OTC 24166

Design of Steel Lazy Wave Riser for Disconnectable FPSO Jingyun Cheng, and Peimin Cao, SBM Offshore

Copyright 2013, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 69 May 2013. 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 disconnectable Floating Production Storage and Offloading system (FPSO) is one of the preferred solutions for deepwater fields in harsh environments and far away from existing pipeline infrastructures. This paper presents a design of steel lazy wave riser (SLWR) system for an internal turret moored disconnectable FPSO in the Gulf of Mexico. The integrated systems of FPSO, disconnectable buoy, riser, and mooring are discussed while focusing on the design challenges of SLWR systems. Due to the complexity of SLWR geometry, a systematic configuration approach is introduced based on buoy payload and riser performance criteria. The study includes the strength and fatigue analysis of production, gas export and water injection risers for the connected, disconnecting, and disconnected conditions. It concludes that SLWR with disconnectable FPSO is a feasible and cost effective solution for deepwater field development in the Gulf of Mexico. The study demonstrates the importance of an integrated design approach, and provides guidance for configuring and design of future disconnectable systems with SLWRs. Introduction The disconnectable Floating Production Storage and Offloading system (FPSO) is one of the preferred solutions for remote fields in harsh environments in the Gulf of Mexico, South China Sea, offshore West Australia, and offshore arctic region from both technical and commercial aspects. The advantage of a disconnectable system compared to a permanent system is that the mooring system does not have to be designed to accommodate the economically penalizing severe loadings associated with hurricane and typhoon conditions. It allows rapid vessel removal for maintenance or upgrade. It also enables phased development due to production uncertainty, which reduces reservoir risk. During the past decades, the disconnectable FPSOs have been deployed successfully using flexible riser and free standing hybrid riser technology in shallow to deepwater developments [Refs.1, 2, 3]. Compared with flexible riser and especially with hybrid riser, steel catenary riser (SCR) is simple and cost effective for deepwater development, particularly under high pressure, high temperature and sour service conditions. It reduces the risks to delivery schedule and lifetime operation due to additional riser components. SCR has been the preferred riser solution for deepwater floating production facilities in the Gulf of Mexico. It has also been used in the permanent moored FPSOs for relatively benign environment offshore West Africa. However, SCR is very sensitive to vessel heave motion, and thus may not be feasible for FPSO in the harsh environment. Steel lazy wave riser (SLWR) has been recently used to decouple vessel motion under such conditions, e.g., offshore Brazil [Ref. 4]. With many lower tertiary fields discovered in the Gulf of Mexico far from the existing oil export pipeline infrastructures, it is important to investigate the feasibility of SLWR for disconnectable FPSO while using existing field proven SCR technology and maintaining commercial competiveness. Internal turret moored FPSO has been considered in this study. External turret system of the MoorSparTM type is a viable alternative to enable SCRs [Ref. 5]. This paper presents a disconnectable mooring and riser system while focusing on the design of SLWRs. An internal disconnectable buoyant turret mooring system (BTM) with full weathervaning capability has been developed to moor an FPSO in the Gulf of Mexico under winter storm and loop current conditions. In the event of hurricane threat, the BTM and risers can be quickly released to a predetermined depth to avoid damage and the FPSO sails away to a safe area. The FPSO will be reconnected with BTM and risers to resume production after passage of the hurricane. The paper focuses on several technical challenges facing SLWR design, such as payload limitation, riser strength and fatigue performance during connected, disconnecting, and disconnected conditions. The disconnecting condition refers to a transient process from buoy

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release till its final equilibrium position. A systematic configuration approach is introduced. Integrated buoy, riser and mooring design is required, especially for disconnecting and disconnected conditions. The design of SLWR system follows API standards and criteria. Its impact on buoy and mooring design is also discussed. System Description and Design Basis The field development scenario investigated in this study is located in the Gulf of Mexico at a water depth of 6,000 ft. The FPSO has a processing capacity of 100,000 BOPD with 1.0 million barrels of oil storage capacity. The system design life is 20 years. The field development consists of four (4) production risers, one (1) gas export riser, one (1) water injection riser and four (4) umbilicals. The maximum production well pressure is 7,500 psi, and the maximum water injection riser pressure is 10,000 psi. The generic central Gulf of Mexico metocean data [Ref. 6] is used in the study as summarized in Tables 1 to 3. Generic long term current data is not available, but the risers are assumed to be covered with sufficient VIV suppression devices. A disconnectable BTM system with 360 weathervaning capability is chosen for the study. Based on the design operating philosophy, the vessel will stay connected with its mooring and riser system up to 1,000-year winter storm or 100-year sudden hurricane, or maximum loop current conditions. For any incoming hurricane threat, the FPSO will disconnect from its mooring and riser system and sail away to a safe area. The planned disconnect seastate criterion is significant wave height of 9.84 ft and peak wave period of less than 10 seconds. Emergency disconnect at high seastate with large vessel excursion is also evaluated. Based on the required production rate, a double hull Suezmax conversion vessel is selected as the candidate FPSO. The main particulars are summarized in Table 4. The turret center is located 15% LPP from the bow. The turret diameter is 22 m. The lower part of turret is connected to a BTM buoy through structure connectors. The BTM buoy keel will be at approximately 65 m below MWL at the disconnected condition.

Table 1 Typical Gulf of Mexico Metocean Data 10 year

Winter Storm

100 year Winter Storm

1,000 year Winter Storm

100 year Sudden

Hurricane

100 year Hurricane

1,000 year Hurricane

Significant Wave Height Hs (ft) 18.1 28.9 36.1 26.2 51.8 65.0 Peak Spectral Period Tp (s) 10.0 13.0 14.0 12.2 15.4 17.2 Peak Enhancement Factor 1.0 2.0 2.0 2.0 2.4 2.4 1-hour Mean Wind Speed Vw (ft/s) 67.0 88.0 108.0 95.5 157.5 196.9 Surface Current Speed Vc (ft/s) 1.8 3.8 4.5 4.8 7.9 9.8

Table 2 Max. Loop Current Data (Associated Wave Hs=4.0 ft, Tp=6 sec)

Water Depth (ft) Current Speed (ft/s) 0 6.90

82 6.90 164 6.82 328 5.02 492 3.74 656 2.94 820 2.40

1,060 1.87 1,148 1.74 1,312 1.54 1,640 1.15 1,968 0.95 2,624 0.53 3,280 0.44

Seabed 0.00

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Table 3 Typical Gulf of Mexico Fatigue Seastates Fatigue

Bin Hs (ft) Tp (s) Vw (ft/s) Vc (ft/s) Probability of

Occurrence (%) 1 0.75 4.5 16 0.5 18.034 2 2.5 5.5 20 0.65 39.241 3 4.5 5.5 24 0.75 11.884 4 4.5 7.5 24 0.75 12.403 5 6.5 6.5 28 0.9 5.907 6 6.5 8.5 28 0.9 5.081 7 8.5 8.5 34 1.1 4.210 8 11.0 9.5 38 1.3 2.162 9 13.75 9.5 46 1.5 0.518

10 17.5 11.5 56 1.8 0.473 11 22.5 12.5 72 3.1 0.068 12 27.5 12.5 88 3.8 0.014 13 32.5 14.5 104 4.4 0.003 14 37.5 14.5 116 5.2 0.003

Table 4 FPSO Main Particulars

Parameter Unit Ballasted Fully Loaded Length between Perpendiculars (LPP) (m) 264.0 Breadth (m) 48.0 Depth (m) 23.2 Turret Location (from midship) (m) 92.4 Draught (m) 8.81 18.83 Displacement (te) 88,977 173,683 Longitudinal Center of Gravity from aft Perpendicular (m) 142.99 132.72 Vertical Center of Gravity (KG) (m) 12.33 14.73 Radius of Gyration in Roll (m) 18.28 15.12 Radius of Gyration in Pitch (m) 75.51 63.82 Radius of Gyration in Yaw (m) 76.20 64.37

A 3 x 3 group mooring system is designed based on API RP-2SK and API RP-2SM. The maximum vessel design offset is 6% and 8% of water depth for intact and one (1) line broken conditions, respectively. In order to reduce the mooring payload, a chain-polyester-spring buoy-chain mooring leg configuration is adopted. One of the most onerous and complex design aspects for the disconnectable system is the reconnection process. Following several design iterations of buoy and mooring, it is decided to limit riser system payload to 1,000 metric tons to assure that buoy design and its reconnection is practical and feasible. From riser design perspective, reconnection is a controlled quasi-static lifting process; therefore, its impact is minimal and will not be elaborated in this paper. SLWR Configuration Philosophy for Disconnectable FPSO The SLWR is a steel catenary riser with syntactic foam modules added to the middle section of the riser to decouple the floater dynamic motion from the touchdown of riser and also to reduce riser payload. As a result, the strength and fatigue performance of the riser is significantly improved. However, deepwater syntactic foam buoyancy modules are very expensive and difficult to install. Therefore, it is a crucial design tradeoff to incorporate the minimum amount of buoyancy relatively close to the sea bed, but still sufficiently decouple vessel motions from riser system. For a disconnectable system, riser payload limit is also a main design constraint to affect the amount of syntactic foam buoyancy required. Due to its complex geometry and additional design parameters, SLWR has attracted continuous efforts from both academic and industries to optimize its configuration. A practical design approach based on intuitive observation is introduced here. This approach allows efficient configuration of a lazy wave shape that meets the riser performance target. A typical SLWR as shown in Figure 1 consists of the four sections: an upper catenary section, a middle buoyancy section, a lower catenary section, and a bottom section. Table 5 lists the parameters and variables used to define the SLWR geometry.

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Table 5 SLWR Parameters & Variables Parameters

Water depth The horizontal force for SLWR system The submerged weight for upper catenary section The submerged weight for middle buoyancy section The submerged weight for lower catenary section 1,6 Segment length for each segment 1,6 Scope for each segment

The upper catenary section length The lower catenary section length Variables

Equivalent riser payload water depth Departure angle The middle buoyancy section length

Figure 1 Sketch of Steel Lazy Wave Riser

The static cable solution of the above SLWR shape can be defined by the following three equations: sinh sinh

(1)

sinh sinh (2)

cosh cosh cosh

(3)

Where ;

;

are the minimum local radii of curvature at the sag bend, arch bend, and touchdown

locations, respectively. The buoyancy ratio is an important design parameter and is defined as 1 1

(4)

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The above system has a determined solution for known departure angle , equivalent payload water depth , and buoyancy section length. The equations also reflect the equilibrium of weight and buoyancy. The net buoyancy of the segment lifts the weight of the segment . The net buoyancy of the segment lifts the weight of the segment . Therefore, the riser payload equals to the weight of segment part of upper catenary riser, which is governed by . To determine the optimal SLWR geometry, an intuitive approach is introduced to quantify the decoupling effect. The approach uses riser touchdown point (TDP) movement per unit horizontal or vertical hang off motion as a measure to predict decoupling efficiency. It is obvious the less the TDP moves, the more efficient the system is. This definition can be better explained by previous studies [Ref. 7] that related TDP movement to the bending stress variation near the touchdown region. To further illustrate the idea, several parametric studies have been performed. Equations 1 to 3 are modified to compute geometry change due to offset at the hang off point. Figure 2 shows the TDP shift comparison between a SCR and a SLWR. The slope of the line defines the TDP movement per unit offset. It is shown that the SLWR has significantly less TDP movement than the SCR for the same departure angle. It can also be seen that both riser configurations will be more sensitive to heave motion than surge motion. This explains why riser fatigue life of the turret moored FPSO is governed by porch heave motion. Figure 3 compares SLWR decoupling efficiency for various buoyancy lengths with the same departure angle and equivalent riser payload water depth. It shows that longer buoyancy configuration provides more decoupling as expected. Figure 4 compares SLWR decoupling efficiency for various equivalent riser payload water depths with the same departure angle and buoyancy length. It shows that deeper equivalent riser payload water depth configuration provides more decoupling. To validate the above hypothesis, wave fatigue analysis is performed for an 8-inch riser with the various SLWR configurations. Figure 5 shows TDP fatigue life from FLEXCOM analysis for various SLWR configurations. The fatigue results confirm that TDP movement is strongly correlated to the fatigue life of touchdown region, and thus a good measure to predict SLWR decouple efficiency. A detailed correlation study can be performed based on the methodology proposed in [Ref. 7].

Figure 2 Decoupling Efficiency between SLWR vs. SCR

Figure 3 Decoupling Efficiency for SLWRs with Various Buoyancy Section Lengths

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Figure 4 Decoupling Efficiency for SLWRs with Different Payloads

Figure 5 TDP Fatigue Life Comparison for Various SLWRs Configurations

Based on the above study, the following procedure has been adopted to configure a SLWR for a disconnectable system. Step 1). The riser hang off angles and azimuth angles are selected to accommodate project specific subsea layout, mooring

offset balance, buoy trim balance, and interference considerations. Step 2). Determine equivalent payload water depth based on riser payload limit allowed by the buoy. Step 3). Perform parametric buoyancy length study to determine the decoupling effect to meet the performance target. Step 4). Other important factors such as maximum hang off angle, VIV fatigue, riser strength and fatigue at hang off and

buoyancy regions need to be confirmed during detail analysis.

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OTC 24166 7

Description of SLWR System The key design data for various SLWRs considered in this study are summarized in Table 6. The riser wall thickness is calculated based on the pressure requirement per CFR and API RP 2RD. The production risers are also covered with thermal insulation layer for flow assurance considerations. All risers will be covered by fairings approximately 90 percent of upper suspended length to suppress VIV. Fairings are selected to reduce drag force from current loading and from buoy descent velocity during disconnect. It is also prudent to add VIV suppress devices near the lower catenary section when strong near seabed current exists. The overall payload from riser and umbilical system is about 1,000 metric tons. The equivalent payload water depth is 75% for production and gas export risers, and 70% for water injection riser. Figure 6 illustrates the riser and umbilical system with FPSO and BTM mooring system.

Figure 6 Illustration of the Disconnectable SLWR System

Table 6 SLWR Design Data

8 Production 6 Gas Export 10 Water Injection Pipe OD (inch) 8.625 6.625 10.75 Pipe WT (inch) 1.0 0.625 1.30 Material Grade X70 X70 X70 Corrosion Allowance (inch) 0.1574 - - Dry Weight Tolerance +7% +7% +7% Thermal Insulation or Coating Thickness (inch) 3 0.018 0.018

Thermal Insulation/Coating Density (lb/ft3) 50.0 90.0 90.0 MAOP (psi) 7,500 3,625 10,000 Content Density (lb/ft3) 55.0 19.8 64.0~69.25 Departure Angle (deg) 6 6 6 Top Termination Unit Flex Joint Flex Joint Flex Joint Equivalent Riser Payload Water Depth (ft) 4,500 4,500 4,200 Upper Catenary Section Length (ft) 5,400 5,190 5,026 Middle Buoyancy Section Length( (ft) 1,800 1,500 2,200 Lower Catenary Section Length ( (ft) 1,398 1307 1,398 Buoyancy Ratio 2.0 2.0 1.8 Buoyancy Material Density (lb/ft3) 36.7 36.7 36.7 VIV Suppression Coverage 90% 90% 90%

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Load Case Matrix The full riser design load case matrix includes connected, disconnecting, disconnected, and reconnecting conditions. Omni-directional environments are considered in this study. To reduce the amount of analysis work, a screening analysis is performed for various vessel drafts, vessel headings, riser porch locations and azimuth angles. In general, ballasted draft condition tends to induce more vessel dynamic motions than fully loaded draft condition. The BTM mooring allows the vessel to weathervane. For simplicity, vessel headings between 0 and 30 degrees are compared. A heading of 30 degrees is found more onerous than the head sea, and is used for riser strength design. Based on the results of the screening analysis, the critical load cases and acceptance criteria for the riser strength design are detailed in Table 7. It is noted that accidental conditions such as buoy or FPSO compartment flooded cases will be considered in the detailed design. The impact of buoy damage can be reduced by its compartmentation design.

Table 7 Riser Strength Design Load Case Matrix

FPSO/buoy Connection Environment Mooring Condition Load Category API Stress Criteria

Connected 10-year Winter Storm Intact Operating 0.67 Connected 100-year Winter Storm Intact Extreme 0.80 Connected Max. Loop Current Intact Extreme 0.80 Connected 100-year Winter Storm One mooring line broken Survival 1.00 Connected 1000-year Winter Storm Intact Survival 1.00 Disconnected 100-year Hurricane Intact Extreme 0.80 Disconnected 1000-year Hurricane Intact Survival 1.00 Planned Disconnect Hs=3m Intact Operating 0.67 SLWR is modeled using nonlinear finite element software FLEXCOM. Riser buoyancy modules are modeled as continuous section with equivalent mass and hydrodynamic properties. Table 8 lists the riser internal fluid properties for various conditions. For emergency disconnect, full bore pressure is assumed.

Table 8 Riser Internal Fluid Properties

Load Case Production Gas Export Water Injection

Density (lb/ft3)

Pressure (psi)

Density (lb/ft3)

Pressure (psi)

Density (lb/ft3)

Pressure (psi)

Operating 50 2,500 19.8 3,625 69.0 10,000 Extreme 50 7,500 19.8 3,625 69.0 10,000 Survival 50 7,500 19.8 3,625 69.0 10,000 Planned Disconnect 50 n/a 19.8 n/a 69.0 n/a

The strength design of SLWRs is implemented in accordance with API RP-2RD. For the connected cases, the analyses for each riser are performed in the near, cross and far riser directions with multiple three (3) hours random dynamic simulations. The vessel motions are input using RAO approach. It is assumed that the hull offset is 5% of water depth for operating conditions, is 6% of water depth for extreme conditions, and is 8% of water depth for damaged and survival conditions. The observed extreme responses from all realizations are used to estimate the extreme expected value. For both disconnected and disconnecting cases, time traces motions generated by the coupled global performance software AQWA are imposed for the riser analysis. The fatigue design of SLWRs should include damages from connected, disconnecting, disconnected, vortex induced vibration, slugging, and installation. This study only focuses on wave loading fatigue from connected, disconnecting, and disconnected conditions. The target system design life for connected condition is conservatively set as 2,000 years including a factor of safety 10. This provides sufficient design allowance for other fatigue damage sources. For disconnecting and disconnected conditions, the system is designed to accommodate two (2) full hurricane events annually. To simplify the analysis scenario, 100-year hurricane condition with 72 hours duration is conservatively assumed. Time domain rain-flow cycle counting approach based on stress cycle (S-N) method is used to estimate the fatigue damage. API X curve with an SCF value of 1.2 is used for pipe to pipe welds. The fatigue damage at critical TDP, buoyancy section, hang off (1st offshore weld between flex-joint extension piece and riser pipe) are reported. Analysis Results The strength analysis results for production, gas export, and water injection risers are summarized in Tables 9, 10 and 11, respectively. It is found that all risers meet the API RP 2RD stress criteria. The main conclusions are discussed as follows:

For the connected condition, some minor compression has been observed at the lower end of upper catenary section for production riser and gas export riser during 1,000-year winter storm. The compression level remains low and

OTC 24166 9

will not cause any overstress and buckle of the riser pipe. This is induced by the large downward heave velocity at about 19 ft/s. The results confirm that maximum heave velocity is the key measure to assess FPSO steel riser feasibility [Ref.8]. The maximum flex joint rotation angle is less than 13 degrees, which is within the 20 degree design allowable. Typical riser von Mises stress distribution and effective tension distribution envelopes in the 100-year winter storm are plotted in Figure 7.

For the disconnected condition, the BTM buoy keel is submerged to a depth of 65 meters below MWL. The effect of the extreme hurricane wave loading is reduced by more than 60% at this water depth. Therefore, riser stress and flex joint angle are relatively low compared with the connected condition. It should be noted that current drag loading will significantly influence the buoy and riser displacement due to low mooring stiffness at this stage.

Buoy release is a complex transient process. Buoy response depends on many factors including buoy/turret configuration, mooring and riser loads, vessel offset, disconnect seastate and release time. Integrated buoy, mooring and riser system is analyzed using coupled hydrodynamic software, which is calibrated by model tests. Figure 8 compares the typical buoy heave and pitch motions for connected, disconnecting, and disconnected seastates. It can be seen that except for the quick initial descent, the buoy tends to have a large inclination angle when it exits from the turret moonpool. Based on the design iterations from the integrated analysis model, the buoy should be designed to limit its descent velocity to less than 13 ft/s and inclination angle to less than 14 deg in order to meet the riser and operation performance requirement. The riser results of planned disconnecting condition meet the above design limits. The vessel offset prior to release significantly affects the buoy velocity and angle. Based on the emergency disconnect evaluation, the vessel excursion limit is 2.5% of water depth. This requirement will be included in the operation philosophy.

Table 9 Strength Analysis Results for 8-inch Production Riser

Min Tension (kips)

TDP Stress (ksi)

/Utilization

Arch Stress (ksi)

/Utilization

Hang-off Stress(ksi) /Utilization

Max. Flex Joint Rotation Angle (deg)

10-yr Winter Storm (Connected) Operating 20.5 24.0 /0.51 27.3 /0.58 24.4 /0.52 2.8 100-yr Winter Storm (Connected) Extreme 9.1 43.0 /0.77 46.0 /0.82 40.6 /0.73 7.5 1000-yr Winter Storm (Connected) Survival -6.8 41.3 /0.59 48.8 /0.70 47.5 /0.68 12.8 100-yr Hurricane (Disconnected) Extreme 21.5 20.4 /0.37 23.2 /0.41 23.2 /0.39 3.1 1000-yr Hurricane (Disconnected) Survival 23.0 20.1 /0.29 23.0 /0.33 23.0 /0.34 4.4 Planned Disconnect 13.2 20.0 /0.43 23.0 / 0.49 20.9 /0.45 2.6 Emergency Disconnect -27.2 42.4 /0.61 44.5 /0.64 64.3 /0.92 14.5

Table 10 Strength Analysis Results for 6-inch Gas Export Riser

Min Tension (kips)

TDP Stress (ksi)

/Utilization

Arch Stress (ksi)

/Utilization

Hang-off Stress(ksi) /Utilization

Max. Flex Joint Rotation Angle (deg)

10-yr Winter Storm (Connected) Operating 9.8 18.6 /0.40 23.2 /0.49 24.7 /0.53 2.4 100-yr Winter Storm (Connected) Extreme 4.1 19.4 /0.35 28.1 /0.50 39.1 /0.70 6.2 1000-yr Winter Storm (Connected) Survival -3.6 22.3 /0.32 37.5 /0.54 60.8 /0.87 10.4 100-yr Hurricane (Disconnected) Extreme 11.5 16.0 /0.29 21.0 /0.37 22.1 /0.39 3.1 1000-yr Hurricane (Disconnected) Survival 10.6 16.0 /0.23 22.0 /0.31 24.2 /0.35 4.3 Planned Disconnect 9.5 18.9 /0.40 21.5 /0.46 19.8 /0.42 2.2 Emergency Disconnect -7.0 18.6 /0.27 32.9 /0.47 63.8 /0.91 13.2

Table 11 Strength Analysis Results for 10-inch Water Injection Riser

Min Tension (kips)

TDP Stress (ksi)

/Utilization

Arch Stress (ksi)

/Utilization

Hang-off Stress(ksi) /Utilization

Max. Flex Joint Rotation Angle (deg)

10-yr Winter Storm (Connected) Operating 44.8 42.8 /0.91 44.3 /0.94 38.0 /0.81 3.0 100-yr Winter Storm (Connected) Extreme 34.2 42.8 /0.76 45.0 /0.80 42.0 /0.75 7.3 1000-yr Winter Storm (Connected) Survival 20.6 43.8 /0.63 46.9 /0.67 45.4 /0.65 11.7 100-yr Hurricane (Disconnected) Extreme 50.4 41.5 /0.74 37.5 /0.67 36.7 /0.65 3.0 1000-yr Hurricane (Disconnected) Survival 48.7 41.1 /0.59 37.9 /0.54 37.0 /0.53 4.1 Planned Disconnect 50.0 24.9 /0.53 26.4 /0.56 23.7 /0.51 3.1 Emergency Disconnect 2.0 42.4 /0.61 43.6 /0.62 49.0 /0.70 16.8

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Figure7 8-inch Production Riser Max. API 2RD von Mises Stress and Effective Tension Envelope

Figure 8 Comparison of Buoy Motion Time Series

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Wave loading fatigue lives are computed for the three different risers based on FPSO ballasted draft (40% time) and fully loaded draft (60% time) for the connected condition. It is assumed the risers are in the near plan and vessel is at 30 degree heading. The estimated unfactored lives at critical locations of touchdown zone, buoyancy arch and hang-off zone are summarized in Table 12. Its found that all risers exceed fatigue target of 2,000 years. In general, fully loaded condition causes less fatigue damage rate than the ballasted draft condition. The touchdown zone is more critical than the buoyancy arch. For hang off zone, the fatigue damage can be mitigated by moving 1st offshore weld location further away from the hang off point. Fatigue life distribution along the 8 production riser is shown in Figure 9. The detailed damage breakdown is listed in Table 13. The majority of wave loading fatigue at the riser touchdown zone is caused by the median seastates, induced mainly by vessel heave and pitch motions. The majority of wave loading fatigue at the riser hang off zone is caused by the median to high seastates, induced mainly by vessel pitch and roll motions.

Table 12 Comparison of Unfactored Wave Loading Fatigue Lives for Connected Condition

Riser FPSO Draft Fatigue Life (yrs) Touchdown Zone Buoyancy Section Hang-off

8" Production Ballasted Draft 2,410 5,666 1,560 Fully Loaded Draft 3,507 8,400 3,092 Combined 2,966 7,040 2,220

6 Gas Export Ballasted Draft 2,791 3,892 3,000 Fully Loaded Draft 4,355 4,860 6,000 Combined 3,557 4,420 4,285

10" Water Injection

Ballasted Draft 2,357 6,332 2,234 Fully Loaded Draft 3,979 8,933 4,970 Combined 3,120 7,672 3,335

Table 13 Comparison of Fatigue Breakdown (8-inch Production Riser / Ballasted Draft)

Fatigue Bin

Hs (ft)

Tp (s)

Probability of Occurrence (%)

Touchdown Zone Buoyancy Section Hang-off (1st Offshore

Weld) Damage

Probability Damage Damage

Probability Damage Damage

Probability Damage

1 0.75 4.5 18.034 0.00% 2.39E-11 0.00% 7.28E-12 0.00% 6.88E-10 2 2.5 5.5 39.241 0.00% 4.91E-09 0.00% 1.52E-09 0.00% 1.35E-07 3 4.5 5.5 11.884 0.00% 1.31E-08 0.00% 4.07E-09 0.00% 3.62E-07 4 4.5 7.5 12.403 0.50% 2.05E-06 0.26% 4.64E-07 0.19% 2.54E-05 5 6.5 6.5 5.907 0.10% 4.33E-07 0.06% 1.14E-07 0.06% 8.11E-06 6 6.5 8.5 5.081 4.98% 2.07E-05 2.27% 4.00E-06 1.06% 1.45E-04 7 8.5 8.5 4.210 9.89% 4.10E-05 4.80% 8.48E-06 2.41% 3.28E-04 8 11.0 9.5 2.162 30.31% 1.26E-04 17.46% 3.08E-05 8.36% 1.14E-03 9 13.75 9.5 0.518 13.22% 5.49E-05 9.93% 1.75E-05 6.02% 8.20E-04

10 17.5 11.5 0.473 37.25% 1.55E-04 49.19% 8.68E-05 47.06% 6.41E-03 11 22.5 12.5 0.068 1.03% 4.26E-06 2.40% 4.23E-06 3.06% 4.16E-04 12 27.5 12.5 0.014 0.30% 1.24E-06 1.19% 2.11E-06 1.60% 2.18E-04 13 32.5 14.5 0.003 1.00% 4.15E-06 4.31% 7.61E-06 9.24% 1.26E-03 14 37.5 14.5 0.003 1.42% 5.89E-06 8.12% 1.43E-05 20.93% 2.85E-03

Sum 100% 100% 100% 100%

12 OTC 24166

Figure 9 8-inch Production Riser Wave Loading Fatigue Life along Riser

Table 14 presents fatigue results for disconnecting and disconnected conditions. Annual fatigue damage includes the total damage for BTM disconnecting, and 72 hours disconnected conditions from two 100-year hurricane events. The reconnection damage is negligible. Coupled motion time traces are used to predict the buoy and riser responses. It can be seen that the fatigue damage is significant and comparable with those from the connected seastates. It should be noted that the storm condition and its duration are conservatively assumed in the study. Further increase of buoy submerged depth can also mitigate riser fatigue damage.

Table 14 Disconnecting and Disconnected Fatigue Results

Riser 8 Production 6 Gas Export 10 Water Injection

Touch Down Zone

Hang Off Zone

Touch Down Zone

Hang Off Zone

Touch Down Zone

Hang Off Zone

Planned Disconnect Event Damage 2.75e-6 1.23e-5 2.12e-6 3.29e-6 2.51e-5 1.50e-5 100-yr Hurricane Disconnected Event Damage for 72 hrs Duration 2.13e-4 3.42e-4 2.81e-4 1.77e-4 1.63e-4 7.15e-5

Annual Fatigue Damage Total (2 events) 4.32e-4 7.09e-4 5.66e-4 3.60e-4 3.31e-4 1.73e-4 Unfactored Fatigue Life (Years) 2,312 1,411 1,765 2,776 3,022 5,779 Conclusions This paper has presented the feasibility design of a SLWR system for a disconnectable FPSO with BTM in the 6,000 ft water depth in the Gulf of Mexico. An integrated approach is required to design the buoy, mooring and riser systems. With the focus on the SLWR configuration and design, the key conclusions are as follows:

An efficient procedure is presented to systematically configure SLWRs for disconnectable FPSOs. SLWR touchdown movement per unit hang off offset is a good measure to predict the decoupling efficiency. It is

strongly correlated to the fatigue life of touchdown region. SLWR for the disconnectable BTM can be configured based on buoy payload and target riser performance. Based on the analysis results, internal turret moored FPSO with SLWR is feasible to be connected under maximum

winter storm and loop current conditions in the Gulf of Mexico. The fatigue life for connected seastates exceeds the design target based on proposed procedure.

The riser system will impose design constraints for buoy descent velocity and inclination angle. For emergency disconnect, the vessels excursion is limited by these design constraints. The fatigue damage during disconnecting and disconnected conditions is significant and comparable with that in the connected seastates.

In summary, the study demonstrates that the disconnectable FPSO with SLWR system is a feasible and cost effective solution for the deepwater development in the Gulf of Mexico. The integrated approach presented in this paper can also be used for similar applications in other areas.

10

100

1000

10000

100000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

Fatig

ueLif

e(yrs)

TDPHangoff

SLWRforDisconnectableFPSO WaveFatigue/BallastedDraft

8"Prod 30degHeading

HangoffTDP

OTC 24166 13

Acknowledgements The authors would like to acknowledge permission from SBM Offshore to prepare and publish this work. The authors would like to thank Carlos Mastrangelo, Jingxi He, and Sherry Xiang for their review and valuable comments. Abbreviation Lists BOPD Barrels of Oil Per Day BTM Buoyant Turret Mooring FPSO Floating Production Storage and Offloading MAOP Maximum Allowable Operating Pressure MWL Mean Water Level LPP Length between Perpendiculars OD Outer Diameter RAO Response Amplitude Operator SCF Stress Concentration Factor SCR Steel Catenary Riser SLWR Steel Lazy Wave Riser TDP Touch Down Point VIV Vortex Induced Vibration WT Wall Thickness Reference 1. Mace A.J, Hunter K.C (1987) Disconnectable Riser Turret Mooring System for Jabirus Tanker-Based Floating

Production System, OTC 5490. 2. Nion G.O, Calo D, Seguin R, Huang S (1990) Innovative Disconnectable Mooring System for Floating Production

System of HZ-21-1 Oil Field at Huizhou, South China Sea, OTC 6251. 3. Masson C, Carter R.H, Streit P, Delepine Y (2011) Cascade and Chinook Subsea Development, The Worlds Deepest

Production Risers, OTC 21857. 4. Hoffman J, Yun H, Modi A (2010) Parque das Conchas (BC-10) Pipeline, Flowline and Riser System Design,

Installation and Challenges, OTC 20650. 5. Banon H, Lavagna P, Connaulte X, (2012) Ultra Deepwater Mooring & SCR Solution for Disconnectable FPSOs, OTC

23500. 6. API Bulletin 2INT-MET (2007) Interim Guidance on Hurricane Conditions in the Gulf of Mexico. 7. Aranha J.A.P, Martins C.A., Pesce C.P. (1997) Analytical Approximation for the Dynamic Bending Moment at the

Touchdown Point of a Catenary Riser, International Journal of Offshore and Polar Engineering. 8. Connaulte X, Lavagna P, Schuurmans S (2009) Steel Catenary Riser Feasibility Prediction for Ultra Deep Water FPSO

Applications, DOT.