SUBMERGED TURRET LOADING OF OIL IN ICE

Transcription

1 SUBMERGED TURRET LOADING OF OIL IN ICE Sveinung Løset 1, Arnor Jensen 1 and Ola Ravndal 2 1 Department of Structural Engineering, Norwegian University of Science and Technology, Trondheim 2 Navion ASA, Stavanger ABSTRACT One of the keys to an efficient loading of oil in the Arctic offshore is probably a subsea solution where the interference with ice is at a minimum. Therefore an attempt to assess the performance of an Arctic Shuttle Barge System including a subsea mooring and loading terminal was done at a modelscale of 1:25 in the Hamburg Ship Model Basin (HSVA) ice tank in The system consists of a barge of about tons loaded displacement and tons ballast displacement ( DWT, length overall L oa = m,) and a pusher/icebreaker of about 8000 tons displacement (2000 DWT, L oa = 86 m). The pusher serves as the main propulsion and connects/disconnects to a notch in the aft of the barge. The operational performance and forces exerted on the barge, the pusher and the mooring system, including a riser, were investigated. The system was pushed by the pusher through level ice or towed through level ice and pressure ridges by the mooring system. The latter simulated the moored condition in drifting ice. This paper describes the test set-up, procedures and performance of the concept when manoeuvring into the loading position in level ice. The maximum ice breaking force was about kn during a ridge event. The paper also elaborates on the use of a wedged plough and ice milling propellers to avoid ice from interfering with the mooring lines and riser. Finally we have a brief discussion on how the transhipment at the ice edge could be done. 1. INTRODUCTION Plans for exploitation of hydrocarbon resources discovered in the European Arctic are still in an early stage. Currently, plans are being made for gas production from the huge gas reserves in the Shtockmanovskoye field in the eastern Barents Sea and oil production from some fields in the Pechora Sea. At the moment there is no oil production in the Barents Sea except for minor production on the Kolguyev Island. This production is based on summer shipping of the crude oil. Onshore, oil has been produced since 1988 from the Kharyaga oil field and recently from the Ardalin oil field, both in Nenets okrug. This oil is shipped Southwest to Yaroslavl through the Transneft pipeline. Offshore, waves and ice loads will govern the design of oil production and off-take systems. For instance, the ice regime makes demands beyond the tremendous challenge the oil industry faced in the North Sea almost three decades ago. Structures and vessels shall apply environmentally sound and cost-effective technologies as well as securing human safety in a hostile environment. Onshore, the infrastructure, including the foundation for pipelines on permafrost and river crossings are major concerns.

2 The paper gives a brief introduction to some of the problems we foresee connected to export of oil from a sea with drift ice present most of the year. Further, the paper elaborates on the major findings from the current study of the Arctic Shuttle Barge System and discuss a possible transhipment at the ice edge. 2. TECHNICAL CONCERNS WITH LOADING OF OIL IN THE ARCTIC OFFSHORE 2.1 History On a larger scale there is no proper experience with production and export of oil and gas from the Arctic offshore using icebreakers and tankers. The petroleum activity in the early 1970's in the Beaufort Sea never came to a stage where real offshore production was a part of the scenario. However, several exploration wells were drilled offshore. Fig. 1a shows the distribution by year of wells drilled by the different types of structures. There is no obvious trend of one structure displacing another, it is rather governed by water depth and ice conditions on the drill site (Masterson et al., 1991). Fig. 1b shows cost indications pertaining to the different types of structures. Further, Masterson et al. conclude that there seems to be a good potential for developing turret moored solutions for deeper water areas such as the Chukchi Sea. A ship shape turret moored system would have the advantage of being capable of operating in severe wave conditions as well as coping with the dynamic conditions of an ice field. (a) (b) Fig. 1. (a) Types of structures by year in the Beaufort Sea, and (b) cost of Beaufort Sea islands. The costs quoted are in US $ (Masterson et al., 1991).

3 2.2 Ice drift The motion of the ice is a crucial question when planning an off-take system. Let us now focus on the drift of four Argos positioned buoys (Buoys 06640, 22435, and 24051) that were deployed on the drift ice in the Pechora Sea during mid-april 1998 (Løset and Onshuus, 1999). The drift of the four buoys is shown in Fig. 2. (a) (b) (c) (d) Fig. 2. Drift of: (a) Buoy 06640, period ; (b) Buoy 22435, ;(c) Buoy 24050, period and (d) Buoy 24051, period The drift is mainly governed by wind, waves, ocean currents and tidal forcing. Let us look for mathematical properties of the motion. On a large time scale the motion is clearly stochastic, and with the exception of periods with rather straight-lined movement, it resembles Brownian motion: Though mathematically attractive, Brownian motion is obviously not suited to describe ice motion on a smaller time scale. Since ice floes are generally large and heavy objects, the direction and absolute value of their speed can not change instantly. But the question is, how fast does it change? Let us study the latitude and longitude position values as functions of time, and assume that they can be expressed as sums of harmonic functions. Denoting longitude X(t), we get

4 m X ( t) = Ai cosσ it + Bi sinσ it i= 1 (1) As this is a deterministic function, we need to guess what are the m frequencies of interest. If the N nt j, X js original longitude time series is given as j =1, the least square approximations of the coefficients A i and B i are N A$ 2 = X cosσ t N i j i j j= 0 N B$ 2 = X sinσ t N i j i j j= 0 (2) With the frequencies and weights at hand, we could turn Eq. (1) into a stochastic model, by changing A i and B i to Gaussian random variables. We can derive a continuous model on the latitude/longitude under the assumption that it is given by the sum of harmonic functions, with frequencies ranging from 10 minutes to 2 months. If we assume that this model is a valid representation of the ice dynamics, Fig. 3 gives an impression of the movements during a 24-hour period. Fig. 3. Modelled movement of the ice drift. Dots every 10 minutes. We see that the model predicts rather steady motion of the ice, but occasionally the ice drift may change to the opposite direction in roughly half an hour. This is a major concern for the conventional loading concept where the tanker, say DWT, is staying in the wake behind the platform/tower as shown in Fig. 4. This situation should call for a subsea loading concept where there is a minimum of interference with the sea ice and the tanker can 'ice-vane' all depending on the movement of the drift ice.

5 Fig. 4. Sketch of a typical loading system where the tanker is located in the wake of the loading platform/tower A new approach to loading and export of oil in ice With the concerns indicated above in mind, recently we have made several efforts to demonstrate that new techniques such as the Submerged Turret Loading (STL) can be utilised for the purpose of loading and export of oil in ice (Løset et al., 1998; Jensen et al., 2000a,b). In open water this concept proves an excellent performance. In Arctic waters, such as the Eastern Barents Sea, the presence of drifting ice implies of course additional challenges such as loads from level ice and pressure ridges. The use of the barge concept for export of oil includes the following four major phases: initial approach to the loading facility final approach and hook-up loading and departure. The physical environment and its rate of change will have impact on each of these operations and especially affect the feasibility, time consumption, and thus the regularity. The initial approach includes the last part of the transit where the tanker is in a more or less straight transit mode heading against or with the ice drift. For this phase it is believed that the shuttle tanker typically will run at 1 to 2 knots in 1.2 m thick level ice, i.e. 2 to 4 nautical miles in 2 hours without icebreaker support (Jensen et al., 2000a). The concerns are then the ice breaking performance of the tanker and the manoeuvrability. The final approach and hook-up include sailing from the end of the initial phase to the loading position. This phase also includes manoeuvring time and hook-up time. Jensen et al. (2000a) suggest that the time consumption will be maximum four hours when unescorted and about one hour when escorted (Jolles et al., 1997). In this phase the major concerns are the horizontal positioning (becoming increasingly important in shallow waters due to less horizontal flexibility of the buoy), ice breaking performance of the tanker and loads on the tanker.

6 3. MANOEUVRING - EXPERIMENTAL SETUP AND PROCEDURES 3.1 The Arctic Shuttle Barge System The system is based on a barge of about tons loaded displacement and tons ballast displacement ( DWT). The main dimensions are as follows: length overall L oa = m, length between perpendiculars L pp = m, breadth B = 38.0 m. The scantling (maximum) draft of the barge is 16 m while the ballast draft is 11.5 m. Further, a pusher/icebreaker serves as the main propulsion and connects/disconnects to a notch in the aft of the barge. The pusher of about 8000 tons displacement (2000 DWT) has the following characteristics: L oa = 86 m, L pp = 80 m and B = 23 m. The maximum draft is 8.5 m. The pusher is equipped with two azimuth propellers and the barge has two retractable azimuthing bow propellers for ice milling and manoeuvring, and one tunnel thruster each at bow and stern. Fig. 5 depicts a side view of the barge/pusher while a plan view is given in Fig. 6. With a model scale of 1:25, the total model length (barge with pusher in the notch) is about 13 m. For this concept we foresee a loading site of minimum 30 m water depth unless some excavation of the sea floor is done at the buoy. In the Eastern Barents Sea (Pechora Sea) pressure ridges may extend m below the sea surface and their presence may therefore exceed the draft of the barge (Løset et al., 1999). Although the ridges are unconsolidated at these depths (loose ice blocks in the lower part of the keels), their possible keel-interference with the mooring lines and riser is a concern. Fig. 5. Sketch of the barge and the pusher, side view. 3.2 Test Set-up The experiments were conducted in the large ice tank at HSVA during the autumn of The tank is 78 m long, 10 m wide and 2.5 m deep. The basin is equipped with a motor driven towing carriage and a movable underwater platform 1.20 m below the water surface. The model-scale was 1:25. In this way the underwater platform served to model a water depth of 30 m. The wheels of the underwater platform ( sea bed ) were hooked on rails mounted on the tank wall about 0.5 m below the water surface. The underwater platform could either stay fixed at a certain position in the tank or be connected to the main carriage and follow its motion. In this way two principle different modes could be run. The fixed position mode is shown in Fig. 6. Sketches of the mooring system are shown in Figs. 7 and 8. Froude scaling is used for scaling the model results (see Ashton, 1986; Løset et al., 1998). The forces are scaled by λ 3 (λ= 25). Speeds and time are scaled by λ 1/2. The scaled results are used when evaluating the feasibility of the tanker concept. The testing was conducted in level ice. A full-scale ice thickness of 1.2 m (h fs = 1.20 m) corresponds to h ms = 48 mm in model-scale. The model ice was of fine-grained columnar type and grown from a

7 sodium chloride solution (about 0.65 % concentration). The procedures and preparation of the HSVA model ice are thoroughly described by Evers and Jochmann (1993). 72 m 11.5 m Service carriage Main carriage Curtains 10 m Z -X 49 mm Fig. 6. Sketch of the test set-up with fixed position of the false bottom. Fig. 7. Sketch of the mooring system, plan view (x-dir. is forward). Fig. 8. Illustration of the mooring lines and buoys hook-up, side view (full-scale). All units in metres.

8 4. MANOEUVRING - TEST MATRIX The test matrix for 'Manoeuvring' testing is shown in Table 1. Table 1. Test matrix for 'Manoeuvring' (all numbers in model-scale). Test Test set-up Description # 1000 Level ice h = 48mm, σ f = 30 kpa Barge in ballast with reamers, pusher. Connecting/disconnecting under load. The turret was located at the 50 m tank mark. Initial position: the barge bow at 25 m (pushed just into the ice sheet) and 1.8 m off the centre-line of the tank. Ice was placed into the notch. The pusher was manoeuvred into the notch and pushed the barge towards and finally above the buoy. The buoy was manually connected to the barge. The false bottom was connected to the main carriage. Then the main carriage moved forward at 0.1 m/s speed and the buoy was dropped at full load after 3-4 m forward movement Level ice h = 48mm, σ f = 30 kpa Barge in ballast without reamers, pusher. Connecting/ disconnecting under load. The procedure was equal to Test FINAL APPROACH, HOOK-UP AND EMERGENCY SHUT DOWN 5.1 Power and thrust in level ice Power and propeller thrust during the final approach through level ice were analysed for Tests 1000 and 2000 within a time window after the first acceleration of the model until the first backing. In both tests the model had to perform a curved track (turning circle) i.e., the azimuth thrusters were operated with significant steering angles (about 30 ). Additional steering forces were applied by one of the azimuth bow thrusters on the barge. The approach was performed with the barge in ballast draft. The average speed and the maximum thruster azimuth angle as well as the actual ice properties in the actual time window are shown in Table 2. The time-traces of the speed are shown in Fig. 9. Table 2. Speed and thruster azimuth angle of the model together with the actual ice thickness, flexural strength and friction coefficient (full-scale values are given). Test number Average speed, v [m/s] Max. thruster azimuth angle, [ ] Ice thickness, h i [m] Flex. strength, σ f [kpa] Friction coeff., f id [-] The average thrust developed by the azimuth thrusters of the pusher was measured by the load cells in the azimuth thrusters on the port and starboard side. The power was calculated from the propeller torque and angular speed. The measured values are corrected for a target ice thickness of 1.20 m, a target flexural strength of 750 kpa and a target skin friction factor of 0.10.

9 The target flexural strength was 750 kpa. Since this is a rather high value, the power and thrust are also estimated for a value of 500 kpa (for correction procedure, see Elvebakk and Lindberg, 1998). The total developed thrust T total and the total delivered power P d total is reported in Table 3. Speed [m/s] f.s Time [s] f.s. Fig. 9. Time-trace for speed during Tests 1000 and Table 3. Power and thrust (in full-scale). Test # Measured in turning motion (σ f = 950 kpa) Corrected for target ice properties in turning motion (σ f = 750 kpa) Corrected for target ice properties in turning motion (σ f = 500 kpa) Corrected for target ice properties and straight motion (σ f = 500 kpa) T total [kn] P d total [MW] T total [kn] P d total [MW] T total [kn] P d total [MW] T total [kn] P d total [MW] Manoeuvring in ice The manoeuvrability of the barge in level ice was demonstrated in Tests 1000 and 2000 and is reported in three different ways: A general impression of the manoeuvrability was obtained from visual observations (and video records). The trace of the barge (Fig. 10) moving from the initial position to the hook-up point at the 50 m tank mark was estimated from the speed and the yaw angle. The turning circle (tactical circle) in undisturbed level ice is calculated and reported in Table 4 (for the same time window as reported in Table 2). Heideman et al. (1996) report from full-scale trials that azimuth thrusters provide a very good manoeuvrability in ice. This is also the impression from the present model-scale tests. The manoeuvring into the hook-up position was easily done both with and without reamers on the barge. However, the operation worked somewhat better with reamers. It was found that the optimum procedure to get into the hook-up position was: first to overrun the buoy, then to back 1-2 L pp and simultaneously widen the broken channel by the propeller wash and finally to manoeuvre into position. In the ice tank the lateral deviation from the target position above the buoy was in the range

10 of 1 to 3 m, full-scale. This is a very good positioning but we should bear in mind that the coupling and manoeuvring into position was done without any lateral ice pressure present. The general impression from the manoeuvring is that the pusher/barge system is able to turn provided that a working mode is chosen where the system is oscillated about 1-2 L pp forward and backwards, and at the same time widening the broken channel with the propeller wash of the azimuth thrusters. The calculated barge trace for Test 1000 (with reamers) and Test 2000 without reamers, is shown in Fig. 10. The calculation is based on the measured speed and yaw angle. Tank position Y [m] f.s. Tank position Y [m] f.s (a) Tank position X [m] f.s (b) Tank position X [m] f.s Fig. 10. Trace of the model movement in the tank: a) Test 1000 and b) Test The minimum turning circle (tactical circle) is calculated from the speed and acceleration in the x- and y-directions via the measured yaw angle and speed. The calculation is done for a time window starting after the first acceleration of the model and ending at the first backing of the system i.e. for undisturbed level ice. The calculations show a relatively large difference between Test 1000 (Barge with reamers) and the other tests without reamers. The minimum turning circle is calculated in areas where the barge is performing with maximum possible steering capacity (30 rudder angle, both azimuth propellers active and side-way use of one front propeller) without stalling. The average circle is calculated in the full time window. The turning circle is reported as multiplicands of the L pp of the barge (255 m). Table 4. Turning circle in level ice. Test number Min. turning circle D min Average turning circle D mean 1000 (with reamers) 30 L pp = 7.5 km 60 L pp = 15 km 2000 (without reamers) 80 L pp = 20 km 125 L pp = 30 km 5.3 Loads on pins Each of the pin connections between the pusher and the barge has a triaxial load cell. From these measurements the maximum, minimum and average loads are reported for each test run. A typical

11 time-trace for the total pin loads and the x-dir. loads during Test 1000 is shown in Fig. 11, see also Table Load [kn] f.s Ft Fx Time [s] f.s. Fig. 11. Full-scale load in the starboard pin connection in Test The pin loads are not corrected for the target value in flexural strength. The statistics is made during test run (see Fig. 10) in both the starboard (st) and port side (ps) load cell, and the pretensioning force in the pins in y-direction is subtracted. Table 5. Pin forces during Test Action Minimum Maximum Average Std. Dev F xps [kn] F yps [kn F zps [kn] F tps [kn] F xst [kn] F yst [kn F zst [kn] F tst [kn] Hook-up, emergency shut down of the buoy The hook-up procedure was the following: manoeuvring into position, clearing of ice from the moon pool, release of the pop-up buoy and connecting of the buoy. During transit and manoeuvring ice may accumulate in the moon pool. This requires an ice clearing system in the moon pool. In these tests a system for pumping water into the moon pool was installed (see Fig. 8). The system performed well. As previously reported, the manoeuvring into position was easily done. In general the barge was located m (1-3 m in full-scale) off the target position above the buoy. A small pop-up buoy was installed on the top of the buoy. The small buoy was released by a remote line and appeared in the moon pool where the buoy was connected to the barge manually. The conditions for this operation in the tank is favourable compared to a real situation where ice drift and wind will be present. For the ice drift situation an area around the target position of about one ship length should be cleared.

12 Moving the barge at 0.5 m/s speed (full-scale) forward in level ice tested an emergency shut-down situation with ice pressure present. Then a rapid disconnection of the buoy was undertaken. In these tests the buoy left the moon pool nicely and went into its idle position in +/ m (0.25 m in fullscale). The performance of the buoy in this situation will very much depend on the stiffness of the mooring system and the submerged weight of the buoy. 5.5 Connecting of pusher and barge in ice An important part of the performance of this concept is the ability of connecting and disconnecting the pusher in various situations. The testing of a connecting situation with ice accumulated in the barge notch was done during Tests 1000 and These tests showed that ice was easily removed from the notch by just entering the pusher bow into the notch. Some power had to be added for this situation and the barge had a slight movement forward. The power/thrust to move the pusher into the notch was rather low, but to adjust the pins (after putting pressure on the pins) when they were not immediately on the correct position, required some steering forces and power. 6. LOADING - TEST MATRIX AND TESTING PROCEDURES The test matrix for the barge in loading condition is shown in Table 6. The tests were performed with the barge towed by the mooring system through level ice and ridges (see Fig. 12). During the tests forces in the mooring lines and the total forces in a triaxial load cell in the buoy were recorded. We measured also the rate of revolution and azimuth angle of the front propellers, and surge movement of the barge. On the false bottom three underwater video cameras were recording the ice situation in the turret and moon pool area. 10 m Service carriage 21 m Z 63.8 m -X 46.5 mm 60 m 23 m Fig. 12. Sketch of the test set-up.

13 Table 6. Test matrix for 'Loading' (model-scale values in brackets). Test Test set-up Description # 3000 Level ice h i = 1.2 m (48 mm) σ f =750 kpa (30 kpa) v = 0.5 m/s (0.1 m/s) 3100 Ridge v = 0.5 m/s (0.1 m/s) 4000 Level ice h i = 1.2m (48 mm) σ f =750 kpa (30 kpa) v = 0.5 m/s (0.1 m/s) Barge moored on location, in ballast condition and no pusher. The bow moved from the 27 m to the 46 m tank mark. Bow thrusters active, washing backwards (45 ) in pulling mode. Consolidated ridge at the 48 m tank mark. Continuation of Test Bow: 46 m - 60 m. Bow thrusters active, washing backward (45 ) in pulling mode. Barge moored on location, in loaded condition and no pusher. Bow: 27 m - 46 m. Bow thrusters active, washing backward (45 ) in pulling mode Ridge v = 0.5 m/s (0.1 m/s) 5000 Ridge v = 0.5 m/s (0.1 m/s) 5100 Ridge v = 0.5 m/s (0.1 m/s) Consolidated ridge at 48 m. Continuation of Test Bow: 46 m - 60 m. Bow thrusters active, washing backward (45 ) in pulling mode. Consolidated ridge at 48 m. Barge in ballast condition and no pusher. Bow: 41 m 54 m. Bow thrusters active, washing backward in the pulling mode. Consolidated ridge at 56 m. Barge moored on location, ballast condition and no pusher. Bow: 54 m - 62 m. Bow thrusters active, washing forward (-45 ) in the pulling mode. 7. PREPARATION OF ICE RIDGES The testing was conducted in level ice with ridges embedded. A full-scale (fs) ice thickness of h ifs = 1.20 m corresponds to h ims = 48 mm in model-scale (ms) with a scale of 1:25. Table 7 reports the actual level ice thickness for Tests 3000 and 4000 and the actual flexural strength (σ f ). Table 7. Level ice thickness and bending strength in full-scale. h ifs [m] σ ffs [kpa] Test 3000 Test 4000 Test 5000 Test 3000 Test 4000 Test The ridges in the tank were prepared by pushing level ice against a transverse beam as shown in Fig. 13. The boom was successively moved forward, each time 0.6 m, until a complete ridge with the desired width and keel was formed (Jensen et al., 2000b).

14 72 m m 6.2 m Service carriage Main carriage Y X mm Crushed ice movement 2 m 2 m 51 m 19.8 m Fig. 13. Principle for preparation of ice ridges. A typical ridge profile is shown in Fig. 14. Table 8 reports the ridge dimensions i.e., keel depth (h k ), sail height (h s ) and the keel width (w k ) Keel Sail Width [m] f.s Sail Keel Fig. 14. Sketch of the profile of Ridge 1, Test All units in metres. Table 8. Summary of ridge parameters in full-scale and model-scale. Test # Model-scale Full-scale h k [mm] h s [mm] w k [m] h k [m] h s [m] w k [m] R R

15 8. BARGE IN LOADING CONDITION 8.1 Level ice Moving the barge, the mooring system and the false bottom altogether through the stationary model ice formations in the tank simulated drifting ice. Figs. 7 and 8 show a sketch of the mooring system. Two tests in level ice were conducted with the barge moving straightforward. In this situation no ice interaction with the buoy and riser was seen and the wedge shaped plough effectively cleared ice from the riser and mooring system. Table 9 reports the loads on the mooring lines. Table 10 comprises the loads recorded by the triaxial load cell together with maximum displacement in the x-y plane. Table 9. Mooring line forces in Tests 3000 and 4000 (full-scale values in [kn]). Test 3000 Test 4000 Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 8 Avg Stdev Max Avg Stdev Max Table 10. Forces in the triaxial load cell and maximum displacement of the turret (full-scale values, forces in [kn] and displacements in [m]). Test 3000 Test 4000 F x F y F z F t D x D y Avg Stdev Max Avg Stdev Max Ridge Fig. 15 shows a typical time-trace of the total forces in the triaxial load cell during a ridge event. Load [N] m.s Fx Fy Fz Ft Time [s] m.s. Fig. 15. Time-trace of the forces in the triaxial load cell during Test Table 11 reports the maximum forces in the triaxial load cell and the three front mooring lines as well as the maximum surge and sway displacement.

16 Table 11. Maximum forces and horizontal displacements during ridge events (full-scale values: forces in [kn] and displacements in [m]). F x F y F z F t Line 1 Line 2 Line 8 D x D y Test Test Test Test Ice in the turret area To reduce the ice interaction with the turret, mooring lines and riser, a wedge-shaped plough at the bow of the barge was introduced. Further, the barge was equipped with two retractable azimuthing bow propellers for ice milling. One of the major conclusions from the tests was that both devices are efficient for clearing of ice from the buoy area. However, with ridges present it is not possible to fully avoid ice interaction in the turret area, especially for ridges with keels that exceed the draft of the barge. This is clearly seen from the video spot shown in Fig. 16. During the ridge testing a variation of three parameters have been done during the four tests: The draught of the barge in Test 4000 was 16 m and in the other tests 11.5 m. The extension between the barge and the mooring hook-up on the buoy in Test 5000 and Test 5100 was 5.75 m while it was 3.5 m in Tests 3000 and The front propellers washed backwards at a 45 o angle in Tests 3000, 4000 and 5000 while in Test 5100 the propeller washed forward. All these parameters have impact on the ice situation in the buoy area. From the underwater videos the interaction between the mooring lines and ice blocks are observed. It is obvious that mooring Line 2 has significantly more impact than Line 1. Fig. 17 shows a time-trace of the mooring forces in Test Areas marked with circles are typical areas where ice interacts with the mooring lines. It appears as ripples on the smooth graph. Only a few spots of ice interaction with mooring Line 1 are seen. To identify the mooring line forces we developed a numerical model of the mooring line system. With input from the global forces measured by the triaxial load cell to this model, we were able to compare the calculated and the measured line loads. Fig. 18 shows six examples of such scatter plots where the measured line forces versus the calculated forces are displayed. Bold marks show the first part of the ridge interaction where no ice is present near the mooring system and light marks for the latter part of the tests where ice clearly interacts with the mooring lines. No ice interaction with the mooring lines gives a linear dependence while a scatter originates from ice that directly interacts with the lines. These figures also demonstrate the effect of the buoy extension. For instance, Test 5000 shows a significant less scatter and thereby less ice interaction with the mooring lines than Test Similar curves can be made for all tests and would give an impression of the ice interaction with the mooring lines during the ridge tests.

17 (a) (b) (c) Fig. 16. Pictures from UW-video in Test 5000: (a) Front camera, (b) near camera and (c) side camera. Fig. 17. Time-trace of forces in mooring Lines 1 and 2 in Test 5000.

18 (a) 3100-L1 (b) 3100-L2 (c) 3100-L8 (d) 5000-L1 (e) 5000-L2 (f) 5000-L8 Fig. 18. Scatter plot of calculated forces (vertical) and measured forces (hor.) for Test 3100 in (a)-(c), and for Test 5000 in (d)-(f). Both the video records and the scatter plots show that the extended mooring line hook-up and the barge draft are important parameters for the ice/mooring line interference. At present the effect of the propeller wash and its direction is not properly quantified. 9. TRANSHIPMENT The Arctic Shuttle Barge System shall export its oil to the market in an efficient way. Rotterdam is the major oil terminal in Europe and is therefore a natural point of destination. The Barge System could certainly bring the oil to this terminal, but it would probably be more economical to have a transhipment point just inside the ice edge and transport the oil to market by 'ordinary' tankers. Such a transfer is indicated in Fig. 19. From the figure we see that the seasonal and annual variation of the sea ice extension is very high with a maximum southern extension in March and a minimum extension in September (Løset et al., 1999). The proposed transhipment is tailored for such a situation since this transhipment can 'follow' the position of the ice edge. The transhipment indicated in Fig. 19 makes use of the Barge System from the loading terminal (oil field) to the ice edge where an ice-strengthened tanker is ready for transhipment side-by-side. The transfer of oil should take place, say a nautical mile or so inside the ice edge. Due the strong attenuating effect the ice has on the waves (Løset et al., 1994), we foresee just minor movements between the tanker and the barge at that position. The procedure of the actual transhipment is illustrated in Figs

19 Fig. 19. Shipping system using transhipment at the ice edge. The sea ice extension is indicated. Fig. 20. The Barge System is heading towards the ice edge.

20 Fig. 21. The pusher has left the barge and is heading for the tanker. Fig. 22. The pusher is assisting the tanker at the ice edge. Fig. 23. The tanker is loading oil side-by-side from the barge.

21 10. CONCLUSIONS The Arctic Shuttle Barge System typically consists of a barge of approximately tons loaded displacement and tons ballast displacement ( DWT). A number of model tests have been performed in the HSVA ice tank in Hamburg at a scale of 1:25. The purpose of the testing was to identify and demonstrate the potential of the concept as well as suggesting modifications that can lead to an optimum design of the concept. The most important results are as follows: The testing showed that the manoeuvring into the hook-up position was easily done both with and without reamers. The operation worked better with reamers on the barge. The general impression of manoeuvring in ice is that the barge is able to turn when the pusher is connected. The minimum turning circle when moving forward in level ice was estimated to: 30 L pp = 7.5 km (with reamers) and 80 L pp = 20 km (without reamers). During transit and manoeuvring ice may accumulate in the moon pool. A system for pumping water into the moon pool was installed and cleared ice from the moon pool effectively. A rapid disconnecting of the buoy under stress was undertaken. In these tests the buoy left the moon pool nicely and went into its idle position in +/ m (0.25 m in full-scale). The total thrust used in 1.2 m thick level ice was about 1900 kn without reamers and 2100 kn with reamers. The major conclusions from two level ice tests and four ridge tests of the Arctic Shuttle Barge System in an oil-loading situation are as follows: The maximum ice breaking force was about kn during a ridge event. The average ice breaking force in 1.2 m level ice was about 1400 kn in ballast condition and 2100 kn in loaded condition. Ice interaction with the riser/mooring lines is an important parameter for this concept. The wedged plough and the ice milling propellers are efficient for clearing of ice from the buoy area. However, with ridges present it is not possible to fully avoid ice in the turret area, especially for ridges with keels that exceed the draft of the barge. Video records and load calculations show that an extension of the mooring line hook-up from the barge hull and the barge draft are important parameters for the ice/mooring line interference. Acknowledgement The authors would like to thank APL for technical assistance in the test set-up and Navion ASA for financial support to the project. We highly appreciate the graphical work done on some of the figures by dr. student Dennis Tazov. Further we would like to thank the Hamburg Ship Model Basin (HSVA), especially the ice tank crew, for the hospitality, technical support and professional execution of the test programme in the ARCTECLAB. The research activities carried out at the Large Scale Facility ARCTECLAB were granted by the TMR Programme from the European Commission through contract N ERBFMGECT

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