INSTALLATION OF A SUBMERGED BUOY FOR SUPPORTING RISERS (BSR) SYSTEM IN CAMPOS BASIN SITE

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1 Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands Proceedings of the 30 th International Conference on Ocean, Offshore and Arctic Engineering OMAE 2011 June 18-24, 2011, Rotterdam, Netherlands OMAE OMAE INSTALLATION OF A SUBMERGED BUOY FOR SUPPORTING RISERS (BSR) SYSTEM IN CAMPOS BASIN SITE Jairo Bastos de Araújo PETROBRAS - E&P Rio de Janeiro, RJ, Brazil jairoba@petrobras.com.br José Carlos Lima de Almeida PETROBRAS - E&P Macaé, RJ, Brazil jcalmeida@petrobras.com.br Antonio Carlos Fernandes LOC & LabOceano/COPPE UFRJ Rio de Janeiro, RJ, Brazil acfernandes@peno.coppe.ufrj.br ABSTRACT The BSR (Buoy for Supporting Risers) concept is composed by a submerged buoy anchored to the sea bottom by tethers and intended to support risers coming from the bottom (probably SCRs Steel Catenary Risers) and going to the floating platform (probably with flexible jumpers). For the case under analysis here, the main dimensions of the BSR prototype are 27.2 m length x 27.2 m width x 5.0 m depth. The paper describes all final full scale installation step so that the BSR may be considered a suitable technology. The installation indeed was the great challenge of this design due the size of the hull. The present work also evaluates numerically and experimentally a specific new manner to install the BSR with the support of auxiliary mooring lines among with the four tethers connected to it. One of the installation premises was to make use of Anchor Handling Supply Vessels instead of Crane Vessels. After this numerical analysis, the work went on by performing model tests that simulates the operation in a deep water model basin using 1:40 scale. The model test anticipated several problems such as the chain stopper weakness in the operation and others as discussed in this paper. As a conclusion the work was devised the most important parameters during the system installation and suggested ways to improve the methodology. In November 2009 the BSR was installed in 500 m of water depth at Congro field location, Campos Basin, offshore Brazil. The tethers were adjusted in January 2010 and in March 2010 two risers were installed. Thenceforward the last edge of this knowledge was considered over passed. INTRODUCTION In the last years the oil extraction in Brazil has occurred in deep and ultra-deep waters (more than 2,000 m). In that condition the risers (even the SCRs) suffer larger tensions due to the weight, current and wave direct actions and the oscillatory motion of the connection point. An alternative to mitigate this problem is to use a submerged buoy system for supporting risers and referred here as BSR. The BSR scheme is shown in Figure 1. Figure 1 - Schematic view showing the BSR system. 1

2 The hull itself is composed of steel cylindrical members, anchored to the sea bottom by tight tethers and the connection to the Floating Production Unit is made by jumpers of flexible lines. It is well suited to support SCRs. The BSR works as an intermediate buoyant element for the hybrid risers system, and avoids the large oscillatory motions at the connection point since the environment action on BSR itself is much smaller since it is below the sea level (75 m for the case study here). This conception has been numerically analyzed and tested since the 90 s. However the BSR installation was considered an open issue until A prototype BSR was fabricated in 2004, as showed in Figure 2. Despite the adequate behavior obtained in the lab tests, numerical analysis one had to close a main issue: how to put a big hull below the sea level and keep it by tethers? So an installation procedure was first developed by PETROBRAS. Then several model testing were performed ocean basin of LabOceano/COPPE/UFRJ, Rio de Janeiro, Brazil, to seek for improvements of installation procedure. The references [1] and [2] present the lab results and in 2005 the first BSR installation takes place in Campos Basin with some good and partial results. DW: Dead Weight. MT: Metric tons. FPU: Floating Production Unit. KS: Hook modified to work like a subsea connector and disconnector. Pelikelo: Subsea connector that can open under tension. BSR DESCRIPTION AND ITS MAIN COMPONENTS The first BSR prototype developed by PETROBRAS is showed in Figure 2. Their main characteristics are presented in Table 1. Table 1 Main Properties of the BSR. Length 27.2 m Breadth Depth Diameter of each cylinder (*) Weight in air Net Buoyancy 27.2 m 5.1 m 2.4 m 287 MT 619 MT Nb. of ballast tanks 24 (*) The stern of BSR is characterized for existing two cylinders. One connected above the other. Chain stopper and chain sheave (see Figure 3) constitute a passive system used to adjust the BSR tethers. The chain stopper has a flap that restricts the chain in one direction and the chain sheave is a pulley with free rotation to support the chain. Figure 2 BSR at the shipyard. In 2008 another installation procedure was prepared and the last lab test and numerical analysis detailing the phases of the procedure were done and presented in references [3] and [4]. Finally in November of 2009 the BSR is installed in 500 m of water depth, at 75 m below the sea level, at Congro field, Campos Basin. The job developed and the final tasks on the BSR installation are discussed afterward. GLOSSARY AHTS: Anchor Handling, Towing and Supply Vessel. SV: Survey Vessel equipped with ROV. ROV: Remote Operated Vehicle. Figure 3 Chain Stopper and Chain Sheave of the BSR. The BSR has a ballast system that is used to compound the BSR weight during the installation. The system was designed to work with compressed air and sea water. Each ballast tank has two valves. 2

3 Two receptacles were installed on the stern and bow of BSR to connect risers and jumpers lines respectively. The BSR has a set of four tethers, anchored at the soil by torpedo anchor. Tethers were used to assist the BSR installation before work with its specified function. Chain and wire rope compounds the tethers and its characteristics are showed in Table 2. The bottom chain of tethers was divided in segments of 25 m and 30 m. All 25 m were pre-set with the torpedo with the KS shackle- pick-up buoy system on the free extremity, as shown in Figure 4. Table 2 Tethers Properties. DESCRIPTION Diameter Length Type Bottom Chain 76 mm 55 m R3 Wire Cable 78 mm 300 m Six Strand Top Chain 76 mm 70 m R3 Studless Four auxiliary mooring lines were used to support the BSR installation and keep it on its right coordinate during the hook up of the tethers. These mooring lines were also anchored at the soil by torpedo anchor and its composition is shown in Table 3. Table 3 - Auxiliary Mooring Lines Properties. DESCRIPTION Diameter Length Type Bottom Chain 76 mm 340 m R3 Studlink Wire Cable 87 mm 850 m Six Strand Top Chain 76 mm 100 m R3 Studless Four dead weights were fabricated and used to pull the BSR below the sea level and towards its installation depth and also to facilitate the hook up of tethers. Each dead weight has 40 MT in the sea water. Figure 4 Bottom Chain (25 m) pre-set. Phase 2: The BSR was towed from Guanabara Bay, Rio de Janeiro, to Campos Basin site (see Figures 5 and 6). Four AHTS and one SV were standing by with all remaining auxiliary mooring lines and tethers components to start the operation. PREPARATION OF THE BSR PRIOR TOWING BSR was prepared on shipyard in order to facilitate its installation and transportation. The main tasks made were: Installation of the towing system. Installation of 08 top chain segments. Two in each corner or 70 m of tether and 30 m of auxiliary mooring line. Ballasting of 06 tanks. Checking the navigation lights. Flushing of the ballast and vent lines. Figure 5 BSR at site for installation. INSTALLATION OF THE BSR For better understanding of the installation procedure the following phases have been devised as described below. Phase 1: Some auxiliary mooring lines and tethers components were pre-installed and laid down at the sea floor. 3

4 The descent of the BSR until its required depth was conducted simultaneously by the 04 AHTS as showed in Figure 8. Some steps that correlated the distance between AHTS and BSR vs. length of the work wires were followed as Table 4. Figure 6 BSR at surface site for installation. Each auxiliary mooring line pre-set was fished on the sea bottom by the AHTS and connected to the BSR through Pelikelo. So the towing vessel was released. The lengths of the top chain were adjusted on the deck of the AHTS. Phase 3: With BSR safely moored with the four auxiliary mooring lines the tethers hook up operation began. Each remaining tethers lengths, as well their respective DW, were on board each of the four AHTS. The tethers loaded in each one of four AHTS were connect in their respective 70 m of top chain pre-installed on BSR and paid out until the 30 m of bottom chain. Then the DW was connected on the 30 m of chain triplate through a sacrifice wire cable of 2 in X 4 m, a KS on the 30 m of chain free end, the AHTS work wire on the DW and the system was launched on the water as showed in Figure 7. Figure 8 AHTS arrangement for the descent of BSR. Table 4 Correlation Distance vs. Work Wire Length. AHTS-BSR Work Wire STEPS Distance (m) Length (m) Once the DWs reached near to the sea bottom, theirs placement were guided by the ROV in order to facilitate the connection between the KS hook and its shackle (see Figure 9). Figure 7 Line configuration for the descent of BSR. Figure 9 KS hook connected on its shackle. 4

5 The ROV carried out the four KS connection, cut the sacrifice wire cable and later on released the four auxiliary mooring lines by opening the Pelikelos. Figure 10 shows the final appearance of the BSR system after its installation. Two months later two risers were installed on the BSR (see Figure 12) to validate the procedure and the technology as presented in reference [5]. Figure 12 Two risers installed on the BSR. Figure 10 BSR installed. It was necessary to adjust the length of 03 tethers due to high trim and heel angles on the BSR after tethers hook up operation. This adjustment was done using 03 DW with the right leg length connected at auxiliary mooring line segment hanging off on BSR corner. When the BSR was in the correct level of trim and heel the 03 tethers top chains were pulled out trough the respective chain stopper, as per Figure 11. CONCLUSIONS This installation procedure developed and carried out, including the equipment, vessels and materials selected, proved the technical and economical feasibility of the BSR technology The installation in full scale experience clearly shows the complete feasibility of the BSR concept. This may explain why the two BSR concepts were chosen by the market in a recent design competion. The pre-salt area in Campos Basin is a scenario for the BSR technology since it is a deep water scenario with a FPSO as the choice for the FPU. The possible trim and heel angles requires care during the descent of the BSR stage. It must be addressed in later stages. The lengths of the auxiliary mooring lines and tethers segments must have a fine control. The adjustment system of the tethers must have proven capabilities. The tolerances for BSR trim and heel angles must be defined in the design phase. The hull of the BSR can be made by steel plate, synthetic foam or a combination of both. Figure 11 Tethers adjustment by pulling the top chain. 5

6 ACKNOWLEDGMENTS The authors acknowledge Petrobras and the CNPq (the Brazilian National Research Council). REFERENCES [1] Fernandes, A.C., Santos, M.F., Sales, J.S., Araújo, J.B., Almeida, J.C.L., Diniz, R. and Rangel, M.; Model Test and Numerical Simulations for the Development of the First Full Scale Riser Support Buoy (BSR) - XVIII International Congress of Mechanical Engineering (COBEM); Ouro Preto, Brazil, November 6-11, [2] Fernandes, A.C., Almeida, J.C.L., Araújo, J.B., Rangel, M., Santos, M.F. and Sales, J.S.; Numerical Simulation and Experimental Analysis for a Riser Support Sub-surface Buoy Rio Oil & Gas, Expo and Conference, Rio de Janeiro, Brazil, IBP241_04, October 04-07, [3] Fernandes, A.C., Almeida, J.C.L., Araújo, J.B., Franciss, R., Rangel, M., Merino, J.A. and Sales, J.S.; Parametric Evaluations of the Buoy Supporting Riser (BSR) Installation - 1 st Marine Operations Specialty Symposium 2008 (MOSS2008); paper MOSS-60, Singapore, March 5-7, [4] Fernandes, A. C., Merino, J. A., Silva, A. R., Araújo, J. B., Almeida, J. C. L., and Franciss, R.; Analysis of the Installation of a Buoy Supporting Risers (BSR) by Numerical Modeling and Model Testing - Dtec Deepwater Offshore Technology Symposium 2008, Shangai, China, November 17-19, [5] Franciss, R., Almeida, J.C.L., Araújo, J.B., Gonzales, E.C., Fernandes, A.C., Alternative Method of Buoy Supporting Riser (BSR) Installation, OMAE paper 49862, June 18-24, 2011, Rotterdam, Netherlands. 6