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.
08/04/2015
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Contents
• Aims
• Jack-Up Rigs
• Semi–Submersible Drilling Units – SSDU
• General Operating Comparison
• Conclusions
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Aims
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 Rig Assessment
• Air Gap
•
Leg Punch Throughs
• Rack Phase Differential – RPD
•
Jack Up Rig Day Rates
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Jack-Up Rigs
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
The legs of such units may
penetrate the sea bed, may be
fitted with enlarged sections or
Different modes of jack-up operation
footings (spudcans) to reduce
penetration, or may be attached
to a08/04/2015
bottom pad or mat
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Jack-Up Rigs
• Jack-up Rig Assessment
Advantages
• 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.
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Disadvantages
• 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
• 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
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Definition of Air Gap
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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
• 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).
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Rack Phase Differentials
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Jack-Up Rigs
•
Jack Up Rig Day Rates
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).
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Semi–Submersible Drilling Units – SSDU
• Semi-Submersible Assessment
• Semi-Submersible Classification
• SSDU Rigs Day Rates
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Semi–Submersible Drilling Units – SSDU
• Semi-Submersible Assessment
Advantages
Disadvantages
• 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
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• 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
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Semi–Submersible Drilling Units – SSDU
• Semi-Submersible Classification
SSDU classified by IMO related to generation
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
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Semi–Submersible Drilling Units – SSDU
• SSDU Rigs Day Rates
SSDU 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.
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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
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General Operating Comparison
• Drilling Operational Comparison
16
14
12
10
8
6
4
2
0
RC 36" Best Op
Days
An SSDU has the time saving option to
explosively cut casing whereas a jackup 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 .
RC36"AVG Op
RC 36"Poor OP
JU
SEMI
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
Jack-Up and SSDU Cost Difference for RC 36" related to the
operation
3000
2000
Runing Contactor 36" B OP Cost
1500
Runing Contactor 36" A OP Cost
1000
Runing Contactor 36" P OP Cost
500
0
JU
Jack-up SEMI
Rig
SSDU
Jack-up and SSDU Overall Cost Difference
14000
12000
x1000 US Dollar $
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).
X1000 US Dollar
2500
10000
RC 36" Cost
12 3/8"Csg Cost
8000
9 5/8"Csg Cost
Total Cost
6000
4000
2000
0
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JU
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SEMI
Jack-up Rig
SSDU
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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
likely08/04/2015
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 YOU
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