Jack-up units

From MarineWiki

1 Introduction
2 Hull
3 Legs and Footing
4 Preload and Soil Penetration
5 Equipment
6 Basic Jack-up Configurations
7 Sharing

Introduction

A Jack-up is an offshore structure composed of hull, legs and a lifting system that allows it to be towed to a site, lower its legs into the seabed and elevates its hull to provide a stable work deck capable of withstanding the environmental loads. A typical modern drilling Jack-up is capable of working in harsh environment like wave heights up to 25m, wind speeds in excess of 100 knots and in water depths up to 150m.

Jack-up units have been used for exploration drilling, tender assisted drilling, production, accommodation, and work/maintenance platforms. The limits imposed on jack-up for their operations include deck load carrying when afloat, load carrying capabilities when elevated, environmental limits, drilling limits, and soil (foundation) limits. There is a constant urge to stretch these limits to explore deeper waters, harsher environments, and areas where soils and foundations may be challenging or even unstable.

Hull

The hull of a Jack-up unit is a watertight structure that supports or houses the equipment, systems, and personnel, thus enabling the Jack-up unit to perform its tasks. When a Jack-up unit is afloat, the hull provides buoyancy and supports the weight of the legs and footing (spud cans), equipment and variable load.

As the dimensions of the hull is increase, like in any typical floating body, it can carry more load (variable deck load and equipment), surely in the afloat mode due to increased buoyancy.

Larger hull will also ensure that there is sufficient space in machinery compartments as well as clear deck areas for operation. The larger hull may have larger preload capacity that may permit increased flexibility in preloading operations.

The disadvantage of larger hulls include higher environmental loads - wind, wave and currents. The higher weight of these hulls also means that it requires larger capacity of elevating unit to lift and hold the unit. The weight also affects the natural period of the Jack-up unit in elevated mode.

The draft of the hull also affect the payload capacity - it increases with draft. It also affects the stability of the unit in afloat condition.

For two identical hulls, the one with deeper draught weighs more. A deeper draught means lower freeboard and hence lower stability in afloat condition.

Legs and Footing

The legs and footing of a Jack-up are steel structures that support the hull when the unit is in the elevated mode and provide the stability to resist the lateral loads. Footing are needed to increase the soil bearing area and provide more strength.

The legs of a Jack-up unit may extend over 150 m above waterline when the unit is being towed with legs fully retracted. The tall and heavy legs above the hull means that the center of gravity of the afloat structure is raised and thereby affects the stability in a detrimental way. For similar hull configuration and draft, the unit with longer and heavier legs that have center of gravity further from the water is more unstable.

The magnitude of wind, wave and current loads is a function of water depth, air gap and distance the footing penetrate into the seabed. Generally, the larger the legs and footing, the more loads would be. Also as the depth increase the distance between support footing and hull/leg connection increases and thereby the Jack-up stiffness decreases.

Preload and Soil Penetration

Jack-ups are preloaded when they first arrive at site to ensure that the soil is capable of withstanding the maximum expected footing reaction (either from extreme weather or operating condition) without experiencing additional leg penetration or soil failure. The amount of leg penetration is dependent on soil properties, vertical reaction of the legs, and footing area. The greater the footing area, the lower the penetration under same circumstances.

The amount of soil penetration should be checked against the footing structural capabilities and scour characteristics of the site. Proper planning with regards to soil information and predicted penetration curves should be done. If during actual preloading the leg penetrations are recorded (penetrations vs. footing reaction curves), then this information can be used to improve upon the prediction penetrations curves methodology.

Equipment

The equipment required to satisfy the mission of a Jack-up unit affects both the hull size and lighthsip of the unit. There are three main groups of equipment on a Jack-up unit - the Mission equipment, Elevating equipment and Marine equipment.

Mission equipment

This refers to the equipment and systems onboard a unit, which are necessary for a unit to complete its mission. These may include derricks, mud pumps, mud piping, drilling control systems, production equipment cranes, combustible gas detection and alarm systems, etc. Some of these items may form a part of the variable deck load and may not always be located onboard the Jack-up unit.

Elevating equipment

This refers to the equipments and systems onboard a Jack-up which are necessary for the unit to raise, lower, and lock-off the legs and hull of the Jack-up.

Marine equipment

This refers to the equipments and systems onboard a Jack-up that are not related to the mission equipment or elevating equipment. There are items found on any sea-going vessel regardless of its form or function. These may include main diesel engines, fuel oil pipings, electrical generation and distribution, lifeboats, communication and navigation equipment, accommodation and galley equipment, etc. Marine equipment are not directly involved with the mission of the unit, however, they are necessary for supporting the mission. All marine equipment is classified as part of lightship.

Basic Jack-up Configurations

The types of designs are based on legs, elevating systems, and load transfer philosophy between the hull and the legs. Some of them are:

Mat footing versus Independent Spud can footing

Most of the Jack-up units have footings. They increase the leg's bearing area, thereby allowing it to operate in soils with lower foundation support. There are two types of footing: Mats and Spud cans

Mat footing connect all the Jack-up unit's legs to one common footing. Mat footings are typically rectangular structures, flat on the top and bottom, and contain buoyancy chambers which are flooded when the mat is submerged.

Advantages	Disadvantages
1. Due to their large size, mat footings exert a lower bearing pressure on the soil than units with spud cans. This is beneficial in areas where the soil cannot support high bearing loads.	1. Mats cannot be used on uneven seabeds or those with large slopes. Sloping or uneven seabeds induce large bending moments on the mat and legs. This would make the mat structure heavy to withstand this.
2. In afloat transit mode, mats provide considerable buoyancy, which may translate to increased variable load carrying capability.	2. Mat units cannot be used on bottoms where there are obstructions such as pipelines, debris, etc.
	3. During transition from afloat to on-bottom operations, the matt must be flooded. The flooding sequence must be done carefully so as not to cause large heeling moments or loss of afloat stability of the unit. Also to pump out water from the mat during re-floating of the unit, it requires additional equipment not needed in spud cans.

In case of units with spud can footings, they can have the same number of spud cans as there are legs. Spud cans are typically conical structures, with sloping tops and bottoms. The sloping top helps in sloughing off mud that may collect on the top of the spud can in the event of deep penetration. The sloping bottom helps to ensure that there will be some penetration, even in very stiff soils. Spud cans are normally designed to be free flooding when submerged, though they can be pumped dry for inspection.

Advantages	Disadvantages
1. Spud cans can be used in a great variety of seabeds. They have operated in sea beds of hard and soft soils, sloping bottoms (though they may be sensitive to large slopes on hard soils), and in areas where there are pipelines or other structures that must be avoided.	1. The smaller area of footing means that the bottom bearing pressure is larger and result in increased soil penetrations when compared to mat units. Such high pressure leaves impression in areas with soft soils. If another Jack-up unit later works in the same are, these impressions may induce horizontal forces on one or more legs if the spud cans slide in to them.
2. Spud can do not require complicated ballasting sequence or equipment and some rigs can retract the spud cans flush into the hull to permit easy dry transport of the unit.

Cylindrical Legs versus Trussed Legs

All Jack-up units have legs. Their purpose is to raise the hull above the storm wave crest; withstand wind, wave, and current loads; and to transmit operational, environmental, and gravity loads between the hull and footings. There are two main leg types: Cylindrical and Trussed.

Cylindrical legs are hollow steel tubes. They may or may not have internal stiffening, and may have rack teeth or holes in the shell to permit jacking of the hull up and down the legs. These are currently found on units operating in water depths less than 100 meters. The newer units operating in water depths of 100 meters or more have trussed legs.

Advantages	Disadvantages
1. Cylindrical legs are beneficial for units operating in shallow waters as they are smaller in size and hence they take up smaller deck area.	1. These require more steel to provide the same resistance to environmental loads as trussed legs.
2. Cylindrical legs are less complicated to construct and require less experience than trussed legs.

Trussed legs consists of chords and braces. In general, the braces provide the shear capacity of the leg while the chords provide the axial and flexural stiffness.

Advantages	Disadvantages
1. They allow for optimal steel utilization and result in lighter stiffener legs with reduced drag loads.	1. Truss legs are larger in size and hence they take up larger deck area and hence not suitable for shallow water or smaller units.
	2. Truss legs are complicated to construct and hence need high degree of expertise.

3-Legged versus 4-Legged Jack-ups

The great majority of Jack-ups in the world have either three or four legs, with three being the minimum required for stability. There are rarely some units built with more than four legs.

Units with three legs have the legs arranged in some triangular form.

Advantages	Disadvantages
1. They eliminate the need to build extra leg(s).	1. Three-legged units require pre-load tankage.
2. For a given hull size, three-legged ones can carry more deck load in afloat mode; and usually have a reduced number of elevating units (pinions, cyliners, etc.), resulting in reduced power/maintenance requirements, and less weight.	2. They have no leg redundancy.

Units with four legs have the legs arranged in some rectangular form.

Advantages	Disadvantages
1. Four-legged units require little or no preload tanks on board. This is because four-legged units can preload two legs at a time using the elevated weight as preload weight. This result in savings of piping and equipment weights, and more usable space within the hull.	1. Four-legged units are stiffer in the elevated mode than a three-legged unit. This apparent advantage may be offset by the fact that the additional leg adds wing, wave and current loads.
2. In the afloat transit mode, the fourth leg is a disadvantage as its weight causes a direct reduction in the afloat deck load compared to an equivalent three-legged unit.

3-Chorded Legs versus 4-Chorded Legs

Trussed legs have either three or four main vertical structural members called chords. All trussed-leg Jack-up units have one of these chord arrangements. The advantages and disadvantages are similar to that described above in the 3 versus 4 legged units except that they do not affect preloading procedures in any way.

Elevating System

All jack-ups have mechanism for lifting and lowering the hull. There are two types - pin and hole system, rack and pinion system. The pin and hole system allows for hull positioning only at discrete leg positions. The rack and pinion system allows for continuous jacking operations.

There are two basic jacking system - floating and fixed. The floating system uses relatively soft pads to try to equalize chord loads, where as fixed system allows for unequal chord loading while holding.

There are two types of power source for fixed jacking systems - electric and hydraulic. Both systems have the ability to equalize chord loads within each leg. A hydraulic-powered jacking system achieves this by maintaining the same pressure to each elevating unit with a leg, taking care that the transmission loss is take care of.

Upper and Lower Guides

All Jack-up units have mechanism to guide the legs through the hull. In case of the one with pinions, the guide protect it from "bottoming out" on the rack teeth. The upper and lower guides are capable of transferring bending moment to the hull to some degree determined by the design. In deep hull there are intermediate guides and these guides only help to maintain the correct distance away from the pinion and do not transfer leg bending moment to the hull.

Opposed Pinion Chords versus Radial Pinion Chords

Jack-up units that have rack and pinion elevating systems have the interface between the racks and pinions in one of two configurations - two opposed pinions or a single radial pinion on each chord. All jacking system exert vertical and horizontal forces on the leg at the pinion/rack interface (as contact area is not horizontal). Opposed pinion system balances these loads and hence no net load on the leg bracing. However, radial pinions exert a horizontal load on the leg bracing due to the pinion arrangement.

Opposed pinion systems have rack and elevating systems on to two opposite sides of the same chord, resulting in chord sections with double symmetry. Radial pinion systems have rack and elevating pinions on one side of the chord only; thereby resulting in chords with only one plane of symmetry and having the net vertical pinion loads inducing bending of the chord due to the eccentricity.

Advantages of Opposed Pinion Systems	Advantages of Radial Pinion Systems
1. There is good load sharing between pinions of a chord.	1. The upper guide are located further away from the lower guides compared to opposed pinion systems. This is because of the greater height of this type.
2. If pinions are arranged on both sides of the same chord, the overall height of the jacking tower is reduced compared to pinion arranged only on one side of the chord.	2. A lower drag coefficient on the leg chord. This is because one rack will cause less hydrodynamic drag than two racks.
3. Reduced height of the jacking tower. This reduced height results in less wind load on the Jack-up as well as reduced weight.