SUBMERGED TURRET LOADING OF OIL IN ICE

Sveinung Løset1, Arnor Jensen1 and Ola Ravndal2

1Department of Structural Engineering, Norwegian University of Science and Technology, Trondheim

2Navion ASA, Stavanger

ABSTRACTOne of the keys to an efficient loading of oil in the Arctic offshore is probably a subsea solutionwhere the interference with ice is at a minimum. Therefore an attempt to assess the performance of anArctic Shuttle Barge System including a subsea mooring and loading terminal was done at a model-scale of 1:25 in the Hamburg Ship Model Basin (HSVA) ice tank in 1999. The system consists of abarge of about 120 000 tons loaded displacement and 80 000 tons ballast displacement (90 000 DWT,length overall Loa = 265.5 m,) and a pusher/icebreaker of about 8000 tons displacement (2000 DWT,Loa = 86 m). The pusher serves as the main propulsion and connects/disconnects to a notch in the aftof the barge. The operational performance and forces exerted on the barge, the pusher and themooring system, including a riser, were investigated. The system was pushed by the pusher throughlevel ice or towed through level ice and pressure ridges by the mooring system. The latter simulatedthe moored condition in drifting ice. This paper describes the test set-up, procedures and performanceof the concept when manoeuvring into the loading position in level ice. The maximum ice breakingforce was about 23000 kN during a ridge event. The paper also elaborates on the use of a wedgedplough 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 earlystage. Currently, plans are being made for gas production from the huge gas reserves in theShtockmanovskoye field in the eastern Barents Sea and oil production from some fields in thePechora Sea. At the moment there is no oil production in the Barents Sea except for minor productionon the Kolguyev Island. This production is based on summer shipping of the crude oil. Onshore, oilhas been produced since 1988 from the Kharyaga oil field and recently from the Ardalin oil field, bothin 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. Forinstance, the ice regime makes demands beyond the tremendous challenge the oil industry faced in theNorth Sea almost three decades ago. Structures and vessels shall apply environmentally sound andcost-effective technologies as well as securing human safety in a hostile environment. Onshore, theinfrastructure, including the foundation for pipelines on permafrost and river crossings are majorconcerns.

The paper gives a brief introduction to some of the problems we foresee connected to export of oilfrom a sea with drift ice present most of the year. Further, the paper elaborates on the major findingsfrom the current study of the Arctic Shuttle Barge System and discuss a possible transhipment at theice edge.

2. TECHNICAL CONCERNS WITH LOADING OF OIL IN THE ARCTIC OFFSHORE

2.1 HistoryOn a larger scale there is no proper experience with production and export of oil and gas from theArctic offshore using icebreakers and tankers. The petroleum activity in the early 1970's in theBeaufort 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 noobvious trend of one structure displacing another, it is rather governed by water depth and iceconditions 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 deeperwater areas such as the Chukchi Sea. A ship shape turret moored system would have the advantage ofbeing capable of operating in severe wave conditions as well as coping with the dynamic conditionsof an ice field. (a) (b)

Fig. 1. (a) Types of structures by year in the Beaufort Sea, and (b) cost of Beaufort Sea islands. Thecosts quoted are in US $ (Masterson et al., 1991).

2.2 Ice driftThe motion of the ice is a crucial question when planning an off-take system. Let us nowfocus on the drift of four Argos positioned buoys (Buoys 06640, 22435, 24050 and 24051)that were deployed on the drift ice in the Pechora Sea during mid-April 1998 (Løset andOnshuus, 1999). The drift of the four buoys is shown in Fig. 2.

(a) (b)

(c) (d)

Fig. 2. Drift of: (a) Buoy 06640, period 17.04-30.06.98; (b) Buoy 22435, 17.04-30.05.98;(c) Buoy24050, period 17.04-10.06.98 and (d) Buoy 24051, period 20.04-23.06.98.

The drift is mainly governed by wind, waves, ocean currents and tidal forcing. Let us look formathematical properties of the motion. On a large time scale the motion is clearly stochastic, and withthe exception of periods with rather straight-lined movement, it resembles Brownian motion: Thoughmathematically attractive, Brownian motion is obviously not suited to describe ice motion on asmaller time scale. Since ice floes are generally large and heavy objects, the direction and absolutevalue 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 canbe expressed as sums of harmonic functions. Denoting longitude X(t), we get

X t A t B ti i ii

m

i( ) cos sin= +=∑ σ σ

1

(1)

As this is a deterministic function, we need to guess what are the m frequencies of interest. If the

original longitude time series is given as t Xj j j

N,n s =1 , the least square approximations of the

coefficients Ai and Bi are

$ cos

$ sin

AN

X t

BN

X t

i j i jj

N

i j i jj

N

=

=

=

=

∑

∑

2

20

0

σ

σ(2)

With the frequencies and weights at hand, we could turn Eq. (1) into a stochastic model, by changingAi and Bi to Gaussian random variables. We can derive a continuous model on the latitude/longitudeunder the assumption that it is given by the sum of 20 000 harmonic functions, with frequenciesranging from 10 minutes to 2 months. If we assume that this model is a valid representation of the icedynamics, 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 maychange to the opposite direction in roughly half an hour. This is a major concern for the conventionalloading concept where the tanker, say 90 000DWT, is staying in the wake behind the platform/toweras shown in Fig. 4. This situation should call for a subsea loading concept where there is a minimumof interference with the sea ice and the tanker can 'ice-vane' all depending on the movement of thedrift ice.

Fig. 4. Sketch of a typical loading system where the tanker is located in the wake of the loadingplatform/tower.

2.3. A new approach to loading and export of oil in iceWith the concerns indicated above in mind, recently we have made several efforts to demonstrate thatnew techniques such as the Submerged Turret Loading (STL) can be utilised for the purpose ofloading and export of oil in ice (Løset et al., 1998; Jensen et al., 2000a,b). In open water this conceptproves an excellent performance. In Arctic waters, such as the Eastern Barents Sea, the presence ofdrifting 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 andespecially affect the feasibility, time consumption, and thus the regularity. The initial approachincludes the last part of the transit where the tanker is in a more or less straight transit mode headingagainst or with the ice drift. For this phase it is believed that the shuttle tanker typically will run at 1to 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 themanoeuvrability. The final approach and hook-up include sailing from the end of the initial phase tothe 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 aboutone hour when escorted (Jolles et al., 1997). In this phase the major concerns are the horizontalpositioning (becoming increasingly important in shallow waters due to less horizontal flexibility ofthe buoy), ice breaking performance of the tanker and loads on the tanker.

3. MANOEUVRING - EXPERIMENTAL SETUP AND PROCEDURES

3.1 The Arctic Shuttle Barge SystemThe system is based on a barge of about 120 000 tons loaded displacement and 80 000 tons ballastdisplacement (90 000 DWT). The main dimensions are as follows: length overall Loa = 265.5 m,length between perpendiculars Lpp = 255.0 m, breadth B = 38.0 m. The scantling (maximum) draft ofthe barge is 16 m while the ballast draft is 11.5 m. Further, a pusher/icebreaker serves as the mainpropulsion and connects/disconnects to a notch in the aft of the barge. The pusher of about 8000 tonsdisplacement (2000 DWT) has the following characteristics: Loa = 86 m, Lpp = 80 m and B = 23 m.The maximum draft is 8.5 m. The pusher is equipped with two azimuth propellers and the barge hastwo retractable azimuthing bow propellers for ice milling and manoeuvring, and one tunnel thrustereach 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 ofthe sea floor is done at the buoy. In the Eastern Barents Sea (Pechora Sea) pressure ridges may extend20-22 m below the sea surface and their presence may therefore exceed the draft of the barge (Løset etal., 1999). Although the ridges are unconsolidated at these depths (loose ice blocks in the lower partof 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-upThe experiments were conducted in the large ice tank at HSVA during the autumn of 1999. The tankis 78 m long, 10 m wide and 2.5 m deep. The basin is equipped with a motor driven towing carriageand a movable underwater platform 1.20 m below the water surface. The model-scale was 1:25. Inthis way the underwater platform served to model a water depth of 30 m. The wheels of theunderwater platform (‘sea bed’) were hooked on rails mounted on the tank wall about 0.5 m below thewater surface. The underwater platform could either stay fixed at a certain position in the tank or beconnected to the main carriage and follow its motion. In this way two principle different modes couldbe run. The fixed position mode is shown in Fig. 6. Sketches of the mooring system are shown inFigs. 7 and 8.

Froude scaling is used for scaling the model results (see Ashton, 1986; Løset et al., 1998). The forcesare scaled by λ3 (λ= 25). Speeds and time are scaled by λ1/2. The scaled results are used whenevaluating the feasibility of the tanker concept.

The testing was conducted in level ice. A full-scale ice thickness of 1.2 m (hfs = 1.20 m) correspondsto hms = 48 mm in model-scale. The model ice was of fine-grained columnar type and grown from a

sodium chloride solution (about 0.65 % concentration). The procedures and preparation of the HSVAmodel ice are thoroughly described by Evers and Jochmann (1993).

11.5 m10 m

72 m

49 mm

Main carriage

Curtains

Service carriage

Z

-X

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.

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,σf = 30 kPa

Barge in ballast with reamers, pusher. Connecting/disconnectingunder load. The turret was located at the 50 m tank mark. Initialposition: the barge bow at 25 m (pushed just into the ice sheet) and1.8 m off the centre-line of the tank. Ice was placed into the notch.The pusher was manoeuvred into the notch and pushed the bargetowards and finally above the buoy. The buoy was manuallyconnected to the barge. The false bottom was connected to the maincarriage. Then the main carriage moved forward at 0.1 m/s speed andthe buoy was dropped at full load after 3-4 m forward movement.

2000 Level ice h = 48mm,σf = 30 kPa

Barge in ballast without reamers, pusher. Connecting/ disconnectingunder load. The procedure was equal to Test 1000.

5. FINAL APPROACH, HOOK-UP AND EMERGENCY SHUT DOWN

5.1 Power and thrust in level icePower and propeller thrust during the final approach through level ice were analysed for Tests 1000and 2000 within a time window after the first acceleration of the model until the first backing. In bothtests the model had to perform a curved track (turning circle) i.e., the azimuth thrusters were operatedwith significant steering angles (about 30°). Additional steering forces were applied by one of theazimuth 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 theactual 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, flexuralstrength and friction coefficient (full-scale values are given).

Testnumber

Averagespeed,v [m/s]

Max. thrusterazimuth angle,

[°]

Ice thickness,hi [m]

Flex. strength,σf [kPa]

Frictioncoeff.,fid [-]

1000 0.42 30 1.17 950 0.112000 0.60 30 1.20 950 0.11

The average thrust developed by the azimuth thrusters of the pusher was measured by the load cellsin the azimuth thrusters on the port and starboard side. The power was calculated from the propellertorque and angular speed. The measured values are corrected for a target ice thickness of 1.20 m, atarget flexural strength of 750 kPa and a target skin friction factor of 0.10.

The target flexural strength was 750 kPa. Since this is a rather high value, the power and thrust arealso estimated for a value of 500 kPa (for correction procedure, see Elvebakk and Lindberg, 1998).The total developed thrust Ttotal and the total delivered power Pd total is reported in Table 3.

0 240 480 720 960 1200 1440

Time [s] f.s.

0

0.514

1.028

Spe

ed [m

/s] f

.s.

1000

2000

Fig. 9. Time-trace for speed during Tests 1000 and 2000.

Table 3. Power and thrust (in full-scale).Test # Measured in turning

motion(σf = 950 kPa)

Corrected fortarget ice properties in

turning motion(σf = 750 kPa)

Corrected fortarget ice propertiesin turning motion

(σf = 500 kPa)

Corrected fortarget ice propertiesand straight motion

(σf = 500 kPa)T total

[kN]Pd total

[MW]T total

[kN]Pd total

[MW]T total

[kN]Pd total

[MW]T total

[kN]Pd total

[MW]1000 3922 47.2 3403 38.5 2910 30.4 2134 25.92000 3891 47.5 3422 39.2 3050 33.0 1881 23.1

5.2 Manoeuvring in iceThe manoeuvrability of the barge in level ice was demonstrated in Tests 1000 and 2000 and isreported in three different ways:

• A general impression of the manoeuvrability was obtained from visual observations (and videorecords).

• The trace of the barge (Fig. 10) moving from the initial position to the hook-up point at the 50 mtank 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 goodmanoeuvrability in ice. This is also the impression from the present model-scale tests. Themanoeuvring 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 optimumprocedure to get into the hook-up position was: first to overrun the buoy, then to back 1-2 Lpp andsimultaneously widen the broken channel by the propeller wash and finally to manoeuvre intoposition. In the ice tank the lateral deviation from the target position above the buoy was in the range

of 1 to 3 m, full-scale. This is a very good positioning but we should bear in mind that the couplingand 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 providedthat a working mode is chosen where the system is oscillated about 1-2 Lpp 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 inFig. 10. The calculation is based on the measured speed and yaw angle.

600 900 12000

50

100

150

200

250

Tank

pos

ition

Y [

m] f

.s.

600 900 12000

50

100

150

200

250

Tank

pos

ition

Y [m

] f.s

.

Tank position X [m] f.s.

Tank position X [m] f.s.

(a)

(b)

Fig. 10. Trace of the model movement in the tank: a) Test 1000 and b) Test 2000.

The minimum turning circle (tactical circle) is calculated from the speed and acceleration in the x- andy-directions via the measured yaw angle and speed. The calculation is done for a time window startingafter the first acceleration of the model and ending at the first backing of the system i.e. forundisturbed level ice. The calculations show a relatively large difference between Test 1000 (Bargewith reamers) and the other tests without reamers. The minimum turning circle is calculated in areaswhere the barge is performing with maximum possible steering capacity (30° rudder angle, bothazimuth propellers active and side-way use of one front propeller) without stalling. The average circleis calculated in the full time window. The turning circle is reported as multiplicands of the Lpp of thebarge (255 m).

Table 4. Turning circle in level ice.Test number Min. turning circle Dmin Average turning circle Dmean

1000 (with reamers) 30× Lpp = 7.5 km 60× Lpp = 15 km2000 (without reamers) 80× Lpp = 20 km 125× Lpp = 30 km

5.3 Loads on pinsEach of the pin connections between the pusher and the barge has a triaxial load cell. From thesemeasurements the maximum, minimum and average loads are reported for each test run. A typical

time-trace for the total pin loads and the x-dir. loads during Test 1000 is shown in Fig. 11, see alsoTable 5.

0 600 1200 1800 Time [s] f.s.

-10000

-5000

0

5000

10000

Load

[kN

] f.s

.

Ft

Fx

Fig. 11. Full-scale load in the starboard pin connection in Test 1000.

The pin loads are not corrected for the target value in flexural strength. The statistics is made duringtest run (see Fig. 10) in both the starboard (st) and port side (ps) load cell, and the pretensioning forcein the pins in y-direction is subtracted.

Table 5. Pin forces during Test 1000.Action Minimum Maximum Average Std. Dev

Fxps [kN] -9307 6180 -3037 2628Fyps [kN -931 3265 927 631Fzps [kN] -2289 1068 -797 701Ftps [kN] 10 068 3748 2114Fxst [kN] -6485 8240 -741 2122Fyst [kN -1160 2349 73 456Fzst [kN] -1220 1678 41 485Ftst [kN] 8731 1824 1476

5.4 Hook-up, emergency shut down of the buoyThe hook-up procedure was the following: manoeuvring into position, clearing of ice from the moonpool, release of the pop-up buoy and connecting of the buoy. During transit and manoeuvring ice mayaccumulate in the moon pool. This requires an ice clearing system in the moon pool. In these tests asystem 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 waslocated 0.05-0.10 m (1-3 m in full-scale) off the target position above the buoy. A small pop-up buoywas installed on the top of the buoy. The small buoy was released by a remote line and appeared inthe 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 driftand wind will be present. For the ice drift situation an area around the target position of about oneship length should be cleared.

Moving the barge at 0.5 m/s speed (full-scale) forward in level ice tested an emergency shut-downsituation with ice pressure present. Then a rapid disconnection of the buoy was undertaken. In thesetests the buoy left the moon pool nicely and went into its idle position in +/- 0.01 m (0.25 m in full-scale). The performance of the buoy in this situation will very much depend on the stiffness of themooring system and the submerged weight of the buoy.

5.5 Connecting of pusher and barge in iceAn important part of the performance of this concept is the ability of connecting and disconnectingthe pusher in various situations. The testing of a connecting situation with ice accumulated in thebarge notch was done during Tests 1000 and 2000. These tests showed that ice was easily removedfrom the notch by just entering the pusher bow into the notch. Some power had to be added for thissituation and the barge had a slight movement forward. The power/thrust to move the pusher into thenotch was rather low, but to adjust the pins (after putting pressure on the pins) when they were notimmediately 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 withthe barge towed by the mooring system through level ice and ridges (see Fig. 12). During the testsforces in the mooring lines and the total forces in a triaxial load cell in the buoy were recorded. Wemeasured also the rate of revolution and azimuth angle of the front propellers, and surge movement ofthe barge. On the false bottom three underwater video cameras were recording the ice situation in theturret and moon pool area.

10 m

21 m

63.8 m

23 m60 m

46.5 mm

Service carriage

Z

-X

Fig. 12. Sketch of the test set-up.

Table 6. Test matrix for 'Loading' (model-scale values in brackets).Test#

Test set-up Description

3000 Level ice hi = 1.2 m (48 mm)σf=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. Bowthrusters active, washing backwards (45°) in pulling mode.

3100 Ridgev = 0.5 m/s (0.1 m/s)

Consolidated ridge at the 48 m tank mark. Continuation ofTest 3000. Bow: 46 m - 60 m. Bow thrusters active, washingbackward (45°) in pulling mode.

4000 Level ice hi = 1.2m (48 mm)σf =750 kPa (30 kPa)v = 0.5 m/s (0.1 m/s)

Barge moored on location, in loaded condition and no pusher.Bow: 27 m - 46 m. Bow thrusters active, washing backward(45°) in pulling mode.

4100 Ridgev = 0.5 m/s (0.1 m/s)

Consolidated ridge at 48 m. Continuation of Test 4000.Bow: 46 m - 60 m. Bow thrusters active, washing backward(45°) in pulling mode.

5000 Ridgev = 0.5 m/s (0.1 m/s)

Consolidated ridge at 48 m. Barge in ballast condition and nopusher. Bow: 41 m – 54 m. Bow thrusters active, washingbackward in the pulling mode.

5100 Ridgev = 0.5 m/s (0.1 m/s)

Consolidated ridge at 56 m. Barge moored on location,ballast condition and no pusher. Bow: 54 m - 62 m. Bowthrusters 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 hifs =1.20 m corresponds to hims = 48 mm in model-scale (ms) with a scale of 1:25. Table 7 reports theactual level ice thickness for Tests 3000 and 4000 and the actual flexural strength (σf).

Table 7. Level ice thickness and bending strength in full-scale.hifs [m] σffs [kPa]

Test 3000 Test 4000 Test 5000 Test 3000 Test 4000 Test 50001.11 1.20 1.25 750 775 1375

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 thedesired width and keel was formed (Jensen et al., 2000b).

38-39 mm

Crushed ice movement

4321

0.6

64 m

19.8 m2 m 2 m

0.6 0.6

6.2 m

Main carriage

Service carriage

Y

X

72 m

51 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 (hk),sail height (hs) and the keel width (wk).

-18.00-16.00-14.00

-12.00-10.00

-8.00

-6.00

-4.00-2.00

0.00

2.00

4.00

0.00

22.5

0

37.5

0

41.2

5

45.0

0

47.5

0

51.7

5

57.7

5

65.0

0

68.0

0

72.2

5

76.2

5

81.2

5

84.7

5

88.7

5

93.7

5

112.

50

Kee

lS

ail

Width [m] f.s.

Keel

Sail

Fig. 14. Sketch of the profile of Ridge 1, Test 5000. All units in metres.

Table 8. Summary of ridge parameters in full-scale and model-scale.Model-scale Full-scaleTest #

hk [mm] hs [mm] wk [m] hk [m] hs [m] wk [m]3000 580 70 7 14.5 1.75 1754000 600 47 3.5 15.0 1.18 88

5000 R1 680 98 3.25 17.0 2.45 815000 R2 695 81 4.0 17.4 2.03 100

8. BARGE IN LOADING CONDITION

8.1 Level iceMoving the barge, the mooring system and the false bottom altogether through the stationary modelice 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 iceinteraction with the buoy and riser was seen and the wedge shaped plough effectively cleared ice fromthe riser and mooring system. Table 9 reports the loads on the mooring lines. Table 10 comprises theloads 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]).

Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 8Avg 1386 1336 993 836 1197 875 1784

Stdev 192 201 184 110 175 145 163Test 3000Max 1840 1756 1337 1190 1844 1346 2270Avg 383 389 255 50 176 869 1590

Stdev 514 676 467 79 223 728 859Test 4000Max 2195 2754 2073 654 1082 2076 2863

Table 10. Forces in the triaxial load cell and maximum displacement of the turret (full-scale values,forces in [kN] and displacements in [m]).

Fx Fy Fz Ft Dx Dy

Avg -1309 -133 2415 2909 -1.38 -0.11Stdev 710 717 54 351 0.11 0.10Test 3000Max -3015 -1848 2633 3990 -1.74 -0.29Avg -2120 -2640 2391 5109 -0.80 -0.60

Stdev 1176 3215 322 1705 0.28 0.73Test 4000Max -4825 -6286 2888 7749 -1.45 -1.42

8.2 RidgeFig. 15 shows a typical time-trace of the total forces in the triaxial load cell during a ridge event.

200 210 220 230 240 250 260 270Time [s] m.s.

-1500

-1000

-500

0

500

1000

1500

Load

[N] m

.s.

Fx Fy Fz Ft

Fig. 15. Time-trace of the forces in the triaxial load cell during Test 5000.

Table 11 reports the maximum forces in the triaxial load cell and the three front mooring lines as wellas the maximum surge and sway displacement.

Table 11. Maximum forces and horizontal displacements during ridge events (full-scale values: forcesin [kN] and displacements in [m]).

Fx Fy Fz Ft Line 1 Line 2 Line 8 Dx Dy

Test 3100 -18596 1198 7656 19546 7623 5148 8003 -5.07 0.25Test 4100 -19896 -4673 5878 20777 7455 2505 8724 -4.90 -1.06Test 5000 -22516 4271 7675 23797 9416 5555 9193 -5.11 0.73Test 5100 -18768 1524 6458 19768 7901 5166 7819 -4.34 0.32

8.3 Ice in the turret areaTo reduce the ice interaction with the turret, mooring lines and riser, a wedge-shaped plough at thebow of the barge was introduced. Further, the barge was equipped with two retractable azimuthingbow propellers for ice milling. One of the major conclusions from the tests was that both devices areefficient for clearing of ice from the buoy area. However, with ridges present it is not possible to fullyavoid ice interaction in the turret area, especially for ridges with keels that exceed the draft of thebarge. 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 4000.• The front propellers washed backwards at a 45o 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 videosthe interaction between the mooring lines and ice blocks are observed. It is obvious that mooring Line2 has significantly more impact than Line 1. Fig. 17 shows a time-trace of the mooring forces in Test5000. Areas marked with circles are typical areas where ice interacts with the mooring lines. Itappears as ripples on the smooth graph. Only a few spots of ice interaction with mooring Line 1 areseen.

To identify the mooring line forces we developed a numerical model of the mooring line system. Withinput from the global forces measured by the triaxial load cell to this model, we were able to comparethe calculated and the measured line loads. Fig. 18 shows six examples of such scatter plots where themeasured line forces versus the calculated forces are displayed. Bold marks show the first part of theridge interaction where no ice is present near the mooring system and light marks for the latter part ofthe 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 icethat 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 themooring lines than Test 3100. Similar curves can be made for all tests and would give an impressionof the ice interaction with the mooring lines during the ridge tests.

(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.

(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 thebarge draft are important parameters for the ice/mooring line interference. At present the effect of thepropeller 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 themajor oil terminal in Europe and is therefore a natural point of destination. The Barge System couldcertainly bring the oil to this terminal, but it would probably be more economical to have atranshipment point just inside the ice edge and transport the oil to market by 'ordinary' tankers. Such atransfer is indicated in Fig. 19. From the figure we see that the seasonal and annual variation of thesea ice extension is very high with a maximum southern extension in March and a minimumextension in September (Løset et al., 1999). The proposed transhipment is tailored for such a situationsince 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 (oilfield) to the ice edge where an ice-strengthened tanker is ready for transhipment side-by-side. Thetransfer of oil should take place, say a nautical mile or so inside the ice edge. Due the strongattenuating effect the ice has on the waves (Løset et al., 1994), we foresee just minor movementsbetween the tanker and the barge at that position. The procedure of the actual transhipment isillustrated in Figs. 20-23.

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.

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.

10. CONCLUSIONS

The Arctic Shuttle Barge System typically consists of a barge of approximately 120 000 tons loadeddisplacement and 80 000 tons ballast displacement (90 000 DWT). A number of model tests havebeen performed in the HSVA ice tank in Hamburg at a scale of 1:25. The purpose of the testing was toidentify and demonstrate the potential of the concept as well as suggesting modifications that can leadto 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 andwithout 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 isconnected.

• The minimum turning circle when moving forward in level ice was estimated to: 30×Lpp = 7.5 km(with reamers) and 80×Lpp = 20 km (without reamers).

• During transit and manoeuvring ice may accumulate in the moon pool. A system for pumpingwater 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 themoon pool nicely and went into its idle position in +/- 0.01 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 kNwith reamers.

The major conclusions from two level ice tests and four ridge tests of the Arctic Shuttle Barge Systemin an oil-loading situation are as follows:

• The maximum ice breaking force was about 23000 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 forridges 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 thebarge hull and the barge draft are important parameters for the ice/mooring line interference.

AcknowledgementThe authors would like to thank APL for technical assistance in the test set-up and Navion ASA forfinancial support to the project. We highly appreciate the graphical work done on some of the figuresby 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 thetest programme in the ARCTECLAB. The research activities carried out at the Large Scale FacilityARCTECLAB were granted by the TMR Programme from the European Commission throughcontract N°ERBFMGECT950081.

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