A Jack Up is a type of offshore drilling rigs. It is made up of a hull, legs, and a lifting system and a jack up rig can be towed to the offshore site, and then lower its legs into the seabed to lift up the hull, providing a stable work deck which is strong enough to cope with the required environmental loads.
Another advantage of Jack Up rigs is that they can operate in high wind speeds and with significant wave heights, as well as in water depths reaching 500 feet. Since the Jack Up will be ultimately supported by the seabed, they are usually preloaded upon arrival at the intended site to simulate the kind of leg loads that they will be exposed to. This ensures that once the rig is fully jacked up and in operation, the seabed will be able to provide a strong foundation for the rig.
The offshore industry has made significant use of Jack Up rigs for over 60 years now. They are especially useful for exploration drilling since they are relatively easy to set up, and also provide ample production, accommodation, and maintenance areas. Over the years, Jack Ups have been pushed to their limits in terms of what they can do. This includes their deck load carrying limits both when afloat and when elevated, their environmental and drilling limits, and the soil, or foundation, limits. By pushing these limits, drilling companies have been able to explore deeper waters, drill deep reservoirs in harsh conditions, and even drill in areas with unstable soils and foundations.
Components of Jack Up Rigs
Each Jack Up unit is made up of three main components: the Hull, the Legs and Footings, and the Equipment used on the Jack Up. Each of these components will be described below, and their functions will also be explained.
Hull
Much like the hull of a boat, a Jack Up unit’s hull is watertight, and houses or supports the equipment, systems, and personnel needed to carry out normal operations. While the Jack Up is afloat, the hull also provides the buoyancy needed to stop the Jack Up from sinking. The parameters of the hull can vary depending on the different modes of operation of the unit.
Figure 1 – Hull
As a rule, the bigger the length and depth of the hull, the more variable deck load and equipment the Jack Up unit will be able to carry. This is especially true while it is in its afloat mode, since there is increased deck space and additional buoyancy. Larger hulls have another advantage in that they provide a bigger space both inside and out to accommodate piping and machinery, and allow for clear work areas. Larger hulls can also have greater preload capacity which allows for more flexibility during preloading operations.
There are some downsides to larger hulls, though. They are often subject to higher wind, current, and wave loads. On top of this, since a larger hull increases the weight of the Jack Up, they will require additional elevating jacks with a larger capacity in order to safely elevate and hold the unit. Finally, additional weight has an impact on the natural period of the Jack Up while it is in its elevated mode.
Another factor which has a direct impact on the amount of variable deck load that can be carried, as well as the afloat stability, is the draft of the hull. This refers to the distance between the afloat waterline and the baseline of the hull. The draft of the hull has an opposing relationship with the hull’s freeboard, which refers to the distance between the afloat waterline and the main deck of the hull. Therefore as the draft of a Jack Up increases, the freeboard decreases by the same amount.
With two Jack Ups with identical hulls, the one with the deeper draft will weigh more. This extra weight could come from either lightship weight or variable deck load. On the other hand, the unit with the deeper draft will suffer from decreased afloat stability compared to its shallower counterpart. However, the most influential factor on a Jack Up unit’s afloat stability is its freeboard. If the hull and leg length of two Jack Ups is identical, then the one with a larger freeboard will have the higher afloat stability.
Leg and Footing
A Jack Up’s legs and footings are made from steel, and serve to support the hull while the unit is elevated, and offer the necessary stability to resist lateral loads. Footings are used to increase the soil bearing area, meaning that the Jack Up can be used in areas with lower soil strength than if there were a smaller bearing area. Both legs and footings have multiple characteristics which affect the way that the unit reacts in both elevated and afloat modes, and it is therefore important to understand these characteristics.
Figure 2 – Legs
A Jack Up unit’s legs can extend up to 500 feet above the water’s surface when the unit is towed, even when they are fully retracted. Larger and longer legs usually have the biggest impact on the afloat stability of a Jack Up unit. Due to both the large wind area of the legs, and the heavy weight which causes a high center of gravity, the afloat stability will be negatively affected. Units with larger legs will be less stable than other units with the same hull configuration and draft.
When the unit is in its elevated mode, its legs will be affected by wave, wind, and current loadings. As well as environmental conditions, the magnitude of these loads is a result of water depth, air gap (the distance between the hull baseline and the waterline), and how far the footings penetrate into the seabed. A general rule is that the larger the legs and footings, the more load will be exerted on them by the wind, waves, and current.
Depending on their design and size, different legs will exhibit different amounts of lateral stiffness. This refers to the amount of load necessary to produce a unit deflection. This stiffness decreases as water depth increases, and in higher depths, flexural stiffness (the chord area and spacing) has a much greater impact than shear stiffness (brace). Leg stiffness is directly related to Jack Up stiffness while the unit is in its elevated mode, which affects the amount of hull sway and the natural period of the Jack Up unit.
Drilling Rig Equipment
Each Jack Up unit will require certain equipment in order to fulfil its purpose. This equipment will therefore have an effect on the hull size and the lightship weight of the overall rig. The equipment used on Jack Up rigs can be split up into three major classifications: Marine Equipment, Mission Equipment, and Elevating Equipment.
The Marine Equipment is everything which isn’t directly related to the mission equipment. This Marine Equipment category therefore encompasses all of the equipment that one might expect to find on an ordinary sea-going vehicle, such as a diesel engine, oil piping, electrical equipment, lifeboats, radar and sonar, communications equipment, and so on. While all of this equipment isn’t directly relevant to the rig’s mission, it is nevertheless essential for supporting personnel aboard the rig, and for allowing it to operate on its own at sea. The Marine Equipment is typically classed as part of the rig’s lightship weight.
Figure 3 – Mission Equipment
As mentioned above, the Mission Equipment covers everything aboard the Jack Up rig that allows it to complete its mission. Naturally, this will vary depending on what exactly this mission is, as well as the individual Jack Up. For instance, two Jack Up rigs which were both being used for exploration drilling might not use exactly the same type of equipment. Mission Equipment covers things like cranes, mud pumps and piping, derricks, drilling control systems, gas detection equipment and alarms, and so on. Unlike Marine Equipment, Mission Equipment is not always classed as part of the lightship weight, since some tools like cement units may not always be situated aboard the Jack Up itself. Finally, Elevating Equipment covers everything which is involved in allowing the Jack Up to raise, lower, and lock-off its legs and hull.
References
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Bork, K. (1995). The rotary rig and its components. Austin: Petroleum extension service. Division of continuing education. University of Texas at Austin.
Davis, L. (1995). Rotary, kelly, swivel, tongs, and top drive. 1st ed. Austin: Petroleum extension service. Division of continuing education. University of Texas at Austin.
