OTC 12971 Floatover Deck Installation for Spars J. V. Maher, I. Prislin, J. C. Chao, J. E. Halkyard, L. D. Finn, CSO Aker Engineering, Inc. Copyright 2001, Offshore Technology Conference This paper was prepared for presentation at the 2001 Offshore Technology Conference held in Houston, Texas, 30 April 3 May 2001. This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract The spar floating production system offers an attractive alternative for drilling, producing and storing oil in deep water depths. Of the spars installed to date, conventional derrick barge methods have been used to install the top deck in multiple lifts and to perform hook up and commissioning operations offshore. Development and engineering have been carried out in developing economical alternatives to install top decks. One such scheme is the float-over deck installation onto a pre-installed spar hull with the deck supported by two barges. Using this method, the top deck will be hooked up and commissioned onshore and installed in one piece to reduce offshore operation time and to avoid the use of a large, expensive derrick barge. The development of such a floatover method is presented in the present paper. Introduction Several spar-based drilling and production platforms have been installed in the deep water of the Gulf of Mexico (GOM) [1,2,3]; a number of other spars are currently being fabricated and designed for the GOM and other deep water regions (Fig 1). The deep draft, intrinsic stability, and low motions of the spar make it an attractive option in many deep water regions throughout the world. All the spars that have been installed to date have utilized dynamically positioned large crane vessels to install the top decks in single or multiple lifts followed by hook up and commissioning at the deepwater site. Day rates of such crane vessels with large lift capacity are high and these vessels are not available at all offshore locations. Furthermore, hook up and commissioning of deck facilities offshore are inherently expensive. CSO Aker Engineering, Inc. (AEI) has been conducting research and engineering in developing alternative methods to install top decks onto a spar hull without using a large expansive crane vessel. One of the methods developed is a self- floating deck and has been described in an early presentation [4]. In this method, all the required drilling and/or production facilities are installed on a barge and hooked up and commissioned inshore; the barge is then towed to the spar site to mate with the pre-installed spar hull (Fig 2). The second method being developed is the float-over deck in which the integrated deck is supported by two small-waterplane-area pontoon barges for mating with the spar hull (Fig 3). The advantage of this method is its reduced motions and higher operable seastates. This method was also presented in an early paper [5]. An alternate float-over deck method is to use, instead of two small-water-plane-area pontoon barges, two ships or barges to support the deck. Again, the top deck is fabricated in one piece with all facilities hooked up and commissioned onshore. The top deck is transported by two support barges to the spar site and positioned over the spar for mating with the spar hull. AEI has carried out theoretical studies and model tests for all the above schemes. In the present paper, the second floatover deck method using two ships or barges will be discussed. Specifically, this paper will discuss the seakeeping characteristics of the barges, the proposed mating operations, model tests and comparison of predictions with model test results. While the procedures have been developed for using either two ships or two barges. We will use "barges" in the following discussion. The float-over method is not new to the offshore industry. The method has been used successfully for mating decks in calm waters (e.g., GBS structures in Norway and Canada), and to a limited extent in open waters (e.g., fixed structures in Australia, Middle East, and West Africa) [6,7]. Floatover Deck Installation Procedures To provide a basis for the following discussion, the entire float-over procedure is given in this section. The base case procedure is assumed to start from the time the top deck fabrication and commissioning are complete (and is ready to be transported to the site) to the final mating operation of the deck with the spar hull. Analysis and design results for the base case procedure will be discussed in the following sections. While the float-over technology was developed for
2 J.V. MAHER, I. PRISLIN, J.C. CHAO, J.E. HALKYARD, L.D. FINN OTC 12971 use in a wide range of application conditions (e.g., Offshore West Africa, Offshore Brazil, and GOM), a GOM installation site is assumed in the following discussion. In addition, the deck is assumed to be loaded onto the support barges at the Aker Gulf Marine (AGM) yard. outside of Corpus Christi. Pre-Install Spar Hull: Upon completion of the spar hull and prior to towing the top deck to the site, the spar hull must be installed at the site and be prepared to receive the top deck (Fig 4). First, the mooring system is installed at the site, then the spar hull is towed to the location, upended, and connected to the pre-installed mooring system. Fixed ballast is then placed, followed by controlled flooding of several selected tanks to bring the spar hull to the desired freeboard. The spar will be at a shallow freeboard to allow the deck to be floated over. To ballast the spar hull, temporary pumps with power supplies will be placed on top of the spar hull or on a surface support vessel. The pumps will be connected to manifolds with control valves to direct the water to the appropriate tanks. Load Deck onto Support Barges: After the deck fabrication, hook-up and commissioning are complete, the deck is ready to be loaded onto the barges. One scheme to load the deck onto the support barges would be to skid the deck directly onto the two barges at the quayside (Fig 5). Next, the top deck is fastened to the barges, the barges are deballasted to the correct towing draft and is ready to be towed to the spar site. Tow Deck/Barges to Installation Site: Fig 6 shows that the deck and support barges are being towed out of the AGM yard and through the ship channel. Fig 7 illustrates the offshore tow of the deck. It is assumed that the final installation site is not far from the AGM yard and the deck can be towed safely in the catamaran configuration. If the final installation site is far from the deck loading yard a temporary mating site could be established at a deep water site not far from the deck loading yard. The deck can be mated at this temporary site and then the spar with its installed deck can be vertically towed to the final installation site. A temporary mating site not far from the loading yard will make the catamaran tow a shorter operation. This will reduce the tow risk as the weather window can be more accurately predicted for a short tow. Maneuver and Position Deck over Spar Hull: When the tow arrives in the field, the support vessel will hip up to one of the barges and together with the tow tugs will maneuver the deck towards the spar hull. As the deck approaches the spar, mooring lines will be connected between the spar hull and the barges to assist with maneuvering of the barges on each side of the spar hull. Fig 8 illustrates such a maneuvering and approach operation. Final positioning of the deck over the spar hull can be made with the assistance of soft lines connecting the spar hull to the barges and a fendering system between the spar hull and the barges. Fig 9 illustrates the positioning and docking of the deck/barge over the spar hull. For the majority of conventional float-over deck installations performed to date for fixed platforms, two basic schemes have been used to position and secure the deck barges. One scheme is to have a clearance between the deck barge and jacket fendering and have a system of mooring lines between the barge and jacket to hold the barge in position. The other approach is to have minimal clearance between the deck and jacket so the barge cannot move laterally. For the spar deck float-over using two barges, the minimal clearance approach is considered to be the most suitable as the barge position can be controlled at all phases of the mating operation. Mate Deck with Spar Hull: Two critical phases of the mating operation in a seaway are: (1) initial contact between the spar hull and the deck, and (2) separation of the deck from the barges. It is during these phases that wave induced motions can induce large loads to the structure. The float-over operation will be controlled from the surface support vessel. After alignment is made between the spar hull and the top deck, mating operation can commence. Mating operation includes lifting of the spar and at the same time ballasting the barge and eventually transfer the deck from the barges to the spar hull. An air system has been described by Chao, et al [5] to provide buoyancy and lift to the spar hull. Deballast the spar and ballast the barge by a large pump system can also be used depending upon the amount of ballast needs to be moved and the available weather window for the mating operation. A mechanical or hydraulic system that will allow a quick separation of the deck from the barges can be incorporated into the deck supports on the barges. After the barges are separated from the deck, they will be safely towed out from underneath the deck. Fig 10 shows four sketches illustrating the mating operation to transfer the deck from the support barges to the spar hull. To ensure and maintain alignment between the deck and the spar hull during the mating operation, the deck will be fitted with stabbing cones that will go down into the hull support columns. Alignment pins inserted into the stabbing cones will ensure that the deck remains properly aligned before and during the weight transfer. Conventional passive elastomeric shock cells, used on most deck float-overs onto jackets, will be incorporated into the stabbing guide system to accommodate any possible impact loads at the start of load transfer. Fig 11 shows one such possible mating units. Floatover System/Components Analysis and Design AEI has conducted an extensive R&D program including analysis tool development and model tests for floatover operations and equipment designs. Technology development, and analyses and designs of major floatover deck components/systems are discussed in this section. Model tests will be discussed in the next section. Technology/Software Development: AEI has modified ABAQUS and MLTSIM to simulate floatover operations. MLTSIM [8] is a time-domain hydrodynamics computer
OTC 12971 FLOATOVER DECK INSTALLATION FOR SPARS 3 program capable of computing motions and loads on multiple floating bodies. ABAQUS is used to model the structures of the floating bodies. The connections between any given two bodies (e.g., between a spar and a deck) are simulated as nonlinear springs. Hydrodynamic inputs to the program are specified by a set of user-defined coefficients for added mass, linear damping, and quadratic viscous drag. Added mass, potential damping, slow varying drift forces, and diffraction forces are computed using programs such as WAMIT, HYDRO3, or similar. Simulating reactions between deck and spar (or barges) using stiff connectors result in a stiff system of differential equations. This requires a special solver to obtain solutions for these differential equations. Top Deck Design: One design issue for the floatover operation is to determine the deck structure required to withstand the loads induced by the floatover operation. Several structure designs have been carried out for floatover decks to determine the increase in structure weight. The structure weight for floatover decks are compared with conventional decks. Floatover decks showed a modest increase around 10% in steel weight. The required additional strengthening involves mostly increased wall thickness of the diagonal bracings. The reasons for this modest increase are: (1) the original structure is designed for an inplace operational load (higher) and 100 year event, (2) the floatover condition (while with a different support configuration) is designed for a lower dry installation weight and a lower design environment (installation seastate), (3) the design stress levels for installation operation (short duration) can be higher. Deck Support Loads on Barges during Transport: Deck supports on the barges need to be designed to withstand the transportation loads. Sea fastening could be required when the dynamic loads become large. A number of analyses and designs have been made to determine the loads induced during transport of the dock to the mating site. Fig 12 shows one support arrangement on the barges used in the analysis. Fig 13 shows the predicted load time history at the most heavily loaded support. This calculation was made for a seastate of 16 ft and for a deck weight of 25,000 st, with a dimension of 230 ft by 210 ft supported by two barges of 262.5 ft x 55.8 ft x 52 ft each. This analysis assumes four supports on each barge. It can be seen that the static load on each support is 6,300 kips; the maximum dynamic load is 3,800 kips indicating that the supports are always under compression. These support loads will be reduced when more supports are used. Floatover Operation: Four-body model (two barges, spar and the deck) was used to simulate the floatover operation. Fig 14 shows the support arrangement for one of the mating simulation; the deck is initially supported on four supports on each barge and the deck weight is eventually loaded onto the spar hull with eight supports. Fig 15 shows the simulation of the entire mating operation from the point when the spar is started below the deck (~ 5 ft), come into contact with deck and finally the deck separates from the barges. It can be seen that the wave induced motion of the spar is very small at the beginning (when the spar is at a deep draft, i.e., low freeboard) while the motions of the deck/barge are relatively large. At the point of spar making contact with the deck; some motions are being imparted to the spar. Afterwards, the deck/barge and the spar move essentially as a single body when ballasting of the barges and deballasting of the spar hull continues. When the top of the spar reaches to around 25 ft from the waterline the deck starts to separate from the barges; finally the deck is completely separated from the barges when the spar is at about 28 ft. It can be seen that the motions of the barges become large after the deck weight is removed and these barges need to be removed from the site quickly. This simulation was made for a seastate of 6.6 ft. The deck and barge dimensions used in the simulation are as discussed above. These results are used to determine the loads for designing the contact pads between the deck and the spar hull supports and between the deck and barge supports. These results are also used to define the seastates that mating operation can be performed. Tables 1 and 2 present additional results from the simulation. Table 1 gives the spar and deck relative motions during mating and Table 2 presents the maximum dynamic loads at the spar support at the time of making contact and the loads at the deck supports at the time when the barges separate from the deck. It can be seen that the loads at the barge supports are larger as the barge motions are higher. These dynamic loads are, however, not large; elastomeric pads (or shock cells) can be designed easily to withstand these loads. The loads will be reduced if the number of supports are increased (e.g., eight). Model Test AEI has conducted a number of model test programs for various floatover concepts including the self-floating deck [9], the small-water-plane-area pontoon barges [10] and the flatbottom barges [11]. In the present paper, only the model test with flat-bottom barges are discussed and are used to compare with theoretical calculations discussed in the above section. The model tests were performed at the Offshore Model Basis in Escondido, California. Model Scale was 1:49.56. The objectives of the model test program are: (1) to validate and calibrate analytical tools used for the analysis of large deck catamaran towing and floatover with a spar platform, (2) to provide data needed for design of equipment required for these operations, and (3) to assist with evaluation of the various phases of transportation and mating operations. Fig 16 is a photo of the model test showing the deck and support barges over the spar hull. The deck modeled is 35,000 st with a dimension of 270 ft by 250 ft. The barges modeled are 485 ft x 105 ft x 30 ft each. The barge weight is 7,000 st each. Two spacer structures are employed on top of the barges to support the deck to provide the required height for floatover. The spar modeled is 158 ft in diameter. Measurements included motions of the various bodies and loads at various contact points. Tests included
4 J.V. MAHER, I. PRISLIN, J.C. CHAO, J.E. HALKYARD, L.D. FINN OTC 12971 seakeeping tests, catamaran tow tests, floatover and mating tests in various wave and current conditions. Comparison of Predictions and Model Test Results Fig 17 shows part of a load measurement made during one of the mating tests. The load time histories are comparable to those discussed above. The test was made for a 6.6 ft quartering seas. Fig 18 compares the predicted and measured load time histories. It can be seen that the predictions match well with the measured values, both in amplitudes and periods. The average (static) simulated loads differ slightly from the measured loads because of slightly different initial conditions set in the numerical model. This, however, does not significantly change the results for the dynamic part of the load. Table 3 presents additional comparison between measured and predicted loads. Fig 19 shows a comparison of measured and predicted load spectra. The location of the main spectral peaks and magnitudes of the spectral peaks agree reasonably well between measured and predicted loads. Economics This study demonstrates the technical feasibility of using barges for float-over deck installation. Costs to modify existing barges for floatover operation were found to be lower than employ a large crane vessel. The exact savings depending upon the location of the installation site (WA, GOM, or Brazil) and deck weight. The economics of the float-over method will become even more attractive when multiple use of the barges becomes possible. Our analysis of the spar projects in the GOM, Brazil and West Africa certainly support this potential. References 1. R. D. Vardeman, S. Richardson and C. R. McCandless, Neptune project: overview and project management, Offshore Technology Conference, Houston (1997). 2. M. De Luca, Seventy US gulf deepwater fields awaiting development; 26 in production, Offshore, (September, 1998). 3. G. Taylor, Genesis spar assembled, positioned offshore, Offshore, (November, 1998). 4. P. N. Stanton, J. L. LeJune, J. C. Chao, and H. G. Kumpis, Deep water drill spar, Offshore West Africa '99 Conference and Exhibition, Abidjan, Cote D'Ivoire, (March, 1999). 5. J. C.. Chao, J. E. Halkyard, J. A. Hinrichs, B. N. Johnson, M. R. Zeringer, H. M. Thompson, G. N. Beitch, Floatover Decks for the Spar Floating Production System, Deep and Ultra-Deep Water Offshore Technology Conference, (March, 1999). 6. J. P. Labbe, V. Allegre, J. Volker and F. Agdern, Ekpe gas compression project-float over deck installation, Offshore Technology Conference, Houston (May 1998). 7. J. H. Sigrist, P. Athomas and J. C. Naudin, Experience in float over integrated deck - flexibility of the concept, Offshore Technology Conference, Houston (May, 1998). 8 Y. S. Hong, J. R. Pauling, A procedure for the computation of wave- and motion-induced forces on three-dimensional bodies at zero forward speed, A report to the American Bureau of Shipping (1979). 9 Universal Spar, Self-Floating Deck Model Test, Deep Oil Technology, Inc. (December, 1997). 10 Float on Deck Model Test, Deep Oil Technology, Inc. (September, 1996). 11 Float Over Model Test, Aker Maritime, Inc., (November, 2000). Hs (ft) TABLE 1 SPAR / DECK RELATIVE MOTIONS Max Vertical Max Spar Support Motion of Deck Col Vertical @ Contact Pt (ft) Motion (ft) Max Relative Motion (ft) 3.1 0.76 0.22 0.36 6.0 1.63 0.55 2.06 8.0 2.29 0.98 3.00 The max motions are based on 30 min duration TABLE 2 MAXIMUM DYNAMIC LOADS DURING DECK TRANSFER Hs (ft) Spar Initiates Contact (kips) Barge Separates from Deck (kips) 6.0 833 970 8.0 1,084 3,000 TABLE 3 COMPARISON OF MEASURED AND PREDICTED FORCES Mean (kips) Min (kips) Max (kips) RMS (kips) Tp (sec) Td (sec) Spar Portside Barge Measured Predicted Measured Predicted 15,780 11,420 27,690 29,310 18,330 13,420 29,570 32,300 12,890 9,166 25,600 27,690 975.8 846.6 672.5 696.5 9.9 9.4 7.8 7.0 4.3 5.7 6.2 5.8
OTC 12971 FLOATOVER DECK INSTALLATION FOR SPARS 5 Fig 1 Classic Spar Fig 2 Truss Spar 40' MWL 36' 46' 50' SPAR Fig 3 Self-Floating Deck Fig 4 Small-Water-Plane Pontoon Barges PRE-INSTALL MOORING SYSTEM AT SITE TOW SPAR HULL TO SITE UPEND SPAR HULL CONNECT MOORING SYSTEM TO SPAR HULL HOOK UP AIR SUPPLY AND BALLAST SUPPLY INSTALL FIXED BALLAST PRESSURE UP SKIRT TANK ADJUST BALLAST TO OBTAIN START BALLASTING CORRECT FREEBOARD Fig 5 Pre-Install Spar Hull at Site
6 J.V. MAHER, I. PRISLIN, J.C. CHAO, J.E. HALKYARD, L.D. FINN Fig 6 Skid Deck onto Support Barges Fig 8 Deck./Barge Offshore Tow Fig 10 Maneuver Deck/Barge over Spar Hull Fig 7 Tow Deck/Barge out of AGM Yard Fig 9 Deck/Barge Approaches Spar Hull Fig 12 Mating Unit (Shock Cell) OTC 12971
OTC 12971 FLOATOVER DECK INSTALLATION FOR SPARS 7 FLOAT DECK OVER BALLASTED SPAR HULL DEBALLAST THE SPAR 3 FEET SHOCK CELL COMPRESSED - 10% OF LOAD TRANSFERRED) 5' 12' 9' 34' 22' UPPER 24' UPPER 29' 30' 10' FREEBOARD LOWER 20' FREEBOARD LOWER LC SPAR CL LC SPAR CL (a) Deck/Barge Floatover Spar & Aligned (b) Contact between Spar and Deck Made DEBALLAST THE SPAR RAPIDLY TO TRANSFER REMAINING 10% OF LOAD AND LOWER SUPPORTS TO CLEAR S BY 5 FEET DEBALLAST THE SPAR TO THE OPERATING FREEBOARD OF 35 FEET - LOAD TRANSFER COMPLETE PULL AWAY FROM SPAR PULL AWAY FROM SPAR 24' UPPER LOWER UPPER LOWER LC SPAR CL LC CL (c) Deck Separated from Barges (d) Tow Barges from Site Fig 11 Deck Mating Operation MLTSIM Model for Catamaran Tow Configuration 1 0E+00-1E+06-2E+06-3E+06 1 force y -4E+06-5E+06-6E+06-7E+06 500 550 600 650 700 750 800 850 900 950 1000 time (s) Fig 13 Deck Supports on Barges Fig 14 Predicted Support Load Time History
8 J.V. MAHER, I. PRISLIN, J.C. CHAO, J.E. HALKYARD, L.D. FINN OTC 12971 ABASIM Model for Mating Operation Configuration 1 40 Spar & Deck/Barge Mating Spar & Deck Displacement (ft) 30 20 10 0-10 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Time (sec) Fig 15 Spar & Barges Supports Fig 16 Mating Operation Simulation Float Over: Dynamic Deck Mating With the Spar Piggyback Configuration (Model Test; Hs=6.6 ft, Tp=7 sec; quartering sea) Deck Load Distribution 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% Portside barge Starboard barge Spar 0% 0 500 1000 1500 2000 2500 3000 3500 time (sec) Fig 17 Model Test Fig 18 Measurements During Mating Test 50% Numerical Simulation: Hs=6.6 ft Tp=7 s (GOM) Vertical Load (% of Deck Weight) 45% 40% 35% 30% 25% 20% 15% 10% 5% Spar Portside barge Starboard barge 0% 200 210 220 230 240 250 260 270 280 290 300 time (s) 50% Model Test: Hs=6.6 ft; Tp=7 s (GOM) Dynamic1 - after mating Vertical Load (% of Deck Weight) 45% 40% 35% 30% 25% 20% 15% 10% 5% Spar Portside barge Starboard barge 0% 3000 3020 3040 3060 3080 3100 time (s) Fig 19 Predicted & Measured Load Time History During Mating Test
OTC 12971 FLOATOVER DECK INSTALLATION FOR SPARS 9 s par Spectrum (sim ulation) PSB Spectrum (simulated) 3.50E+ 11 3.00E+11 PSD (unit2) ^2/sec 3.00E+ 11 2.50E+ 11 2.00E+ 11 1.50E+ 11 1.00E+ 11 5.00E+ 10 0.00E+ 00 PSD (unit2) ^2/sec 2.50E+11 2.00E+11 1.50E+11 1.00E+11 5.00E+10 0.00E+00 0 5 10 15 20 0 5 10 15 20 Period (s) Period (s) Ports ide barge Spectrum Spar Spe ctrum (Mode l test) (measured) 3.50E+ 11 3.00E+11 PSD (unit2) ^2/sec 3.00E+ 11 2.50E+ 11 2.00E+ 11 1.50E+ 11 1.00E+ 11 5.00E+ 10 0.00E+ 00 PSD (unit2) ^2/sec 2.50E+11 2.00E+11 1.50E+11 1.00E+11 5.00E+10 0.00E+00 0 5 10 15 20 Period (s) 0 5 10 15 20 Period (s) Fig 20 Comparison of Predicted & Measured Force Spectra
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