sizing of semi

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Sizing a Semi-Submersible for SCR Feasibility

Prakash Mokkarala & Vivek JaiswalGranherne, KBR

Houston, Texas, USA

ABSTRACT

Often field development studies include a preliminary feasibility of the

vessel and riser combination. For economics, a steel catenary riser(SCR) is the first choice to carry the hydrocarbons along with semi-submersible that can accommodate topsides and risers. Traditionally,hull sizing is performed for accommodating topsides size and weight,

air gap and stability. The riser engineer finds out at a later stage that thevessel motion was not good enough and discards several concepts thatmay include semi-submersible with SCR. In this paper, a methodologywill be shown that optimizes the vessel upfront for semi motions that

govern SCR performance. The key indicators for SCR feasibility arethe strength at extreme, survival and fatigue life. Critical locations for

SCR stresses are at the touchdown and at the hang-off. An example thatdemonstrates how the vessel RAOs are calculated by optimizing heavefor both strength and fatigue are shown. The methodology uses key

indicators like heave velocity which relates to the SCR touchdownstress.

KEY WORDS: SCR, fatigue, Semi-submersible, optimization,extreme, RAO

NOMENCLATURE

FEED Front End Engineering Design

GM Metacentric heightHs Significant Wave HeightRAO Response Amplitude OperatorSCR Steel Catenary Riser

SITP Shut-in Tubing PressureTp Peak Period

TDP Touchdown PointTDZ Touchdown Zone

WADAM Wave Analysis by Diffraction and Morison Theory

INTRODUCTION

SCRs are currently installed on several types of floaters namely Spar,TLP, Semi and FPSO. Floater choice depends up on the field

development. Water depth of the field is a major consideration in

choosing a Spar, TLP or semi whereas the availability of thinfrastructure is a key consideration for choosing a FPSO. Floatevertical motion at the SCR hang-off is very small in the case of Spar o

TLP. In the case of semi, the hang-off heave motions are large and thiin turn affects the SCR touchdown stress. In this paper, SCR with semi is the concept that will be discussed.

Semi-submersibles are being used in many geographical regions anunder different water depths (Figure 1). SCR with semi is a populachoice where the water depths are larger than 5000 ft. Challenges fo

the SCR remain in water depths less than 5000 ft, i.e. fatigue at thetouchdown and at hang-off. Many of the fields for hydrocarbodevelopment do lie with in this water depth and they select expensiv

flexible riser or more expensive 'hybrid' riser solutions to work with semi-submersible. In this paper, emphasis is on making the semfeasible for a SCR at a water depth of around 5000 ft.

Figure 1 Semi-submersible with Mooring Lines and SCRs

9

Proceedings of the Twenty-fir st (2011) In ternational Of fshore and Polar Engineeri ng Conference

Maui, Hawaii, USA, June 19-24, 2011

Copyri ght 2011 by the International Society of Of fshore and Polar Engineers (I SOPE)

ISBN 978-1-880653-96-8 (Set); ISSN 1098-6189 (Set); www.isope.org


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Figure 2 Semi-Submersible Deck Equipment

PROCESS OF SIZING A SEMI

Initial footprint of the semi is based on the required area for the decklayout (Figure 2). Simple geometry, square columns and ring pontoon,

is assumed. An excel spreadsheet is used select the sizes of pontoonand the column particulars. The sizing of semi is based on requirementsof airgap and various functional requirements. However, the SCR

motion requirements at the porch are not included. In the following, itwill be demonstrated that as the SCR motion constraints at the vesselhang-off are included, the weight penalty increases. The size of thesemi that can make SCRs feasible can be determined quickly using an

excel spreadsheet with an optimizer. Thus, the vessel designers canstart the detailed design of the semi which can also makes SCRsfeasible.

Hull Design Criteria

Semi-submersible: Conventional 4-column designDeck box or Truss: Depends on the topsides weight which is in-turndepends on the subsea systems, station keeping, topsides equipment.Topsides integration: to reduce the costs, quayside integration is

preferredWater depth: 5,000 ftAir gap: positive value or per local regulations for 100-year and 1000-year return periods

Stability: Max. heel 12 degrees for 100-year hurricaneDesign environment: 100-year hurricaneStorage requirements: oil, water etc., ballastLiving Quarters: 100

Topsides design group then determines the weights and arearequirements to size the overall semi-submersible facility. The design

considerations from topsides include topsides arrival temperatures,pigging, chemical injection requirements, helideck, cranes, riser pullingequipment etc.

Riser and Mooring loads are also needed for the semi sizing. These are

estimated from past experience.

Ballast

The spreadsheet outputs include ballast weight and check on thavailable space for ballast. Excessive ballast prediction can b

controlled by providing limits.

Manufacturability (quayside integration)

Current GoM facilities prevent drafts exceeding 40m due to dry dockavailability or water depth limits during inshore tows. The draft value iuser controlled and was not varied for a set of design inputs.

Methodology

A simplified method was used to develop semi sizing spreadsheet andwas validated with benchmark semis built throughout the world. ThSpreadsheet calculates heave, roll and pitch RAOs based on estimateweight distribution. Figure 3 shows the methodology in a flow chart.

Figure 3 Methodology Used in Semi Optimization

OPTIMIZATION PROBLEM

An optimization problem was set up that includes objective functiondesign variables and design constraints. Excel's built-in tool 'Solverwas used to solve the optimization problem.

Objective Function

Main objective is to reduce the weight of the semi-submersible.

Design Variables

Design variables are pontoon width, and height, column size, antransverse (longitudinal) spacing between the columns.

Constraints

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Constraints include GM, deck area, sizing ratios. A database of semi-submersibles was compiled from the public domain data (Offshore,2008) and certain ratios like the pontoon width to height, column width

to pontoon width were developed to size the semi.

Motions

Additional constraints were added to tighten the problem, namely theheave period, maximum heave RAO between 10 to 18s, and maximum

pitch RAO values. These values were derived from riser engineers,based on SCR analysis.

Heave period at least 20sHeave RAO (10-18s) 0.35Pitch RAO (10-18s) 0.15 deg/m

Guidance

The spreadsheet has guidance for infeasible solutions by means ofsuggesting if the draft needs to be increased or some variables (columnsize) need to be adjusted.

Iterative process

The optimization problem is solved based on Linear Programming. Thesolution is found by linearly varying the design variables a small

amount. The solution may depend upon the starting values which meanthat the solution may be only a local optimum. This can be improvedlater by using a MATLAB based powerful optimizer.

Verification

Once the 'Solver' identifies an optimized solution, RAOs were verifiedthrough a detailed WADAM model. Consistently, WADAM model

showed a 30% higher heave motions than excel based tool. Still, thetool was considered a quick turn around utility.

PRELIMINARY SCR DESIGN

Steel Catenary Risers (SCR) are initially designed to withstand theinternal pressure, external pressure, burst criteria per API RP 1111 andother applicable codes. A static analysis with offset (static offset anddynamic offsets, low frequency motion) in near, far, and cross

directions can be used to configure the riser hang-off angle. A flex jointor stress joint will be used at the hang-off to reduce the vessel bendingstresses at the hang-off. If the riser is straked, higher stresses can beexpected at the hang-off and at the TDZ due to higher drag coefficient.

Large vessel heave motion at the riser hang-off causes high stresses atSCR hang-off and at the TDP. Fatigue of the SCR also depends on thesoil stiffness at the TDP. Assuming rigid or softer sea bed stiffness hasimplications on riser design. In this paper, minimizing the vessel heave

motion is the main focus. Other methods can be employed bymodifying SCR to improve fatigue performance are not discussed.

EXAMPLES

Below are some examples to illustrate the workings of the semispreadsheet optimizer. The intent of the rest of the section is to producevessel designs for the SCRs feasible from strength and fatigue point of

view at a Pre-FEED stage.

Semi-Submersible, Design 1

A semi is to be designed in a water depth of 1524m (5,000 ft). Tosides weigh about 15,000 short tons. There are 20 SCRs that includ

production, export, etc. Risers are connected to the semi with flex join

or stress joint. The SITP for the production SCR is 15,000 psi. The fielis located in Gulf of Mexico. A ring pontoon semi will be sized. 10year hurricane (Hs 15.5m and Tp 14.8s) environment was used.

Deck area (footprint) required to place the topsides equipment is 7,00m2. Airgap (freeboard) requirement is 17.5m. Starting with a minimum

draft of 26m (85.3ft), heave motion at the riser porch will be calculatedThere are no constraints or limits imposed on the vessel motion fromSCR point of view. Thus, these motions can be termed as resultinfrom the topsides and stability requirements. Semi optimizer tries t

minimize the weight by varying the design variables that includcolumn spacing, column width and pontoon dimensions. Limitimposed for the semi design are shown in Table 1 .

Table 1 Design Variable Limits

User Constraint

Variables and Constraints: Min Max

Outer Column Width & Length (m) 10.40 22.1

Transverse Column Spacing (Ysp) (m) 52.00 110.5

Longitudinal Column Spacing (Xsp) (m) 52.00 110.5

Pontoon Height (all) (m) 6.50 12.5

Longitudinal Outer Pontoon Width (m) 14.00 20.8

Longitudinal Inner Pontoon Width (m) 14.00 20.8

Cross Pontoon Width (m) 14.00 20.8

Table 2 shows the resulting output from the spreadsheet. In additiontransverse GM and longitudinal GM of 4.6m and a transit draft GM o4.3m was obtained. A heave natural period of 25.2 s was obtained. Al

ratios, pontoon width to height and column width to pontoon width etcpassed the set criteria.

Table 2 Semi-Submersible with Topsides Area Constraint

Summary of Principal Dimensions

Outer Column Width & Length (m) 16.0

Transverse Column Spacing (Ysp) (m) 85.0

Longitudinal Column Spacing (Xsp) (m) 85.0

Pontoon Height (all) (m) 9.0

Longitudinal Outer Pontoon Width (m) 16.0

Longitudinal Inner Pontoon Width (m) 16.0

Cross Pontoon Width (m) 16.0

Draft (m) 2

Vessel Displacement (tonnes) 68027.

Ballast weight calculated was 7,802 tonnes. Hull weight estimated to b29,390 tonnes. Figure 4 shows the plan view and the elevation of thdesigned semi. Figure 5 shows the heave RAO calculated at 18s to b

0.53 by the spreadsheet. Figure 6 shows pitch and roll RAOs.

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26m

34.5 m0 m

16.00m

53m

8.00m

9.00m

43.00m

8.50m

ELEVATION VIEW

PLAN VIEW OF PONTOON

60.5m

101m

85m

101 m

85 m

16 m

16 m

16m

26.00m

Figure 4 Semi-Submersible Design Design 1

Heave RAO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 5 10 15 20 25 30 35

Period (sec)

Heave(m/m)

Hdg=0 Hdg=90

Figure 5 Heave RAO Design 1

Roll & Pitch RAO's

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0 5 10 15 20 25 30 35 40

Period (sec)

AngularMotion(rad/m)

Pitch, Hdg=0

Roll, Hdg=90

Figure 6 Pitch and Roll RAOs Design 1

For the semi in Design 1, SCR stresses at the touchdown are expecteto exceed the allowables. SCR touchdown stress for the extreme evenis directly proportional to the porch heave velocity. Heave velocit

limit for an assumed riser located on the inside of the pontoon for 100-year extreme event (Hs=15.5m, Tp=14.8s) is about 1.62m/s. Thilimit was derived from the API RP 2RD allowable stress, 80% of th

material yield (X-65 material is assumed), for an extreme event baseon past studies. For the semi-submersible in Design 1, a WADAMmodel was run to calculate the maximum heave velocity at the risehang-off. Using statistics (Ochi, '81), the maximum heave velocitarrived was 3.8 m/s. Therefore, the semi designed purely from th

topsides consideration would make the SCR infeasible, since the stresutilization equals 3.8*100%/1.62 = 234%.

Now, additional constraints will be introduced to the optimizer throug

heave RAO limits in the extreme and for fatigue.


First, try to resize the semi for meeting the SCR heave velocity limit fothe extreme sea state.

Heave RAO limit for the extremes, and for X-65 SCR, is derived a

follows:

For GoM, 100-year event wave height can be considered as 1.86 timeHs. Assuming a regular wave, inputs and outputs are sinusoids

Maximum vessel heave response at the extreme wave is proportional tthe maximum wave amplitude. Calculate heave RAO (at the hang-offneeded such that the maximum TDP stress of the SCR is within th

allowable.

ampH

VRAOTherefore,

velocityheavevesselisVwheret)(RV

wavetheofamplitudetheis

ampHandheavevesselisRwhereampRAO*HR

heightwaveiset) wher(A

=

=

=

=

sin

cos

Heave RAO = 1.62m/s / ([Hs*1.86*0.5]*2*pi*(1/14.8))

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= 0.264

Table 3shows the results of the excel optimizer with the new heavemotion constraint. It can be observed that the column size was

increased to accommodate reduced heave RAO requirement. Alsopontoon width and height were reduced.

Table 3 Semi-Submersible with extreme heave constraint

Summary of Principal Dimensions








Draft (m) 30

Vessel Displacement (tonnes) 74795

It is clear, by comparing Table 2 and Table 3, that to design a semi-submersible for SCR to be feasible at extreme, it needs to be 7,000

tonnes heavier. The additional weight is mainly due to increased draftfrom 26m to 30m. Ballast calculated by the spreadsheet was 11,802tonnes. Figure 7 shows the resulting heave RAO of 0.265 at 14s. Figure8 shows pitch and roll RAOs. Notice the maximum amplitude for pitch

RAO dropped almost 50% from Design 1. Figure 9 shows theoptimized semi shape having a larger area around the columns.Maximum heave velocity calculated (statistically) as 2.7 m/s at a riser

porch location inside the pontoon. This value is still higher than the

desired 1.62 m/s. The discrepancy between excel calculated RAOs toWADAM predicted RAOs can be attributed to the approximationsmade in the spreadsheet based calculations. Another round of iterationis needed to bring the RAO to the desired limit.

Heave RAO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20 25 30 35 40

Period (sec)

Heave(m/

m)

CG, Hdg=90


Roll & Pitch RAO's

0.000

0.005

0.010

0.015

0.020

0.025

0 5 10 15 20 25 30 35 40

Period (sec)


Spreadsheet Pitch, Hdg=0

Spreadsheet Roll, Hdg=90


26m

31.63 m24.61m7.02m0 m

14.5

8m

48.6

8m

10.2

0m

6.5

0m

49.5

0m

8.5

0m

ELEVATION VIEW


64.5

m

98.2

4m

83.6

6m

104.08 m

83.67 m

20.41

14 m

20.4

1m

14m

30.0

0m

Figure 9 Semi-Submersible Design Design 2

SCR may be feasible from extreme stress point of view. However, thfatigue response may be unacceptable.


To make the SCR feasible from the fatigue point of view, one shoullook at the scatter diagram and select sea states that are dominating thfatigue damage. Low frequency motions were not considered. B

comparing the wave frequency damage, let us assume the maximum

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damage occurs for Hs of 2.5m and Tp of 6.9s. The heave RAOresponse from Figure 7 is 0.036. Using the narrow band fatigue

damage, RMS stress was calculated for the sea state that causes 70% ofthe damage. Using the heave velocity limit, an acceptable heave RAOlimit was derived for the critical fatigue sea state and this happened to

be at 0.023.

Re-analyzing with the heave limit for fatigue and the extreme resultedin new design as shown in Table 4.

Table 4 Semi-Submersible design with extreme and fatigue heave

constraint








Draft (m) 36

Vessel Displacement (tonnes) 77396

Figure 10 shows elevation and plan view of the optimized semi design.It is interesting to note that the design with heave constraint at fatigue

sea state resulted in narrower columns and a thicker pontoon. Thedisplacement of the semi was increased slightly to 77,396 tonnes fromDesign 2. The weight increase can be attributed to the higher draft,from 30m to 36m. Ballast predicted to be 13,511 tonnes with Design 3.

Hull weight is estimated to be 33,050 tonnes. Figure 11 and Figure 12show the heave and pitch/roll RAOs respectively.

A WADAM model was built to verify the RAOs and the values ofheave RAOs are 0.35 and 0.021 at 15s and 7s respectively. Spreadsheet

RAO are off by about 32% for the extreme and therefore, additionaliteration is required till the desired result is obtained. Maximum heave

velocity calculated from WADAM (statistically) is 2.22 m/sec for theextreme and 0.064 m/s for the fatigue. This process was verified on arecent in-house project which showed improved fatigue lives andextreme strength for an SCR in a harsher environment.

26m

32.69m22.49m10.2m0 m

14.36m

51.04m

9.14m

7

.44m

54.56m

8.50m

ELEVATION VIEW


70.5m

98.04m

83.68m

101.92m

83.65 m

18.27

14 m

18.27m

14.26m

36.00m

Figure 10 Semi-Submersible Design 3

Heave RAO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20 25 30 35 40

Period (sec)

Heave(m/m)

H dg= 0 H dg= 90


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Roll & Pitch RAO's

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0 5 10 15 20 25 30 35 40

Period (sec)


Pitch, Hdg=0

Roll, Hdg=90


CONCLUSIONS

In this paper, an approach was presented for a semi-submersible design

from SCR feasibility. Vessel motion at riser porch directly affects SCRstrength and fatigue. By setting heave velocity targets for the semi

design based on extreme and critical fatigue sea states is highlyrecommended.

Three semi-submersible designs were presented. Design 1 was thebaseline for a semi design from topsides footprint requirements. Design

2 includes consideration for extreme environment by limiting heaveRAO. Design 3 further optimizes the semi design by limiting the heavemotion at a critical fatigue sea state. The trend was increase of draft asthe heave RAO limits are reduced. For the fatigue sea state at low

period, however, the column footprint was reduced and the pontoothickness was increased. The problem was solved with an exce

spreadsheet and with a built-in optimizer. It can be concluded that thapproach suggested here that represents best upfront guidance that ca

be given to vessel designer in terms of semi sizing for making SCRfeasible.

ACKNOWLEDGEMENTS

Many thanks to Richard D'Souza, Vice President, Granherne, fomaking the semi sizing spreadsheet available and thanks to Bob Gordo

for his technical guidance.

REFERENCES

1. 'Design of Risers for Floating Production Systems (FPSs) amTension-leg Platforms (TLPs), API RP 2RD

2. ' World wide Survey of Semi-FPSs and FPUs', Offshore MagazinPoster, 20083. 'Design, Construction, Operation, and Maintenance of Offshor

Hydrocarbon Pipelines (Limit State Design)', API RP 11114. 'SESAM USER MANUAL', HYDROD, WADAM, POSTRESP5. Ochi, M.K., 'Principles of Extreme Value Statistics and thei

Application', SNAME Extreme Loads Response Symposium

Arlington, VA, October 19-20, 1981.

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sizing of semi

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