SISTEMA DE NUEVE BARRAS

The criterion to be used for damage stability as per ABS 2005 is: • Design wind speed for all topside modules is 50 knots

TAMU Team South China Sea 25 Final Report

• The minimum extent of weather tight integrity line must be greater than the first intersection of the righting moment and heeling moment (static angle)

• Horizontal penetration should be at least 1.5m

• Longitudinal damage extent should be 1/3L2/3 _{or 14.5 m, whichever is less}

• Damaged compartments are completely filled

• The designed KG must not exceed the maximum allowable KG

For damage conditions the minimum extent for water tight integrity angle known as the downflooding angle has to be greater than the first intersection of the righting moment and heeling moment. Figure 19 shows the ABS representation of what is required.

Figure 19: ABS MODU 2005 Damage Stability Curve

The damage stability curve represents the worst case scenario that the vessel must forego until proper repairs can be made. The vessel was constructed of 3 meter deep ballast tanks to fulfill the requirement of the double hull design. The double hull design ensures that the extent of the damage will not penetrate the crude oil tanks. ABS codes also specify that the damage encountered must be at least 14.5 meters long. In this case, the worst case scenario will be damaging 2 ballast tanks as if the damage occurred at the intersection of the tanks. This ensures that the worst case scenario is achieved. Through several iterations of damaging a combination of ballast the worst case can be seen by damaging tanks 7 and 9 either on the port or starboard sides, which fulfill the ABS MODU criteria for damage. These tanks are shown in Figure 20.

Figure 20: Damaged Ballast Tanks - 2 total From the damaged analysis the stability curve in Figure 21 could be analyzed.

Figure 21: Damage Stability Curve

From the damage stability curve, the downflooding angle must not cross the first intercept at 9.50 degrees. As shown, the worst case scenario passes the established criteria.

Table 14 shows the conditions that must be satisfied. ABS MODU rules stated that the angle of inclination when flooded must not exceed 25°. The optimum angle is 12.72°, which passes this criterion. Also, the input KG of 17.74 m must not be greater than the least allowable KG calculated from StabCAD. The least allowable KG is 19.20 m which is greater than 17.74 m.

Table 14: Damage Conditions to Satisfy

(M) (Deg) (Deg) (Deg) (Deg) (Deg) For Input KG = 17.74 3.51 9.21 9.51 54.79 Heeling Arm = Righting Arm 19.40 12.72 0.47 12.25 12.71

Static Angle = 15.00 19.57 12.72 0.00 12.72 13.24 28.82 Area Ratio = 1.00 19.22 12.72 0.88 11.84 12.25 32.79 RM/HM Ratio = 2.00 19.20 12.72 0.92 11.80 12.21 32.98 Static Angle 2nd Intercept 1st Intercept

Condition To Satisfy

Allowable _{KG Tilt Angle}Optimum Range Of _Stability

Overall, both the intact and damaged stability of a 100% loaded vessel passed the ABS rules and regulations. For an intact hull, the range of stability was found to be 15.1 degrees before the first downflooding point was reached by the water line. The design and calculated vertical center of gravity was less than the maximum allowable KG as dictated by ABS. The maximum angle of inclination was found to be less than 25 degrees, satisfying all conditions of a stable vessel.

For the damaged stability, the minimum requirements were met as well. The stability curve calculated a downflooding angle at 12.72 degrees which is greater than the first intercept between the righting moment and heeling moment at 9.5 degrees. The range of stability for the damaged condition was found to be 3.51 degrees. For a draft of 22.57 meters, the input vertical center of gravity of 17.74 meters was less than the maximum allowable KG of 19.2 meters. Throughout the analysis, a maximum stability analysis was run, and the vessel proved stable if all of the ballast tanks were damaged proving a optimum vessel design.

5 Local and Global Loading

In order to determine the strength of main girder required to fulfill ABS standards of strength, loading conditions must be determined. Figure 22 below shows the local load conditions starting at the stern and taking measurements every 15 meters forward from that point for the empty, ballasted, and full storage tank cases:

Local Loading

0 500 1000 1500 2000 2500 0 15 30 45 60 75 90 ₁₀5 120 135 150 165 180 195 210 225 240 255 270 285 300 315 333

Distance from Stern (m)

Load (

m

t) _empty

ballasted full

Figure 22: Local Loading

This figure shows that the largest amount of the load comes from filling the storage tanks. Also the elevated loading in the forward section of the ship represents the effect of the permanent trim tanks that were added to the vessel.

While this data can help to determine which loading condition is most likely to cause the largest deflections, shears and moments, it is not complete enough to get accurate data. In order to model the loads more completely, a global loading profile, which can be seen in Figure 23, was created. This profile will provide more accurate values for the displacement, shear, and bending moments that the hull experiences.

Global Loading

0 500 1000 1500 2000 2500 0 50 100 150 200 250 300 350

Distance From Stern (m)

L o ad (m t) full ballasted empty

Figure 23: Global Loading

6 General Strength and Structural Design

Using the 2005 ABS Rules for Building and Classing Steel Vessels (section 3.2.1) the minimum section modulus and moment of inertia for the beam were calculated and shown in Table 15.

Table 15: ABS 2005 Regulations

Parameter Limiting Value

Hog/Sag ± 0.5 m Maximum

Section Modulus 72.25 m^3 Minimum Moment of Inertia 722.5 m^4 Minimum

After determining these values, visual analysis was used to analyze the ship hull. This was accomplished by modeling ship hull as a beam with the specifications above. Next, the global loading profile was entered as a set of distributed loads oriented in the negative y direction. Then the buoyant force was applied in several different configurations in order to determine the maximum levels of hog, sag, shear and bending moments. The section modulus of the beam had to be increased after the initial runs due to excessive sagging and hogging of almost 4 meters. The section modulus of the beam had to be increased to 2400 m4 it meet the deflection requirements.

The first of the configurations analyzed was the evenly distributed buoyant load show in Figure 24 below:

Figure 24: Evenly Distributed Buoyancy Case

This case will represent conditions where wave action is small and the ship is floating level. The results from the analysis of this case can be seen in Figure 25. The linear springs in the model are used to represent the force the water below the ship as it resists the downward bending of the vessel. The springs’ stiffness was found my calculating the amount of force required to deepen the draft of the vessel by one meter. This force was then distributed evenly among the springs. Finally, the rotational spring located at the location of the internal turret represents the force exerted by the water in response to the ship pitching around the turret.

Next, two cases were designed to simulate the worst case waves that our ship will experience. This wave had a height of 13.5 meters and a wavelength of 169 meters. The maximum hog case was found to occur when the wave troughs were at the ends of the vessel. Figure 26 and Figure 27 show the input and beam deflection for this load case. The positive deflection present at the bow of each of these configurations is due to the fact that the ship was fixed at the location of the turret instead of at the beam. However in each of the conditions this end beam displacement was minimal.

Figure 26: Maximum Hog Buoyancy Case

Figure 27: Maximum Hog Buoyancy Case Deflection

Finally, a case representing the design wave from above with a crest at each end of the ship was implemented. This case was chosen because the lacks of buoyant force in the center of the ship lead to an extreme case of sagging. The input and deflection from this load case can be seen in the following Figure 28 and Figure 29. While this loading case has more than twice the displacement then the evenly distributed buoyant force, the deflection is still within the limit of half a meter.

Figure 28: Maximum Sag Buoyancy Case

Figure 29: Maximum Sag Buoyancy Case Deflection

The next step in the analysis was to find the wave induced shear and moments generated by the hag and sag loading conditions. This was done by taking the total shear and moments for those conditions and subtracting the Stillwater shear and moment from them. This value was then compared to the shear and moment envelopes determined using the ABS 2005 Rules for Building and Classing Steel Vessels (sections 3.5.2 and 3.5.3). Figure 30 and Figure 31 below show the wave induced shear and bending moment under the max hag and sag conditions compared to the calculated envelope.

Wave Induced shear

-150000 -100000 -50000 0 50000 100000 150000 0 50 100 150 200 250 300 350

Distance from Stern (m)

Moment (k

N/m)

Positive Envelope Negative Envelope Sag Worst Case Hog Worst Case

Wave Induced Moment -15000000 -10000000 -5000000 0 5000000 10000000 15000000 0 50 100 150 200 250 300 350

Distance from Stern (m)

Mo men t ( kN/ m) _{Hog Envelope} Sag Envelope Sag Worst Case Hog Worst Case

Figure 31: Wave Induced Moment

The figures above show that the vessel meets the ABS requirements for both wave induced sheer and moment. It can be noted that if the weight near the stern of the ship were distributed differently, the shear in that area of the ship could be improved. It would be recommended that more weight be added to the stern section of the vessel in order to offset the much larger buoyancy force found in the sag and still- water conditions.

7 Wind and Current Loading

In order to obtain the environmental loads, the areas of the vessel and components for both above and below the draft must be found. The draft was determined to be 22.57m from the keel. This draft line was modeled in AutoCAD and the resulting surface areas were calculated as shown below. Figure 32 shows how the area calculations and coefficients were distributed about the ship shape. Table 16 displays the calculated areas of each designated A.

Figure 32: AutoCAD Area Distribution Table 16: Area Calculations of Ship Shape

A1 2674.7 m² A2 303.63 m² A3 303.63 m² A4 303.63 m² A5 303.63 m² A6 303.63 m² A7 303.63 m² A8 303.63 m² A9 188.46 m² A10 431.74 m² A11 15.75 m² A12 1160 m² A13 454.14 m² A14 7005.1 m² A15 1287.5 m²

Once these areas were established, the actual environmental loads could be calculated. Wind, current, and the mean wave drift force will all have an effect on the vessel. Both wind and current speeds, along with the significant wave height employed to determine this was verified to be the following values as seen in Table 17.

Table 17: Wind, Current, & Significant Wave Height Implementation

Wind Speed (m/s) 35.7 Current Speed (m/s) 1.71 Significant Wave Height (m) 8

These values were used to figure the total environmental force for bow, beam, and quartering seas on the vessel as seen in

.

Table 18: Total Environmental Forces on Vessel

Force (kN) Bow Seas Beam Seas Quartering Seas

Wind 1987.4 7063.7 3635

Current 10.8 269.9 78.9

Mean Wave Drift Force 77.7 423.1 256.2 Total Force (kN) 2075.9 7756.7 3970.1

The quartering seas force depicted above was calculated with a set angle of 22.5 degrees. When this angle increases the total force will also increase. This being known, it is unlikely that this vessel will be subject to environmental conditions at this angle since a single point mooring design has been chosen. This single point mooring design will allow the vessel to weathervane in order to avoid extreme environmental loads.

These hand calculated results were compared with the environmental external forces obtained through Mimosa, and are discussed in the Mooring/Station Keeping section. With the aid of the mooring software, the wind, current, and wave data was projected for the bow of the vessel to be 1291.9 kN, 264.9 kN, and 1308.2 kN. These values differ somewhat drastically to the bow seas calculations shown above in . Therefore, there must be a discrepancy in the hand calculated forces, as the majority of the confidence lies in the accuracy of the mooring results. In going through the process of determining the wind, current, and wave through hand calculations, one assumption was strictly implemented. Due to limited resources, the coefficients used to determine these loads were characteristics of an average drill ship. Seeing as drill ships have an overall length that would be considerably less than an FPSO and a more hydrodynamic bow which is able to cut through the environment, this assumption can be agreed upon to be the cause of the inconsistency.

8 Mooring/Station Keeping

An integrated motion-mooring analysis was performed for the South China Sea FPSO facility. The environmental conditions considered are shown below in Table 19.

Table 19: Environmental Conditions Considered

Hs (m) Hmax (m) Ts (s) Tz (s) 1yr Typhoon 8 13.5 10.4 8.6 10yr Typhoon 12.5 20.9 13.3 11.1 100yr Typhoon 14.9 24.9 14.7 12.2 1yr non-Typhoon 2.7 4.5 6.6 5.5 10yr non-Typhoon 5.1 8.5 8.8 7.3 100yr non-Typhoon 7.3 12.3 10.4 8.6

Speed (m/s) Direction Speed (m/s) Direction 1yr Typhoon 35.7 67.5 1.72 225 10yr Typhoon 55.4 67.5 2.32 247.5 100yr Typhoon 65.9 67.5 2.75 247.5 1yr non-Typhoon 21.9 45 0.57 225 10yr non-Typhoon 29.8 45 0.85 225 100yr non-Typhoon 33.7 45 1.1 225 Wave Data

Wind Data Current Data

Environmental Loads were calculated both by hand and using MIMOSA. Table 20 shows a comparison of the environmental forces. The loads found through hand calculations are much smaller, because the wave coefficients used in those calculations were based on a drill ship. The wave force in MIMOSA is thought to be high due to the shallow operating depth.

Table 20: Environmental Load Comparison

Wind (kN) Wave (kN) Current (kN) Total (kN) 1291.9 1308.2 264.9 2864.9 Wind (kN) Wave (kN) Current (kN) Total (kN)

1987.4 77.7 10.8 2075.9 Collinear Environmental Forces From MIMOSA

ENVIRONMENTAL LOADS

Hand Calculated Environmnetal Forces

Mimosa, a program used for the design and analysis of mooring systems for offshore vessels, is used to conduct all mooring analyses/design. The referenced API guidelines for measuring the factors of safety in the analysis are shown below:

API RP-2SK Mooring Code:

• Dynamic Intact FOS = 1.67 • Dynamic Damaged FOS = 1.25

The maximum vessel offset, as recommended by API for intact condition, is 8% of the water depth for rigid risers and 10% of the water depth for flexible risers. For damaged condition, API allows for an offset of 12% to 15% of the water depth. Vessel offsets are controlled by adjusting the pretension and total line length associated with each leg.

For single point mooring systems ABS (section 3.3.1.7 of the Guide for Building and Classing Floating Production Installations) dictates that there are a minimum of three wind, wave, and current configurations must be analyzed. The first of these is when all three of the conditions are collinear and acting on the bow of the ship. The second consist of wind and current collinear and both at 30 degrees to

the waves. The third configuration has the wind at 30 degrees to the waves and the current at 90 degrees to the waves.

Three different mooring systems were selected as potential candidates for the design of a FPSO to be located in the South China Sea. Each mooring system will be developed and drawn out using the MIMOSA program within the SEASAM package. Then, each system will be analyzed using MIMOSA and then revised until it meets current design criteria. Mooring of the FPSO will be accomplished by utilizing one of three systems listed below.

1. An external disconnectable turret with a single point mooring system 2. An internal disconnectable turret with a single point mooring system 3. A spread mooring system

The mooring system is configured to meet certain design characteristics with respect to maximum allowable offsets and specified safety factors for both intact and damaged conditions. Since the FPSO will be designed to operate in a water depth of around 100 m, the maximum allowable offset for operating conditions will be 10m, which is 10% of the water depth. The minimum safety factors that must be met are 1.67 for intact conditions and 1.25 for damaged conditions. It is required that the mooring system be able to allow the FPSO to operate in a 1 year non-cyclonic storm and survive on location a 100 year non- cyclonic storm or a 1 year cyclonic storm.

The mooring systems to be considered for the FPSO design are an internal and external disconnectable turret mooring systems. Advantages of the turret type mooring systems are the vessels ability to weathervane, or move with the environmental forces to minimize forces on the mooring system and vessel. Another advantage of the turret type mooring system is the availability of a disconnectable turret. With the disconnect-able turret system, a vessel can shut-in the well and disconnect when there is a threat of severe weather, such as typhoons that are frequently encountered in the South China Sea. Some disadvantages of the turret type mooring system is the limited space inside the turret for risers and the additional cost of the turret system. If a turret is designed to house 10 risers, it can hold a maximum of 10. Depending on the type of environment that the FPSO will be moored in, the extra cost and limited production ability of the turret design could outweigh the risk of a spread mooring system.

Figure 33 (courtesy of SBM IMODCO) shows internal turret and an external turret mooring systems on ships. It is important to notice how the vessel’s ability to weathervane is affected with the internal turret. The further astern the turret is moved, the lesser the vessel’s ability to weathervane. This is effective because when the vessel is disconnected from the system, the turret sinks down to an equilibrium point where it is well below the force of wind and waves.

Figure 33: Internal and External Turret Mooring Systems

A single point, internal, disconnectable turret was chosen for this FPSO application. The reasoning associated with this decision can be viewed in Table 21. According to the weighted mooring system selection chart, the most beneficial type of mooring system to utilize for the environmental conditions and water depths given is the internal turret with a disconnectable feature. The internal turret will allow the vessel to weathervane to minimize the forces acting on the vessel due to the environment, reducing the force that the mooring system has to comply with. With the disconnectable mooring system, if severe weather threatens the vessel, the mooring system can easily be dropped and the vessel either move under its own power or be towed out of the area until the threat of the severe weather has passed. Once the weather has passed the production site, the vessel can be placed back over the production field, reconnect to the mooring system and resume production in a relatively short period of time.

Table 21: Weighted Mooring System Selection Chart

Magnitude Score Value Magnitude Score Value

Need to weathervane 0.3 % time vessel is headed into current/winds 99 5 1.5 99 5 1.5 Ability to disconnect 0.3 Disconnect Time (hrs) 6 4 1.2 4 5 1.5 Reconnection Efficiency 0.2 Reconnection Time (hrs) 10 3 0.6 6 4 0.8 Vessel motion 0.1 Effective increase on vessel motion (%) 40 3 0.3 40 3 0.3

Cost 0.1 Cost (million) 25 1 0.1 25 1 0.1

3.7 4.2

OVERALL UTILITY VALUE

Internal Disconnectable Turret

Objective Weight Parameter External/Internal Permanent Mooring

The mooring system for the FPSO was designed through an iterative process where a system was designed then analyzed then improved upon many times. The mooring system is comprised of 12 lines of chain with 4 groups. The 4 groups are spread out evenly 90° apart with a 5° spacing between each line within the 4 groups. The lightest possible chain that was selected for the system was 142 mm R4

In document UNIVERSIDAD NACIONAL DEL CENTRO DEL PERU (página 78-87)