Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering

Transcription

1 Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE2011 June 19-24, 2011, Rotterdam, the Netherlands OMAE OMAE YEARS OLD JACKETS REMOVED AS A SINGLE PIECE Jochum C.G. van Hoof Heerema Marine Contractors Leiden, the Netherlands Ruben de Bruin Heerema Marine Contractors Leiden, the Netherlands ABSTRACT Heerema Marine Contractors (HMC) have removed and will be removing various platforms from the North Sea. For these projects Heerema has developed an unconventional method to remove and transport jackets: Jackets are lifted as one single piece and transported to the recycling yard whilst being suspended from both cranes of the Heavy Lift Vessel (HLV) Thialf. Two purpose built structures at the stern restrain the jackets from horizontal motions during transport. In summer 2010 three eight-legged jackets were removed and transported using this method (see Figure 1). The jackets weighed approximately 5,000 metric tonnes each and stood in 70-80m of water. The jacket removal resulted in load cases that were never considered during jacket design. Jacket strength appeared very marginal when cutting the jacket into several sections, but by lifting the jacket as one single section, all members remained connected and ensured a stable structure. Other benefits were reduction of the offshore project duration, the subsea cutting scope and the required vessel spread. Risk on weather downtime was reduced and safety improved by preventing back loading operations in an offshore environment. The transport distance with the jacket suspended from both Thialf cranes ranged from nautical miles (1½-2½ days sailing). The final cuts and the jacket lifting required relatively low sea states. The wave climate for transport was determined with the assumption of a preceding weather sensitive operation, which is different to a transport that assumes a start at a random time. Model tests for these design sea states have been performed to accurately assess the Thialf dynamic behavior at its shallow transit draught. Additional analyses were performed to confirm the vessel-jacket dynamic interaction. During transport the so called restraints gripped around the jacket corner legs, restraining the structure horizontally and preventing side loads on the cranes. During the three transports, motions have been measured and dynamic behavior corresponded well with the analyses. Removing and transporting these jackets as a single piece was a unique operation. The method worked well and resulted in a predictable, safe and time efficient jacket removal. This paper will address the removal method, including structural aspects, model tests and full scale verification. Figure 1 Heavy Lift Vessel Thialf transporting eight-legged jacket as a single piece, June 2010 (photo HMC) 1 Copyright 2011 by ASME

2 INTRODUCTION In the next twenty years many of the over five hundred installations in the North Sea will have to be removed partially or entirely due to international legislation [ref. 1]. Heerema Marine Contractors are currently contracted for one of the most extensive decommissioning projects to date. This project includes the decommissioning, removal, disposal and recycling of various platforms in the Central North Sea. For this project, Heerema s fleet of heavy lift vessels is used for the offshore removal operations that started in 2009 and are to be concluded in Topsides of three of these platforms were removed in 2009 by reversed installation. While the topsides were being removed, inspections and preparations were performed for the jacket removal scope. The year after, 2010, the three jackets were removed and transported to the recycling yard in Norway. To complete this scope, Heerema Marine Contractors developed an unconventional method to remove and transport these jackets as a single piece. The first part of this paper presents background information on the jackets including the structural aspects. The single piece removal and transport method is described next. Then the design sea state and the model tests for these design conditions are discussed. Finally the actual project execution is described including the full scale verification. NOMENCLATURE C.o.G. Centre of Gravity DGPS Differential Global Positioning System DNV Det Norske Veritas DP Dynamic Positioning F Cumulative probability distribution function GM Distance C.o.G. to metacenter HLV Heavy Lift Vessel HMC Heerema Marine Contractors H s Significant wave height MRU Motion Reference Unit mt metric tonnes P non Probability of non-exceedance RAO Response Amplitude Operator RP Recommended Practice Operation Reference Period T R JACKET INFORMATION All three jackets have eight legs arranged in a 2 x 4 grid vertically braced in the two longitudinal and four transverse planes. Horizontally the jackets are braced at six elevations (see Figure 2). The jackets were installed in the 1970s in meter of water. The jackets were launched from a barge and thereafter upended. After placement onto their position, piles were driven through and grouted to the jacket legs to ensure a sound connection to the seabed and to support the topsides. The piles and grout significantly increased the weight to approximately 5,000 metric tonnes per jacket as presented in Table 1. Combined with later modifications to the jacket, removal resulted in load cases that were never considered during jacket design. Jacket removal and transport was modeled and analyzed using a finite element structural analysis suite. The models included all structural and non-structural jacket items. Detailed sub sea surveys were performed during the preparation phases and all anomalies found were included in the model. Structural integrity calculations clearly showed that the jackets are quite weak structures, which have never been designed with removal in mind. If lifting the jacket in several sections, structural strength during lift and transport appeared very marginal. By lifting the jacket as one single vertical section, all members remained connected and ensured a stable structure. Table 1 Average Jacket Properties Property Value Unit Jacket base dimension 63 x 35 m Jacket top dimension 43 x 16 m Jacket Height 91 m Jacket Leg Diameter 1,400 mm Total Dry Weight 5,000 mt Structural Steel (excl. piles) 2,000 mt Piles and Grout Weight 2,000 mt Ancillary Items (e.g. caissons, 500 mt risers, anodes, boat bumpers) Marine Growth 500 mt Figure 2 Typical eight-legged jacket - Isometric view 2 Copyright 2011 by ASME

3 SINGLE PIECE REMOVAL AND TRANSPORT METHOD The jacket removal has been designed as a dual crane lift. The lifting equipment consisted of four doubled grommets, four lift points and two spreader bars that were welded to the top of the four corner jacket piles. The jacket piles were cut two meters below seabed. Subsequently the entire jacket was lifted above sea level and transported to shore whilst being suspended from both cranes of Heavy Lift Vessel Thialf (see Figure 3). During transport the jacket surge and sway motions were restrained to prevent side loads on the cranes. The so called restraints are purpose built structures on the stern that gripped around the corner legs (see Figure 16). The restraints were designed to transfer all horizontal transportation loads that could occur during this unrestricted transport. Transport distance Nm Figure 4 Transport route Figure 3 Jacket arriving in Norway (photo HMC) Benefits of removing and transporting these jackets as a single piece were: Shorter offshore project duration; Reduced subsea cutting scope; Subsea lift tools not required; Smaller marine spread; Improved safety due to fewer offshore operations and by preventing back loading operations to a barge at open sea; Less risk on weather downtime. On the other hand the onshore decommissioning and recycling scope increased as the jackets were delivered as one large section. DESIGN SEA STATE The transport distance with the jacket suspended from both cranes ranged from nautical miles (see Figure 4). The transport has been designed for removal in the period from May to August and involved approximately 1½ to 2½ days sailing. Unlike a transport scheduled for platform installation, this removal transport would be preceded by a period of relatively good weather to perform the final cut and lift of the jacket. DNV-RP-H102 [ref. 2] states that for unrestricted operations with duration less than five days (T R 120 hours), the design wave height and wind speed can be reduced using pure statistical methods. As successive sea states are not fully independent but show some persistence, the design wave height may very well be reduced. The persistence is captured in measured and modeled time series of wave heights. These time series were used to calculate the design wave height based on the condition that the transport can only start directly after jacket lifting (with a sea state below a certain value). The design sea state was based on extreme value statistics. A 10% probability of exceeding of the design value is acceptable during the operation reference period. The coinciding probability that the design value will not be exceeded during a 3 hour period for a 72 hour exposure is: 3 72 P = ( 1 0.1) = 99.56% (1) non and H design s 1 = F ( P ) (2) H s non 3 Copyright 2011 by ASME

4 In which F Hs is a cumulative distribution function approximated as a Weibull distribution using regression analysis. The design values were based on a 2-parameter Weibull regression on top 30% values. For clarity it shall be noted that for a 72 hour exposure the 10% exceedance value is equivalent to a 1 year monthly return condition. An operation period less than 72 hours shall not be used in this respect. The probability distribution was determined for a sea state population that meets following conditions (see also Figure 5): Sea state The operability limits for the preceding operation (jacket final cut, lifting and securing) have not been exceeded during the planned operation period of that preceding operation; Sea states need to be within reference period of the transportation; All possible arrival dates are simulated. For each arbitrary start time of the lift operation the first possible weather window is used. Wave data after the lift window are used for transport statistics. Threshold (precondition) Sea state time trace Restricted operation (final cut / lift) Unrestricted transport (with precondition) The transport distance; The considered season for transport. The average reduction of the design wave height for the single lift transport appeared to be approximately 10% for the summer period. The associated wave period range was calculated according to DNV [ref. 4]. FROM MODEL TESTS TO DESIGN CRITERIA The single lift transport was performed at a shallow vessel draught with the floaters of the heavy lift vessel just submerged. Numerical computations using diffraction theory will not result in accurate motion statistics of the Thialf at this draught. In order to accurately assess the transportation forces during the design sea state, it was therefore decided to perform sea keeping model tests (see Figure 6 and Figure 7). The vessel loading condition, vessel and jacket mass, roll and pitch inertia and the GM values were included in the model and calibrated prior to the model tests. The Thialf and jacket were modeled as one rigid body which combined with some post-processing, forms an accurate representation of the vessel - restrained jacket system. The scale of the Thialf model was 1:40 and sea states were of the JONSWAP spectrum. Various wave conditions were analyzed for five different headings. An interesting observation concerned the large damping for beam seas, possibly due to water running on and off the submerged floaters. At transit draught the floaters of the semi submersible column stabilized unit Thialf were just submerged (~0.9 m of water on top of the floaters). Time start 1 start 2 Extreme value analysis input Figure 5 Simulating unrestricted transport after a weather restricted operation (pre-condition) start n Calculating the design sea state using a pre-condition has lead to reduced design values, whilst the transport is still an unrestricted operation. The reduction in wave height varies pending: Figure 6 Sketch wave tank (Océanide) Duration of the preceding weather restricted operation 1 ; The limiting sea state for the preceding operation; 1 Durations of weather restricted operations are limited to the maximum reliable weather forecast which is currently set at 72 hours. DNV suggests that forecasts ranging up to 120 hours may be considered reliable [ref. 3]. 4 Copyright 2011 by ASME

5 The design accelerations were direct input in the finite element structural analysis suite. The jacket structural model included rigging and the correct spring stiffness for the restraints and crane tips. The finite element model was used to verify jacket structural integrity during lift, transport and setdown and to calculate design loads for the lift equipment and the restraints. Figure 7 Thialf sea keeping model tests (photo HMC) Figure 8 shows the RAO s for roll for different significant wave heights. The RAO s are lower for higher wave heights. This means that roll responses of the Thialf at this draft do not increase linearly with wave height. This non-linear behavior makes it impossible to predict motions using linear panel methods. TRANSPORT INCLUDING FULL SCALE VERIFICATION A motion monitoring system was installed on board the Thialf including wireless motion sensors that were installed on top of the jackets. This system provided the opportunity for real-time monitoring of jacket accelerations (six degrees of freedom) and angles (roll, pitch and yaw). In addition various other vessel sensors were connected to this system so this data could be monitored as well (see Figure 9). Besides real-time monitoring, data was logged for evaluation purposes. Roll RAO amplitude for beam seas for different significant wave heights RAO 0 Hs1 Hs2 Hs3 Hs4.3 Wave Period [s] Figure 8 Model test RAO s for roll motion Relative water elevation and jacket accelerations for the design sea states were therefore extracted from the model tests as directly as possible: Motions of the model at the location of the jacket C.o.G. were differentiated twice to yield the accelerations. Most probable maximum values were derived from the resulting acceleration time traces. On top of the accelerations derived from the model test results, following accelerations were added: Static wind heel angle (transverse acceleration); Relative pitch acceleration between jacket and vessel. The restraints prevented relative horizontal jacket motions between jacket and vessel. Relative pitch between jacket and vessel was however not restrained. Rigging was selected in such a way that the natural period for jacket pitch was only 4 seconds. As the Thialf will transfer very little energy at this frequency, relative pitch accelerations are limited. A dynamic analysis was performed to quantify this relative jacket pitch. Work Station on Bridge to log and display: Vessel information: o DGPS; o Gyro Compass; o Wind; o MRU Thialf DP system Motion sensors (wireless): o On jacket (2x); o On Thialf stern (1x) ; o On Thialf bridge (1x). Figure 9 Motion monitoring system on board HLV Thialf The weather during all three transports was relatively mild so the design sea state was not encountered. Nevertheless some higher sea states occurred which showed that the measured Thialf RAO s and the model test RAO s were very similar. RAO s calculated based on the motions measured during the offshore campaign are shown together with the model tests RAO s (Figure 10 to Figure 12). The full scale RAO s were calculated using the hindcasted wave spectrum. Note that the Thialf was sailing in near head-seas (15 degrees) during the full scale measurements. Roll motions were therefore very small which leads to a relative large influence of noise. 5 Copyright 2011 by ASME

6 Amplitude Heave motion Measured offshore Model test Hs = 2, Heading = 150 deg Model test Hs = 2, Heading = 180 deg Figure 10 Heave motion RAO Period [s] Roll motion The relative jacket pitch motions appeared very small which was in accordance with the dynamic analysis. Preparing the jackets, cutting the piles, lifting the jackets, engaging the jackets into the restraints and transporting the jackets was all executed according to the planning. The average duration of the offshore operations was around 15 days per jacket. This duration excludes (de)mobilization and weather downtime, but includes other removal activities which were performed simultaneously with the jacket removal (e.g. module support frame removal). Contingency procedures proved to be adequate, all restricted operations were finished well within the anticipated weather windows and the unrestricted transport delivered no surprises. Photos of the offshore execution are included in Annex A (Figure 13 to Figure 17). Amplitude Measured offshore Model test Hs = 2, Heading = 150 deg Model test Hs = 2, Heading = 180 deg Figure 11 Roll motion RAO Amplitude Figure 12 Pitch motion RAO Period [s] Pitch motion Measured offshore Model test Hs = 2, Heading = 150 deg Model test Hs = 2, Heading = 180 deg Period [s] CONCLUSION Removing and transporting jackets as a single piece resulted in a predictable, robust, safe and time efficient jacket removal. Heerema Marine Contractors successfully transported three jackets to a recycling yard with the jacket restrained horizontally whilst being suspended from both cranes of Heavy Lift Vessel Thialf. REFERENCES 1. OSPAR Commission (July 1998): Convention for the Protection of the Marine Environment of the North-East Atlantic, Annex III, Article 5, Sintra, Portugal. 2. Det Norske Veritas (April 2004): Marine Operations during Removal of Offshore Installations, DNV-RP-H102, Høvik, Norway. 3. Det Norske Veritas (June 2007): DNV Marine Operation Rules, Revised Alpha Factor JIP Project, rev. 03, Høvik, Norway. 4. Det Norske Veritas (April 2007): Environmental Conditions and Environmental Loads, DNV-RP-C205, Høvik, Norway. 6 Copyright 2011 by ASME

7 ANNEX A: PHOTOS OFFSHORE EXECUTION Figure 13 Welding lift points on top of jacket leg (photo HMC) Figure 14 Lift points installed: Jacket ready for single piece removal and transport (photo HMC) 7 Copyright 2011 by ASME

8 A B C D E Figure 15 A) Subsea diamond wire cutting final jacket leg; B) Rigging hook on; C) Final jacket leg cut jacket lift-off; D) Happy Heerema crew; E) Jacket lift-off (photos HMC) 8 Copyright 2011 by ASME

9 A B C Figure 16 A) Restraint connected to Thialf stern; B) Engaging jacket leg into restraint; C) Jacket leg secured to the Thialf (photos HMC) 9 Copyright 2011 by ASME

10 Figure 17 Heerema transports eight legged jacket as a single piece to Norway for recycling (photo HMC) 10 Copyright 2011 by ASME