OTC 21551

Design of Deepwater Tower Riser for West of Africa Application Jing Cao, CNOOC Research Institute, Beijing, China

Copyright 2011, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 25 May 2011. 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 As one of the field proven riser concepts, tower riser is a popular concept in particular for deepwater application in the West of Africa (WoA). This paper addresses on the design exercise of a deepwater tower riser in WoA. A brief presentation of deepwater hybrid riser history and evaluation is given first. Secondly, design basis for the tower riser study is described. Thirdly, design of the tower riser has been focused on including logistical design procedure, major design issues, and critical factors affecting the design, global configuration design, connection design, and local component design. To verify the design and to provide input for local component sizing, global analysis was performed and results were presented as the fourth part of this paper using riser tool and ABAQUS. Lastly, conclusions are drawn from this study and recommendations are made.

1. INTRODUCTION In recent years, exploration and production activities have increased dramatically in deep and ultra-deep water.

The targeted water depths for oil and gas developments in areas such as the Gulf of Mexico (GoM), West of Africa (WoA), and Brazil are increasing every year.

As one of the key elements in offshore infrastructure, deepwater riser technology plays an important role in offshore development. Among the field proven riser concepts, which include steel catenary risers (SCRs), top tension risers (TTRs), flexible risers (FRs), and hybrid risers (HRs) as shown in Figure 1-1, hybrid risers offer unique advantages over other concepts for specific applications and requirements. Even though hybrid riser concept initiated and started in the GoM, its popularity in deepwater development starts in the WoA with the deployment of the Girassol tower riser followed by Kizomba single line hybrid riser, Greater Plutonio tower riser, etc.

Figure 1-1: Deepwater Riser Concept Illustration

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In Oct. 2007, the first hybrid riser was successfully developed and deployed in the Campos Basin in Brazil as oil export for the P-52 PDET project. In early 2010, the five disconnectable hybrid risers were deployed successfully for the Cascade and Chinook field development in the GoM [Ref. 3]. Hence, the hybrid riser concept has gained wide recognition within the deepwater offshore industry over the last years and it is likely to remain one of the most attractive field proven riser concepts.

Although there are different versions of the hybrid riser and its configuration has been modified through the years, the key technical benefit of this concept remains that the major rigid vertical riser is offset from the floating production unit (FPU) using a top flexible jumper as connection. Hence the rigid riser is decoupled from FPU motion and is thus fatigue insensitive. Since the fatigue design of deepwater risers is a common challenge, the decoupling effect enhances hybrid riser fatigue performance significantly.

The hybrid riser either single line or tower offers advantages over other field proven deepwater riser concepts for particular applications and has been used mostly in the West of Africa. This concept has evolved in the last few years and now is drawing more attention in other deepwater provinces worldwide. It is basically a combination of steel and flexible risers with utilization of their advantages and hence offers unique features including:

Decoupling of riser from FPU motion Insensitivity to fatigue Pre-installation before floater on site Minimizing payload on floater Isolation of flowlines from riser loads Flexibility for future field expansion Retrievability without affecting flowlines Tower riser fabrication and tested onshore No need of professional installation vessel Offshore towing installation for tower riser using tugs Increase in local content

The original tower riser concept was derived from pipeline bundle technology where all lines were bundled

together in one tower, fabricated and tested onshore. The tower riser, e.g. Girassol type, consists of a center core tubular surrounded with production, water injection, gas injection/lift, and service lines enclosed with thermal insulation and syntactic foam buoyancy modules. This tower concept has been used for deepwater fields mainly in WoA such as Girassol, Rosa Lirio, and Greater Plutonio [Ref. 4]. All these tower risers were fabricated onshore, towed to sites, upended, and pulled-down into position.

The hybrid consists of a vertical rigid pipe anchored to the seabed via a foundation (e.g. suction pile) and tensioned by means of a near-surface buoyancy can that provides the required uplift force. Flexible jumper connects the rigid riser via a gooseneck assembly to the FPU. The connection of the riser to seabed is by means of mechanical connector (e.g. tie-back connector or roto-latch connector). A riser base jumper, either flexible or rigid, connects riser offtake spool and PLET. A typical example of such tower riser is the Girassol tower concept, which is illustrated in Figure 1-2 [Ref. 5].

Figure 1-2: Overall of Girassol Tower Riser

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This paper presents the engineering design and analysis aspects of a deepwater tower riser for application in the WoA based on actual offshore field data. First of all, a general description and comparison of the tower riser system is presented. Advantages of this concept are summarized. Secondly, the procedure for engineering design and analysis of tower riser is discussed and presented in more detail. Global configuration design with consideration of riser performance and its effect on component design is presented followed by major design considerations for individual components of the system. Attention is paid to the key component design such as buoyancy can, cross-section design, spacer, and interface design. Thirdly, the analysis requirement and methodology is discussed in detail with focus on global response analysis, vortex induced vibration (VIV) analysis, motion fatigue analysis, interface loads, and component analysis such as buoyancy can, top riser termination assembly, flexible jumper configuration, and riser foundation pile. The relationship between engineering design, fabrication and installation is discussed. An example of the tower riser design for the application in the WoA is given at the end of this paper.

2. TOWER RISER DESIGN METHODOLOGY 2.1 Design Basis

Similar to other type of risers, the first step of tower riser engineering is to set up the design basis, which is the foundation for on-going tower riser engineering work. It provides a set of consistent data for tower riser engineering as well as analysis methodology and design acceptance criteria.

Table below summarizes the primary design basis used in the engineering design of the tower riser.

Table 2-1: WoA Tower Riser Design Basis

Parameter Unit Production Water Injection Gas Injection

Water Depth m 1,500

Number -- 2 1 1

OD inch 10.75 8.625 8.625

Design Pressure bara 387 245 442

Design Temperature C 102 48 38

Corrosion Allowance mm 3.0 3.0 1.5

Fluid Density kg/m3 790 1025 360

FPSO Offset (intact / damaged) WD 6% / 8%

Table 2-2: Typical Metocean Data in West of African

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In addition to the rigid risers, there are one control and one service umbilicals, which are bundled within the riser tower.

2.2 Riser Tower Design Flowchart

Considering the unique challenges imposed by the Cascade & Chinook FSHR, complexity of the system and interface with fabrication and installation, there is a logical sequence to be followed for FSHR engineering.

The flowchart in Figure 2-1 shows the procedure and major issues to be considered during the engineering design of the Cascade & Chinook FSHR.

Figure 2-1: FSHR Engineering Design Flowchart

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2.3 Design Codes Due to the complexity of the tower riser system involving not only riser string, but support structure (e.g core pipe, offtake spool), and connector (e.g. gooseneck, roto-latch), different design codes are needed to address individual components based on their functionality, while the system design must satisfy API RP 2RD as a primary design code.

Table 2-3 below lists some of the design codes and applicable local components used for the Cascade & Chinook design.

Table 2-3: Hybrid Riser Design Code Summary

Code Title Application

API RP-2RD Design of Risers for Floating Production Systems (FPSs), and Tension Leg Platforms (TLPs)

Primary, General

API RP 2A-LRFD

Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms- Load and Resistance Factor Design

Suction Pile Design

API RP 2SK Design and Analysis of Station Keeping Systems for Floating Structures

Suction Pile Design

API Bulletin 2U Stability Design for Cylindrical Shells BC Design

API Bulletin 2V Design of Flat Plate Structures BC Design

DNV-RP-E303 Geotechnical Design and Installation of Suction Anchors in Clay Suction Pile Design

DNV-RP-B401 Cathodic Protection Design General

DNV RP-C203 Fatigue Design of Offshore Steel Structures Fatigue Analysis

DNV Rules DNV Rules for Planning and Execution of Marine Operations Offshore engineering

. 2.4 Software

Commercial software is available for deepwater riser analysis. Some programs are developed for riser analysis and some are generic FEA tools. Primary analyses may be conducted using the industry-standard analysis software packages such as: ABAQUS ANSYS FLEXCOM ORCAFLEX SHEAR7 Etc.

2.5 Global Configuration Design As the first step of the tower riser design, its global configuration needs to be engineered considering the following factors: Functional requirements In-place performance Metocean criteria FPSO excursions Subsea field layout

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Flexible jumper performance Field interference Fabrication specification Installation methodology

The following parameters have been identified governing the global configuration design: Depth from MWL to the top of buoyancy can; Riser foundation offset to riser hang off location at FPSO; Overall flexible jumper length; Flexible jumper departure angles at FPSO and riser ends; Stress joint interface between buoyancy can and riser top; Towing engineering requirement

3. TOWER RISER DESIGN AND ANALYSIS 3.1 Objectives

The objectives of a tower riser analysis are identified as follows: To obtain the riser global response by applying environmental events to the hybrid riser system. To demonstrate that the design complies with relevant specifications and industrial codes. To provide input for mechanical component design.

3.2 Global Configuration Design

As the first step of the design of tower riser, the following need to be considered during its global

configuration design.

Functional requirement In-place performance Metocean data FPSO offset Subsea layout Interference Fabrication

In general, the following factors govern the global configuration design of the tower riser:

Distance between top of the buoyancy can to the MWL Buoyancy can offset to the riser base Overall length of flexible jumpers Connection method between buoyancy can and riser strings Fabrication Towing and upending

Based on above considerations, the global configuration design results are summarized below and is also illustrated in the Figure 3-1, too.

Buoyancy can top to MWL: 70 m Riser base offset to FPSO hang off location: 200 m Flexible jumper overall length: 330 400 m

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Figure 3-1: Tower Riser Global Configuration Arrangement

3.3 Material Selection and Wall Thickness Sizing of Riser Pipes

For the specific application, different materials can be available for riser pipes. The selection of the material grade depends upon the following factors:

Functional requirements (pressure, temperature, service, thermal performance, etc.) Material property Weldability Corrosion resistance Weight requirement Market availability

Optimization of material section can be performed on the condition that above requirements are met.

For tower riser, it is very much preferred to have higher material grade, which can reduce the weight of

the riser system. But such consideration needs to be coupled with weldability, which affects the fatigue

performance of the welds.

Different loading conditions need to be considered for wall thickness sizing of riser pipe, such as

operating condition and temporary condition. In general, the hoop stress and tensile stress governs

the wall thickness sizing of the tower riser pipes.

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Table below summarizes the material selection and wall thickness sizing results for the tower riser

pipes.

Table 3-1: Tower Riser Pipe Wall Thickness Sizing Results

Function No. OD Material

Grade (ksi)

Min WT mm

Selected WT mm mm inch

Production 2 273.05 10.75 65 18.609 20.625

Water Injection 1 219.08 8.625 65 9.696 11.125

Gas Injection 1 219.08 8.625 65 16.86 18.263

Central Core Pipe 1 457.20 18 65 18.00

19.05

3.4 Cross Section Design

The following are considered during the cross-section design of the tower riser:

Functional requirement In-place performance Onshore fabrication and assembly requirement Offshore towing and installation requirement

For tower riser, its unit weight is pretty heavy due to inclusion of large variety of riser pipes as well as

umbilicals. Hence, a central core pipe is needed to support the riser pipes and to transfer buoyancy

can tension to the riser system. The requirements for tower riser cross section design are summarized

below:

Tension provided by the buoyancy can needs to be supported by a central core pipe Distributed buoyancy module is designed to make the the unit submerged weight of the

system as neutral as possible

End termination spacer is needed to ensure the integrity and security of riser pipe at both ends.

Length of distributed buoyancy module should accommodate the fabrication and assembly requirements.

Among above requirements, maximization of symmetric profile is a key to ensure that no unacceptable offset of the tower riser for onshore assembly, towing and in-place performance, as well as security of peripheral lines to the central core pipe.

Figure 3-2 below shows the cross section design results of the tower riser.

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Figure 3-2: Tower Riser Cross Section Design

Figure 3-3: Tower Riser Cross-Section Illustration

3.5 Buoyancy Can Design and Analysis

Buoyancy can design is probably one of the most complicated and important task in the tower riser

engineering due to its criticality and complexity. The primary task of buoyancy can design is to determine

its principal dimensions including length, diameter, number of compartments, interface design, and

contingency design.

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Based on the study, the following are assumed to conduct buoyancy can sizing:

1) Top tension factor: 1.5

2) Weight in air to displacement ratio: 0.3

3) Buoyancy contingency reservation: 15%

4) No. of contingency compartment: 1

5) No. of chamber flooded accidental: 1

6) No. of compartment flooded intentionally: 1

Considering fabrication, operation, and offshore installation, as well as above assumptions, the designed

buoyancy can results are tabulated below.

Table 3-2: Summary of Buoyancy Can Design

Description Unit Value Overall Length m 26 Top Portion Length m 18 Top Portion OD m 5.8 Transition Cone Length m 5 Bottom Portion Length m 3 Bottom Portion OD m 2.5 Max Net Uplift Force te 285 Weight te 173 Normal Net Uplift Force te 236 Bottom Tension te 236

Figure 3-4 and 3-5 provide the overall dimension design results of the buoyancy can and artist

illustration respectively.

Figure 3-4: Buoyancy Can Overall Dimensions Sizing

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Figure 3-5: Buoyancy Can Design Illustration

In addition to normal operating condition, the design of buoyancy can must accommodate accidental damage case, which means one compartment could be accidental flooded. In this case, if it happens, it will not lead to the tower riser systemic failure. Hence, the buoyancy can is compartmenized with 8 chambers in total. The bottom chamber will be flooded from beginning and can be dewatered later if one chamber is accidental flooded. The design net uplift force can be restored. To verify the buoyancy can design, detailed finite element analysis (FEA) was performed using ABAQUS. Figure 3-6 below is the global mesh modeling of the buoyancy can, while Figure 3-7 shows the local model between the interface of taper stress joint and lower compartment of the buoyancy can.

Figure 3-6: Buoyancy Can Global FEA Modeling ABAQUS

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Figure 3-7: Buoyancy Can Local FEA Modeling ABAQUS

Combined stress contour for buoyancy can in-place strength check are shown in Figure 3-8 and 3-9 below, showing that the strength check meet design code requirements.

Figure 3-8: Buoyancy Can Global Stress Contour Distribution In-Place Condition

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Figure 3-8: Buoyancy Can Local Stress Contour Distribution Ring Stiffener

Figure 3-10: Buoyancy Can Local Stress Contour Distribution Brackets

3.6 End Termination Spacer Design

At both ends of the tower riser, riser pipes as well as umbilicals need to re-distribute to follow the buoyancy can or offtake spool. Additional force due to such redistribution will be introduced. Security of the pipes to the central core pipe can be maintined by introducing end termination spacer. It should be noted that such spacer design requires no weld to riser pipe and should be used easily to attach riser pipe as well as providing security fuction. Figure 3-11 shows the end termination spacer design. The horizontal plates are welded to the central core pipe by fillet welds. Four riser pipes are attached to the spacer using conventional flange.

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Figure 3-11: End Termination Spacer Design

4. CONCLUSION

As a popular riser concept in the WoA, the design study was performed for the tower riser based on an actual field in the WoA. Focus is given the overall system design as well as critical local component design, e.g. buoyancy can. The proposed design methodology has been demonstrated successfully through the design of the tower riser. Key design issues associated with the design of tower riser are addressed including global arrangement, cross-section design, and buoyancy can design. 5. REFERENCES 1. API RP 2RD (1998): Design of Risers For Floating Production Systems (FPSs) and Tension-Leg Platforms

(TLPs) 2. DNV OS F201: Dynamic Risers, 2001 3. Song, R, Stanton, P, Zhou, X: Engineering Design of Deepwater Free Standing Hybrid Riser, Proceedings of

OMAE 2010, OMAE Paper #20600, Shanghai, China 4. Djia, F, et al: Design of the Greater Plutonio Riser Tower, Proceedings of OMAE 2009, OMAE Paper #79015,

Hawaii, USA 5. G. Chapin: Inspection and monitoring of Girassol hybrid riser towers, Proceedings of OTC 2005, Houston,

USA, OTC Paper # 17696

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