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

*Station KeepingThe 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 - contd 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.

*Station KeepingWhen investigating or designing a mooring system, the following criteria should be considered:

*Operational Stage1. 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 Connected2. 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:

Displacement > 10% of water depth

*Station Keeping - contdExample

Water Depth = 1,000 ft

Drilling: 50-60 ft

Connected:100 ft max1,000

*Environmental Forces Acting on the Drilling Vessel(i)Wind Force

(ii)Current Force

(iii)Wave ForceThese forces tend to displace the vessel

*The Station Keeping SystemMust be designed to withstand the environmental forces

Two types:

Mooring System (anchors) Dynamic Positioning (thrusters)

*(i) Wind ForceThe following equation is specified by the American Bureau Shipping (ABS) and is internationally accepted:

*Wind ForceWhere:

*Table 3-1. Shape Coefficients

*Table 3-2. Height Coefficients

*(i) Wind Force - exampleVA = 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 - exampleVA = 50 (wind velocity, knots) 1 knot = 1 nautical mile/hr = 1.15078 statute mile/hr1 nautical mile = 1/60 degree = 1 minute = 6,076 ft

*

Where:

(ii) Current Forcelbf

*Fc = 1 * 1 * 22 * 30 * 400

Fc = 48,000 lbf = 48 kips(ii) Current Force - exampleVc = 2 (current velocity, ft/sec)Cs = 1 (shape coefficient)A = 30 * 400 (projected target area, ft2)

*(iii) Wave Forces - (a) Bow Forces:T = wave period, secL = vessel length, ftB = vessel width, ftH = significant wave height, ft

*Bow Forces - contdNOTE: Model test data should be used when available

*(iii) Wave Forces - (b) Beam ForcesNOTE: API now has Recommended Practices with modified equationsWhere D = vessel draft, ft

*Beam Forces - contd

*Figure 3-1. The catenary as used for mooring calculations.Floating Drilling: Equipment and Its UseThe Mooring LineT

*The Mooring Lines Resist the Environmental Forces

*The equations used for mooring calculations for one single weight line are:The Shape of the Mooring Line:THVqcosh z = (ez + e-z)/2

*More equations used for mooring calculations:

*Where:

*and:

*Station KeepingTable 3-4. Example of Single Line Restoring ForcesTry to duplicate this Table

*TqH

*Figure 3-2. The effect of changing line weight--single-line calculations.Too HardLooks OKToo SoftSingle Line Restoring Force, kips Offset - Percent of Water Depth

*Figure 3-3. The effect of changing initial tension only--single-line calculations.Effect of Initial TensionWater Depth - 500 ft Chain - 2 in., 42.6 lb/ftInitial Tension ( KIPS )Single Line Restoring Force, kips Offset - Percent of Water Depth

*Figure 3-4. The effect of changing water depth only; single-line calculations.Effect of Water DepthInitial Tension - 30 KIPSWater Depth , ftWire Rope 3 in. 18.6 lb/ftSingle Line Restoring Force, kips Offset - Percent of Water Depth

*Station Keeping1. 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 - contd4. 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 - contd6. Once the riser is disconnected, the vessel heading may be changed to decrease the environmental forces on the vessel.

*Station KeepingTypical Mooring Patterns for Non-Rectangular Semis

*Typical Mooring Patterns for Ship-Like Vessels and Rectangular Semis

*Typical 8-line Mooring Pattern

*Table 3-5. Effects of Mooring Line Patterns

*Figure 3-8. Drag anchor nomenclature.crownAnchor shackleflukestockCrown pad eyeshank

*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:Mooring Program

*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.

Mooring Program should...

*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.Mooring Program should...

*Setting Anchor with WorkboatAnchor before touching bottomDrilling vessel winching-in cablePendantMooring LineFluke

*Fluke Tip Touching BottomMud Pressure Holds Fluke Open

*Figure 3-12. The sequence of setting an anchor with a workboat.Anchor Set and Digging InPendant Slacked(What is Piggyback?)

* 6 strands, 19 wires per strand Strand Construction for Mooring Lines ( IWRC - Independent Wire Rope Core )

*Table 3-7. Wire Rope Specifications6 x 37 Bright

*Wire Rope Specifications6 x 37 BrightDiameterin

1233.5Weightlbs/ft

1.857.3916.622.7Strengthtons

49.1190414555

*Fatigue Life of 3/4 Wire Rope Load = 30% of breaking strength: Life = ~105 cyclesLoad = 20% of breaking strength: Life > 4*106 cycles

*Figure 3-15.Chain Nomenaclature.Stud Link Chain Stud keeps chain from collapsing

3 chain has breaking strength > 1,000 kips! WireDia.Pitch

*Chain Quality InspectionChain 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

*Table 3-10. Table for Renewing Stud-Link ChainFor 3 chain, renewal dia. = 2 11/16

*A chain is only as strong as its weakest linkFigure 3-18. Typical wire rope connection to chain.

*Wire Rope location for barge Wire LineTensiometers Mooring Winches OutboardFairlead Read tension while moving slowly

*Station KeepingFigure 3-20. Drum Capacity and minimum drum-to-sheave spacingRd > 200 dwire

q = 1.5o (smooth) q = 2.0o (grooved)Rd

*Figure 3-21. Deck machinery arrangements for ship-like vessels.Chain mooring requires a wildcat & chain stopper.Tension is usually measured with a load cell.ChainStopperDualWildcat

*Figure 3-22. Typical chain wildcat and fairlead locations on a semi.

*Dynamic PositioningDynamic positioning uses thrusters instead of mooring linesto keep the vessel above the wellhead.

Glomar Challenger used dynamic positioning as early as 1968.

The Ocean Drilling Program (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

*Simple position-referencing systemWH1 = WH2 = WH3WH1 = WH3 WH2 > WH1 , WH3WH1H2H3

*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. Acoustic Position Referencing

*Diagram of controller operations.