Otc 23221

OTC 23221 An Efficient Hull for Cost-Effective Power Solutions Dominique Roddier, Marine Innovation & Technology Christian Cermelli, Marine Innovation & Technology

Copyright 2012, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2012. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract In this paper, updated conceptual designs and preliminary schedule estimates for the MiniFloat-III power generation hull will be presented. In this study, the MiniFloat hull is sized to support power generation capacity ranging from 5 to 100 MW. The MiniFloat platform is a small semi-submersible with excellent motion characteristics. It has been under technical development since 2003. The design philosophy has been to optimize the economical benefits by minimizing the total cost of the project. This is achieved by addressing all areas of the development, and by favoring simplicity in the engineering, fabrication, towing, installation and commissioning of the unit. Technical qualification of the MiniFloat includes hydrodynamic, structural and model tests results. The MiniFloat was sized specifically for power generation equipment to support subsea equipment in marginal field developments. One variation of the MiniFloat concept is the WindFloat, a hull supporting a large (2-10 MW) floating wind turbine. The WindFloat is the most advanced form of the MiniFloat, with the first unit fabricated in the spring 2011 and commissioned at the end of the summer. Again, simplicity in all phases, from turbine commissioning at quay side to a wet tow to the deployment site has significant commercial advantages. Applicability of those learnings to the subsea power generation unit will be addressed in the manuscript. Large subsea developments are highly sensitive to project CAPEX and amount of power delivered to the sea bed. Being able to deliver constant, reliable power to the subsea, in all type of wave environments, while minimizing the cost of the platform supporting the power production equipment is crucial to this industry. It will enable the US to develop its marginal fields in the GOM, when other solutions may not be technically feasible. This has tremendous benefits in today’s world economy where there is significant desire to decrease the dependence on foreign oil. Introduction Background Information The MiniFloat platform as shown in Figure 1 is a small semi-submersible with excellent motion characteristics. It has been under technical development since 2003. The design philosophy has been to optimize the economical benefits by minimizing the total cost of the project. This is achieved by addressing all areas of the development, and by favoring simplicity in the engineering, fabrication, towing, installation and commissioning of the unit. Technical qualification of the MiniFloat concept has been presented in various conference papers [1], [2], [3], [4]. These include hydrodynamic, structural and model tests results. The MiniFloat was then sized specifically for power generation equipment to support subsea equipment in marginal field developments. The results were presented at DOT [5], and Offshore Magazine summarized the work in a general audience article [6] . Variations of the MiniFloat concept include:

• the CellFloat, to place telecommunication equipment far and remote from shores,

• the MiniFloatel, a 100-person capacity accommodation unit and

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• the WindFloat, a hull supporting a large (2-10 MW) floating wind turbine. A summary of the technical development required for the WindFloat is presented in [7]. The WindFloat is the most advanced form of the MiniFloat, with fabrication of the first unit performed in the spring 2011 and commissioned late September the same year. Again, simplicity in all phases, from turbine commissioning at quay side to a wet tow to the deployment site has significant commercial advantages. A picture of the WindFloat in the drydock is shown on the left of Figure 2, and being towed to site on the right.

Figure 1: MiniFloat III Platform, Designed for Remote Power Generation for Deployment of Subsea Technologies in Deepwater

Marginal Fields

The MiniFloat hull configuration aims to minimize hull weight and cost while providing excellent motion characteristics. It is an ideal fit for the application of this study because:

• It is well suited to the full range of payloads and water depths considered and a wide range of metocean conditions.

• It meets functional requirements while minimizing substructure weight and construction cost

• The configuration and small size increase fabrication option. Shipyards in the Gulf of Mexico, have performed feasibility analaysis and expressed interest in local fabrication.

• It involves very low technical risk due to minimal technology extension. The most novel features of the concept, the water entrapment plates, have been deployed on other concepts (e.g. Heave plates on spars). The MiniFloat has been through rigorous analysis and model tests. A similar hull (WindFloat) was constructed in the spring 2011, and deployed late September offshore in the north of Portugal.

Design Basis The generic design basis for the MiniFloat in this study is summarized in Table 1. Six cases are investigated, varying the power generation between 5, 50 and 100 MW, and the water depth between 5000 and 10,000 feet. This study assumes power generation as the main function. However, at an incremental cost, the platform could also be designed to provide subsea control, pig launching, chemical injection and other functions to support a remote subsea development. The study assumes a Gulf of Mexico development. Because of the significant economic advantages of a wet tow, it is further assumed that the construction will take place in a gulf coast yard, rather than a far-east construction and transport. In previous studies, Gulf coast shipyards have been approached and have confirmed the feasibility and interest in such a fabrication.

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Figure 2: WindFloat pictures in the dry-dock (left) and being towed to site (right)

The metocean criteria for a typical GOM site can be found in [8] and is summarized in Table 1. In this preliminary sizing, the design (airgap, motions, etc) has not been validated yet against extreme events, but in general, the platform would be expected to survive the 1,000 year storm. The payload requirements and deck space area are summarized in Table 2. Additionally, based on the MiniFloat generic design basis, the following criteria were imposed on the design:

• No more than 10 ft draft variation between the extreme cases of having the diesel tanks full and empty

• Draft at quayside < 35 ft, for a wet tow. Additionally, a stability requirement check on GM to be greater than 3 feet in transit configuration (no permanent ballast).

• Minimum operating draft at least 75 ft

• Minimum airgap at least 55 ft to top of column from MWL, to survive the 1000 year storm.

• Minimum operating GM=20 ft, an initial stability criteria

• Heave period > 20 sec, to ensure good motion performance.

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Table 1: Generic Design Basis

Remote Power Platform

Gulf of Mexico

CASE DESCRIPTION 5MW

POWER DEEP

50MW POWER DEEP

100MW POWER DEEP

5MW POWER ULTRA-DEEP



Case ID Number A –WD1 B–WD1 C–WD1 A–WD2 B–WD2 C–WD2

GENERAL

PLATFORM FUNCTION Power Generation for Remote Subsea

Production System Power Generation for Remote Subsea

Production System

DESIGN LIFE (yrs) 25 25

LOCATION GOM 5000’ WD GOM 10,000’ WD

FACILITIES

FACILITY DESCRIPTION:

No. of Well Slots None None None None None None

PROCESSING & STORAGE FACILITIES:

Oil / Prod. Water / Associated Gas Systems

None None None None None None

Gas Lift System: None None None None None None

Gas Injection System: None None None None None None

Water Injection System: None None None None None None

Crude System:

DIESEL STORAGE IN HULL (4 DAYS) (BBLS)

1550 7380 9670 1550 7380 9670

POWER GENERATION FACILITIES:

Power Requirement (MW) 5 50 100 5 50 100

Sparing 2 @ 100% 3 @ 50% 4 @ 33% 2 @ 100% 3 @ 50% 4 @ 33%

Fuel Dual Fuel:

Gas / Diesel Dual Fuel:





Gas / Diesel

Mooring System:

Type Chain-Poly-

Chain Chain-Poly-

Chain Chain-Poly-

Chain Chain-Poly-

Chain Chain-Poly-

Chain Chain-Poly-

Chain

Number of Lines TBD TBD TBD TBD TBD TBD

Anchor Type VLA VLA VLA VLA VLA VLA

PIPELINES, FLOWLINES & RISERS

RISERS:

Gas Supply Risers - No. x Size - Type

1 x 4.5" - SCR

1 x 6.625" - SCR

1 x 8.625" - SCR

1 x 4.5" - SCR

1 x 6.625" - SCR

1 x 8.625" - SCR

Power Cable - No. x Size 2 x 4MW (~5"OD)

13 x 4MW (~5"OD)

25 x 4MW (~5"OD)

2 x 4MW (~5"OD)

13 x 4MW (~5"OD)

25 x 4MW (~5"OD)

METOCEAN DATA (return Period in years) 100 1000 10000 100 1000 10000 Significant Wave Height (ft) 51.8 65 72.5 51.8 65 72.5

Maximum Wave Height (ft) 91.5 114.5 128.3 91.5 114.5 128.3

Storm Surge and Tide (ft) 4.0 5.1 6.0 4.0 5.1 6.0

Associated Wind (10 min, ft/s) 178.8 228 258.2 178.8 228 258.2

Associated Current (ft/s) 7.9 9.8 11 7.9 9.8 11

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Table 2: Payload Requirement Summary

Cases A–WD1 B–WD1 C-WD1 A–WD2 B–WD2 C–WD2

Water Depth (ft) 5,000 5,000 5,000 10,000 10,000 10,000

Payloads (st)

Topsides Op Wt (st) ‐ Incl. Deck Steel 1 2,601 4,400 5,154 2,601 4,400 5,154

Topsides Op Wt (st) ‐ Excl. Deck Steel 1 1,180 2,408 2,985 1,180 2,408 2,985

Gas Supply Riser & Power Cables (st) 146 850 1,633 306 1,692 3,204

Payload Subtotal (st) ‐ Incl. Deck Steel 2,747 5,250 6,787 2,907 6,092 8,358

Payload Subtotal (st) ‐ Excl. Deck Steel 1,326 3,258 4,618 1,486 4,100 6,189

Diesel Stored In Hull (st) 230 1080 1420 230 1080 1420

Deck Area (sf) 19,080 27,636 30,897 19,080 27,636 30,897

Number of Power Cables (4MW each, 18.1 lb/ft wet weight)

2 13 25 2 13 25

1 Incl. 25% Weight Contingency

Note on water depth. It must be noted here, and can be seen in the next section, that the MiniFloat carries a substantial amount of permanent water ballast. The additional tensions that the mooring and SCR add onto the structure when going from 5,000 to 10,000 feet of water depth can be easily accommodated by reducing the amount of water in the ballast tanks. Additionally, this tension improves slightly the stability performance (lower apparent KG). The increase in cost is minimal, and is only due to the additional length of hardware and possibly longer installation operations.

Size and Weight Summary of the MiniFloat-III Platforms For each case of power generation capacity, 4 design conditions were considered. As the power requirement increases, so does the equipment and the payload. The hull displacement therefore increases as well. However, as mentioned previously, the same size platform can be used for both 5,000 and 10,000 feet of water depth, by reducing the amount of water ballast in the columns. Therefore for a given power generation case, the 4 design conditions are:

I. Full: with diesel, with a maximum draft change of 10 feet over the empty case, in deep water (5,000 ft) – No active ballast included, draft variation kept to under 10 feet, to accommodate mooring and power lines / SCRs.

II. Full: with diesel, in ultra-deep water (10,000 feet) III. Empty: No diesel, in deep water IV. Transit: Before wet tow, a draft requirement of 35 feet.

Mooring The mooring is similar to what was presented in [5] and is composed of a segment of 300 feet grade 4 chain on the platform, connected to a long segment of polyester rope (roughly 6” in diameter) across the water column, then another segment of chain on the seabed, long enough to cover the touchdown point, and finally a VLA (roughly 100 ft2). Six mooring lines are used for the 5 MW version, and nine for the larger two versions. Orcaflex simulations were performed in [5] to size the mooring components. Deck Area The deck is assumed to be of a triangular shape with apex resting on the center of the column outboard. In Case B and C, the deck area being smaller than required, the deck is slightly increased with an overhang to match the target.

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Table 3: MiniFloat III Major Dimension for all 3 Power Generation Capacity Cases

Cases A B C

Design Basis Requirement 5 MW 50 MW 100 MW

Payload Subtotal (Incl. Deck Steel) 2,747 5,250 6,787 st

Diesel Stored In Hull 230 1080 1420 st

Target Deck Area 19,080 27,636 30,897 ft2

MiniFloat Particulars

operating draft: 78 84 85 ft

airgap: 55 55 55 ft

Area

deck length edge 208.0 235.1 248.6 ft

Deck Area with overhang 21,632 27,636 30,901 ft2

Mooring

No of Mooring lines: 6 9 9

Approximate breaking strength 1,500 1,500 1,500 kips

Chain Diameter (grade 4) 3.5 3.5 3.5 in

top and bottom Chain length 300 300 300 ft

Horizontal / water depth scope 1.2 1.2 1.2

Displacement 9,628 14,497 16,657 st

Case A: 5 MW Table 4 shows the weight distribution summary of the MiniFloat for 5 MW for all 4 design conditions studied. Table 4: Weight Summary for 5 MW MiniFloat-III

Design Condition A-I A-II A-III A-IV

Power Generation 5 5 5 5 MW

Operational Mode full full empty transit

Water Depth 5,000 10,000 5,000 5,000 ft

Weight Summary

Hull 2592 2592 2592 2592 st

Deck+Facility+Live loads 2601 2601 2601 2601 st

Tethers (mooring +risers) 782 1027 782 0 st

Diesel storage in hull 230 230 0 0 st

Water ballast 3423 3179 3423 603 st

TOTAL WEIGHT 9628 9628 9398 5795 st

MASS (EXCL. TETHERS) 8846 8601 8616 5795 st

DISPLACEMENT 9628 9628 9398 5795 st

Table 5 highlights the different load contribution elements in the sizing, as well as draft conditions and heave period. The deck steel is not estimated as part of this exercise, but is specified in the design basis.

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Table 5: Main Characteristic Summary for 5 MW MiniFloat-III

Design Condition A-I A-II A-III A-IV



Water Depth 5,000 10,000 5,000 5,000 ft

Structure:

Deck primary steel 779 779 779 779 st

Deck secondary steel 642 642 642 642 st

Column steel 1482 1482 1482 1482 st

Pontoon steel 450 450 450 450 st

Plate steel 560 560 560 560 st

Appurtenances 100 100 100 100 st

Total structure 4013 4013 4013 4013 st

Deck Equipment:

Total 1180 1180 1180 1180 st

Hull liquids:

Diesel 230 230 0 0 st


Total 3653 3409 3423 603 st

Risers:

Total 146 306 146 0 st

Mooring:

Total 636 721 636 0 st

Estimated Main Characteristics:

Estimated draft change with diesel use 2.58 2.58 0.00 0.00 ft

Displacement at 35 ft draft 5795 5795 5795 5795 st

Weight at quayside 5193 5193 5193 5193 st

Minimum operating draft 75.42 75.42 75.42 35.00 ft

Minimum airgap 55 55 57.58 98 ft

Minimum GM - check value with no diesel 23.10 24.11 22.69 11.54 ft

Deck area 21632 21632 21632 21632 ft2

Heave period 20. 20. 20. 20. sec

Case B: 50 MW Table 6 shows the weight distribution summary of the MiniFloat for 50 MW for all 4 conditions studied. Table 7 highlights the different load contribution elements in the sizing, as well as some preliminary estimates of draft conditions and heave period.

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Table 6: Weight Summary for 50 MW MiniFloat-III

Design Condition B-I B-II B-III B-IV



Water Depth 5,000 10,000 5,000 5,000 ft

Weight Summary

Hull 3671 3671 3671 3671 st





TOTAL WEIGHT 14497 14497 13416 8247 st


DISPLACEMENT 14497 14497 13416 8247 st


Design Condition B-I B-II B-III B-IV



Water Depth 5,000 10,000 5,000 5,000 ft

Structure:



Column steel 2216 2216 2216 2216 st





Deck Equipment:

Total 2408 2408 2408 2408 st

Hull liquids:

Diesel 1080 1080 0 0 st


Total 4621 3652 3541 176 st

Risers:

Total 850 1692 850 0 st

Mooring:

Total 955 1082 955 0 st








Deck area 27636 27636 27636 27636 ft2


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Case C: 100 MW Table 8 shows the weight distribution summary of the MiniFloat for 100 MW for all 4 conditions studied. Table 9 highlights the different load contribution elements in the sizing, as well as some preliminary estimates of draft conditions and heave period. Table 8: Weight Summary for 100 MW MiniFloat-III

Design Condition C-I C-II C-III C-IV



Water Depth 5,000 10,000 5,000 5,000 ft

Weight Summary

Hull 4141 4141 4141 4141 st





TOTAL WEIGHT 16657 16657 15237 9541 st


DISPLACEMENT 16657 16657 15237 9541 st


Design Condition C-I C-II C-III C-IV



Water Depth 5,000 10,000 5,000 5,000 ft

Structure:



Column steel 2491 2491 2491 2491 st





Deck Equipment:

Total 2985 2985 2985 2985 st

Hull liquids:

Diesel 1420 1420 0 0 st


Total 4584 2885 246 3163 st

Risers:

Total 1633 3204 1633 0 st

Mooring:

Total 1146 1273 1146 0 st








Deck area 30901 30901 30901 30901 ft2


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Fabrication & Installation Fabrication The MiniFloat-III is designed as to be fabricated in a gulf-coast yard. The hull and the deck can be fabricated in the same yard or in two separate facilities. There are multiple fabrication scenarios that can be envisioned and selection will be influenced by contractual constraints and commercial optimizations. In general terms, the hull would be fabricated vertically to minimize the construction of heavy pieces high above ground. This can be done either in a graving dock or at quay side. The deck would be fabricated and brought near the hull at the time of integration. In the case of the graving dock or a drydock, the deck would be lifted onto the hull prior to opening the flood gates. In the case of a lift, the hull would be loaded out first and brought near the heavy lift crane, which would then place the deck onto the 3 columns in a protected environment, as shown in Figure 3. This is favorable if the deck is not fabricated in the same yard and brought over to the integration location by sea way.

Figure 3: Quay side integration of the deck

Transport Once the unit is fully assembled, it floats at a draft of 35 feet, which is shallower than most yard channels (usually around 40-45 feet). It is then wet towed to the site using a couple of small tugs, or by an AHV already on contract to the project (Figure 4). This was performed successfully on the WindFloat installation in the Portuguese project (Figure 5, right, where a Bourbon Liberty 200 AHV was used for laying anchors and wet tow). The platform can be partly ballasted as it reaches the open water to improve the tow stability performance in waves. No buoyancy modules are necessary.

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Figure 4: MiniFloat being towed fully assembled at transit draft away from the yard

Figure 5: WindFloat being towed fully assembled at transit draft away from the yard

Installation & Commissioning As the final assembly and towing of the unit is taking place, the mooring system is prelayed at the site. The total installation consists solely of ballasting, mooring and cable / SCR hookup. Unlike deeper draft platforms, the MiniFloat does not require some of the most challenging offshore operations such as platform upending and deck mating in the open waters. These operations additionally require extremely mild weather window, and very expensive vessels to be mobilized. Coming to site fully assembled has tremendous cost savings opportunities.

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Level 0 Schedule Table 10 shows a preliminary project schedule for the MiniFloat platform. A 24 month schedule is achievable. The schedule includes a 6 month FEED period, and an 8 to 10 months fabrication. Three budget commitment milestones are included and are necessary to move forward. Any delays in the decisions supporting these commitments would impact the schedule and are not included. Table 10: Project Schedule

Year Year 1 Year 2 Quarter Q1 Q2 Q3 Q4 Q4 Q6 Q7 Q8

Month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Project Start Budget commitment 1

FEED Budget Commitment 2

Detail Design Budget Commitment 3

MTO / Steel Order Shop Drawings

Fabrication 5 MW Fabrication 50/100 MW

long lead item Order Mooring prelay Platform Tow

Platform hook up Project Delivery

Acknowledgments The authors are grateful for Chevron Energy Technology Company for sponsoring part of this study, and in particular to Deanna Mangin and Mark Balcezak. WindFloat pictures are courtesy of Principle Power, the organization commercializing the MiniFloat for offshore renewable applications.

References [1] C. A. Cermelli, D.G. Roddier and C.C. Busso, MINIFLOAT: A novel concept of minimal floating platform for marginal

field development, Proc. 14th Int. Offsh. & Polar Engrg. Conf., Toulon, France, May 2004

[2] C. A. Cermelli, D.G. Roddier, Experimental and numerical investigation of the stabilizing effects of a water-entrapment plate on a deepwater minimal floating platform , Proc. 24th International Conference on offshore Mechanics and Arctic Engineering, Halkidiki, Greece, June 2005

[3] Alexia Aubault, Christian A. Cermelli, Dominique G. Roddier, Structural Design of a Semi-Submersible Platform with Water-Entrapment Plates Based on a Time-Domain Hydrodynamic Algorithm Coupled with Finite-Elements, Proceedings of the Sixteenth International Offshore and Polar Engineering Conference, San Francisco, California, USA, May 28-June 2, 2006

[4] Alexia Aubault, Christian A. Cermelli, Dominique G. Roddier, Parametric Optimization of a Semi-Submersible Platform With Heave Plates, Procs. of OMAE’07, 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, USA 11-16 June 2007

[5] C. Cermelli, D. Roddier, A. Aubault, Remote Power Generation for Deployment of Subsea Technologies in Deepwater Marginal Fields, Deepwater Offshore Technology Conference (DOT) Proc. Houston. 2008.

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[6] C. Cermelli, D. Roddier, A. Aubault, Remote Power Generation for Deployment of Subsea Technologies in Deepwater Marginal Fields, Offshore Magazine, January 2008.

[7] D. Roddier, C. Cermelli, A. Aubault, A. Weinstein, WindFloat: A Floating Foundation for Offshore Wind Turbines , Journal of Renewable and Sustainable Energy, Vol.2, Issue 3, 2010

[8] American Petroleum Institute, “Interim Guidance on Hurricane Conditions in the Gulf of Mexico”, API Bulletin 2int-Met, MAY 2007

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