Installation Alternatives Installation of a Wind Turbine in One Step

Transcription

1 Installation Alternatives Installation of a Wind Turbine in One Step Dutch Offshore Wind Energy Converter project DOWEC-F1W2-SH /00-C Name: Signature: Date: Written by: S.A.Herman, ECN 05 November 2002 H.J.T. Kooijman, ECN version Date No of pages 0 Nov Initial Issue

2 Contents 1 INTRODUCTION DESCRIPTION OF ALTERNATIVES GENERAL HUNG OWEC BOTTOM-HELD OWEC ESTIMATION OF MASS AND OVERALL DIMENSIONS OF OWEC DEFINITION OF TYPE OF SUPPORT STRUCTURE GENERAL DIMENSIONS AND MASS OF 6MW TURBINE INSTALLATION PROCEDURE GENERAL TRANSPORT AND INSTALLATION PERFORMED BY ONE VESSEL (ALTERNATIVE 1) TRANSPORT AND INSTALLATION PERFORMED BY DIFFERENT VESSELS (ALTERNATIVE 2) REQUIREMENTS TO THE OFFSHORE LOCATION SEA BOTTOM LEVELLING SCOUR PROTECTION ESTIMATION OF TRANSPORT AND INSTALLATION TIME TRANSPORT AND INSTALLATION PERFORMED BY ONE VESSEL TRANSPORT AND INSTALLATION PERFORMED BY DIFFERENT VESSELS ESTIMATION OF TRANSPORT AND INSTALLATION COSTS TRANSPORT AND INSTALLATION PERFORMED BY ONE VESSEL TRANSPORT AND INSTALLATION PERFORMED BY DIFFERENT VESSELS CONCLUSIONS REFERENCES Page 2 of 14

3 1 Introduction This report describes an alternative installation of the 6 MW DOWEC wind turbine. It is part of the DOWEC (Dutch Offshore Wind Energy Converter) study, concerned to task 7 of work package 1. The objective of this task is to analyse the installation of the wind turbines. The baseline configuration of the DOWEC consists of a monopile support structure. This requires an installation procedure of at least two steps: the installation of the support structure followed by the pre-assembled OWEC tower, nacelle and rotor together. Installation of a wind turbine in one step is conceivable if a jacket support structure is used instead. In the situation of the DOWEC wind turbine, the envisaged installation vessel Svanen of Ballast Nedam has a very large lifting capacity and lifting height. This makes it perfectly capable for the installation of the turbine with its attached support structure (tripod), in one step. Two alternatives for the installation of a complete OWEC in one step are addressed in this document. The first alternative is lifting the OWEC with a high reaching crane. Furthermore, this alternative is referred to as hung OWEC. The second alternative is holding the whole OWEC upright from its base in a way similar to a drinker should hold a glass of wine. This alternative is named bottom-held OWEC in this report. The aim is to determine the lowest transport and installation costs, by saving time and number of offshore handling activities. The costs savings must be compared with extra costs involved with capacity enlargement of the installation vessels, fabrication of a new installation vessel and other. Page 3 of 14

4 2 Description of Alternatives 2.1 General Two alternatives are considered feasible as installation options for a complete OWEC: - To transport and install a complete OWEC including its support structure with a high reaching crane on the transport/installation vessel. (Hung OWEC). - To transport a complete OWEC held upright from the lower side of its support structure, similar to a wineglass, and install it from this position. (Bottom-held OWEC). Both installation alternatives require specific design considerations with respect to loading during transport and installation. The installation of a complete OWEC according to the alternatives implies that the support structure is not a monopile, but must be a jacket construction or a gravity base. In this report, only a tripod support structure will be analysed, which is a special type of jacket structure. The two alternatives also affect the type of transport and installation vessel(s) to be used. The main requirements for the transport and installation vessels are presented in section 4. This report will not discuss the design of a special vessel in detail, but will merely give an indication of the installation costs of such a vessel, compared to the installation procedure and costs of the DOWEC baseline. 2.2 Hung OWEC The installation procedure considered so far in the DOWEC project comprises the transport and installation of the support structure and transport followed by the installation of the preassembled turbine tower, nacelle and rotor. However, a complete offshore wind turbine including its support structure can be transported and installed in one step. This can take place if the support structure considered is a jacket, gravity base or similar. A complete OWEC vertically transported may be free of interaction with the sea (above the sea level) or semi-submerged. The second option implies that the hook level for the transport does not need to be placed higher than the level used for the installation of the baseline design. It might though be necessary to adapt the existing vessel. In any case, the loading on the crane boom and connecting structure will probably be significant compared to a low point of the attachment. Figure 1. Hung OWEC with a tripod. Page 4 of 14

5 2.3 Bottom-held OWEC Instead of hanging the complete OWEC from a point above the nacelle, the wind turbine could be transported upright and held in position by a mechanism that holds the OWEC next to the transport vessel (see for instance Figure 2). Several mechanisms could be designed to realise this transport by holding the complete wind turbine at its base (support structure) or somewhere in the middle of the tower structure. To install the OWEC, the same or a second mechanism could perform the installation. The fixating mechanism considered could be mounted on the bow or along the side of the transport/installation vessel. Similarly as for a hung OWEC, a bottom-held OWEC may be transported free of interaction with the sea or semi-submerged. The latter method would also induce transport loads from the water that need to be addressed. Figure 2. Bottom-held OWEC. Example of mechanism attached to the bow of the vessel. Page 5 of 14

6 3 Estimation of Mass and Overall Dimensions of OWEC 3.1 Definition of Type of Support Structure The following support structures are considered feasible for the transport and installation of complete wind turbines: - Tripod structures, including sleeves for driven piles; - Tripod structures, including suction piles for anchoring purposes; - Lattice towers, including sleeves for driven piles; - Lattice towers, including suction piles for anchoring purposes; - Gravity bases. In this section, only a tripod structure fastened to the sea-bottom by means of driven piles is analysed. 3.2 General Dimensions and Mass of 6MW Turbine The estimated dimensions and mass of the DOWEC 6 MW wind turbine are summarised below. Dimensions and mass of wind turbine tower above support structure, nacelle and rotor are taken from reference [1]. Dimension and mass of tripod structure and foundation piles are taken from reference [2]. Note that in this case the transition piece is not required. Support Structure Type : Tripod with pile sleeves Base radius from tower centreline : 20 m Height (from seabed up to top) : 30 m Mass (excluding piles) : 306 to 445 MT 1 Pile length : 36 m Pile outer diameter : 1.05 m Mass of piles, three per turbine : 82 MT Wind Turbine Tower (taken up from top of support structure) Type: Tubular shaped Base diameter : 6.0 m Top diameter : 3.5 m Length : 80 m Mass : 225 MT Nacelle Height : Mass: Rotor Diameter : Mass (including hub): 4.8 m 189 MT 129 m 83 MT Complete OWEC Height (sea bottom to hub height) : Lifting mass (= total mass excluding piles) : ~ 115 m ~ 810 to 950 MT 1 Depending on the variable water depth, between 21 and 36 metres. Page 6 of 14

7 4 Installation Procedure 4.1 General For the analysis of the installation procedure, the following conditions are assumed: - The OWEC consists of a tripod foundation with pile sleeves. - An auxiliary vessel performs the piling process while the installation vessel holds the complete OWEC in upright position. The most likely procedure for the attachment of the support structure to the sea bottom is that the special purpose design vessel places the OWEC in position while the auxiliary vessel performs the piling process. As an alternative, the special installation vessel could perform the piling process itself. This would probably be the case for at the most two of the three required driven piles. This option is disregarded of further analysis. If pre-assembly of the OWEC at the quayside is considered, two installation procedures for a complete OWEC are possible: - a special purpose design vessel transports and installs the complete OWEC, or - a standard vessel performs the transport and the special purpose design vessel performs the installation. In both cases, onshore crawler crane(s) at the quayside are required to assemble the OWEC before it is transported to the site. In the second case, at least one relatively big crane onshore must lift a complete OWEC and bring it to a pontoon. The crane does not need to be of the crawler type, but could be similar to portal cranes of a container terminal. The pre-assembly of the OWEC may then be achieved by the same crane or by a crawler crane next to it. The latter option seems more logical. Pre-assembly of a complete OWEC does not necessarily have to take place at the quayside. The wind turbine components could be pre-assembled offshore. The aim of doing so is to avoid extremely large onshore cranes at the quayside and to use standard vessels for transport rather than the special installation vessel. On the other hand, the pre-assembly of the OWEC offshore may be economically prohibitive. The installation procedure for this scenario could be as follows: - Onshore crane lifts OWEC components from quayside onto transport barge(s). Three driven piles are transported on the deck for each OWEC on the barge(s) as well; - Transport vessel sails OWEC components and piles to site; - Special vessel lifts OWEC components from barge, performs pre-assembly and holds the complete assembled OWEC on position while auxiliary vessel drives the piles; - Transport vessel returns to harbour. Special vessel and auxiliary (piling) vessel remain offshore. Simultaneously a second transport vessel with new wind turbine components arrives at the site. The time the special vessel requires to perform the pre-assembly works offshore is expected to be large. This time and efforts are comparable with the transport time of the special vessel from the harbour, whereas extra transport facilities would be required. Therefore, this scenario will not be considered any further. 4.2 Transport and Installation Performed by One Vessel (alternative 1) The installation procedure for this scenario is as follows: a. Pre-assembly of OWEC at the quayside; b. Special vessel lifts a complete OWEC from quayside together with three driven piles; c. Special vessel transport OWEC and piles to the site; d. Special vessel holds complete OWEC in position while auxiliary vessel drives the piles Page 7 of 14

8 into position; e. Special vessel returns to harbour. The auxiliary vessel may remain offshore to complete the piling. The requirements of the (offshore) equipment for this scenario are given below: Pre-assembly Onshore crawler crane, minimum lifting capacity 2700 kn (mass of turbine tower = 225 MT), at a height of 120 m and a reach of approximately 25 m. Furthermore, the harbour facilities must be such that the special vessel can approach the quayside and moors to it. Transport and Installation The special purpose design vessel must comply with the following characteristics: Estimated weight of OWEC + piles = 7500 kn Required vessel water displacement (not including own weight) = 800 m 3 Lifting capacity OWEC without piles, including a dynamic amplification factor of 1.2 (see reference [3]) = 8500 kn Approximate crane reach = 25 m Note that if a special shaped vessel like the Svanen (ref. [4]) is used instead, the lifting/holding crane during the installation procedure does not need to satisfy a minimum reach: the load is hanging under the crane hook. The minimum height of the crane hook of the special vessel depends on the installation alternative (see 2.1) Hung OWEC free of sea-interaction: construction height 2 = 115 to 125 m free space between crane hook and top structure = 10 m free space above sea level = 5 m Total distance required between crane hook of vessel and MSL = 130 to 140 m Hung OWEC, semi-submerged: construction height 2 = 115 to 125 m free space between crane hook and top structure = 10 m submerged height (under sea level) = 15 m Total distance required between crane hook of vessel and MSL = 110 to 120 m For bottom-held OWECs, no special crane hook height characteristics for the installation vessels are required, but a holding/locking mechanism, located approximately at deck level, is required instead. The holding mechanism must also allow the complete OWEC structure to descend until seabottom is reached. This means that the OWEC must descend between 20 and 35 metres from sea level. The following requirements to the special vessel are optional: - Own propulsion system instead of using towing tugs - Dynamic positioning system instead of anchoring Piling An auxiliary vessel is required for the piling operation. The vessel must comply with the following characteristics: - Onboard crane with minimum capacity of approximately 200 kn at 10 m reach for the lifting of the piles; 2 Depending on the water depth Page 8 of 14

9 - A pile hammer with enough piling capacity; - Submarine supervision equipment or ROV (Remote Operating Vehicle) during the pile driving process. 4.3 Transport and Installation Performed by Different Vessels (alternative 2) The difference with the previous scenario is that the special vessel may remain offshore during the installation period, while transport barges loaded with the OWECs sail constantly between the quay and the site. The advantage of this procedure is that the travelling time of the special vessel is kept to a minimum. The installation procedure for this scenario is as follows: a. Pre-assembly of OWEC at the quayside; b. High capacity onshore crane lifts a complete OWEC from quayside onto transport barge. Three driven piles are transported on the deck of each barge as well; c. Transport vessel sails OWEC and piles to site; d. Special vessel lifts complete OWEC from barge and holds it in position while auxiliary vessel drives the piles; e. Transport vessel returns to harbour. Special vessel and auxiliary (piling) vessel go to the next location where a second transport vessel delivers new wind turbine components. Requirements for the (offshore) equipment: Pre-assembly of OWEC and Lifting onto Pontoon - Onshore crawler crane, minimum lifting capacity 2700 kn, at a height of 120 m and a reach of approximately 25 m. - Portal (mobile) crane (like a container terminal) with a capacity of 8500 kn and 120 m height. Transport - Transport barge with a minimum water displacement of 800 m 3 and minimum dimensions of approximately 25 m width and 40 m length (1x complete OWEC). - Towing tug, one for each transport barge. Installation Similar requirements as described in section 4.2. Piling Similar requirements as described in section Requirements to the offshore location 5.1 Sea Bottom Levelling Sea bottom levelling is not considered. 5.2 Scour Protection For analysis of scour protection of tripod structure, see reference [2]. Page 9 of 14

10 6 Estimation of Transport and Installation Time 6.1 Transport and Installation Performed by One Vessel This procedure (alternative 1, see 4.2) assumes that only one special vessel performs the transport and the installation. The following installation steps are identified: Steps Required equipment Estimated working time Pre-assemble OWEC Onshore crane 24 hr Load OWEC onto vessel Special vessel 4 hr Load J-tubes onto vessel Special vessel 2 hr Sail to offshore location Special vessel 10 hr Place OWEC in position Special vessel 6 hr Place 3x piles Special vessel 24 hr Auxiliary piling vessel Installation J-tube Special vessel 12 hr Sail to quay for next OWEC Special vessel 10 hr Once an OWEC is pre-assembled, the special vessel takes it from the quay and installs it offshore. The working time of the special vessel amounts to 68 hours, excluding downtime. In the mean time, a new OWEC is being assembled at the quayside. To achieve this installation procedure, a benign weather window of 62 hours is required for the special vessel (between departure from and return to the harbour). This is also the critical path (see also Figure 3). Considering a probability of benign weather window of 60% and an inefficiency factor of 20% [5], the estimated total installation time equals = 68/0.6*1.2 = 136 hours per turbine. A wind farm of 80 turbines would then take hours to be installed. Assuming that the available hours in one year equals 4416 hours [5], it would take 2.5 years to perform the whole installation. Figure 3. Installation of a complete OWEC, planning for installation with one vessel. 6.2 Transport and Installation Performed by Different Vessels This procedure (alternative 2, see 4.3) assumes that one special vessel performs the installation while two combinations of a pontoon pulled by a tug perform the transport to the site. The following installation steps are identified: Steps Required equipment Estimated working time Pre-assemble OWEC Onshore crane 24 hr Load OWEC onto vessel Onshore crane, Pontoon, Tug 4 hr Load J-tubes onto vessel Onshore crane, Pontoon, Tug 2 hr Page 10 of 14

11 Steps Required equipment Estimated working time Sail to offshore location Pontoon, Tug 10 hr Place OWEC in position Special vessel, Pontoon, Tug 6 hr Place 3x piles Special vessel, Auxiliary piling 24 hr vessel, Pontoon, Tug Installation J-tube Special vessel, Pontoon, Tug 12 hr Sail to next OWEC location Special vessel, Auxiliary piling 10 hr vessel Sail to quay for next OWEC Pontoon, Tug 10 hr Using above values, the critical path for the installation of two OWECs consecutively is found to be approximately 5 days (120 hours) for procedure (b), excluding possible downtime. This period of time includes the transport of the first turbine to the wind farm offshore, see Figure 4. From the estimated installation planning, presented in Figure 4, it can be seen that if no OWECs are held on stock at the quayside, the maximum pre-assembly time for one OWEC will be no more than 48 hours. Figure 4. Planning of Installation of two OWECS using separated vessels for transport and installation. If only the working time of the special vessel is considered (see Figure 5), the installation Page 11 of 14

12 time per OWEC equals 52 hours, downtime excluded. A minimum of 30 consecutive hours is needed to fasten one OWEC to the seabed. Considering a probability of benign weather window of 70% for 30 consecutive hours and an inefficiency factor of 20% [5], the estimated total installation time equals 52/0.7*1.2 hours per turbine. A wind farm of 80 turbines would then take 7200 hours to be installed. Assuming that 4416 hours are available for installation in one year [5], it would take less than 2 years to perform the installation of the complete wind farm. Figure 5. Procedure (b), effective use of the special vessel. Page 12 of 14

13 7 Estimation of Transport and Installation Costs 7.1 Transport and Installation performed by One Vessel Based on the transport and installation times found in section 6, the following transport and installation costs are estimated for the procedure presented in 4.2 (alternative 1): The procedure takes 68 hours per turbine and the total installation time is hours (~2.5 years effectively). Offshore Equipment Properties Mobilisation Costs Estimated rent price Special vessel Lifting weight = 800 MT per day Auxiliary (piling) vessel Estimated costs of installation alternative Onshore equipment investment Mobilisation costs = 3x (500, ,000) Rent of offshore equipment = (10,880/24) x (250,000+25,000) Total estimated costs of alternative Transport and Installation performed by Different Vessels per day = = = Based on the transport and installation times found in section 6, the following transport and installation costs are estimated for the procedure presented in 4.3 (alternative 2): The procedure takes 52 hours per turbine and 7200 hours for the entire installation (less than two years). Onshore Equipment Onshore crane Properties Lifting load = 800 MT Own mass = 400 MT Total = 1200 MT Estimated buy price Unit price Estimated rent price N.A. Offshore Equipment Properties Mobilisation Costs Estimated rent price Special vessel Lifting load = 800 MT per day Auxiliary (piling) Piling capacity vessel per day Pontoon 40 x 25 m deck area 800 m 3 displacement per day Tug N.A. per day Estimated costs of installation alternative Onshore equipment investment = Mobilisation costs = 2x {500,000+2x100,000+10,000} = Rent of offshore equipment = (7200/24)x {250,000+25,000+15,000+10,000} = Total costs of alternative 2 Page 13 of 14

14 8 Conclusions - Compared to the installation procedure defined in the baseline [4], the installation of a complete OWEC including support structure seems to take a longer installation time if only one special vessel performs the installation (alternative 1). The installation costs seem to be relatively high (no installation costs are defined for the baseline). Installation costs may be reduced if the same special vessel performs the piling process, avoiding the utilisation of an auxiliary (piling) vessel. - Compared to the installation procedure defined in the baseline [4], the installation of a complete OWEC including support structure seems to take a shorter installation time if besides a special vessel a transport pontoon is mobilised (alternative 2). The installation costs again seem to be high. Installation costs may be reduced if the same special vessel performs the piling process, avoiding the utilisation of an auxiliary (piling) vessel. Compared to the alternative installation 1, alternative 2 appears to be more advantageous with respect to both time and costs. 9 References 1. Kooijman, H.J.T., Lindenburg, C. and Winkelaar, D. Aero-elastic modelling of the DOWEC 6 MW pre-design in PHATAS. ECN-CX Petten, January Zaaijer, M. Tripod Support Structure. DOWEC-F1W2-MZ Delft, May Det Norske Veritas. Marine Operations, Part 2: Recommended practices. RP5, Lifting. June Ballast Nedam Engineering. Installation method 2.75 MW OWEC. Doc. nr Nieuwegein, April Ballast Nedam Engineering. Offshore Workspace. Doc. nr Nieuwegein, May Page 14 of 14