TAMU Pemex Offshore Drilling

1. 1 TAMU PemexOffshore Drilling

2. 2 Lesson 2 - Station Keeping Environmental Forces Mooring Anchors Mooring Lines Dynamic Positioning

3. 3 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?

4. 4 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 and is neglected in most mooring calculations.

5. 5 Station Keeping When investigating or designing a mooring system, the following criteria should be considered:

6. 6 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.

7. 7 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.

8. 8 Disconnected 3. The riser is disconnected from the wellhead and the BOPs, and the vessel can be headed into the seas: Displacement > 10% of water depth

9. 9 Station Keeping - cont’d Example Water Depth = 1,000 ft Drilling: 50-60 ft Connected: 100 ft max

10. 10 Environmental Forces Acting on the Drilling Vessel (i) Wind Force (ii) Current Force (iii) Wave Force

11. 11 The Station Keeping System Must be designed to withstand the environmental forces Two types: Mooring System (anchors) Dynamic Positioning (thrusters)

12. 12 (i) Wind Force The following equation is specified by the American Bureau Shipping (ABS) and is internationally accepted:

13. 13 Wind Force Where:

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18. 18 Where:

19. 19 Fc = 1 * 1 * 22 * 30 * 400Fc = 48,000 lbf = 48 kips

20. 20 (iii) Wave Forces - (a) Bow Forces:

21. 21 Bow Forces - cont’d

22. 22 (iii) Wave Forces - (b) Beam Forces

23. 23 Beam Forces - cont’d

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28. 28 Where:

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35. 35 Station Keeping

36. 36 Station Keeping - cont’d 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.

37. 37 Station Keeping - cont’d 6. Once the riser is disconnected, the vessel heading may be changed to decrease the environmental forces on the vessel.

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43. 43 This author has developed a mooring program for use on a portable calculator (HP-97). The advantage of this program is that it can be carried with the calculator and used in remote locations, even on the rig.There are other programs available. All programs should have the following capabilities:

44. 44 1. Be able to calculate the total restoring force and tension in the most loaded line vs. offset.2. Be able to handle a minimum of ten mooring lines.3. Be able to handle composite line data for wire rope and chain.

45. 45 4. Have iteration limits such that the worst error for calculating forces in a line will be less than 0.1% of the smallest value anticipated.5. Include stretch in both the wire rope and the chain. Errors of over 30% have been encountered when chain stretch was not included.

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54. 54 Chain Quality Inspection

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61. 61 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. The Ocean Drilling Program (ODP) uses dynamic positioning.

62. 62 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

63. 63 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

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65. 65 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.

66. 66 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.

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