Dual string handling feasibility study

Dual string handling Feasibility study

TU Delft report nr. OvS 98/10

IHC Gusto document nr. 6204.1000.302

jf

Dual string handling

feasibility study

GUSTO

Dual string handling

feasibility study

E.F.J. van Leeuwen

M. Spilker

Schiedam, 1998

TU Delft, Maritieme Techniek, Report nr. OvS 98/10

IHC Gusto Engineering BV, Document nr. 6204.1000.302

Preface

This report forms part of our graduation work at TU Delft, Maritieme Techniek., The feasibility study on dual string, handling is performed by order of IHC Gusto

Engineering BV.

We would like to express our gratitude to Dr. Ir. C. v/d Stoep and Jr. J. Lusthof of Gusto Engineering for their support, especially on the use of DYNFLEX.

Schiedam, June 25 1998 E.F.J. van Leeuwen M. Spilker

Dual string handling

feasibility study

E.F.J. van Leeuwen

M. Spilker

Schiedam, 1998

TU Delft, Maritieme Techniek, Report nr. OvS 98/10

IHC Gusto Engineering BV, Document nr. 6204.1000.302

11 I

Preface

This report forms part of our graduation work at TU Delft, Maritieme T'echniek. The feasibility study on dual string handling is performed by order of IHC Gusto

Engineering BV.

We would like to express our gratitude to Dr. Jr. C. v/d Stoep and Jr. J. Lusthof of Gusto Engineering for their support, especially on the use of DYNFLEX

Schiedam, June 25 1998 E.F.J.. van Leeuwen M. Spillcer

Table of contents

Summary

Introduction

2

1 Theory

3

2 Lay out

4

3 Excitation forces

5 3.1 Current 5 3.2 Waves 5 3.3 Ship motions 5

4 Individual load calculation

6

4.1 Conclusions 7

4.2 Suggestion to reduce loading on riser 7

5 Collision occurrence

8 5.1 Configurations 8 5.2 Direction distribution 9 5.3 Case identification 10 5.4 Collision considerations 11 5.5 Simulation input 12 5.6 Simulation assessment 12 5.7 Results 13 5.8 Conclusions 14

6 Critical surge amplitude

15

6.1 Results 15

6.2 Conclusions 15

7 Conclusions

16

Summary

The technique of simultaneously assembling the riser string with the BOP stack attached, while drilling riserless on a template is under consideration for present very

deep-water drilling programs. Significant timesaving can be accomplished for this particular operation by removing actions out of the critical path.

Dynamic motion calculations lead to the following conclusions and recommendations:

Dual string handling is only feasible if environmental conditions are known and constant during the operation. The current profile must be known. In areas were eddy currents can be expected, like GUM, dual string handling is not practicable. If dual string handling is to be applied, emphasis should be put on station keeping ability of the vessel.

Introduction

This report deals with the feasibility of running dual drilling operations from a floating vessel.

The marine riser and drillstring, as suspended below the vessel, are subjected to subsurface currents, wave forces and motions of the vessel. This may lead to

collisions between both strings. Several different scenarios will be investigated for a complete operation. We assume the vessel will be dynamically positioned in a waterdepth of 3000 m. Dynamic motion calculations are performed with DYNFLEX. Firstly, the responses of the riser and the drillstring on the individual excitation forces will be examined. Next, we will investigate collision occurrence of 4 operational configurations on 4 loading conditions, resulting in 16 cases. Thirdly, we will determine how sensitive the motions of the strings are to changes in the vessel's excursion related to the low frequency surge period.

1 Theory

The computer program DYNFLEX will calculate dynamic behaviour of the marine riser and the drillstring. DYNFLEX is a general-purpose time domain simulation program to compute the 3D behaviour of submerged pipe systems excited by end motions, waves and current. Analysis of line tensions and motions are carried out for arbitrary upper-end motions. Though the program has been developed for flexible

risers, it may also be applied to many other submerged slender structures such as

pipes, flowline bundels, hoses, umbilicals and mooring lines.

The program is based on a discrete element technique known as the "Lumped Mass

Method" (LMM). In the LMM the mass and all forces are concentrated ina finite

number of nodes. These nodes are interconnected by linear springs. The bending stiffness is represented by bending springs in the nodes (see figure 1.0.

The model is created by the program STAFLEX. This program calculates the static equilibrium condition, which can be used as a start 'configuration for calculations with DYNFLEX.

Both programs are developed by MARIN.

linear spring bending spring

Model confirm Lumped Mass Method Figure 1.

2 Lay out

A dual RamRig will handle the marine riser and drillstring. In Maritime Tentech's design of the RamRig the distance between both

strings is 101m.

The drillstring will be made up of the following parts:

2700 rii 5?' drilflpipe

180 m hevi-wate

nom

7" drillcollars

The riser will be fitted out with buoyancy packages over its entire length. The outside, diameter of the riser string thus becomes

1,12 m. A 250 tons BOP is connected to the end of the riser string.

As it is expected that the riser string will show the highest offset due to current loads, the initial configuration will have the

drillstring placed forwards of the riser: Detailed input data can be found in the appendix.

-t

m waterdepth 3000m 4 drillpipe 10 A

3 Excitation forces

This investigation will be carried out according to Gulf of Mexico (GOM) weather design data.

3.1 Current

The applied current is typical during a 10-years return period winter storm. The profile of figure 3.1 will be applied.

3.2 Waves

Wave amplitude 3 m.

Zero crossing period 10 s.

3.3 Ship motions

The top end of both strings is excitated by ship motions due to wave forces in both x- and y-direction. First and second order motions are modelled as harmonic motions.

Excitation at top end of strings Table 3.1

I

sway surge Current profile Figure 3.1 5

High frequency Low frequency

Amplitude (m) Period (s)

Amplitude (m) Period (s)

Surge (x) 1 12 20 150

25 20 -a IS I. 10 5

4 Individual load calculation

Firstly, the responses of the riser and the drillstring on the individual excitation forces are calculated. The results are presented below. The displacement of the string is given along its length, from the ship downwards.

Drillstring Riser

Excitation by ship motions (II surge amplitude 20 m, b.C. surge amplitude 1 m)

Excitation by ship motions Drillstring

5.09 5.06

I

Excitation by waves Drilistring

average

mamma

in ixuiiiuim

Excitation by current Drillstring

1000 2001)

waterdepth (m)

Excitation by ship motions Riser

1000 2000 uliteniepth 311110 average Haab/aim maxamm

Excitation by wave forces (wave amplitude 3 m, Tz = 10 s.)

3000

maximum

Excitadun by wines Riser

0 ion X.00

mterdepth (m)

Excitation by current (see profile figure 3.1)

Excitation by current Riser

waterdepth (m) 6 average 1111/1111711 1111701MM maximum 20 10 average 11 0 linininunnaxinwin -10 .a -20 -30 4.97 - .5' 4.94 -5.2 4.91 -5.3 1000 2000 31100 walerdepth 0 2000 3000 -47 48 49 Si, I 40

I

30 10 0 (nth

-4.1 Conclusions

The maximum displacement caused by ship motions is bigger for the riser than for the drillstring. This can be attributed to the higher inertia of the heavy riser string. The maximum displacement caused by wave forces also is bigger for the riser. Due to the large diameter, the inertia force component of the waves becomes

significant (Morison: Fi = Cm '/4 it p du/dt).

The maximum displacement of the riser caused by current forces is 2,5 times as big as that of the drillstring.

The effect of wave forces on the displacement of both strings is insignificant compared to the effect of current.

4.2 Suggestion to reduce loading on riser

Excursions in the magnitude as high as caused by the current are not acceptable in case of dual string handling because a small deviation in current direction will then cause a collision between both strings. To restrain the offset, we propose to

increase the riser tensioner capacity. In current drillship designs, riser tensioner capacity is approximately 1100 tons. The axial stress in the most upper riser joint can be calculated as:

11.000kN

cr = = ,--113N/mm2

A 7-/-/4* (0,542 _0,412)

If the riser tensioner capacity is upgraded to 2000 tons, the outside diameter of the buoyancy packages can be reduced from

1,12m to 0,93m.

Further investigation should be carried out to prove practical attainability.

Less buoyancy forces on riser Increase riser tensioner capacity Smaller buoyancy packages required

Less current & wave excitation Offset of riser decreases 7 D2

-S Collision occurrence

5.1 Configurations

\\\\\\. \\\N

\\\\\\\\\\

\\N\\\\\\\\\x

configuration 1 configuration 2 configuration 3

e ,

0

\\\\\\\\\\N

configuration 4

Drillstring Marine riser configurations

Figure 5.1

8

5.2 Direction distribution

The directional difference between waves, current and wind can ascend to 900.

The investigated combinations are represented in figure 5.2.

Environmental forces Figure 5.2 9, 1 direction I Ii waves i I(

--->

,

current direction 2 current waves

C

, direction 3 waves current

(

_.)

direction 4 waves

_..>

III

5:3 Cizseddentification

EN

-1.

_{warn am1Z.2}

configurationt configuration2 configuration 3 configuration 4

direction 1 direction 2 II, waves. direction 3 waves current current waves direction 4 current waves a 0 i direction 1 2 3 4

c

io %it 1 cz 9. 40 ti-, 1 c o

°

I

I case 1 case 2 case 3 case 4

case 5 case 6 case 7 icase 8

case 9 case 10 case 11 case 12

1 case 13 case 14 case 15 case 16

2

3

4

5.4 Collision considerations

The elements of both strings are of equal length. Consequently a collapse between the strings will be observed when the distance between two nodes at the same elevation '(z-co-ordinate) approaches a value of zero. A time domain simulation of the motions,

as carried out with DYNFLEX, provides the position of each node at each time step during the entire simulation period. It's possible that the collision takes place between two nodes and therefore isn't noticed (see figure 5.3, point A). This can be

encountered by selecting a proper element length. Since the angles are very small and the time step is also small relatively to the speed of the strings, the chance of not noticing a collision is diminutive.

is direction,

Distance between nodes Figure 5.3 y direction 2 6 4 combined ii«xir-x2)2±(yt-Y2)2)

The criteria for collapsing will be set at a distance of 1 m. This makes allowance for interaction between the strings at close distance, accuracy of the calculation process and secures that all collisions are noticed.

5.5 Simulation input

time step 0,005 s.

simulation time 500 s.

swell up time 200 s.

Detailed input data can be found in the appendix.

5.6 Simulation assessment

The motions of the strings, as calculated by DYNFLEX, are post-processed in order to judge the results. This is done in two ways:

Visual judgement of the simulation. The successive position of all elements, at a time step of 0,5 s, is printed on the computer's monitor, side-, front- and top-view. This is a very powerful method to evaluate the dynamic behaviour of the strings.

Collision calculation. By checking the distance between nodes at the same

elevation at each time step, as explained on the preceding page, a collision will be noticed and filed. At the end of this process, a list is composed containing node numbers of collision and time of collision.

12

I.

1000m 2000m 3000m 1000 m 2000 m 3000 m 5.7 Results

The dynamic behaviour can be judged using the visual simulation. As an example of such a simulation, some samples of case 12 are printed below.

225s 250s 275s 300s 325s

(

13 direction 1 2 3 4 = o 1 no no no yes 1,5 = ? _{no no no yes} LTto -i-o ,..) 3 no no no yes no HO no, yes 4 100s 125s 150s 175s 200s 350s 375s 400s 425s 450s Collision occurence n000 in 2000m 3000 tn

5.8 Conclusions

According to the quasi-static approach, the displacement of the marine riser is much bigger than the displacement of the drillstring. Consequently the marine riser should be located in off-stream direction of the drillstring.

If the current direction is constant over waterdepth and time, it has a positive effect on collision occurrence.

Dynamic analysis points out that the low-inertia drillstring accelerates much faster then the high-inertia riser, which may lead to collisions.

The vessel's low frequency oscillation period and amplitude in x-direction are main drivers in collision occurrence.

Current from stern is by no means acceptable.

6 Critical surge amplitude

As the vessel's low frequency oscillation period and amplitude in x-direction are main drivers in collision occurrence, further emphasis will he put on this subject. For case 12 we will determine how sensitive the motions of the strings of pipe are to changes in the vessel's excursion and low frequency period. At the following periods the critical vessel excursion will be determined.

100 s 150 s

200 s 250 s

6.1 Results

Critical surge amplitudes are calculated at integer values for each period. The results are presented graphically below.

50 100 150 200 250 300

low frequency surge period (s)

6.2 Conclusions

At larger low frequency surge periods, higher amplitudes are tolerable.

If dual string handling is to be applied, emphasis should be put _{on station keeping}

ability of the vessel.

15 19 -collision area 18 " 17 = 16 non-collision area 15

7 Conclusions

The riser should be placed in off-stream direction of the drillstring.

The vessels DP system has to be designed in such a manner that the vessel low frequency surge amplitude is between 0,5 and 0,6% of the water depth (depending on the period).

Dual string handling can only be applied if the environmental conditions such as current profile and wave direction are known and constant for a longer period. If the vessels' heading has to be changed due to altering conditions large problems can be encountered. Considering these constraints it is questionable if the gains are high enough to justify the investment of an extra rotary. Less speculative timesaving may be accomplished when the secondary derrick is only used to assemble stands of drillpipes or casing joints.

Appendix

Input DYNFLEX

5-- drillpipe

ENTER -> confirm, ESC -> quit, F3 -> edit, F10 -> delete segment

5- hevi-wate drillpipe

Ms-DOS - st

ENTER -> confirm. ESC -> quit, F3 -> edit. F10 -> delete segment :

STAFLX U95_0a STATIC ANALYSIS OF 3-D FLEXIBLE RISERS not saved CASE0012

segment id 9

segment type : PIPE

pipe name : drillstring

5-pipe mass per unit length pipe polar moment of inertia inner diameter

outer diameter hydro static outer diameter hydro dynamic

axial stiffness : 0.5000E+11 [N]

bending stiffness : 0.1600E+07 [N.m-2]

torsion stiffness : 0.1200E+07 [N.m-2]

normal drag coefficient 1.30 [-] (Or)

tangential drag coefficient 0.20 [-] (OF)

rotational drag coefficient 0.00 [-] (OF)

normal inertia coefficient 2.00 [-] (DF)

tangential inertia coefficient 1.20 [-] (OF)

STAFLX U95_0a STATIC ANALYSIS OF 3-0 FLEXIBLE RISERS not saved CA5E0012

segment id

segment type : PIPE

pipe name : hevi-wate drillpipe

pipe mass per unit length pipe polar moment of inertia

inner diameter

outer diameter hydro static outer diameter hydro dynamic

axial stiffness : 0.2000E+11 [N]

bending stiffness : 0.2340E+07 [N.m-2]

torsion stiffness : 0.1800E+07 [N.m-2]

normal drag coefficient 1.30 [-] (OF)

tangential drag coefficient _{0.20 [-] (OF)}

rotational drag coefficient 0.00 [-] (OF)

normal inertia coefficient 2.00 [-] (OF)

tangential inertia coefficient 1.20 [-] (OF)

t73Ms-DOS - sf _MOM 13 45.00 0.50 0.100 0.130 0.130 74.40 1.00 0.076 0.127 0.130

7" Drill collars

ENTER -> confirm, ESC -> quit, F3 -> edit, F1O -> delete segment :

Marine riser with buoyancy nnekages

STAFLX 1)95 Oa STATIC ANALYSIS OF 3-D FLEXIBLE RISERS not saved _CASE0012

ENTER -> confirm. ESC -> quit, F3 -> edit, F10 -> delete segment :

STAFLX U95_0a STATIC ANALYSIS OF 3-0 FLEXIBLE RISERS not saved CASE0012

segment id 12

segment type : PIPE

pipe name : drill collar

pipe mass per unit length pipe polar moment of inertia inner diameter

outer diameter hydro static outer diameter hydro dynamic

axial stiffness : 0.2000E+11 [N]

bending stiffness : 0.1600E+08 [N.m-2]

torsion stiffness : 0.1200E+08 [N.m-2]

normal drag coefficient _{1.30 [-] (OF)}

tangential drag coefficient 0.20 [-] (OF)

rotational drag coefficient _{0.00 [-] (OF)}

normal inertia coefficient _{2.00 [-] (OF)}

tangential inertia coefficient 1.20 [-] (OF)

segment id 8

segment type : PIPE

pipe name

pipe mass per unit length pipe polar moment of inertia inner diameter

outer diameter hydro static outer diameter hydro dynamic

axial stiffness : 0.2000E+12 [N]

bending stiffness : 0.5800E+09 [N.m^2]

torsion stiffness : 0.4400E+09 [N.m-2]

normal drag coefficient _{1.30 [-] (DF)}

tangential drag coefficient _{0.20 [-] (OF)}

rotational drag coefficient 0.00 [-] (OF)

normal inertia coefficient _{2.00 [-] (OF)}

tangential inertia coefficient _{1.20 [-] (OF)}

Ot. Ms-DOS - st _Num

Ms-DOS - st

eon

224.00 1.00 0.070 0.203 0.210 1080.00 150.00 0.400 1.120 1.128

BOP

STAFLX U95_Oa STATIC ANALYSIS OF 3-D FLEXIBLE RISERS not saved CASE0012

ENTER -> confirm. ESC -> quit, F3 -> edit, F10 -> delete segment :

Marine riser suspended with a BOP attached and disconnected (LINE) 'IMs-DOS -st

STAFLX USS_Oa STATIC ANALYSIS OF 3-0 FLEXIBLE RISERS not saved CASE0012

begin end

id cotype

2 UESSEL

4 FREE

name line/vessel NR arc length [m]

riser to drillship 1

bop free t3000

ENTER -> confirm, ESC -> quit, F3 -> edit, F10 -> delete line :

segment id 11

segment type : PIPE

pipe name : BOP

pipe mass per unit length : 25000.00 [kg/m]

pipe polar moment of inertia 40000.00 [kg.m-2]

inner diameter 3.900 [m]

outer diameter hydro static 4.000 [m]

outer diameter hydro dynamic 4.000 [m]

axial stiffness : 0.1000E+14 [N]

bending stiffness : 0.1300E+12 [N.m-2]

torsion stiffness : 0.1000E+12 [N.m-2]

normal drag coefficient 1.50 [-] (OF)

tangential drag coefficient 0.20 [-] (OF)

rotational drag coefficient 0.00 [-] (OF)

normal inertia coefficient 2.50 [-] (OF)

tangential inertia coefficient 1.20 [-] (OF)

Line name : riser line number

2

number of components : 2

id type name length [m] NR elem

8 PIPE riser 2940.00 49

11 PIPE BOP

moo

1

Drillstring connected (LINE)

STAFLX U95_0 STATIC ANALYSIS OF 3-D FLEXIBLE RISERS not saved CASE8812

ENTER -> confirm. ESC -> quit, F3 -> edit, F10 -> delete line :

Drillstrina disconnected (LINE)

STAFLX 1)95 Oa STATIC ANALYSIS OF 3-0 FLEXIBLE _{RISERS saved CASE0014}

ENTER -> confirm. ESC -> quit, F3 -> edit, F18 -> delete line :

Line name : drillstring line number : 1

number of components : 1

Id type name length [m] NR elem

9 PIPE drillstring 5- 3000.00 50

Id cotype name line/vessel NR arc length [m]

begin : 3 VESSEL string to drillship 1

end : 7 GLOBAL strin. rotatin.

Line name : drillstring line number : 1

number of components : 4

id type name length [m] NR glom

9 PIPE drillstring 5- _{2640.00 44}

13 PIPE hevi-wate drillpipe 180.00 3

12 PIPE drill collar 120.00 2

12 PIPE drill collar 10.00 1

id cotype name line/vessel NR arc length [m]

begin : 3 UESSEL string to drillship

1

end : 5 FREE stria free t3000

tq', Ms-DOS - st _EOM

Waves

-wave frequency : 0.628 [rad/s]

wave length 156.131 [m]

wave number 0.040 [-]

".ENTER -> confirm. ESC -> quit, 13 -> edit

Current

STIOX 1)95_0a STATIC A

0.00 2850.00 2900.00 2940.00

neo.oe

MALY

number of data points : 5 current profile

height above seabed

[N]

ENTER -> confirm, ESC -> quit, 13 -> edit :

PIIMPIL ME_

IS OF 3-0 FLEXIBLE RISERS not saved CASi9014

direction velocity [deg] Cm/s1 90.00 0.20 90.00 0.20 90.00 0.30

woe

0.40 schoo 0.60

ST AFL X 095_0a STATIC ANALYSIS OF 3-0 FLEXIBLE RISERS not saved C

wave data

toggle (yin)

wave direction : 180.000 [deg]

wave amplitude : 3.000 [m] wave period 10.000 [s] E3 IS MsDOS -%Ms-DOS

ENTER -> confirm. ESC -> quit. F3 -> edit :

Simulation

?:11; Ms-DOS - sf ISE313

STAFLX U35_0a STATIC ANALYSIS OF 3-0 FLEXIBLE RISERS not saved CASE0014

ime step 0.00500 [s] simulation input

tension tolerance 0.1000E-05 [-]

maximum number of iterations 50000 [-]

swell up steps 40000 I-] 200.00 [s]

simulation steps 99999 _E-] 499.99 [s]

steps wave update (transfer.) 100 [-] 0.50 [s]

steps wave update (Interpol.) 100 _(-1 0.50 [s]

Analysis method 2 [-] 1 = Transient