4 Platforms & Floaters ASSESSMENT & Comparison

The following chapter will attempt to compare the main functions, characteristics, strength & weakness of the different floater concepts available for the deepwater (>500m) and ultra-deepwater (>1500m) fields developments.

4.1 Main Functions Comparison

Platforms and Floaters main function capabilities are compared in the following Table:

Table 4.1 - Floaters Functions Comparison

	*CPT*	*SPAR*	EDP⁽⁷⁾	FPSO ⁽⁶⁾	*TLP*	Semi-Submersible *DDSS*	*SSP*
Dry Trees	Yes	Yes	Yes	No	Yes	Possible⁽⁵⁾	No
Production	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Storage	No	Yes ⁽⁸⁾	No	Yes	No	Yes ⁽⁸⁾	Yes
Drilling	Yes	Yes	Yes	Yes ⁽¹⁾	Yes	Possible⁽⁵⁾	No
Workover	Yes	Yes	Yes	Yes ⁽¹⁾	Yes	Possible⁽⁵⁾	No
SCRs utilisation	n/a	Yes	Yes	Possible⁽²⁾	Yes	Possible⁽²⁾	Possible⁽⁴⁾
Max. Oil Production ⁽³⁾	280 kBPD	160 kBPD+	200 kBPD	330 kBPD	370 kBPD	360 kBPD	120 kBPD+
Water Depth Limitation	500m	3000m+	3000m+	3000m	1500m	2500m	1500m

Notes

Drilling FPSO (FDPSO) for mild environmental conditions (West of Africa) : AZURITE FDPSO (Tullow, Congo).
Depending on water depth & environmental conditions, or long lazy wave SCR to be considered.
kBPD = 1000 Barrels Per Day.
Under investigation in GOM
Qualified for DDSS
FPSO category includes ship shaped FPSO, FPU and FLNG
Concept still under development
Limited storage capacity

4.2 Main Characteristics Comparison

The following Tables aim to summarise and compare the main characteristics of deepwater production concepts.

Table 4.2 - Platforms & Floaters Main Characteristics

Concept	Main Characteristics
Compliant Tower	Custom designed for site specific application. Single drilling centre. Surface completed wells. Integral drilling/work-over facilities. No oil storage; export pipeline or FSO. Tensioned rigid risers for production. Flexibles or steel catenary risers for import/export. Insensitive to topside load. Long development schedule.
SPAR	Custom designed for site specific application. Well options: - Single drilling centre, surface completed wells, integral work-over. - Remote wells completed subsea by specialist vessel. Export options: - Integral oil storage, export via offshore loading unit. - No oil storage, pipeline or FSO. Tensioned risers, flexibles or steel catenary risers. Medium development schedule. Topside float-over or offshore lifts
*FPSO*	New-build or tanker conversion. Remote wells, normally completed subsea. Drilling/work-over requires specialist vessel. Integral oil storage & off-loading. Flexible risers, SCR (possible), hybrid risers Less sensitive to topside load. Short development schedule. Topside integration at quayside by heavy lift cranes
*TLP*	Custom designed for site specific application. Single drilling centre. Surface completed wells. Integral drilling/work-over facilities. No oil storage; pipeline or FSO. Tensioned rigid risers for production. Flexible or steel catenary risers for import/export. Sensitive to topside load. Relatively long development schedule. Topside integration at quayside by heavy lift cranes
Concept	Main Characteristics
Semi-Submersible	New-build or conversion. Well option 1: Remote subsea wells with work-over by specialist vessel. Well option 2: Wells below with integral drilling/work-over facilities. No oil storage; pipeline or FSO. Sensitive to topside load. Flexible risers – large number possible. Short to medium development schedule. Topside integration at quayside by heavy lift cranes or by float over
SSP	New-build. Remote wells, normally completed subsea. Drilling/work-over requires specialist vessel. Integral oil storage & off-loading. Flexible risers. Insensitive to topside load. Short development schedule. Topside integration at yard or offshore lift
*EDP*	Custom designed for site specific application. Well options: - Single drilling centre, surface completed wells, integral work-over. - Remote wells completed subsea by specialist vessel/rig. Export options: -Limited oil storage, export via offshore loading unit. - No oil storage, pipeline or FSO. Tensioned risers, flexible or steel catenary risers. Medium development schedule. Topside load >50 000 tonnes Topside integration at quayside by skidding

4.3 Platforms & Floaters Motions Comparison

Wave motions of floaters induce different loads and fatigue to their associated mooring and riser systems of which heave motion is a main design criteria. The following Figure presents the relative heave motion versus the wave period of the main floater concepts for deepwater application.

Figure 4.1 - Deepwater Floater Concepts Relative Heave in Waves Response

The following values present typical motions amplitudes for SPAR and TLP in the Gulf of Mexico environmental conditions:

SPAR

Heave:

0.5 feet (0.15m) Annual Winter Storm.
12 feet (3.6m) 100 year Hurricane.

Pitch:

0.5° mean plus 1.5° dynamic in Annual Winter Storm.
5° mean plus 5° dynamic in 100 year Hurricane.

Surge:

6% of Water Depth below Keel in 100 year Hurricane.

TLP

Heave: - Max. 1 foot.

Pitch: - Zero.

Surge: - 10% of Water Depth.

4.4 Platforms & Floaters Strengths & Weaknesses

The following section summarise the main perceived strengths and weaknesses of the different floater concepts:

SPAR (Traditional market: deepwater & ultra-deepwater dry tree facilities and high payload).

Strengths:

Dry trees.
Good platform for drilling.
Low cost for additional payload.
Excellent stability (e.g. GoM hurricanes).
Well suited to ultra deep waters

Weaknesses:

Offshore topsides installation/integration necessary.
High initial ‘capex’ costs (fabrication and installation).

FPSO (Traditional market: offshore oil storage requirement and wet trees).

Strengths:

Storage areas with no/little infrastructure.
Flexible - defined by storage requirements rather than payload.
Traditionally considered to be cheaper than other floater concepts.
First choice for wet tree-type developments, e.g. subsea completion.

Weaknesses:

No dry trees.
Cannot use SCRs except in exceptionally benign conditions, e.g. West of Africa.
Integration/interfaces complex.

TLP (Traditional market: deepwater dry tree facilities).

Strengths:

Dry Trees.
Proven solution for deepwater (<1500m).
Riser system simple.

Weaknesses:

Typically more expensive in deeper water.
Coupled design - involved design spiral, e.g. payload, hull size, tendon size, etc.
High degree of complexity in design and fabrication.
Drilling systems can have high downtime.
Unstable if one tendon fail.

Semi-Submersible (Traditional market: multiple purposes drilling, production although not combined with dry trees).

Strengths:

Proven solution.
Less steel required for given payload.
Large deck space.
Topside integration at quayside or by float over.

Weaknesses:

High initial ‘capex’ costs (fewer tons are not necessarily cheaper tons).
Traditional viewpoint is that motions are too high for use of SCRs, e.g. lazy wave SCR required.

Extended Draft Platform (Potential market: deepwater dry or wet tree facilities in remote areas with marginal onshore facilities, good local content).

Strengths:

Quayside topsides integration.
Dry trees.
Large deck space.
Good platform for drilling.
Low cost for additional payload.

Weaknesses:

New design not field proven.
High initial ‘capex’ costs.

4.5 Floaters Selection Criteria

The selection of the floater concept will depend greatly on the selected Field Architecture (Ref.) which is the result of analyses on key issues and technical topics, as follows (but not limited to):

Economics (tax regime, financing, production sharing agreement, etc).
Field size, reservoir geometry, number of wells.
Process requirement (capacity, nature of treatment).
Oil storage requirement, oil & gas export strategies (shuttle tanker, subsea pipelines).
Water Depth.
Development strategy (fast-track, phased development).
Regional context (local content, access to support services, export infrastructure).
Political risk.
Experience/preference.

Figure 4.2 - Floater Choice (dry or wet trees) versus Reservoir Geometry Criteria

4.6 Design Cycle & Concept Selection

The picture below shows a TLP design cycle, which is a typical engineering process applicable to the design of most floater concept with following variations:

Drilling facilities to be considered only for dry tree floater concepts (TLP, SPAR, EDP) and semi-submersible.
Mooring line system (Chapter 6) to replace TLP tendons.

Figure 4.3 - Typical TLP Design Cycle

4.7 Platforms & Floaters Water Depths & Productions Comparisons

The following Figures summarise the current status and the future potential of Deepwater Production Concepts based on three key topics:

WaterDepthRange, Figure 4.4, “Water Depth Range Comparison by Deepwater Production Type (by Courtesy of Offshore Mag.)”.
Production Capacity Mboe/d (million barrels oil equivalent per day), Figure 4.5, “Throughput Capacity Ranges by Deepwater Production Type (by Courtesy of Offshore Mag.)”.
Well Capability, Figure 4.6, “Well Capability by Deepwater Production Type - Includes Production & Water Injection Wells (by Courtesy of Offshore Mag.)”.

Figure 4.4 - Water Depth Range Comparison by Deepwater Production Type (by Courtesy of Offshore Mag.)

Figure 4.5 - Throughput Capacity Ranges by Deepwater Production Type (by Courtesy of Offshore Mag.)

Figure 4.6 - Well Capability by Deepwater Production Type - Includes Production & Water Injection Wells (by Courtesy of Offshore Mag.)

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OFFSHORE PLATFORMS & FLOATERS AND MOORING SYSTEMS