1. 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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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 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 Rig Assessment
Advantages
Disadvantages
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
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
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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).
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
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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16. Jack-up and SSDU achievement days related to the operation
General Operating Comparison
Drilling Operational Comparison
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 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 HP tugs or anchor handling tugs. For the third generation class, which is propulsion assist, the rig requires 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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19. General Operating Comparison
Economical Comparison
Jack-Up and SSDU Cost Difference for RC 36" related to the operation
Jack-up Rig SSDU
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).
Jack-up and SSDU Overall Cost Difference
Jack-up Rig SSDU
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20. 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.
Accident statistics for jack-up units
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).
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Conclusions
Most of the existing SSDU second generation are old and need to be replaced.
For safety and efficiency, operators want new rigs. Older SSDU’s cannot approach this target.
Assuming equal conditions and comparing downtime/trouble time, an SSDU will drill quicker than a jack-up rig regardless of water depth.
The overall well cost at the same water depth is more expensive if drilled by an SSDU than a jack-up rig.
The jack-up rig contract daily rates in deep water up to 500 ft will be more economical than an SSDU.
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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International Marine and Offshore Engineering Conference (IMOC 2014)