OTC-25840-MS - 2H Offshore

OTC-25840-MS

Frontier Deepwater Developments - The Impact on Riser Systems Design in Water Depths Greater than 3,000m N. Saglar, B. Toleman, and R. Thethi, 2H Offshore Inc.

Copyright 2015, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 4–7 May 2015. 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 offshore industry anticipates the need for production riser systems in ultra-deepwater fields where water depths are

between 3,000m to 4,500m. The development of ultra-deepwater fields leads to many challenges on the selection of the riser

concept and in some instances such applications may require extending riser technology beyond its current limits.

Consequently, there is a need to understand the feasibility of riser systems in such ultra-deepwater applications and the

technology gaps that exist.

In ultra-deepwater, long suspended riser lengths will significantly increase the riser weight potentially leading to

challenges with offshore installation related to laying vessel capability. High external and internal pressure on the riser will

lead to the need for heavy wall pipes. Thick-wall riser pipe will bring about riser design challenges in fabrication of pipes,

riser pipe welding, riser hang-off system selection, long-term fatigue design, and fabrication of specialty riser joints. By

looking into these challenges, it is very important to select the most appropriate riser concept for the ultra-deepwater fields

and understand the current pipe manufacturing limits, enabling technology needed for such systems, and technology gaps

considering the critical points mentioned above.

This paper addresses the key riser design issues considering wall thickness sizing, top tension, axial dynamics, selection

of pipe material, design of key components, and installation issues. This paper evaluates feasibility of a number of production

and export riser configurations for ultra-deepwater applications based on existing technology, identifies current technology

limits, and determines technology gaps that exist. Methods of advancing the current capability to meet the requirements of

frontier deepwater applications are also proposed in the paper.

Introduction Subsea riser design is one of the most challenging engineering aspects of a deepwater field development. The primary

challenge in riser design emanates from the fact that these are dynamic structures highly susceptible to environmental and

operational loads. As the global demand for hydrocarbons has increased, offshore projects have moved into increasingly

deeper water and riser design has become more challenging than ever before, stretching existing technologies and materials.

Deepwater block leases are now been seen in 3,000m (9,842ft) water depth or more [1].

This paper describes an evaluation of riser technology conducted for ultradeep water wet tree production and export riser

systems. Water depths from 3,000m (9,843ft) to 4,500m (14,764ft) and design pressures from 5ksi to 15ksi are considered for

steel catenary riser (SCR) configurations. Riser pipe sizing and tension requirements are determined for 8inch, 10inch, and

12inch production nominal riser diameters. Oil export riser configurations with pipe diameters ranging from 16inch to 24inch

are also evaluated. The risers assessed in this paper include X65 and X70 grade conventionally welded API 5L line pipe [2].

The material presented in this paper provides a review of the current industry technology boundaries with respect to the

use of riser systems in ultra-deep water depths. Key challenges and technology limitations are assessed considering wall

thickness sizing, riser top tension requirements, fabrication and welding limits, and installation vessel capacities. Riser axial

dynamic behavior in terms of strength capability is also determined considering a generic deep draft semi-submersible vessel.

Design Data The field development considered in this work is located in the central GoM. The riser functions and associated pipe sizes

considered are listed in Table 1. The riser are assumed to be constructed from conventionally welded X65 and X70 steel

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2 OTC-25840-MS

pipes with minimum yield strengths of 65ksi and 70ksi respectively. Three different water depths; 3,000m (9,843ft), 3,750m

(12,303ft), and 4,500m (14,764ft) are considered in the riser sizing calculations.

Riser OD (inches) Pipe Type and Fabrication Method

8inch Production 8.625

API 5L, X65 and X70 grade, Seamless, Conventionally welded 10inch Production 10.75

12inch Production 12.75

16inch Oil Export 16.0

API 5L, X65 and X70 grade, DSAW, Conventionally welded 20inch Oil Export 20.0

24inch Oil Export 24.0

Table 1 - Riser Pipe Sizes

The riser internal fluid properties of the different risers are given in Table 2. Five different design pressures ranging from

5ksi to 15ksi are used for the production risers. The oil export riser is evaluated with a design pressure of 5ksi. The design

pressures are defined at the mean water level (MWL). The maximum design temperature is assumed to be lower than 250°F;

hence no temperature derating factor is included in the wall thickness calculation in accordance with API RP 1111 [3]. The

production risers are assumed to be insulated with a multi-layer coating of thickness 2.5inch and an estimated density of 52

lb/ft3. 3mm corrosion allowance is used for the production risers only. The maximum pipe ovality is assumed to be 0.5% for

the risers.

Parameter Production Riser Oil Export Riser

Design Pressure (ksi) 5ksi; 7.5ksi; 10ksi; 12.5ksi; 15ksi 5ksi

Location of pressure definition MWL MWL

Internal Fluid Density (lb/ft3) 55 55

Design Temperature (°F) <250 <250

Table 2 - Riser Internal Fluid Properties and Design Pressure

The production and export risers are assumed to be in an SCR configuration. An example of an SCR configuration is

illustrated in Figure 1. A generic deep draft semi-submersible vessel is considered as a host vessel. Risers are assumed to be

attached to the host vessel with a flexible joint.

Figure 1 – Riser Schematic (SCR)

Riser strength analysis is conducted for selected riser configurations. The flexible joint rotational stiffness for strength

analysis is assumed as 20 kip-ft/degree. The riser top angle used is 12°. The risers are assumed to include 100% strake

coverage. Strakes are assumed to be neutrally buoyant with a base thickness of 0.7inch. The drag coefficient and added mass

coefficient of the straked risers are taken as 1.6 and 2.0 respectively. A typical deepwater Gulf of Mexico soil is selected and

used in this study. The vertical elastic soil stiffness of 27,000 lb/ft/ft is used for dynamic strength analysis. A coefficient of

0.5 is used for axial and lateral soil friction.

The riser strength assessment is conducted in 10-year winter storm and 100-year hurricane environments. Design

robustness is also checked in a 1,000 year hurricane. The wave and current data is obtained from API RP 2MET [4] for the

central GoM and summarized below in Table 3. For the purpose of this study, the current direction is assumed collinear with

the wave direction. When assessing the SCR systems with dynamic sensitivity, ±2sec variation in wave period is also

considered.


OTC-25840-MS 3

Parameter 10 year Winter Storm 100 year Hurricane 1,000 year Hurricane

Hmax (ft) 38.1 91.5 114.5

Tmax (s) 10.0 13.9 15.5

Hs (ft) 21.0 51.8 65.0

Tp (s) 11.1 15.4 17.2

Current Profile

Depth (ft)

Speed (ft/sec)

Depth (ft)

Speed (ft/sec)

Depth (ft)

Speed (ft/sec)

0 1.8 0 7.9 0 9.8

246 1.8 164 7.9 164 9.8

Seabed 0.3 330 0.3 413 0.3

Seabed 0.3 Seabed 0.3

Table 3 - Design Wave Data

For the strength analysis, the response vessel operators (RAOs) are utilized to generate the vessel motions. A generic deep

draft heave optimized semi-submersible vessel, which is specifically constructed for the SCR application is used as the host

production vessel. The generic semi has a heave natural period above 20 seconds and its heave response in environments with

wave periods in the region of 8 seconds or lower is insignificant. The semi-submersible used in this study has also optimized

roll and pitch motions. Yaw response of the typical semis is insignificant.

Design Criteria The wall thickness sizing is carried out per API RP 1111 for both the production and export risers. The export riser wall

thickness is also checked in accordance with the 49 CFR 195 [5]. The BSEE guidance for alternate compliance, NTL 2009-

G28 [6], states that API RP 1111 code can be used for wall thickness sizing in lieu of the Barlow equation in 30 CFR

250.1002(a) [7] if the pipeline design pressure is equal to or greater than the maximum source pressure (MSP) for all line

pipe and riser pipe and the external hydrostatic pressure is not used to offset or reduce the minimum pipeline test pressure

required by 30 CFR 250.1003(b)(1). Therefore, wall thickness sizing is carried out based on the alternate compliance case

and not carried out per 30 CFR 250. Wall thickness sizing is conducted based on internal pressure (burst), external pressure

(collapse), and longitudinal and combined load design. Riser wall thickness is also checked based on buckling due to

combined bending and external pressure criteria. The wall thickness required to resist buckling propagation is not considered

assuming other measures to be set into action to avoid damages by propagation.

The allowable von Mises stress in the strength assessment follows from the recommendations in API-RP-2RD [8].

According to API-RP-2RD guidance, the primary membrane stress should be less than 0.67*σy for operating, 0.8*σy for

extreme, and less than 1.0*σy for abnormal/survival load conditions.

Design Methodology The wall thickness calculations use API RP 1111 while meeting NTL 2009-G28 requirements. For the burst pressure check,

the riser design pressures are defined at the surface and applied along the entire riser. Riser burst sizing is independent of

water depth for this methodology. For the collapse pressure check, the riser is considered empty and collapse external

pressure is calculated at the seabed. The effective tension of the risers due to static primary longitudinal loads are calculated

based on seawater filled condition and limited to 60% of yield in accordance with API RP 1111. Combined load design due

to primary longitudinal load and differential pressure load is also checked. Buckling of the risers due to combined bending

and external pressure is checked assuming a maximum installation bending strain of 0.005 including a safety factor of 3.33

and a maximum in place bending strain of 0.003 including a safety factor of 2.0. 3mm corrosion allowance is subsequently

added to the derived wall thickness for the production risers.

Strength analysis is carried out for selected riser configurations. Regular wave analysis is conducted using generic semi-

submersible motion RAOs. The riser analysis is primarily carried out using the nonlinear time domain analysis program

Flexcom-3D [9]. For the strength analysis, the SCR heading angles are assumed to be in the surge direction of the semi-

submersible with 45° increments, as shown in Figure 2. Risers are hung-off the semi from the inside of the pontoon with a

flexible joint with a top angle of 12° from vertical. The riser hang-off depth is assumed to be 120ft below the mean water

level (MWL). Riser porch location is assumed to be 80ft and 40ft away from the center of the semi along the surge and sway

directions respectively. Strength analysis loading conditions are given in Table 4. 10-year winter storm with the operating

pressure of 3,500psi is assessed as an operating condition. 100-year hurricane with dead oil at a pressure of 300psi is

evaluated as an extreme condition. 100-year hurricane with the shut-in pressure is considered as a survival condition. 1,000-

year hurricane with dead oil at a pressure of 300psi is also checked as a robustness case. The environmental loading is

applied along the riser in the near (towards riser heading) and far (opposite to riser heading) directions. The summary of the

mean vessel offsets for all loading conditions are given in Table 5. The regular wave analysis is carried out for a minimum of

5 wave cycles with wave loading ramped up over one wave period. All results are taken from the final two wave cycles (as

minimum) when the riser dynamic response has stabilized. Effective tension and bending moments are obtained and the

maximum von Mises stress is determined along the length of the riser. The maximum von Mises stress is compared with the

allowable stress criteria to determine the acceptability of riser response. Possible compression (negative tension) along the

riser is also checked.


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Figure 2 – Plan View of Generic Semi-Submersible and Riser Headings

Load Category Environment Pressure (psi)

(Surface) Mooring Condition Weather Direction

Allowable Stress/Yield

Operating 10-year winter storm 3,500 (operating)

Intact Far

0.67 3,500 (operating) Near

Extreme 100-year hurricane 300 (dead oil)

Intact Far

0.80 300 (dead oil) Near

Survival 100-year hurricane Shut-in

Intact Far

1.00 Shut-in Near

Robustness 1,000-year hurricane 300 (dead oil)

Intact Far

Robustness 300 (dead oil) Near

Table 4 – SCR Strength Analysis Load Case Matrix

Environment Mooring Condition Mean Horizontal Offset

(% of Water Depth) Mean Heel Angle (deg)

10-year winter storm Intact 0.5% 0.50

100-year hurricane Intact 2.5% 0.75

1,000-year hurricane Intact 3.3% 1.00

Table 5 – Generic Semi-Submersible Mean Offset Summary for Strength Analysis

Riser Wall Thickness Sizing

Production Risers

SCR wall thickness sizing is initially carried out considering X65 line pipe. For the design pressure of 5ksi, the wall thickness

of the production riser is primarily driven by collapse due to external hydrostatic pressure. For the design pressures above

5ksi, the wall thickness sizing is governed by burst due to the high internal pressure and hence wall thickness does not change

with the water depth, as shown in Figure 3. The maximum wall thicknesses required for 8inch, 10inch, and 12inch pipes are

1.51inch, 1.85inch, and 2.17inch respectively and are driven by burst due to the 15ksi internal design pressure. It should be

noted that these wall thicknesses are purely to resist the burst and collapse pressures. The longitudinal load and combined

load design checks are performed after the sizing is performed for collapse and burst. When considering the longitudinal load


OTC-25840-MS 5

from the static tension in the flooded condition at the top of the riser in water depths of 3,750m and above, the effective

tension exceeds the 60% of yield tension capacity, as shown in Figure 3. A similar conclusion is also drawn for the combined

load design check. Therefore, for ultra-deep water, the longitudinal load design criterion becomes a limiting requirement. For

3,000m water depth, the production risers meet both the longitudinal load and combined load design criteria. Buckling due to

combined bending and external pressure is also checked with the calculated wall thicknesses. The allowable bending strains

of all the production risers are determined to be greater than the assumed maximum bending strain of 0.5%.

The field proven thickest wall to date for conventionally welded X65 single pipe wet tree production riser systems is

1.65inch (42mm) [10] [11]. The weldability of X65 line pipe with a wall thickness of 1.81inch (46mm) has been tested for

offshore fatigue sensitive applications [12]. The industry is currently working on determining the weldability of X65 line

pipes up to 1.9inch (48.2mm). Therefore, in this paper, the maximum wall thickness that can be welded for offshore fatigue

sensitive riser systems is assumed to be 1.9inch (48.2mm). This wall thickness size is specified as an existing technology

limit and is governed by challenges in terms of pipe fabrication, weldability, cladding application technology and successful

AUT to achieve the required fatigue performance. As shown in Figure 3, only the wall thickness required for 12inch

production riser with the design pressure of 15ksi exceeds the 1.9inch limit.

Figure 3 – Production Riser, Wall Thickness Sizing Summary, X65 Line Pipe


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Export Risers

SCR wall thickness sizing is initially carried out considering X65 line pipe with a design pressure of 5,000psi. The wall

thickness of the export riser is first calculated to resist the burst and collapse pressures. It is determined that the wall

thickness required for collapse is higher than the sizing due to burst for all water depths and ODs. However, the wall

thickness driven by collapse does not meet the 0.5% allowable bending strain considering combined bending and external

pressure criterion. Bending in the presence of external pressure is experienced by risers during their installation and also

subsequently during their operation. In installation methods such as S-lay, J-lay, and reeling, the pipe may be bent under

relatively high hydrostatic external pressure. This interaction through ovalization is amplified and can lead to local buckling

in the pipe. This is because the higher external collapse pressure utilizes the majority of the capacity of the pipe and does not

leave any extra capacity for bending. To be able to meet the buckling due to combined bending and external pressure

criterion in API RP 1111, the wall thicknesses of the export risers are increased to maintain the allowable bending strain of

0.5% in the pipe. Therefore, the wall thickness of the export risers is primarily driven by buckling due to combined bending

and external pressure criterion. The wall thicknesses of the export risers are shown in Figure 4. The maximum wall

thicknesses required for 16inch, 20inch, and 24inch pipes are 1.55inch, 1.95inch, and 2.32inch respectively and occur at the

4,500m water depth.

When considering the longitudinal load from the static tension in the flooded condition at the top of the riser in water

depth depths of 3,750m and above, the effective tension exceeds the 60% of yield tension capacity, as shown in Figure 4. A

similar conclusion is also drawn for the combined load design check. Therefore, for ultradeep water, the longitudinal load

design criterion becomes the limiting requirement. For 3,000m water depth, the export risers meet both the longitudinal load

and combined load design criteria. As it is shown in Figure 4, the wall thicknesses required for 20inch export riser at the

water depth of 4,500m and 24inch export riser at the water depths of 3,750m and above exceed the 1.9inch limit.

Figure 4 – Oil Export Riser, Wall Thickness Sizing Summary, X65 Line Pipe

Water Depth Limitation After wall thickness sizing is carried out, an optimum water depth is calculated for each SCR configuration while meeting all

the API RP 1111 criteria. Water depth limitations of each riser are determined considering both X65 and X70 steel grades.

The maximum water depths that the production risers can reach for each design pressure while meeting the API RP 1111

criteria are shown in Figure 5. X70 grade pipes result in thinner wall thicknesses compared to riser with X65 grade. X70

grade also results in a lighter static riser weight at the top of the riser due to a thinner wall thickness and hence extends their

water depth limit as shown in Figure 5. The maximum water depth for the X65 8inch, 10inch, and 12inch production risers

considering 5,000psi design pressure are 3,617m, 3,495m, and 3,422m respectively. As the riser design pressure increases,

the water depth limit decreases. The maximum water depth for the production risers is 3,903m considering X70 8inch OD

5,000psi design pressure. It should be noted that this water depth limits are obtained for the riser top angle of 12°. For higher

riser top angles, the water depth limits would be lower. None of the production risers reach 4,000m water depth considering

API RP 1111 requirements. 12inch production riser with the 15,000psi design pressure is not feasible with any water depth

due to pipe manufacturing and welding limits.

The maximum water depths that the oil export risers can reach with 5,000psi design pressure while meeting the API RP

1111 criteria are shown in Figure 6. Similar to production risers, higher water depths can be achieved with X70 pipe with a

maximum water depth of 3,340m. The maximum water depth does not change significantly for different ODs. It should be

noted that this water depth limits are obtained for the riser top angle of 12°. For larger riser top angles or higher pipe

ovalities, the water depth limits should be lower. None of the oil export risers are reaching 3,400m water depth considering

API RP 1111 requirements.


OTC-25840-MS 7

Figure 5 – Production Riser, Water Depth Limitation, X65 & X70 Line Pipe

Figure 6 – Oil Export Riser, Water Depth Limitation, X65 & X70 Line Pipe

SCRs are constructed from conventionally welded line pipe. A typical steel grade used in the SCR applications is X65.

However, X70 pipe is also used in some SCR projects in the GoM [13]. X80 welded line pipe has not been used to date for

the conventionally welded SCR systems. Weldability of thick wall X80 line pipe offshore is not qualified yet in the industry

for sour service conditions, hence it is not considered in this paper.

Riser wall thickness sizing based on API RP 1111 indicates that an advantage can be gained by increasing wall thickness

along the top section of the riser instead of specifying a single wall thickness along the entire riser. Thicker wall pipe at the

top regions of the riser (up to the pipe manufacturing limit of 1.9-inches) increases the yield tension capacity of the riser and

hence API RP 1111 requirements can be met at deeper water depths. However, varying wall thickness along the riser may

lead to additional work during fabrication, qualification, and installation phases.

Installation Assessment Installability of the risers is evaluated based on the top tension of the SCRs. The weight of thick wall SCRs is an issue for

installation. There are only a limited number of installation vessels able to install thick wall SCRs in deepwater and the

number starts to diminish rapidly as water depth and pipe wall thickness increase.

The riser weight during installation is considered based on both empty and fully flooded cases. The suspended length of

the riser is calculated using catenary equations as a function of hang-off angle and water depth. The assessment considers the

risers with 4°, 8°, and 12° installation departure angles from vertical. For the purpose of the assessment, a dynamic


8 OTC-25840-MS

amplification factor of 1.25 is applied on the static riser weight during installation to account for dynamic loading from vessel

motions during installation.

15ksi design pressure and X65 grade pipe are considered, corresponding wall thicknesses are then used to calculate the

top tension of the production risers. Production riser top tension requirements vs. water depth are plotted in Figure 7 for a

range of riser departure angles. SCRs are typically installed by the following three pipe lay methods: S-lay, J-lay, and reel-

lay. Feasibility of using reel-lay diminishes as the water depth exceeds 3,000m as the top tension increases over reel lay

vessel tension capacity of 800mT. The J-lay method is very suitable for deepwater as the riser leaves the lay system in an

almost vertical position. The riser is only bent once during J-lay installation (at the seabed) which is advantageous for

installing risers that are sensitive to fatigue. S-lay is more suitable for riser installation in shallow and intermediate waters.

Larger water depths are equally possible but require a very long stinger, turning S-lay into a less practical and efficient

solution. For the purpose of this assessment, the J-lay installation method is considered. The tension capacity of J-lay

installation vessels is considered to be 2,000mT [14]. Hence, the feasible SCR applications are identified using vessel

payload of 2,000mT. The top tension calculated for the riser installations cases are found to be within the tension capacities

of the large J-lay installation vessels at all departure angles and water depths for 8inch and 10inch production SCRs. The top

tension required for the 12inch SCR is found to be within the tension capacities of the J-lay installation vessels for both

departure angles of 4° and 8°. The top tension of 12inch SCR starts exceeding 2,000mT for the water depths above 4,000m if

flooded and 12° departure angle is considered. However, it should be noted that the wall thickness required for 12inch

production riser with 15ksi design pressure is 2.17 and exceeds the current pipe manufacturing limit. The top tension of the

SCRs with a design pressure of 12.5ksi and below is within the tension capacities of the vessels at all water depths. The hang-

off tension of the SCRs is below 2,000mT in all configurations if X70 grade pipe is used.

Similarly, the oil export riser installation tension requirements are evaluated. The 16inch oil export with a departure angle

of 4°, results in top tensions below 2,000mT for all assessed water depths. The 20inch oil export with a departure angle of 4°

is limited to a max water depth about 3,600m. The 24inch oil export is not feasible for any water depth while the riser is filled

with seawater. Plots of the riser top tension vs. water depth are shown in Figure 7.


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Figure 7 – SCRs Installation Tension Requirements

Hang-off System Assessment Typically, two hang-off systems have been used in SCR applications with semi submersibles. They are flexible joints and

tapered stress joints. In this study, a flexible joint is selected as the hang-off system for the riser configurations. Flexible

joints have been used as a hang-off system of the SCR configurations in many development projects using SCRs worldwide.

Flexible joint design depends on several factors including design pressure, pressure variation during the service life of

riser, long term and extreme loads, thickness of rubber layers, type of rubber compounds, etc. Up to date, conventional

flexible joints have been provided to maximum riser tension ranges from 300kips to 3,000kips for high pressure applications

(up to 12.0ksi). Conventional flexible joints can carry tension loads up to 4,000kips for larger ODs with relatively lower

pressures (up to 5.0ksi).

Axial tension capacity of the flexible joint is assumed to be 3,000kips for the production SCRs. Although there is

currently no flexible joint design for pressures up to 15ksi, for the purpose of this assessment, it is assumed that the flexible

joint is capable of being designed and qualified within a reasonable timeframe to stretch to a 15ksi design pressure. As shown

in Figure 8, the 12inch high pressure production SCR may not be feasible with a conventional flexible joint system due to its

high axial tension requirements. Static top tension of 8inch and 10inch flooded production risers remain within the limits of

flexible joint.

Similarly, axial tension capacity of the flexible joint is assumed to be 4,000kips for export risers. As shown in Figure 9,

24inch oil export risers are not feasible with conventional flexible joint system due to high axial tension requirements. 20inch

oil export risers are feasible up to 3,400m water depth. Conventional flexible joint hang-off system can be used for 16inch oil

export risers for water depths up to 4,500m.


10 OTC-25840-MS

Figure 8 – Production Riser, Static Top Tension vs. Water Depth

Figure 9 – Oil Export Riser, Static Top Tension vs. Water Depth

Thermal Insulation Assessment Thermal insulation systems are used to reduce fluid cooling while maintaining a minimum temperature and preventing

formation of hydrates, waxes or water separation. For single pipe flowlines and risers, the mechanical loads as well as the

thermal insulation requirements normally increase with deeper waters. Hence, the traditional thermal insulation foam

technology used in shallow waters and the associated design and test methodology may not be applicable to deep water

projects. Considering higher external pressures due to hydrostatic head in deep waters beyond 3,000m, a novel thermal

insulation system will be required. Multi-layer solid polypropylene thermal insulation systems have recently been developed

for ultra-deep waters and can be employed in water depths up to 4,500m [15].

Strength Assessment Strength analysis is carried out for the following three riser configurations:

X65, 8inch production SCR, 0.67inch wall thickness, 5ksi design pressure, 3,617m (11,000ft) water depth;

X65, 10inch production SCR, 1.85inch wall thickness, 15ksi design pressure, 3,283m (10,623ft) water depth;

X65, 16inch oil export SCR, 1.17inch wall thickness, 5ksi design pressure, 3,100m (10,170ft) water depth.

The summary of the strength analysis results for a range of wave heights and periods is given in Table 6. Preliminary

analysis shows that the SCRs exceed the allowable stress ratio of 0.67 in 10yr winter storm (operating condition). This is

because 60% of the pipe stress utilization is already taken by the static effective tension of the riser. With the addition of


OTC-25840-MS 11

bending moments due to vessel dynamic motions and hoop stress due to operating pressure, the SCRs do not meet API RP

2RD criteria in the maximum operating environment. The SCRs do not meet the allowable stress ratios in the 100yr hurricane

extreme condition. For the 100yr hurricane survival condition, only the 8inch production SCR does not meet the criteria.

During 1,000yr hurricane, the stress in the riser exceeds the yield capacity of the pipe in the TDP due to high bending

moment induced by compression loads, as shown in Figure 10. The SCRs experience compression in the touch down zone

due to the high heave motions of the deep draft semi at the longer wave periods in a 1,000yr hurricane, as given in Figure 11.

In addition, 8inch production SCR is a relatively light weight riser and extends into 3,617m water depth which makes the

riser very slender and susceptible to the heave motions of the floater. Because of this, excessive compression and stresses

above yield are observed in the touch down zone of 8inch production SCR. Local buckling behavior would need to be further

analyzed for this riser.

Based on the dynamic response of the risers, it is seen that an X65 grade SCR attached to a deep draft semi-submersible

with a flexible joint presents a significant challenge to provide acceptable strength behavior in water depths above 3,000m.

The key challenges are due to high compression load along the TDZ and resulting high stresses induced by these

compression loads. Compression along the riser could be minimized with implementation of optimized weighted sections,

such as high density insulation, clump weights or weighted plates in the TDZ.

Load Category Environment Maximum Utilization Ratios (%)

8inch Production SCR

10inch Production SCR

16inch Export SCR

Operating 10-year winter storm

with operating pressure 130% - 157%

(2) 96% - 103%

(2) 98% - 106%

2)

Extreme 100-year hurricane

with dead oil pressure 215% - 226%

(2) 101% - 99%

(2) 100% - 96%

(2)

Survival 100-year hurricane

with shut-in pressure 183% - 192%

(2) 93% - 92%

(2) 90% - 88%

(2)

Robustness 1,000-year hurricane

(1)

with dead oil pressure 1.98 - 2.33

(2) 0.90 - 1.05

(2) 1.20 - 1.24

(2)

Note 1/ 1,000-year hurricane is a robustness check and the maximum von Mises stress over yield strength is provided instead of a utilization ratio. Note 2/ Stress range includes the period sensitivity obtained with ±2sec variation in wave period.

Table 6 – Summary of Strength Analysis Results, SCR

Figure 10 – Maximum Von Mises Stress Ratio along Riser – Conventional SCR


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Figure 11 – Minimum Effective Tension along Riser

Steel Lazy Wave Riser Configuration Due to the challenges with the traditional SCR’s to stretch into water depths beyond 3,400m and provide acceptable strength

behavior, an assessment is performed using a steel lazy-wave riser (SLWR). A SLWR is a special SCR with a segment of its

length equipped with external buoyancy modules, where its upward buoyancy force in water is greater than its downward

gravity force. Due to this buoyancy force, the top tension of the riser at the vessel hang off reduces significantly.

Water depth limitations are assessed for the 10inch production riser with 15ksi design pressure and the 16inch oil export

with 5ksi design pressure using the SLWR configuration. Subsea buoyancy modules made of ultralight foams which are

capable of resisting external collapse pressures up to 4,500m are used in the lazy wave riser configuration [19]. The in-air

density of buoyancy modules are 35lb/ft3. Due to the lower tension along the riser, API RP 1111 requirements can be met

with the lazy wave riser configurations in water depth of 4,500m for the selected riser configurations, as shown in Figure 12.

It should be noted that in order to reach 4,500m water depth, longer length of buoyancy segments through mid-depth in the

water column are required, as illustrated in Figure 13. Longer buoyancy sections in mid-depths may experience relatively

high current speeds which may impact the fatigue response of the riser.

Figure 12 – Water Depth Limitation, SCR vs. Lazy Wave Riser


OTC-25840-MS 13

Figure 13 – Water Depth Limitation, SCR vs. Lazy Wave Riser

Steel Lazy Wave Riser Strength Assessment Similar to the SCR strength analysis, a preliminary strength analysis is also carried out with an SLWR configuration for the

10inch production riser with 15ksi design pressure in 4,500m water depth. A buoyancy section of 5,250ft is installed above

the TDP which isolates the porch heave motions from the TDP. Based on the analysis results, the steel lazy wave riser meets

the strength design criteria for all load conditions considered, as it is shown in Figure 14. The maximum von Mises stress

does not exceed the yield strength for a 1,000 year hurricane event, demonstrating the robustness of the lazy wave riser

system. No compression is observed along the lazy wave riser.

Based on the dynamic response of the risers, it is seen that SLWR configuration leads to more viable global response in

water depths exceeding 3,000m due to its lower top tension and its ability to control the dynamic heave response along the

riser.

Figure 14 – Maximum Von Mises Stress Ratio along Riser – Steel Lazy Wave Riser

Preliminary Fatigue Assessment A preliminary fatigue analysis is conducted using the 10inch SLWR configuration. The SLWR configuration selected for

fatigue analysis is based on the strength assessment showing it can reach a 4,500m water depth. A concern in the oil industry

with ultra-deep water depths is that the riser mixed modes (bending+axial or bending+torque) start to move into the range of

wave loading frequencies. This is shown by conducting modal analysis on the 10inch conventional SCR as shown in Figure

15 and Figure 16. A modal analysis is also performed on the SLWR to determine if any mixed modes exist in longer wave

periods. Out-of-Plane Mode 84, as shown in Figure 17, has been identified as a potential excitation mode, which occurs at a

period of 3.4sec. Wave loading with an Hs = 2.3ft and Tp = 3.4sec is applied to the SLWR in the far, near, and perpendicular

directions with respect to the riser span. 3hour long random wave analysis is conducted and the stress cycles are tabulated


14 OTC-25840-MS

with the rainflow counting method and the DNV 05’ E Class SN curve. The resultant unfactored fatigue lives along the

length of the riser assuming the near direction loading is applied for 100% of the time is shown in Figure 18.

The fatigue lives presented do not include a safety factor or a knockdown factor for possible sour service. Assuming a

value of 10 for both safety factor and knock down factor, the fatigue analysis results still result in an acceptable fatigue life

considering this one seastate alone. Note that the semi-submersible floater used in this analysis does not move in the heave

direction for the period 3.4sec. This may not be the case for other types of floaters and could result in lower fatigue lives if

the mixed mode frequency of the riser coincides with larger heave motions of the floater at these low periods.

Figure 15 – Maximum Curvature vs. Frequency for 10inch SCR with Different Water Depths (Out of Plane)

Figure 16 – Maximum Curvature vs. Frequency for 10inch SCR with Different Water Depths (In Plane)


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Figure 17 – Maximum Curvature vs. Frequency for 10inch Steel Lazy Wave Riser (Out of Plane)

Figure 18 – Unfactored Fatigue Lives along Length of Riser with Near Direction Wave Loading

Conclusions A number of riser configurations for different pressures, diameters, and material grades are developed and evaluated at water

depths ranging from 3,000m to 4,500m in this paper. Wall thickness sizes, carried out in accordance with API RP 1111, for

the production risers with a design pressure of 7.5ksi and above are burst controlled and independent of water depth. The

12inch OD production risers constructed from X65 and X70 grade pipe with a design pressure of 15ksi result in wall

thicknesses greater than 2inch and are not feasible for a welded line pipe solution. None of the conventionally welded

production SCRs reaches beyond 4,000m water depth due to large static longitudinal loads exceeding 60% of yield capacity

of the pipe. Similarly, 24inch OD export risers lead to wall thicknesses greater than 2.0inch due to high external collapse

pressures and very high static axial loads, and hence are not feasible for conventionally welded SCR applications in ultra-

deep water depths. The maximum water depths obtained by large OD (16 to 24-inch) export risers with conventionally

welded SCRs are below 3,400m. It is determined that lazy wave riser systems are a feasible option for water depth beyond

the reach of SCR due to additional uplift forces obtained from the buoyancy modules.

Installation assessment considers 2,000mT as the maximum tension limit of a J-lay installation vessel. The production

SCRs in 8 to 12-in OD can be installed down to 4,500m water depth. Due to the oil export risers’ high top tensions,

installation limiting water depth is much less in the 3.500m range compared to the production SCR.

The use of a flexible joint is evaluated at the SCR hang-off. High pressure 12inch production SCR, 20inch and 24inch

export SCRs in water depths ranging from 3,000m to 4,500m may exceed the current axial tension capacity of conventional

flexible joint technology. Therefore, tapered stress joints need to be considered at the hang-off interface.


16 OTC-25840-MS

Dynamic strength analysis is conducted using conventional deep draft semi-submersible floater. Based on the assessment,

it is determined that during extreme environments, the SCR experience high stresses exceeds the API-RP-2RD code

allowables. In addition, compression and yielding is observed in the touch down zone of the risers during hurricane

conditions, which may affect the integrity of the riser. On the other hand, the lazy wave configuration results in acceptable

strength response in all environments considered. Lazy wave risers provide a good alternative to conventional SCRs and may

need to be used with high motion floaters in these water depths to meet the code requirements.

As the water depth increases, the mixed mode axial and bending excitation frequencies of the risers tends to move into the

range of wave energy and increases the likelihood of exciting the risers in small period seastates that tend to drive long term

fatigue life. If a floater has a heave response even with small amplitudes at a mixed mode excitation frequency matching that

of the riser, it would be sufficient to induce significant dynamic motions in a suspended riser and increase fatigue damage.

This phenomenon needs to be further evaluated with different floaters.

Overall, conventionally welded SCR systems with high motion floaters are challenging and this preliminary assessment

shows that they do not provide a robust solution where water depths are between 3,000m to 4,500m. Unconventional

approaches will be required for these extreme depths to reduce the compression response of the SCR including additional

weight in the sag area or the use of buoyancy mid water to generate a lazy wave configuration.

Nomenclature σy = Material minimum yield strength, defined for steel or titanium as the tensile stress required to produce a total

elongation of 0.5% of the test specimen gauge length

API = American Petroleum Institute

AUT = Automated Ultrasonic Testing

GoM = Gulf of Mexico

Hs = Significant Wave Height

mT = Metric Ton

MWL = Mean Water Level

OD = Outer Diameter

RAO = Response Amplitude Operators

SCR = Steel Catenary Riser

SLWR = Steel Lazy-Wave Riser

TDP = Touch Down Point

TDZ = Touch Down Zone

Tp = Peak Period

References [1] Bureau of Ocean Energy Management, “Offshore Statistics by Water Depth Database”.

[2] American Petroleum Institute, 2007, “Specification for Line Pipe”, API Specification 5L, Forty-Fourth Edition.

[3] American Petroleum Institute, 1999, “Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines

(Limit State Design)”, API RP 1111, Third Edition.

[4] American Petroleum Institute, 2014, “Derivation of Metocean Design and Operating Conditions”, API RP 2MET, First Edition.

[5] Code of Federal Regulations, 2009, “Transportation of Hazardous Liquids by Pipeline”, 49 CFR 195.

[6] U.S. Department of Interior-Minerals Management Service, 2009, “Alternate Compliance and Departure Requests in Pipeline

Applications”, NTL No. 2009-G28.

[7] Code of Federal Regulations, 2010, “Oil and Gas and Sulphur Operations in the Outer Continental Shelf”, 30 CFR 250.

[8] American Petroleum Institute, 1998, “Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms

(TLPs)”, API RP 2RD, First Edition

[9] MCS International, 2009, “Flexcom-3D: Three Dimensional Nonlinear Time Domain Offshore Analysis Software”, Version 7.9,

MCS.

[10] Tenaris Media and Publications, News, 2013 “Tenaris Provides Heavy Wall Line Pipe for the Julia Project in the Gulf of Mexico”,

www.tenaris.com/en/MediaAndPublications/News/2013/July/JuliaProjetc_Mexico.aspx.

[11] Novelli, P., Armengol, M., Darcis, P., Motta, L., Anelli, E., Fazackerley, B., Kristiansen, P., Blockhus, D., J., 2011, “Cascade and

Chinook Project: Pipes and Bends: Material Design and Production”, Proceedings of the OTC Conference, Paper OTC21832,

Houston, USA.

[12] Darcis, P, P, Mota, N, Garcia, E, Marines, I, Quintanilla, H, M, Quintanilla, Kan, W, C, Kan, Visco, T. Visco, and Ghosh, R, 2013,

“Fatigue Qualification of Heavy Wall Line Pipe and Girth Weld for High Pressure Applications”, Proceedings of the 32nd ASME

OMAE Conference, Paper OMAE2013-10853, Nantes, France.

[13] Burk, J., D., Ribardo, C., L., 2010, “Thunder Horse – Materials, Welding and Corrosion Challenges and Solutions”, Proceedings of

the OTC Conference, Paper OTC20401, Houston, USA.

[14] Graaf, J., Zandwijk, K., 2014, “Deepwater Construction Vessel ‘Aegir’ Shifting the Frontiers of Reeling”, Proceedings of the OTC

Conference, Paper OTC25101-MS, Houston, USA.

[15] Socotherm, Products, “Five Layer Solid Polypropylene (5LSPP) for Ultra Deep Water”, http://www.socotherm.com/en/products

/wetisokote/wetisokote_5LSPP.html.

[16] MCS International, 2009, “Modes-3D: Three Dimensional Mode Shapes and Natural Frequencies Analysis Software”, Version 7.9,

MCS.


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[17] Li, S, and Nguyen, C, 2010, “Dynamic Response of Deepwater Lazy Wave Catenary Riser.” Proceedings of the DOT Conference,

Amsterdam, Netherlands

[18] Deka, D, Campbell, M, Kakar, K, and Hays, P, R, 2010, “Gulf of Mexico Wet Tree Deepwater Riser Concepts with Sour Service.”

Proceedings of the OTC Conference, Paper OTC20437, Houston, USA.

[19] Seamark, M, J, 1991, “Use of Syntactic Foams for Subsea Buoyancy” Cellular Polymers International Conference, London, UK.

[20] Jackson, A, Jackson, P, Wan, E, and Hegdal J, P, 2009, “A Novel Thermal Insulation System for Bonded Subsea Single Pipe Wet

Insulation Applications”, 18th BHR Conference on Pipeline Protection, Antwerp, Belgium.

[21] Det Norske Veritas, 2005, “Fatigue Design of Offshore Steel Structures”, DNV-RP-C203.


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