Comparison between the Characteristics of Jack-Up and Semi-Submersible Rigs

Welaya, Y. M. and Elhewy, A. H.

Department of Naval Architecture and Marine Engineering, Alexandria University, Egypt.

Hegazy, M. M.

Seawolf Oilfield Services Ltd, Nigeria.

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ContenContentsts• AimsAims

• Jack-Up Rigs Jack-Up Rigs

• Semi–Submersible Drilling Units – SSDUSemi–Submersible Drilling Units – SSDU

• General Operating ComparisonGeneral Operating Comparison

• ConclusionsConclusions

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AimsAims

This paper defines and specifies the jack-up rig modes, advantages and disadvantages. In addition, the air gap requirements, leg punch throughs and the rack phase differentials are discussed in detail. As far as semi-submersibles are concerned, the advantages and disadvantages are critically reviewed, and then a comprehensive comparison between the two rigs is carried out in terms of the daily rate, economical aspects and operating conditions.

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Jack-Up Rigs Jack-Up Rigs

• Jack-up Rig Assessment

• Air Gap

• Leg Punch Throughs

• Rack Phase Differential – RPD

• Jack Up Rig Day Rates11/04/23 International Marine and Offshore Engineering Conference

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Jack-up rigs or self-elevating units have hulls with sufficient buoyancy to safely transport the unit to the desired location, after which the hull is raised to a predetermined elevation above the sea surface on its legs, which are supported on the sea bed

Jack-Up Rigs Jack-Up Rigs

The legs of such units may penetrate the sea bed, may be fitted with enlarged sections or footings (spudcans) to reduce penetration, or may be attached to a bottom pad or mat

Different modes of jack-up operation

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Jack-Up Rigs Jack-Up Rigs • Jack-up Rig Assessment

Advantages Advantages DisadvantagesDisadvantages• Mobile , easier to move than other

units • Stable when installed.• Low initial cost.• Leasable with lower daily rate, which

is very competitive.• No mooring required.• Wells and risers are of conventional

type.

• Sensitive for the weather windows for placement (jacking up or down).

• Seafloor scour. • Limited to areas where soil permits

satisfactory support of the legs.• No storage capability.• Blowout can cause collapse of

platform due to wall fluidization.• Rack Phase Differential

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Jack-Up Rigs Jack-Up Rigs • Air Gap

The air gap is defined as the clear distance between the hull structure and the maximum wave crest elevation and may be calculated according to the still water level (SWL) and the highest astronomical tide including storm surge. The air gap is not to be less than 10 per cent of the combined astronomical tide

Definition of Air Gap 11/04/23 International Marine and Offshore Engineering Conference

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Jack-Up Rigs Jack-Up Rigs • Leg Punch Throughs

When a Jack-up is being preloaded, it is important to be prepared to act in the event of rapid penetration of one or multiple legs. Because of the increased demands on Jack Ups (i.e., larger water depths and higher environmental loads) resulting in higher elevated weights during preload, the consequences of a punch through are increasingly more pronounced as shown in the Figure. Punch through

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Jack-Up Rigs Jack-Up Rigs • Rack Phase Differential – RPD

When reacted by the upper and lower guides, high bending moment acting on a truss leg can cause the leg braces within the guides to buckle. This distortion, manifested in the form of differential vertical displacement of the leg chords with respect to a reference horizontal plane, is termed Rack Phase Differential (RPD).

Rack Phase Differentials11/04/23 International Marine and Offshore Engineering Conference

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Jack-Up Rigs Jack-Up Rigs • Jack Up Rig Day Rates

Too many factors control the jack up daily rate. For example, operation water depth, drilling equipment capabilities, safety equipment including BOP, rig condition (building year, maintenance program on board, crew safety record, etc).

Jack-up rig Day Rates

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Semi–Submersible Drilling Units – SSDUSemi–Submersible Drilling Units – SSDU

•Semi-Submersible Assessment

•Semi-Submersible Classification

•SSDU Rigs Day Rates

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Semi–Submersible Drilling Units – SSDUSemi–Submersible Drilling Units – SSDU•Semi-Submersible Assessment

Advantages Advantages DisadvantagesDisadvantages• Semi-submersibles can achieve good

(small) motion response and, therefore, can be more easily positioned over a well template for drilling.

• Semi-submersibles allow for a large number of flexible risers

• Large deck area

• High initial and operating costs.• Pipeline infrastructure or other means is

required to export produced oil.• Building schedules for semi-submersibles

are usually longer than jack-up rigs. • Limited deck load (low reserve buoyancy). • Structural fatigue.• Expensive to move large distances.• Limited dry-docking facilities available.• Difficult to handle mooring systems and

land BOP stack and riser in rough seas 11/04/23 International Marine and Offshore Engineering Conference

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Semi–Submersible Drilling Units – SSDUSemi–Submersible Drilling Units – SSDU•Semi-Submersible Classification

Semi-submersible Drilling rig construction has historically occurred in boom periods and therefore 'batches' of drilling rigs have been built. Offshore drilling rigs have been classified by IMO in nominal 'generations' depending upon the year built and water depth capability as shows in the Table

SSDU classified by IMO related to generation

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Semi–Submersible Drilling Units – SSDUSemi–Submersible Drilling Units – SSDU•SSDU Rigs Day Rates

The day rates shown in the Table are the current day rates for SSDU. These figures which include both competitive and non-competitive rigs are updated on a daily basis. In the current work the emphasis is placed on the second and third generations of SSDU which are able to drill in up to 1500 ft WD.

SSDU Day Rates

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General Operating Comparison

•Drilling Operational Comparison

•Moving on and off location

•Well Control and Well Heads

•Economical Comparison

• Safety Comparison between JU and SSDU Rigs 11/04/23 International Marine and Offshore Engineering Conference

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General Operating Comparison •Drilling Operational Comparison

02468

10121416

JU SEMI

Days

RC 36" Best Op

RC36"AVG Op

RC 36"Poor OP

An SSDU has the time saving option to explosively cut casing whereas a jack-up must use casing cutters and lay down all pipe from the sea bed up to the cellar deck and wellhead area. An average for the operation discussed, the SSDU will finish the well at a minimum of 3.2 to 6.2 days less than the JU, as shown in the Figure .

Jack-up and SSDU achievement days related to the operation

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General Operating Comparison •Moving on and off location

Large jack-ups generally require three tugs with a minimum of 4200 HP each or in excess of 12000 HP to tow safely. The three tugs are generally required in the event of one brake down during a storm and thus sufficient HP is available to hold the rig into weather. For SSDU, depending upon its hull and towing requirements usually the tow package is smaller. For the first generation class the towing requirements are 2-5600 HP tugs or anchor handling tugs. For the third generation class, which is propulsion assist, the rig requires 1-5600 HP tug.

The time to moor up an SSDU, particularly in shallow water depth, can run as little as 6 to 8 hours but will average 12 to 16 hrs

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General Operating Comparison •Well Control and Well Heads

The chief well control advantage of a semi-sub over a deep water jack-up is that the SSDU will use only one size blowout preventer (usually18 3/4", 10,000 or 15,000 WP) through the entire well whereas the jack-up will have to swap BOP's generally three or more times, namely, the thirty inch annular diverter system, twenty inch BOP and the 13 5/8" high pressure BOP.

The one single advantage that a jack-up has over a semi-sub is repair and change of rams on SSDU BOP stack to the surface.

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General Operating Comparison •Economical Comparison

Assume the new equipment has been installed in the shipyard during the jack up building. Jack-up rigs are capable of operating in over 250 ft of water depth while contracted for $140,000 per day or more whereas shallow water SSDU are contracted for $253,000 per day (6).

0

500

1000

1500

2000

2500

3000

JU SEMI

X10

00 U

S D

olla

r Runing Contactor 36" B OP Cost

Runing Contactor 36" A OP Cost

Runing Contactor 36" P OP Cost

Jack-up Rig SSDU

0

2000

4000

6000

8000

10000

12000

14000

JU SEMI

x1000 U

S D

ollar

$

RC 36" Cost

12 3/8"Csg Cost

9 5/8"Csg Cost

Total Cost

Jack-up Rig SSDU

Jack-Up and SSDU Cost Difference for RC 36" related to the operation

Jack-up and SSDU Overall Cost Difference

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General Operating Comparison • Safety Comparison between JU and SSDU Rigs

The SSDU has the worst safety record due to subsea BOP, stability, anchoring operations in rough weather and sea movement problems if compared with the jack-up rig. The loss of stability is considered the main issue for most of SSDU accidents.

The main findings are that one third of jack-up rig accidents were associated with foundation problems, summarizing their analysis of 51 international foundation led incidents. Punch-through failures represent 53% of all foundation accidents. Uneven seabed/scour/footprint interaction was the next most likely cause, covering 15% of all incidents (13).

Accident statistics for jack-up units

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Conclusions

1. Most of the existing SSDU second generation are old and need to be replaced.

2. For safety and efficiency, operators want new rigs. Older SSDU’s cannot approach this target.

3. Assuming equal conditions and comparing downtime/trouble time, an SSDU will drill quicker than a jack-up rig regardless of water depth.

4. The overall well cost at the same water depth is more expensive if drilled by an SSDU than a jack-up rig.

5. The jack-up rig contract daily rates in deep water up to 500 ft will be more economical than an SSDU.

6. As far as the risk during the rig move operation is concerned, the SSDU has a much higher risk probability than a jack-up rig.

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THANK YOUTHANK YOU

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