Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering
Transport Technology

L.R. Brouwer Offshore Jacking Systems.
Literature survey, Report 2003.TL.6835, Transport Engineering and Logistics.

Worldwide many jack-up platforms, commonly known as jack-ups, are used for oil and gas recovery. These jack-up platforms are also used to maintain and provide accommodation to other fixed platforms at sea. The jack-ups are equipped with special lifting devices to raise the platform so that it is not affected by current and wave movement. In the lifted position the platform can be compared with a fixed platform, ready to perform its duty in offshore conditions. There are many solutions for the applied lifting devices, called jacking systems, but there is little combined documentation on these systems. The purpose of this essay is to create an overview of the different jacking systems.

To give a clear overview of the subject the reader is referred to following morphologic overview. In the overview the jacking system is divided into five parts: For each part there are several possible applications. Not all possible connections given in the overview are feasible; most applications are linked to each other.

The rack and pinion type jack is the most commonly used jacking system. Compared to the other types this system is the less complicated. It consists of a number of pinions positioned on racks mounted on the legs. These pinions are driven by gear boxes and an electric or hydraulic motor. The number of teeth on the pinions is reduced by approximately four teeth compared to the ideal number. Using fewer teeth, of still the same size to cope with the loading will result in a smaller pinion diameter. This reduction results in a lower driving moment, which will save costs. Another result is higher required teeth strength. The teeth have to be over-dimensioned because of the fluctuating transmission created. Other reasons for over-dimension are the load sharing variations between the pinions and the backlash between racks and pinions. But the costs of over-dimensioning the teeth are less than the cost savings made by reducing power.

Racks are practicable in single rack and double rack models. A single rack is one rack on a chord parallel to the leg center. Apart from exerting vertical force, it exerts high horizontal force on the chords. A double rack has two opposite racks on one chord. In this way the resulting horizontal force is omitted and the leg is loaded with only vertical loads.

The following are the referred rack and pinion systems with a brief description: The jacking cylinder jack type generally consists of two ring constructions individually connected to the leg and connected to each other by jacking cylinders. The platform or leg movement is caused by repeating the following operation: Release one ring and extend the cylinders before fixing the ring again, then release the other ring and retract the cylinders and fix this ring again. Repeating the operation will result in a discontinuous but strong movement.

The following are the discussed systems working with jacking cylinders with a brief description: Another recent development is the application of winches for jacking a platform. An example of this is the Jumping Jack belonging to Mammoet van Oord. The four boxed typed legs are each handled with three winches: two winches to lift the Jack-up and one winch to create counter-tension and jack the legs up. The winches are positioned in the middle of the platform, via guide sheaves the wires are led to the head or foot of the leg. At the head and foot at two opposite sides there are sheave blocks each containing eight guide sheaves. The head contains two sheave blocks on either side positioned above each other. To avoid wire contact the upper sheave blocks have larger sheave diameters. The foot contains one sheave block on each side. In the hull are the accompanying sheave blocks each containing seven guide sheaves. The two wires of the two platform lift winches are guided to the sheave blocks in the head of the leg and lead back again to the blocks in the hull fifteen times just on one side of a leg. Next the wires are lead through the head of the leg to the other side of the leg. On that side the wires are also led fifteen times between the head and the hull. At the end the wires are fixed to the platform deck. The wire of the third winch has the same path but it leads to the foot of the leg. Pulling the lifting winches will cause an upright movement of the Jack-up or a downward movement of the legs. Pulling the counter-tension winch will give tension in the other wires to avoid slack and it will be able to lower the platform back into the water. During transit the legs are fixed with sixteen hydraulic cylinders per leg. Four cylinders positioned above each other will push the corners of the leg against the leg guide. The cylinders will create enough friction to hold the legs in place.

The jacking system design is connected to the whole jack-up design because the jack-up capacity partly depends on the jacking system and the leg load capacity. In the first stage of the jack-up design, the designer sets up a jack-up concept that is based on the client'as demands. These demands consist of: This jack-up concept enables the designer to appoint the global weights of the main components and to determine the jack type and the required jacking capacity. The important directives in the jacking system design are: To ensure that a proper design is produced it is important that the design is made according to a prescribed classification agency guideline. The best known agencies are: The investigated number of jacking systems is too small to give definite conclusions about the specific applications for a jacking system. On the other hand, a jacking system has several parameters which can easily be changed to make a system more suitable for a specific situation. The following table shows the most suitable jack type per specific situation, based upon the findings made during the investigation. The parameters used are the moving frequency and the water depth.

For a high moving frequency jack-up the jacking time is an important factor. The jack-up needs to move on a daily or weekly basis. Even so, the low moving jack-up stays in a specific working area for a longer period, therefore the jacking time is less important.

The water depth has a direct influence on the leg types which also influences the jacking system. With larger water depths the legs needs to extend. Truss-type legs are the solution, they have relatively more diameter compared to the closed typed legs of the same weight. A truss-type leg also generates less friction in the water, for large water depths the more expansive truss-type legs are more beneficial. For this type of leg the rack and pinion jacking system is commonly used. Less water depth provides a good working area for closed typed legs. The environment like the current, waves and wind are also important factors in the leg type choice and indirectly also in the jack type choice. After the table a short description per jack type is given.

Winches: This jack type is very suitable for high moving frequencies and shallow water depths. The system has a high speed compared to other jack types. At high water depths this type loses it advantages, this situation requires extremely long hoisting cables or the number of winches will have to be increased. The winch jack type becomes too expensive.

Jacking cylinders: This jack type is very suitable for average and shallow water depths. These jacking systems are strong and stable.

Rack and pinion: When the jacking speeds are important and the water is deep this jack type is much more convenient.

New concepts for jacking system designs are still being presented. For example, there is the recently designed winch jacking system and the continuous jacking cylinder design from OSL. The new double working jacking cylinder jacking-system for the Mayflower Resolution also promises an almost continuous jacking cycle. It can be seen that the jacking system design is still developing.

This essay presents a good understanding in the basic principles of jacking systems. It also points out that further investigation is required. There are still many jacking systems that have not yet been investigated and many companies were not willing to share their information. Deepening out more parts will also result in a better application overview of the different jacking systems.

Reports on Transport Engineering and Logistics (in Dutch)
Modified: 2004.06.02; , TU Delft / 3mE / TT / LT.