can to determine the amplitude ratio. The results given in Figure 12 show that largest air can motions occurred at 90 degree flow angle at Ur between 7 and 14 (corresponds with current velocities between 0.5 m/s and 1.0 m/s). For 0 degree flow angle, air can motions increase with increasing current velocity. Smallest air can response is observed at 45 degree flow angle.

The measured responses at the receptacles were compared with the results carried out by MARIN research tests on an individual SLOR, refer to Wilde (2007). The comparison results given in Figure 12, clearly show that the air can motions of Grouped SLOR are significantly lower (more than two times) than that of an individual SLOR.

Figure 12 Air Can Response between Grouped SLOR and Individual SLOR Installation Assessment To confirm the flexibility and feasibility of installing the Grouped SLOR system by towing, reel laying and J-laying, Subsea 7 has carried out detailed installation strategies coupled with installation analysis. The results confirm that the three methods of installation are feasible with stress and fatigue within allowable limits. The analysis results also define the installation window, rigging arrangements, and vessel and crane requirements.

Foundation Installation

The single suction pile required for each SLOR foundation and the two smaller suction piles required for each guide frame can be installed from either Subsea 7’s vessel or a local support vessel. The foundations are positioned such that the SLORs splay out at the seabed, increasing the 5 m spacing at the frame to approximately 25 m spacing at the seabed. This increases stability as well as allowing for suction pile relocation if required. The 25 m spacing allows for ease of installation of flowline PLETS and rigid spools and avoids the congestion typically associated with hybrid riser bundles where the riser base is heavily congested.

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Guide Frame Installation The guide frame complete with buoyancy tanks, riser stem receptacles and tethers as depicted in Figure 2 will either be towed out to site or lifted off the back of the barge.

Once the guide frame is at the required position, it is lowered into the water by the main crane. The tethers, lead chain, ballasting chain and other necessary riggings are subsequently attached on the guide frame from the installation vessel and tug boat in preparation to lower the frame to the specific depth (typically between 50 m to 200 m below mean sea level). The descent of the frame is controlled using the length of the ballasting chain until the required depth is achieved, and an ROV is used to connect the tether with chain attachment to the suction pile as illustrated in Figure 13. Since the buoyancy tanks are sealed, there is no requirement for additional ROV intervention and the ballast chain can simply be released from the frame.

Figure 13 Installation of Grouped SLOR Frame

SLOR/COR Installation

The SLOR/COR can be installed by one of the three methods, namely towing, J-laying and reel-laying without compromising the integrity of the riser system.

Towing of SLOR/COR

The towing method is divided into three operations, i.e. surface tow to the field location, followed by removal of the buoyancy modules along the riser length, and finally the upending operation.

The surface tow operation requires either one or two tug boats depending on environmental conditions and tug boat limitations, while the upending operation requires two tug boats working in close coordination. This method has the main advantages of assembling, inspection, checking and testing the complete SLOR (including COR) system on the spool base (or assembling base) before loading out. This significantly reduces the overall cost on installation by saving on vessel time, riser assembling, welding and testing as compared with J-lay and reel lay. Subsea 7 has the added advantage of bundle towing

Step 1 Step 2

Step 3 Step 4

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knowledge which will add confidence to the project. The additional fabrications, such as buoyancy modules and piggy-back blocks, all of which being recyclable, will also be highly favourable in terms of the local content contractual agreement.

The disadvantages of this method of installation are the tug boat, rigging and buoyancy module requirements, potential fatigue issues and the location of the spool base relative to the installed field location. Planning of tow route access has to be carried out and clearance obtained from the port authority.

J-Laying of SLOR/COR Subsea 7’s Seven Seas installation vessel can be employed for J-laying the SLOR/COR system by lowering the riser system through the moonpool using the abandonment and recovery (A&R) wire. Cross-haul operation (if required) is subsequently carried out to transfer the riser system to the side of the vessel by utilising the main crane and auxiliary crane as shown in Figure 14. Supply boat is required to bring the pipes and components onto the vessel

Figure 14 Cross Haul Operation during J-Laying

The riser assembly will be constructed by welding the double-joint pipes and associated components. Welding, field joint coating, non destructive test (NDT), anode attachment, etc. are carried out on the vessel, which will impact the installation time. Installation of the COR system will add to the complexity of this method of construction.

The main advantage of J-laying is that the installation field is not restricted to the location of the spool base, which may influence the riser stress and fatigue. It also has the flexibility of installing large diameter risers, without any issues on straightening and ovality. No route access is required from the port authority. The main disadvantages of this method of installation are the time taken and associated cost to assemble the riser system.

Analysis carried out shows that optimisation of the riser system to obtain an effective tension of the order of 90 Te for the COR system is required to avoid snatch load (compression) on the A&R wire.

Step 1 Step 2

Step 3 Step 4

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Reel Laying of SLOR/COR The reel lay operation is carried out using the Subsea 7’s Seven Oceans installation vessel as depicted in Figure 15. The operation is similar to installing a flowline or a steel catenary riser (SCR), except with the attachment of bottom and top assemblies and air cans.

The pipelines are assembled on the spool base and reeled, complete with field joint coating, non destructive examination (NDE), qualification assessment and checks and other pre-commissioning requirements.

Reeling assessment has to be carried out to ensure that the pipe wall thickness is within the allowable strain based on the project’s agreed specifications. No problems are envisaged for reel laying the SLOR. However, for COR (effectively a pipe-in-pipe configuration) system, additional precautions will be necessary to ensure the limits of the straighteners and tensioning system are not exceeded.

The main advantage is that assembling and reeling of the riser pipes can be carried out in a controlled manner, thereby reducing the risk of damaging the pipes and field joint. Fatigue is also not envisaged to be an issue based on preliminary analysis performed to date. There will be high usage of local content and manpower in the assembling and reeling the riser. The disadvantages are the crane limitation, viability of reeling a COR and potential constraints in sizes of top and bottom assemblies.

Figure 15 Reel Laying of SLOR

Mating of SLOR/COR with Guide Frame The SLOR/COR is mated with the guide frame in stages as shown in Figure 16. A pre-installed portable or permantly mounted sheave is attached to the guide frame ready for mating the SLOR (including COR) to the guide frame. An anchor handling tug (AHT) deploys the pull-in wire (with ballasting chain at appropriate length) over vessel stern. The ROV then connects the pull-in wire onto the riser (above the air can) as shown in Figure 16, and ensuring the pull-in wire is placed onto the guide frame’s sheave. The pull-in wire is continuously paid out until the ballasting chain starts weighing down on the sheave and pulling the SLOR towards the guide frame. When the SLOR is in the frame receptacle, the ROV secures the receptacle clamp. The operation continues until all the SLORs are in the guide frame receptacle. This operation is simple without compromising the integrity of the Grouped SLOR system.

Flexible Jumper Installation

Once all SLORs (including CORs) are installed in the guide frame via the receptacle, the flexible jumpers are installed. The pre-installable benefit of the Grouped SLOR is then apparent. By installing all SLORs within the frame, and creating the Grouped SLOR arrangement without the flexible jumper, the system can be put in place prior to the arrival of the production vessel (typically FPSO), thus taking the riser off the critical path.

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The gooseneck complete with flexible jumper and bend stiffener is then lowered using a vessel and an ROV to activate the connector. The flexible jumper is then reeled out and passed to the FPSO where it is pulled in to the I-tube or J-tube and terminated.

The Grouped SLOR design offers significant FPSO payload reduction and operational benefits compared to other free hanging arrangements. The FPSO is only required to support part of the weight of the flexible jumper as the riser is self-supporting, thus increasing the available FPSO deck capacity. The de-coupling of the vessel with the riser via a flexible jumper also results in increased riser stability and fatigue life, and reduced riser stress during extreme storm events.

Figure 16 Mating SLOR/COR with Buoyant Frame Conclusions The Grouped SLOR system is a well developed arrangement and provides an attractive riser solution to congested fields where space is limited but high productivity is paramount. It offers all the benefits of the freestanding SLOR (including COR), whilst incorporating the multi-line advantages of the hybrid bundle without adding to the complexity of the subsea layout, improving both flowline routing and the capacity for future tie-ins.

The qualification programmes have explicitly confirmed the robustness in the design and concept of the Grouped SLOR, and the installation feasibility. The results have shown that the stability and performance are much better than a stand alone SLOR without any clashing and interference issues. With the field proven record in stress response, fatigue and operational performance of individual SLOR, the Grouped SLOR offered further field architecture optimisation to accommodate complex and congested seabed layouts in the ever growing trend of deep and ultra deepwater environments.

Step 1 Step 2

Step 3 Step 4

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The model test of the complete Grouped SLOR assembly indicates the following conclusions: • There were no instabilities, i.e. large amplitude motion and galloping were not observed. • The riser VIV and air can VIM are largely uncoupled since they occur in distinctly different natural period ranges,

i.e. 10 seconds and 100 seconds respectively • Vortex induced response of the air cans for the Grouped SLOR assembly was significantly lower (more than two

times) as compared with individual SLOR due to the presence of guide frame and tether system • The test campaign confirms the robustness of Grouped SLOR concept and design, which is applicable for deep to

ultra deepwater harsh environments (e.g. Gulf of Mexico).

References Bridge C., Dale N., Hatton S. and Karunakaran D. 2007; “Hydrodynamic Properties of the Grouped SLOR using CFD and Model Tests”,

Proceedings of ISOPE-2007, Lisbon, Portugal, 2007 Dale N., Karunakaran D. and Hatton S. 2007; “The Grouped SLOR – Design and Implementation”, Proceedings of OMAE 2007-29461, San

Diego, 2007. Karunakaran D., Lee D., Hatton S., Dale N. and Mair J. 2007; “Grouped SLOR Deep Water Riser System and Installation Assessment”,

Proceedings of Deep Offshore Technology Conference. Stavenger, Norway, 2007. Karunakaran, D., Lee D, Mair, J and Wilde, J. 2008; “Model Test of Complete Grouped SLOR Deep Water Riser System”, Proceedings of

Deep Offshore Technology Conference. Perth, 2008. Wilde J., “Model Tests on the Vortex Induced Motions of the Air Can of a Free Standing Riser System in Current”, Proceedings of Deep

Offshore Technology Conference. Stavanger, Norway, 2007.