Submerged Turret Loading of Oil in Ice
Total Page：16
File Type：pdf, Size：1020Kb
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 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 model- scale of 1:25 in the Hamburg Ship Model Basin (HSVA) ice tank in 1999. The system consists of a barge 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 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 23000 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. 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). 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, 24050 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 17.04-30.06.98; (b) Buoy 22435, 17.04-30.05.98;(c) Buoy 24050, 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 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 m (1) X (t) = å Ai coss it + Bi sins it i=1 As this is a deterministic function, we need to guess what are the m frequencies of interest. If the N t , X original longitude time series is given as n j j s j =1 , the least square approximations of the coefficients Ai and Bi are N $ 2 Ai = å X j coss it j N j =0 N (2) $ 2 Bi = å X j sins i t j N j =0 With the frequencies and weights at hand, we could turn Eq. (1) into a stochastic model, by changing Ai and Bi 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 20 000 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 90 000DWT, 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. Fig. 4. Sketch of a typical loading system where the tanker is located in the wake of the loading platform/tower. 2.3. 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.