1. Introduction

Rio Oil & Gas Expo and Conference 2020

ISSN 2525-7579

Conference Proceedings homepage: https://biblioteca.ibp.org.br/riooilegas/en/

Technical Paper

Simulation of the cargo transfer vessel offloadingoperation in a Brazilian oil field

Eduardo Aoun Tannuri 1

Alex S. Huang 2

Asdrubal N. Queiroz Filho 3

Marcos D. Ferreira 4

Fabio M. Passarelli 5

Paulo Moreira de Carvalho 6

Pål B. Korneliussen 7

Petter Stuberg 8.

1. UNIVERSIDADE DE SãO PAULO, TPN-USP, . SãO PAULO - SP - BRASIL, [email protected] 2. UNIVERSIDADE DE SãO PAULO, TPN-USP, . SãO PAULO - SP - BRASIL, [email protected] 3. UNIVERSIDADE DE SãO PAULO, TPN-USP, . SãO PAULO - SP - BRASIL, [email protected] 4. PETROBRAS, CENPES, . RIO DE JANEIRO - RJ - BRASIL, [email protected] 5. PETROBRAS, PETROBRAS, . RIO DE JANEIRO - RJ - BRASIL, [email protected] 6. PPSA, PPSA, . RIO DE JANEIRO - RJ - BRASIL, [email protected] 7. KONGSBERG, KONGSBERG, . TRONDHEIM - NORUEGA, [email protected] 8. KONGSBERG, KONGSBERG, . TRONDHEIM - NORUEGA, [email protected]

Abstract

In a new concept of offloading operation, the oil from a spread moored FPSO is transferred, directly, to a conventionaltanker with an intermediate Cargo Transfer Vessel – CTV equipped with a Dynamic Positioning – DP system. Thisoffloading operation will not require: i) a Dynamic Positioning Shuttle Tanker – DPST and ii) a Ship to Ship transfer (DPSTto conventional tanker) with a reduced total export cost. This paper presents the results of a comprehensive set of Real-Time Maneuvering simulations carried out at the University São Paulo– USP Simulation Center. The study wasperformed with participation of DP Operators and Captains, to verify critical conditions, taking into account the humanfactor. Since the operation involves 4 different vessels (FPSO, CTV, conventional Tanker and pull-back tugboat), it is veryimportant to execute simulations with all the participants, in order to verify not only the technical aspects of each vessel(such as power requirements and limiting motions), but also how the multiple-vessel system will behave when operatingcooperatively. The simulations were executed under typical and extreme environmental conditions of the Libra Field,Brazil. Normal operation and failure scenarios were tested. The actual Kongsberg’s dynamic positioning system of theCTV were integrated to the simulator (hardware in the loop architecture) which provided an extra level of realism in thesimulation. The real-time simulations showed that the common operational maneuvers such as convoy approach andconnection, re-positioning, offloading station change, disconnection and departure could be safely executed in presenceof typical and severe Santos Basin weather conditions. The novel offloading solution showed to be feasible with asatisfactory level of operability. The next steps will be the execution of further risk assessment, with the evaluation of theconsequences of different failures and contingency procedures. Real field tests, with no oil transfer, is also necessary andrecommended.

Keywords: Offloading Operation. FPSO. Dynamic Positioning. Real Time Simulation

Received: February 19, 2020 | Accepted: Jun 06, 2020 | Available online: Dec 01, 2020Article Code: 156Cite as: Rio Oil & Gas Expo and Conference, Rio de Janeiro, RJ, Brazil, 2020 (20)DOI: https://doi.org/10.48072/2525-7579.rog.2020.156

© Copyright 2020. Brazilian Petroleum, Gas and Biofuels Institute - IBP. This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2020, held between

21 and 24 of September 2020, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the final

paper submitted by the author(s).The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum,

Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2020

Proceedings.

https://biblioteca.ibp.org.br/riooilegas/en/

https://orcid.org/0000-0001-7040-413X

Simulation of the cargo transfer vessel offloading operation in a Brazilian oil field

Rio Oil & Gas Expo And Conference, 2020. | ISSN 2525-7579 2

1. Introduction

With the recent discoveries of large oil fields in the Brazilian pre-salt area and the installation of new FPSOs, Brazil exportation of crude oil significantly increased. There is an increasing demand for more DP-STs (Dynamic Positioned Shuttle Tankers), making the offloading operation one of the critical steps in the oil exportation logistics. The crude oil must be transferred from the DP-ST to a conventional Tanker, since the daily rate of the DP-ST is too high, and it is not economically viable to be applied to international navigation.

The industry has considered two options. The solution adopted so far is based on the offloading operation executed by a DP-ST, followed by a Ship to Ship (STS) transfer to a conventional tanker. The STS is carried out at berths in the existing ports, in sheltered bays with the receiving vessel anchored or in an offshore area with both vessels underway.

As a second option, the industry is searching for new solutions for transferring the oil from the Spread-Moored FPSOs directly to a conventional Tanker, still keeping the risk level of the offloading operation acceptable. This option would reduce the dependency on the DP-ST and the overall time required for the oil exportation since the STS operation would not be necessary.

We here focus on a new concept of offloading operation, in which the oil is transferred from the Spread-Moored FPSO to the conventional Tanker by an intermediate Cargo Transfer Vessel (CTV) equipped with a DP system (Syvertsen, and Smedal, 2017). The convoy composed of the CTV, Tanker and Tugboat would approach the FPSO bow or stern offloading station already connected (hawser, between CTV and Tanker, and tug line, between Tanker and Tugboat) and aligned with the resultant environmental force. During this approach phase, the CTV tows the whole convoy, the Tanker main propeller is off, and the tugboat only maintains a minimum tension on the tug line. Once the CTV and Tanker reach the operational sector and the hose is connected (from the FPSO to the CTV), the loading phase begins (

Figure 1).

Eduardo Aoun Tannuri, Alex S. Huang, Asdrubal N. Queiroz Filho,Marcos D. Ferreira, Fabio M. Passarelli, Paulo Moreira de Carvalho, Pål B. Korneliussen, Petter Stuberg


Figure 1: Offloading operation of a SMS FPSO with a conventional Tanker

intermediated by the CTV with support of a tugboat.

Source: produced by the authors.

During the offloading, the CTV DP system enables the CTV to rotate around the FPSO operational sector following the Tanker environmental alignment while keeping the vessels inside the operational sector. The tugboat applies an adequate pull-back force to assist the position-keeping of the Tanker. Figure 2 exemplifies the CTV and Tanker operational sector for the FPSO stern offloading station. Finally, when the offloading ends, the vessels disconnect and depart. The Tanker is ready to export the oil and the CTV to execute another offloading operation. Two units of the Cargo Transfer Vessel have already been constructed and are operating in experimental tests.

Figure 2: FPSO stern offloading station operational sector.



Source: (SeaLoading, 2018)

This paper presents the results of a comprehensive set of Real-Time Maneuvering simulations carried out at the University São Paulo– USP Simulation Center (TPN-USP,2020). The study was performed with the participation of DP Operators and Captains to verify critical conditions, taking into account the human factor. Since the operation involves 4 different vessels (FPSO, CTV, conventional Tanker and pull-back tugboat), it is important to execute simulations with all the participants. The objective is to verify not only the technical aspects of each vessel (such as power requirements and limiting motions) but also how the multiple-vessel system behaves when operating cooperatively.

2. Vessels and Location Characteristics

The CTV main properties are shown in

Table 1 and Figure 3 depicts its main dimensions. The CTV propulsion system is composed of 4 azimuth thrusters: 2 × 4.2 MW (79 tf each) located at CTV's stern and 2 × 2.2 MW (37 tf each) located at CTV's bow. There is another 0.7 MW (11 tf) tunnel thruster, located at CTV's bow, which is only used during port maneuvers; since it is not used by the DP system, its dynamical parameters will not be given. The capability analysis of the CTV DP System is presented by Hu et al. (2019).

The tugboat adopted in this study is based on the Maersk Handler, and its main properties are presented in Table 2. The vessel has the following thrusters: 2ൈ2.5 MW stern azimuth (47 tf each) and 1ൈ0.7 MW bow thruster (11 tf).



This study considers a conventional Tanker of the Suezmax class whose main properties are shown in Table 3 for all the 3 loading conditions (Ballast, Intermediate and Full).

Table 1 Main properties of the CTV

Parameter Dimension

LOA / LBP (݉ ) 90.0 / 80.0

Beam (݉ ) 20.0

Draft (݉ ) 6.5

Displacement (t) 8150

Frontal Windage Area (݉ଶ) 452.0

Lateral Windage Area (݉ଶ) 1436.0

Figure 3: Dimensions of the CTV and positioning of its thrusters.

Source: (SeaLoading, 2018)

Table 2: Main properties of the tugboat



Parameter Dimension

LOA / LBP (݉ ) 80.0 / 69.3

Beam (݉ ) 18.0

Draft (݉ ) 6.6


Table 3: Main properties of the Suezmax conventional Tanker

Parameter Ballast Intermediate Full

LOA / LBP (݉ ) 278.5 / 262.0

Beam (݉ ) 48.0

Draft (݉ ) 8 11 16,5

Displacement (t) 76412 113390 169460

Frontal Windage Area (݉ଶ) 1626 1482 1243

Lateral Windage Area (݉ଶ) 5425 4591 3197

The production unit considered in the analysis is a typical FPSO based on converted VLCC vessels located in the Santos Basin in Spread Moored anchoring. For all the analysis, we consider a VLCC hull shape with 14m draft (intermediate loading), with the following properties:

Table 4: Main properties of the FPSO

Parameter Dimension

LOA / LBP (݉ ) 321.5 / 310.0

Beam (݉ ) 58.0

Draft (݉ ) 14.0




Frontal Windage Area (݉ଶ) 1505

Lateral Windage Area (݉ଶ) 7622

Average heading related to North (deg)

195

The properties of the hawser (that connects the CTV to the Tanker) and the tug line (that connects the Tanker to the tugboat) are shown in the Table below.

Table 5: Hawser and Tug line parameters

Parameter Hawser Tug line Gain Nylon Nylon Length (݉ ) 150 150 Diameter (݉ ݉) 184 120 Weight air (݇ ܰȀ݉) 0.19 0.091 Weight water (kܰ Ȁ݉) 0.02 0.098 Axial Stiffness EA (kN) 1.68ൈ ͳͲସ 2.5ൈ ͳͲସ MBL (tf) 763 234

The offloading operations are located in the Libra Field (in the Santos Basin pre-salt area). We used environmental data during a period of ten years between Jan/01/2000 to Jan/01/2010, for a total of 29225 cases. Although the detailed description of the environmental data is confidential, general information about the conditions are shown in this section.

Figure 4 presents the average condition for the basin.

There are two typical scenarios depicted in



Figure 5. The NE Condition is the most common, in which the wind blows from NE, associated with a local sea wave and with the current going SW. During most of the time, there is also a swell coming from S (bimodal sea state). The SW condition is associated with the southern cold front. The wind and the local sea come from the SW direction and induce a modification of the surface current, rotating to the W. In this scenario, the swell and local sea come from similar directions (S quadrant), the swell being the wave component with a higher peak period. The transition from the NE to the SW scenarios occurs almost weekly (more frequently during the winter) and is associated with a sudden change in the wind direction (also known as wind rotation).

Figure 4: Libra Field - Average environmental conditions


Figure 5: Libra Field - Typical environmental conditions

Swell (Tp>10s)

Hs=1.94m

Tp=12.1s

Dir = 172o

Local Sea (Tp<10s)

Hs=1.48m

Tp=6.8s

Dir = 89o

V=0.34m/s

Dir = 241o

V=7.7m/s

Dir = 64o

N

E

Libra Field

Average Conditions




3. Real-Time Simulation

The main purpose of the Real-Time simulation was to evaluate the offloading operation for the Spread-Moored FPSO, anchored in the Santos Basin (Libra Field), employing conventional oil Tankers with the support of the DP2 vessel CTV and a tugboat.

Real-Time simulations were executed, with the vessels being controlled by certified captains of tankers, tugboats and the CTV, with support from DP System developer (Kongsberg). Those simulations were performed during 5 days at the Ship Maneuvering Simulation Center of TPN-USP (TPN-USP, 2020), with 3 simulation cabins (2 of which are of the Full-Mission type,

Figure 6).

N

E

Libra Field

NE Condition

N

E

Libra Field

SW Condition



The following scenarios were investigated: connection between CTV and Shuttle Tanker; convoy approach and connection to the FPSO; offloading station change; operation under mild and severe weather conditions (including wind rotation and wind squall events) and the disconnection after a blackout of the tugboat.

Figure 7 shows the simulations of the convoy approaching the bow offloading station in the typical Santos Basin NE condition. The CTV was able to tow the convoy, maintaining the approximation velocity (about 1 kn maximum). During the approach, the tension in the hawser and in the tug line was kept within the operational limits.

Figure 6: Simulators used in the real-time simulation campaign.




Figure 7 – Convoy approaching to the bow offloading station (detail of a 3D bird-view of the operation).


In the second test, the SE wind increased from 12kn to 21kn during the loading phase, forcing the disconnection from the bow station and the reconnection to the FPSO stern station. Figure 8 presents the main screen of the Kongsberg’s DP station showing the convoy moving to the FPSO stern. Despite the severity of the operation considering that the Tanker was partially loaded with a large inertia, no additional difficulty was reported by the CTV captain.

Figure 8 – Kongsberg’s DP station screen showing the convoy moving to the stern offloading station.




WaveHs=2.4mTp=6.5sDir = 167o

V=0.42m/sDir = 252o

V=12kn 21knDir = 165o

N

E



Figure 9 shows the Kongsberg’s DP station screen after a wind rotation event, which consisted in rotating a 20 kn wind from NE to S in 40 minutes. This event led the CTV closer to the limit of the operational sector forcing a disconnection. After the disconnection, during the 0.1 kn depart maneuver, peaks of 70 tf in the hawser were verified, while the average tension on the tug line was around 20 tf.



Figure 9 – Kongsberg’s DP station screen showing the motion of the convoy due to a

wind rotation event.


Figure 10 – Tension on the tug line (left) and hawser (right).


In Figure 11 the Kongsberg’s DP screen shows the motion of the CTV and the Suezmax Tanker after a wind squall event. The squall event consisted of a drop in the wind velocity from 20 kn to 5 kn and a direction change from NE to SE in 5 minutes and then an increase in the velocity up to 50 kn and back again to 20 kn in 5 minutes. During the squall event, peaks of tensions above 100 tf were

4000 6000 8000 100000

20

40

60

80Cable ID:

F (

ton)

Tempo (s)4000 6000 8000 10000

0

20

40

60

80Hawser ID:

F (

ton)

Tempo (s)Time (s) Time (s)

HawserTug line

F (t

f)

F (t

f)



observed in the hawser and in the tug line. A disconnection operation would have started when wind velocity reached 40 kn in real life, but the captains decided to keep the vessels connected in the simulation to verify the behavior of the system. Even in this extreme situation, the CTV DP System was able to keep the vessel inside the operational sector, following the fast lateral displacement of the Tanker due to the sudden wind variation.


wind squall event.




Figure 12 and Figure 13 show the collision avoidance maneuver after the Tugboat blackout. During the blackout event, the strong 31kn wind reached the convoy by the stern, and the disconnection operation started. After the blackout was identified, the CTV captain started to pull away the Tanker bow from the FPSO. When the Tanker was in a safe position, it started its main engine and both the CTV and the Tanker started to move away from the FPSO. At this point, a high degree of coordination between the captains was required since all the three vessels remained connected. The tension on the hawser reached values of 200 tf and although this value is less than the MBL of the hawser, the tanker bollard could be damaged since it usually does not support such a high load. At the end of the simulation, the vessels were in a safe position.

Figure 12 – Convoy moving away after disconnection during Tugboat blackout.





wind squall event.




4. Conclusions

The analysis of all the scenarios led to some conclusions: the approach and connection operations were safely executed (even with partially loaded vessels) in typical and strong Libra Field weather conditions. In the offloading operation, under mild weather conditions (in which the weathervane heading is a weak attractive, stable position), the tugboat force could keep the vessels aligned and inside the operational sector. It is important to highlight that, in this case, the tugboat is essential to maintain the whole convoy aligned. The strength of the Tanker bollards must be verified since simulations demonstrated peaks of tension of more than 120 tf in the hawser and tug line.

An emergency disconnection operation and a possible offloading station change could be safely carried out due to a wind rotation event, even with the wind blowing from the stern of the convoy; also, a wind squall event did not make the disconnection operation unviable.

The tugboat with 140 tf of bollard pull was satisfactory to keep the Tanker aligned to the CTV in all the tests. A few simulations were carried out with the tug bow thruster disabled and, in these scenarios, the tugmaster reported difficulties in maintaining the heading and in executing the maneuvers ordered by the CTV captain; therefore, the tugboat bow thruster was mandatory to keep the tugboat in the correct position during the tests. The coordination between the three captains is also mandatory; some simulations demonstrated that the definition of the responsibility chain must be clear. During the simulations, we verified that the orders to the tugmaster should come from the CTV Captain, since he is the one with a better knowledge of the whole situation.

Regarding the Tugboat blackout test, it was shown that an evasive maneuver could be safely executed; once more, the importance of precise coordination between the CTV and the Shuttle Tanker captains became evident. It is recommended, however, to carry out a Risk Assessment analysis with a specific simulation campaign to identify and to test contingency actions to mitigate potential failures that could cause damage and severe consequences to the system.

5. Acknowledgments

The first author thanks the CNPq (process 304784/2017-6) for the research grant. The authors acknowledge Cefront for providing the data of the vessel and for the fruitful technical discussions.

Referências

SeaLoading, & Autor 1. (2018). “Outline Operation Procedure.” Technical Report 200-SL-100-O-PRC-00016:

(Confidential).

Syvertsen, K., & Smedal, A. (2017). “Cargo Transfer Vessel”, United States Patent US 9,764,801 B2. United States:

Patent.

TPN-USP, & Autor 1. (2020). TPN-USP Ship Maneuvering Simulation Center. São Paulo, Brazil:

http://www.tpn.usp.br/simulador/.

1. Introduction

Documents

Transcript of 1. Introduction