DP Offloading Operation: a Numerical Evaluation of Wave Shielding Effect

Proceedings of the 8th IFAC International Conference on Manoeuvring and Control of Marine Craft September 16-18, 2009, Guarujá (SP), Brazil

DP Offloading Operation: a Numerical Evaluation of Wave Shielding Effect Asdrubal N. Queiroz Filho*. Eduardo A. Tannuri.** * Dept. of Telecommunication and Control Engineering, University of São Paulo, (e-mail: [email protected]) ** Dept. of Mechatronics Engineering, University of São Paulo, (e-mail: [email protected]) Numerical Offshore Tank TPN-USP Abstract: The propose of this paper is to discuss the advantages obtained by the simulations of offloading operations in the presence of waves, detailing the effects that arise when body’s interactions are taken into account. Effects are sensed in the equilibrium position, on the drift’s forces and the necessary power to dynamic position system. On this paper a simulation of a real offloading operation is presented. Results are shown with and without the shielding effect. Keywords: simulation, dynamic position system, offloading operation, shielding effect, zero shielding method, exact shielding method, FPSO, shuttle ship. 1. INTRODUCTION Nowadays, a great number of offloading operations is held using a dynamic position (DP) system. The DP system allows the position between the shuttle tanker (ST) and the FPSO platform (Floating Production Storage and Offloading), to be maintained constant, bringing some several advantages to the operation. In this kind of operation the two vessels must stay very close in the sea, increasing significantly the hydrodynamic interaction effects between them. When a body is floating at the sea, it becomes into a barrier to the wave propagation, disturbing the wave field around it. This effect is called shielding effect or shielding area. The shielding area depends on the wave length, the size of the body and the wave direction. During oil offloading operations, the connection between the ST and FPSO is executed at the distance of approximately 80m, and the oil transfer is performed at distance of approximately 160m. Due to proximity and size of the vessels, the shielding effect is very important. In this paper, attention is focused on the influence that the wave shielding effect causes in the wave forces experienced by the shuttle tanker, especially concerning the power demanded from the DP system to perform the station-keeping task. In the present paper, a numerical simulator that correctly evaluates the shielding effect is used, and a real offloading operation in Campos Basin is considered. Results are shown with and without the shielding effect. A detailed comparison between the results is presented, stressing the necessity of incorporating such hydrodynamic effect during DP design and power dimensioning. 2. NUMERICAL EVALUATION OF WAVE EFFECTS ON OFFSHORE OPERATIONS The traditional simulation method to predict motions of multi-body systems simplifies the hydrodynamic (wave and current) interactions between bodies. In the present paper, a simulator that correctly evaluates the wave loads in a 978-3-902661-51-7/09/$20.00 © 2009 IFAC

multibody system is used. Such simulator uses the computational kernel code and the hardware facilities of the Numerical Offshore Tank (TPN), a 216 processor cluster with the ability to perform detailed simulations considering bodies and lines dynamics very efficiently. Such system has been developed at the University of São Paulo since 2001, in cooperation with the oil state company Petrobras, as presented in Nishimoto, et al. (2003). Furthermore, the simulator integrates the commercial wave analysis software WAMIT (WAMIT, 2000), which performs a frequencydomain calculation of potential wave effects on the floating bodies. WAMIT software is able to predict loads on a multibody system, since it considers the radiation and diffraction of waves due to each body, and the influence of such effects in the overall wave field. The outcomes of the frequencyanalysis performed by WAMIT are drift coefficients, added masses, first order forces and the RAOs (Response Amplitude Operators). This data is used as input by dynamical simulators, including the TPN. In traditional simulation method, WAMIT software is executed only once, before the simulation itself begins, in order to evaluate all the coefficients used by the simulator (drift coefficients, excitation forces, added masses and RAOs). The execution of the WAMIT must consider a fixed and predefined configuration of the bodies, which generally is chosen as the initial condition of the simulation or an estimated mean position. In such case, the simulator uses the same wave coefficients along all simulation, even when the relative position of the bodies changes significantly. However, during the simulation, the relative position of the bodies changes, due to transient behaviour or persistent oscillations, altering the wave field incident on each body and the forces on it. Some times, for the sake of simplification, WAMIT software is executed considering each body isolated, disregarding, in this case, all interaction effects that may appear in the real system. This method is also called the zero shielding method, as presented in Orozco (2003).

382

10.3182/20090916-3-BR-3001.0015

8th IFAC MCMC (MCMC 2009) September 16-18, 2009, Guarujá (SP), Brazil

The integration of the TPN kernel and the WAMIT solves such problems. During the simulation, body positions are monitored, and, when a significant alteration is detected, the simulation is interrupted and the WAMIT software is executed, considering this new configuration. This is called the exact shielding method and is the one that brings bests results as presented Orozco (2003). All wave coefficients in TPN input data are, then, updated, and the simulation is restarted. Due to the necessary computational effort required by WAMIT software, this approach is only feasible in a highperformance computer cluster. A modified version of WAMIT was specially developed for such integration, with paralleling processing (each wave-frequency is executed in one node of the cluster) and inter-process communication (WAMIT sends/receives messages to/from the TPN). Next section presents a technical description of the simulator, emphasizing the differences of a conventional offshore system simulator and the main features of the parallel processing. A detailed description of the integration between the TPN and WAMIT was presented in Tannuri et al. (2003). The simulation approach is represented in Fig.1. In each time step, bodies’ positions are evaluated and the relative positions between bodies and wave headings are verified. If the variation in such values is superior to a pre-defined value, TPN simulation is interrupted, all WAMIT input files are generated considering the new configuration of the bodies and WAMIT is started. After the execution, WAMIT output files are read by TPN, in order to update the wave coefficients used in the simulation, which is then restarted. The interaction between bodies are determined by the relative positions are ∆X and ∆Y and the vessel headings Ψ1 and Ψ2. Being l the index for the last time step in which WAMIT was executed, and i the actual time step, the Eq (1) is used to determine if a new WAMIT execution must be carried on. The definitions of coordinate system are given in Fig. 2.

Intíca irot S

Reegaa dd ai nd p C a rr o su td d e al i tnah:a s , c oyefd icr oi ednytens. hCi dor eo fs H d i n.,â L m iinc e oss , aW e r oadvine âamni cdosWe i n d de oDnad ta as

t _t_naotuwa l = = 00

 ∆ Xi − ∆ Xi > ∆ X max   or (1)  if  ∆ Yi − ∆ Yi > ∆ Y max  or   max( Ψ 1, i − Ψ 1, l , Ψ 2 , i − Ψ 2, l ) > ∆Ψ max

Fig. 2. Configuration of a two-body system. Obviously, the utilization of small values for the predefined variations ∆Xmax, ∆Y max and ∆ Ψ max causes a large number of WAMIT executions, and the simulator will use more accurate wave coefficients for each configuration, with a better prediction of interaction effects. However, it increases the simulation time. On the other hand, large values decrease the number of WAMIT executions and the simulation time. The interaction effects are, in this case, less accurately predicted. Such trade-off is illustrated in the next section, by means of the case study. TPN and WAMIT codes are adapted for a parallel computing cluster. TPN parallelization creates individual processes for the finite element method calculation of each mooring line, hawser line and hose, besides the main process that is responsible for dynamical simulations of each vessel and evaluation of environmental forces. The processes are then distributed among the nodes (computers). WAMIT parallelization creates processes for each wave frequency, with a reduction in processing time (relative to a single computer processing) approximately equal to 1/n, being n the number of frequencies. For a typical analysis of offshore bodies, n varies between 20 and 50. The parallel version of WAMIT code was developed by Petrobras. 3. CASE STUDY - DESCRIPTION

t t_ _ antuoawl < t_ u lta a çl ã o < st i_mt o

N Não o C á lc uCl oa ldcaus l a f otirço ans odfe v id as r enntot/L coW r rei nn dte/C zau, rve e ilni neh a s

F o rce s

CC á lca ul cl o. o d af swf ao v rçea sfodrecveidsas a o n d a s , u t il iz a n d o o s uco s ienf g dr if t c o ef . a n d ie n te s d e d e ri v a e e x c i t. fo r c e s f ro m fo rç a s d e e xc i ta ç ã o o b ti d o s a p a rtl airs dt a W c oAn M fi gIuTr a ç ã o e x ienci cuiatilo n

A p li cAapç pã loi cdaet iino te n goraf ç ã o n u m éRr iuc n a g(R e -uK n guett- a K u tta ) arg a roabtiteor na m s neoth vaos d i np te p o to s iç õoebst ad ions nc e o rwp o s f lu tu a n te s

Y es S im

p o s i ti o n s

HSoi g u ve n i fiucmaan ta ltaerl te a çr a ã ot iod n as p o s iiçnõ rees l a retil avtiv e aps om s iatii o rnqsu e a d a) ?) (toEl eqra( 1

NNãoo

YSes im

era E xGe e cn u ta Wti o A nM o ITf W c oAmM nIT o va c oi nnpfiug tu fi r al eç sã ow dit ohs acc ot u rpaol s c o n fi g uf lu r atu t ioa n ns te a s n d run

A tuUapl izd a t ceode ri fi ft c iecnot eef.s , d e d e ri veaxc , fo i t.r çFaso rcd ee se a xcnidta ç ã o eRRAAOOs' s

t _naotuwa l == tt__antu a l ++ t_ ow titimm ee__ ss te tepp

In the present section, the TPN is used to evaluate the influence of the wave shielding effect in the wave forces experienced by the shuttle tanker, especially concerning the power demanded from the DP system to perform the stationkeeping task. A typical offloading configuration is adopted, with a SMS (Spread Mooring System) moored VLCC-FPSO (Floating Production, Storage and Offloading System) connected to a DP Suezmax Shuttle Tanker (ST) (Fig. 3). The FPSO is in the full loaded condition and the ST is in the ballasted condition. Main characteristics of each vessel are presented in Table 1.

G e n e r a t io n o f o u t p u t f ile s

Im p re s s ã o d o s r e s u l ta d o s

EF n i md

Fig. 1. Flowcharts TPN and WAMIT integrated. 383

8th IFAC MCMC (MCMC 2009) September 16-18, 2009, Guarujá (SP), Brazil

1 Stern Azimuth P=2.2 00kW T~ -242kN / +374kN X=-100m; Y=0m

Table 1. FPSO and ST characteristics Properties Mass (M) Length (L) Draft (T) Breadth (B)

ST 75694 ton 260 m 8,0 m 44,5 m

FPSO 310720 ton 320 m 9,0 m 54,5 m

1 Bow Azimuth P=2.20 0kW T~ -242kN / +374kN X=100m; Y=0m

Main Propeller + Rudder P=18.891kW T~-1200kN / +2200kN X=-120m; Y=0m

2

4

1

3

5

The shuttle ship is required to be positioned at the green zone, defined in Fig. 4. The relative position with reference to the FPSO is maintained by a DP system. Only wave action is considered in the present analysis. Three wave incidence directions are considered, shown in Fig. 3. The wave significant height is considered to be 3.0m, with peak periods from 7 to 13s, modelled by JONSWAP spectra. The ST is required to keep the position indicated in Fig. 3 (60o related to FPSO longitudinal axis). It must be stressed that in Brazilian waters, 98% of registered wave presents significant height smaller than 3.0m.

1 Bo w Tu nnel P=2.000 kW T~+/-280k N X=+110m; Y=0m

1 Stern Tun nel P=1.600kW T~+/-224 kN X=-110m; Y=0m

Fig. 5. Propeller positions in meters (related to local vessel axis centered at CG), thrusters power (P) and estimated maximum and minimum thrusts (T) Several simulations were carried out, considering the following combinations: 1) Wave direction of 270o, 240o or 210o related to FPSO; 2) Wave peak period of 7, 9, 11 and 13s; 3) Shielding effect i.

le utt Sh

60o

Without (zero shielding method);

ii.

With shielding effect, during oil transfer stage, (d=160m)

iii.

With shielding effect, during connection stage, (d=80m)

FP S O

d

For each simulation, a full time domain analysis is carried out. As an illustration, the next figures present the full set of results obtained in the simulation of one case (wave direction 270o, peak period of 9s, without shielding effect). Trace plot of the whole system, a bar graph showing the mean utilization for each thrust and the relative distance between the vessels are presented in Fig. 6. ST position and heading time series are presented in Fig. 7. It is also obtained the force time series of each thrust (Fig.8), and of the resulting DP forces on each degree of freedom (Fig.9). Finally, it is presented a table containing mean thrust’s forces and standard deviation (Table 2).

270o 240o 210o

Fig. 3. System configuration.

100

500

90

400

80 Us ed Mean Thrus t (% )

300 200 100 0

70 60 50 40 30

-100

20 -200

10 -300 -300

-200

Fig. 4. System configuration.

-100

0

100

200

300

400

0

500

1

2

Distance bet ween hawser connec tion points (m) - max = 163. 8m

3 Thrust er

4

5

min = 131.3m

160

ST propellers’ position, power and estimated thrusts are shown in Fig.5. The system consists of one tunnel and one azimuth thruster in the bow and the same arrangement in the stern portion of the vessel. The main propeller and a conventional rudder are also used by the DP system. To installed DP power is 8000kW (not considering the main propeller power).

150 140

5000

5500

6000

6500

7000

7500

Hawser Tension (tonf) - max = 88.9t onf

8000

8500

min = 0. 0t onf

9000

9500

mean = 0. 0tonf

Fig. 6. Trace plot, mean thrust utilization and relative distance

384

8th IFAC MCMC (MCMC 2009) September 16-18, 2009, Guarujá (SP), Brazil

ST MidShi p X Pos. (m)

standard procedure would result in under-estimating the required DP power. Fig.13 shows that the ST is in a zone where the waves are amplified by approximately 1.2, due to the presence of the FPSO.

o

ST MidSh ip Y Pos. (m)

ST Yaw Ang le ( )

340 -12 0

305.5

338 -13 0

336

305

334 -14 0

304.5

332 304

-15 0

330 303.5

328 -16 0 50 00 6000 7 000 8000 9000

5000 6000 700 0 8000 90 00

Conne c. Po int X Pos. (m)

-60

Conn ec. Po int posi ti on (m) 234

232

232

230

230

228

228

Y(m)

-50

226

-70

-80

226

224

224

222

222

50 00 6000 7 000 8000 9000 Ti me (s)

The main conclusion of this case is that for oil transfer operation (d=160m) under wave condition of Hs=3.0m and Tp from 5s to 13s, the shielding effect may increase the DP required power of up to 589kW. During the connection (d=80m), such difference increases to 687kW. Simulation errors are 23% and 27% respectively if the shadow effect is not considered.

5000 60 00 7000 8 000 9000

Con nec. Point Y Pos. (m) 234

5000 6000 700 0 8000 90 00 Time (s)

-80

-70

-6 0 X(m)

-50

T hrus. 4

Fig. 7. Shuttle ship position.

For the typical shuttle tanker DP layout (Fig. 5), these differences represent 8.6% of the total DP installed power. DP power dimensioning is normally done by static analysis using simple hydrodynamic models, including approximately 20% of extra power to compensate for dynamic loads and other non-modelled effects. The present analysis showed that a part of such extra power may be “consumed” by the wave hydrodynamic interaction effects between both vessels.

300 200 100 0 -100 5000

5500

6000

6500

7000

7500

8000

8500

9000

9500

5500

6000

6500

7000

7500 Time (s)

8000

8500

9000

9500

T hru s.5

100 0 -100 5000

Fig. 8. Delivered forces on each thrust (4 and 5). Total DP power - 270 degrees Tot al Control Forces Surge (kN)

400

3500

200 0

3000

-200 -400 5000

5500

6000

6500

7000

7500

8000

8500

9000

2500

9500

Sway (kN)

160m

500

1500

0 -500

80m

1000

5000

5500

6000

6500

7000

7500

8000

8500

9000

9500

500

4

x 10 Yaw (kN.m)

without shielding

2000

1000

0

2

Tp=5

Tp=7

Tp=9

Tp=11

Tp=13

0 -2 5000

5500

6000

6500

7000

7500 Time (s)

8000

8500

9000

Fig. 10. Total DP power – 90o Wave direction

9500

Fig. 9. Resulting DP forces on shuttle ship

Table 2. Statistics on each thruster Fmed (kN) 5.79442e+001 6.76859e+001 1.13304e+002 1.28713e+002 1.63003e+001

0 1.4

Std. Deviation (kN) 6.35642e+001 8.29730e+001 6.68610e+001 7.49627e+001 5.49069e+001

Y(m)

Thrust P1 P2 P3 P4 P5

-100

1.2

-200

1 0.8

-300 0.6

4. RESULTS

-400

Fig. 10 shows the total DP power for the 270o wave direction, considering the peak period from 5s to 13s. As expected, the DP power decreases as the wave period increases, due to smaller drift forces for larger wave periods. For 5s and 13s wave periods, one can notice that there is a small variation in DP power considering the simulation with or without wave shielding. In the case of 5s, the ST is located outside the mains shadow zone (Fig. 11). For 13s wave period (Fig. 12), the shadow effect is not important due to the large period.

0.4 -500

-600 -600

0.2

-500

-400

-300

-200 -100 X(m)

0

100

200

300

Fig.11 Wave field for a regular wave, with 1m height, 270o direction and 5s period.

In this case, an unexpected behaviour is observed. The power obtained by the simulation with shielding effect is larger that the one obtained with a conventional simulation (without shielding effect). The dimensioning the DP system by a

385

8th IFAC MCMC (MCMC 2009) September 16-18, 2009, Guarujá (SP), Brazil

tanker DP layout, those differences represent approximately 1.4% of the total DP installed power. 0

Y(m)

1.4 -100

1.2

-200

1

For 11s to 13s wave peak period, it is observed that the DP power substantially increases as the ST approaches to the FPSO. This behaviour can be probably explained by change of wave direction propagation induced by the FPSO irradiation. Since for these wave periods the FPSO is not a relevant barrier to the wave propagation, and the shielding effect is not so pronounced, the irradiation effect became relevant, comparing to others wave periods. In fact, the wave elevation map for a 1m height regular wave of 13s period (Fig.16) shows that the shielding effect is not important, and the ST is maintained in a position such that the wave elevation varies only from 0.85m to 0.88m.

0.8 -300 0.6 -400 0.4 -500

-600 -600

0.2

-500

-400

-300

-200 -100 X(m)

0

100

200

300

Total DP power - 240 degrees

Fig.12 Wave field for a regular wave, with 1m height, 90o direction and 13s period.

1400 1200 1000

0

without shielding

800

1.4

160m 600

-100

1.2

Y(m)

200

1

-200

80m

400

0 0.8

Tp=5

-300

Tp=7

Tp=9

Tp=11

Tp=13

Fig. 14. Total DP power – 240o Wave direction

0.6 -400 0.4 -500

0.2

0 1.4

-600 -600

-500

-400

-300

-200 -100 X(m)

0

100

200

300

Y(m)

Fig.13 Wave field for a regular wave, with 1m height, 270o direction and 9s period. Fig.14 shows the total DP power for the 240o wave direction. In this case, the wave direction relative to the ST is 180o (disregarding the disturbing wave field induced by the FPSO). It is interesting to notice that for simulation without shielding, there is a maximum DP power for peak period of 9s, that coincides with a maximum value of the surge drift coefficient. For wave periods from 7s to 9s, the power obtained by the simulation with shielding effect is smaller that the one obtained with a conventional simulation (without shielding effect). In such case, dimensioning the DP system by a standard procedure would result in over-estimating the required DP power. Fig.15 shows that, indeed, the ST is in a zone where the waves are reduced by approximately 0.8, due to the presence of the FPSO (for a regular wave with 9s period). In those cases, the simulation without shielding effect results a larger DP required power of up to 113kW (for oil transfer stage) and 70kW (for connection stage). Simulation errors are 11.1% and 6.8% respectively. Considering the typical shuttle

-100

1.2

-200

1 0.8

-300 0.6 -400 0.4 -500

-600 -600

0.2

-500

-400

-300

-200 -100 X(m)

0

100

200

300

Fig 15. Wave field for a regular wave, with 1m height, 120o direction and 9s period. Fig.17 shows the total DP power for the 210o wave direction. In this case, the ST is positioned almost at the middle of the shielding area, as illustrated in the Fig.18 and Fig. 19. Due to this fact, it is verified a pronounced reduction in the DP power for the simulations with shielding effect, compared to the non-shielding simulations. The difference reaches 2190kW for oil transfer stage (d=160m) and 2435kW for connection stage (d=80m). Simulation errors of almost 52% are verified if the simulation does not take the shielding effect into account. Those values represent up to 30% of total DP power.

386

8th IFAC MCMC (MCMC 2009) September 16-18, 2009, Guarujá (SP), Brazil

0

0 1.4

-100

1.2

-100

1.2

-200

1

-200

1

Y(m)

Y(m)

1.4

0.8 -300

0.8 -300

0.6

0.6

-400

-400 0.4

0.4

-500

-500

0.2 -600 -600

-500

-400

-300

-200 -100 X(m)

0

100

200

-600 -600

300

Fig 16. Wave field for a regular wave, with 1m height, 120o direction and 13s period.

0.2

-500

-400

-300

-200 -100 X(m)

0

100

200

300

Fig. 18 Wave field for a regular wave, with 1m height, 210o direction and 11s period.

Total DP power - 210 degrees 3500

0 1.4

3000 2500

-100

1.2

-200

1

without shielding

2000

160m 1500

80m Y(m)

1000 500

0.8 -300 0.6

0 Tp=5

Tp=7

Tp=9

Tp=11

-400

Tp=13

0.4

o

Fig. 17. Total DP power – 210 Wave direction

-500

5. CONCLUSIONS

-600 -600

On this paper a real offloading operation was presented. It is demonstrated that for accurate simulation results of offloading operations, it is necessary that commercial marine simulators consider the shielding effect. Commercial simulators normally do not consider the shielding effect, causing expressive errors in the results. The results indicated that the shielding effect normally reduces the DP power demand if the shuttle tanker is kept inside the shielding area. In this case, conventional simulations (that does not take into account such effect), over estimates the DP power. Considering a wave with 3.0m significant height, those differences may represent up to 30% of total DP power of a typical DP ST. In However, there are some zones around the FPSO that the diffraction and irradiation effects increase the wave field close to the ST. In those cases, the DP power required to keep position is larger when compared to the simulations without shielding effect. In this case, conventional simulations (that does not take into account such effect), under estimates the DP power.

0.2

-500

-400

-300

-200 -100 X(m)

0

100

200

300

Fig. 19 Wave field for a regular wave, with 1m height, 150o direction and 7s period. 6. REFERENCES Nishimoto, K. et al. (2003). Numerical Offshore Tank: Development of Numerical Offshore Tank for Ultra Deep Water Oil Production Systems, Proceedings of the 22nd International Conference on Offshore Mechanics & Arctic Engineering (OMAE 2003), June, Cancun Mexico. Orozco, J. M. (2003). Offloading Operability JIP – Wave shielding effect. Technical Note, Bureau Veritas. Tannuri E. A., et al. (2004). Dynamic simulation of offloading operation considering wave interaction between vessels, Proceedings of the 23rd International Conference on Offshore Mechanics & Arctic Engineering (OMAE 2004), June, Vancouver Canada. WAMIT, 2000. Wave Analysis Program: Reference Manual, WAMIT, Inc.

387