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Petroleum Engineering 406 Lesson 9b Station Keeping Station Keeping • Environmental Forces • Mooring • Anchors • Mooring Lines • Dynamic Positioning Station Keeping The ability of a vessel to maintain position for drilling determines the useful time that a vessel can effectively operate. Stated negatively, if the vessel cannot stay close enough over the well to drill, what good is the drilling equipment? Station Keeping - cont’d Station keeping equipment influences the vessel motions in the horizontal plane. These motions are: surge, sway, and yaw. Generally, surge and sway are the motions that are considered. Yaw motion is decreased by the mooring system but is neglected in most mooring calculations. Station Keeping When investigating or designing a mooring system, the following criteria should be considered: Operational Stage 1. The vessel is close enough over the well for drilling operations to be carried out. This varies between operators, but is usually 5% or 6% of water depth. Later, other criteria, based on riser considerations, will be discussed. Non-operational but Connected 2. The condition from the operational stage up to 10% of water depth. Drilling operations have been stopped, but the riser is still connected to the wellhead and BOPs. Disconnected 3. The riser is disconnected from the wellhead and the BOPs, and the vessel can be headed into the seas. Station Keeping - cont’d Example Water Depth = 1,000 ft Drilling: 50-60 ft 1,000’ Connected: 100 ft max Environmental Forces Acting on the Drilling Vessel (i) Wind Force (ii) Current Force (iii) Wave Force These forces tend to displace the vessel The Station Keeping System Must be designed to withstand the environmental forces Two types: – Mooring System (anchors) – Dynamic Positioning (i) Wind Force The following equation is specified by the American Bureau Shipping (ABS) and is internationally accepted: FA 0.003388V * Ch * Cs * A 2 A Wind Force Where: FA wind force, lb VA wind velocity, knots CS shape coefficien t from Table 3 - 1, dimensionless C h height coefficien t from Table 3 - 2, dimensionless A projected area of all exposed surfaces, ft 2 . This area changes w ith both heel and yaw. Table 3-1. Shape Coefficients Table 3-2. Height Coefficients (i) Wind Force - example FA 0.003388VA * Ch * Cs * A 2 VA = 50 (wind velocity, knots) Ch = 1 (height coefficient) Cs = 1 (shape coefficient) A = 50 * 400 (projected target area, ft2) Then FA = 0.00338 * 502 * 1 * 1 * 50 * 400 FA = 169,000 lbf = 169 kips (i) Wind Force - example FA 0.003388VA * Ch * Cs * A 2 VA = 50 (wind velocity, knots) 1 knot = 1 nautical mile/hr = 1.15078 statute mile/hr 1 nautical mile = 1/60 degree = 1 minute = 6,076 ft (ii) Current Force Fc 2 g c Cs Vc A Where: Fc current drag force, lb C s drag coefficien t, dimensionless. Same as the wind coefficien t (Table 3 - 1) Vc current velocity, ft/sec A projected area, ft 2 lbft * sec2 lbf g c 1 4 ft (ii) Current Force - example Fc 2 g c Cs Vc A Vc = 2 (current velocity, ft/sec) Cs = 1 (shape coefficient) A = 30 * 400 (projected target area, ft2) Fc = 1 * 1 * 22 * 30 * 400 Fc = 48,000 lbf = 48 kips (iii) Bow Forces: for T 0.332 L 2 2 0.273 H B L Fbow 4 T T = wave period, sec L = vessel length, ft H = significant wave height, ft Where: T wave period, sec F wave force, lb H significan t wave height, ft L vessellength, ft B vesselbeam length, ft D vesseldraft, ft Bow Forces: for T 0.332 L 2 2 0.273 H B L Fbow (0.664 L T) 4 NOTE: Model test data should be used when available Beam Forces: for T 0.642 B 2D 2 2 2.10 H B L Fbeam 4 T NOTE: API now has Recommended Practices with modified equations Beam Forces: for T 0.642 B 2D 2 2 2.10 H B L Fbeam (1.28 B 2D T) 4 Floating Drilling: Equipment and The Mooring Line Its Use Figure 3-1. The catenary as used for mooring calculations. The Mooring Lines Resist the Environmental Forces Station Keeping 1. In shallow water up to about 500 feet, a heavy line is needed, particularly in rough weather areas. 2. Chain can be used (but may not be advisable) to water depths of about 1,200 feet. 3. Composite lines may be used to ~ 5,000 feet. Station Keeping 4. Beyond about 5,000 feet, use dynamic positioning 5. Calm water tension should be determined to hold the vessel within the operating offset under the maximum environmental conditions specified for operation. Station Keeping, Continued 6. Once the riser is disconnected, the vessel heading may be changed to decrease the environmental forces on the vessel. Station Keeping Typical Mooring Patterns for Non- Rectangular Semis Typical Mooring Patterns for Ship- Like Vessels and Rectangular Semis Typical 8-line Mooring Pattern Figure 3-15. Chain Nomenaclature. Stud Link Chain Pitch Wire Dia. Stud keeps chain from collapsing 3” chain has breaking strength > 1,000 kips! Chain Quality Inspection Chain quality needs to be inspected periodically, to avoid failure: (i) Links with cracks should be cut out (ii) In chains with removable studs, worn or deformed studs should be replaced (iii) Check for excessive wear or corrosion Dynamic Positioning Dynamic positioning uses thrusters instead of mooring lines to keep the vessel above the wellhead. Glomar Challenger used dynamic positioning as early as 1968. ODP uses dynamic positioning. Advantages of Dynamic Positioning (i) Mobility - no anchors to set or retrieve - Easy to point vessel into weather - Easy to move out of way of icebergs (ii) Can be used in water depths beyond where conventional mooring is practical (iii) Does not need anchor boats Disadvantages of Dynamic Positioning (i) High fuel cost (ii) High capital cost (?) (iii) Requires an accurate positioning system to keep the vessel above the wellhead. Usually an acoustic system - triangulation Fig. 3-23. Simple position-referencing system H1 H2 H3 WH1 = WH2 WH1 = WH3 = WH3 WH2 > WH1 , WH3 W Acoustic Position Referencing To understand the operating principles of acoustic position referencing, assume that: 1. The vessel is an equilateral triangle. 2. The kelly bushing (KB) is in the geometric center of the vessel. Acoustic Position Referencing 3. The hydrophones are located at the points of the triangular vessel. 4. The subsea beacon is in the center of the well. 5. No pitch, no roll, no yaw and no heave are permitted. Diagram of controller operations.
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