Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
  • B63B35/44—Floating buildings, stores, drilling platforms, or workshops, e.g. carrying water-oil separating devices
  • B63B35/4413—Floating drilling platforms, e.g. carrying water-oil separating devices

Definitions

• the invention relates to a floating platform for offshore drilling or production of hydrocarbons, comprising a topsides with drilling and/or production equipment, and substructure comprising a lower pontoon and columns connecting the pontoon to the topsides, where during its operation the platform is exposed to wave forces which cause heave motion and roll and pitch motion of the platform.
• the floating structure On account of wave action, floating structures of all types will experience motion in the water. Waves in the sea are a highly complex phenomenon, and the structure is exposed to an excitation from waves in different directions and with different oscillating periods.
• the floating structure partly acquires a drifting motion, i.e. a movement of the structure, and partly an oscillating motion.
• the oscillating motion can be divided into resiprocating linear motion along three axes, i.e. the two horizontal directions and the vertical direction, and reciprocating rotating motion about the same three axes, thus giving a total of six independent motion components.
• Floating platforms are normally designed in such a manner that heave, roll and pitch motion partly occur at the waves' excitation period, and partly at the platform's natural periods for the respective motions. There is usually little wave excitation at the natural period for roll and pitch. Nevertheless, roll and pitch could be activated at the natural period for roll and pitch due to wind and the influence of heave motion.
• SPAR platform In order to minimise the motion of the floating platform in the water and satisfy requirements for space and load capacity, two main types of floating platforms have been developed.
• SPAR platform comprises an elongated, vertical subsea body extending deep down into the water.
• the SPAR platform has little heave motion, but has relatively substantial roll and pitch motion in anti-phase with corresponding wave motions.
• the other type of floating platform is the column platform where three or more columns connect the platform's topsides with one or more pontoons.
• This type of platform normally has a length/width which is considerably larger than its draught, in order to provide a substantial amount of space and load capacity.
• the column platforms have greater heave motion than the SPAR platforms, and have roll and pitch motion in phase with the waves.
• US 4 934 870 describes a floating structure with limited heave oscillations.
• An elongated element has a lower end which is connected to the seabed, and an extendible tension device is connected between a platform deck and the upper end of the elongated element.
• the tension device comprises devices which exert forces which counteract the heave motions.
• the object of the invention is to provide a floating platform which has little motion in the water, while at the same time having a substantial amount of space and considerable load capacity.
• the object is achieved with a floating platform of the type mentioned at the beginning which is characterized by the features which are indicated in the claims.
• the invention therefore relates to a floating platform for offshore drilling or production of hydrocarbons, comprising a topsides with drilling and/or production equipment, and a substructure comprising a lower pontoon and columns connecting the pontoon to the topsides, where during its operation the platform is exposed to wave forces which cause heave motion and roll and pitch motion of the platform.
• a platform with dimensions and dimensional ratios according to the invention encounters little heave, roll and pitch motion.
• the substructure has a draught which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres.
• the ratio between the distance between the columns' centre axes, measured along a side of the substructure, and the substructure's draught is between 1.0 and 1.5, preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35.
• Fig. 1 illustrates a floating platform for offshore drilling or production of hydrocarbons, comprising a topsides 1 with drilling and/or production equipment 8, and a substructure comprising a lower pontoon 3 and columns 4 connecting the pontoon 3 to the topsides 1.
• the platform is lying in the water 23, with the waterline indicated by reference numeral 10.
• the topsides 1 may include one or more decks with equipment and installations for carrying out a number of functions which are necessary in connection with a floating platform, for example living quarters, hoisting cranes and electrical generators.
• the columns 4 are connected to the pontoon via transition portions 7.
• the columns 4, the transition portions 7 and the pontoon 3 are provided with buoyancy tanks (not shown) and ballast water tanks which can be filled with water in order to adjust the platform's position in the water 23, and possibly storage tanks for hydrocarbons.
• the platform may be of a type which is connected to the bottom by means of approximately vertical tension legs, it may be connected to the bottom via slanting, slack moorings, or it may be kept almost immobile in the water with dynamic positioning, by means of positioning propellers controlled by an electronic control system. How the platform is moored or kept immobile is not within the scope of the invention and is not illustrated in the figures.
• Fig. 2 illustrates a cross section viewed from above through the columns 4, through the waterline 10. It shows that the pontoon 3 is octagonal, and in the middle has an octagonal opening 24. It also shows that the columns 4 are four in number, and that the platform's topsides 1 is rectangular. The number of columns and the shape of the pontoon and the topsides are partly chosen on the basis of sizing criteria and could have been different.
• the platform's motion is partly dependent on the design of the platform, such as the platform's mass, damping and rigidity, and partly the driving forces, i.e. wind and wave forces.
• the driving forces for the pitch and roll motion have many contrubutors.
• the wave forces can be divided into mass forces and pressure forces.
• the water particles in a wave will constantly have cyclic accelerations.
• a body which is located in a wave is influenced by forces which can be calculated on the basis of the water particles' accelerations and displaced liquid volume. These forces are called mass forces.
• the mass forces and the pressure forces are indicated in fig. 1 by arrows with reference numeral 9.
• the forces 9 from the waves influence the external surfaces of the pontoon 3, the transition portions 7 and the portions of the columns 4 which are located under water.
• the wave motion is a highly complex phenomenon, and the forces 9 are continuously changing and acting in different directions.
• the sum of all the forces 9 represents the waves' total influence on the platform.
• the forces give rise to moments which can be divided into the following components:
• Fig. 3 also illustrates a curve 32 for a platform's heave motion as a function of oscillating period t.
• the curve 32 is a transfer function for the heave motion with the unit m/m, and illustrates the platform's heave motion per wave amplitude of the excitation waves. It is seen that the curve 32 has a pronounced top 33 for an oscillating period t z of approximately 22 seconds. This oscillating period is called the platform's natural period t z for heave motion.
• the weight of the topsides 1, the design, displacement, number and position of the columns 4, and the design and displacement of the pontoon 3 should be adapted so as to give the platform a natural period t z for heave motion which is at least 1.1, preferably at least 1.2 and most preferably at least 1.3 times the maximum period for significant oscillation excitation from the waves, i.e. 20 seconds, and therefore the platform's natural period t z for heave motion should be at least 22, preferably at least 24 and most preferred at least 26 seconds.
• the substructure has a draught 11, i.e. the distance from the waterline 10 to the bottom of the pontoon 3, which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres. At these depths the vertical influence of the waves is insignificant, thus giving the platform a small heave motion.
• the weight of the topsides 1, the design, displacement, number and position of the columns 4 and the design and displacement of the pontoon 3 must be adapted in order to give the platform a natural period for roll and pitch motion which is different to the waves' excitation periods.
• the ratio between the distance 16 between the columns' centre axes 25 and the substructure's draught 11 is between 1.0 and 1.5, preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35, since it has been shown that the various components of the wave forces then cancel each other out, and the resulting moment from the mass forces is therefore very small, and the waves' excitation of roll and pitch motion is substantially eliminated.
• a platform which both fulfils the requirement that the substructure should have a draught 11 which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres, and the requirement that the ratio between the distance 16 between the columns' centre axes 25 and the substructure's draught 11 should be between 1.0 and 1.5, preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35 provides a particularly advantageous floating platform, where both the heave motion and the pitch and roll motion are slight.

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• Engineering & Computer Science (AREA)
• Architecture (AREA)
• Civil Engineering (AREA)
• Structural Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• Ocean & Marine Engineering (AREA)
• Earth Drilling (AREA)
• Liquid Carbonaceous Fuels (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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• 2000
  • 2000-06-23 NO NO20003307A patent/NO20003307D0/no unknown
• 2001
  • 2001-06-22 AT AT01961431T patent/ATE267113T1/de not_active IP Right Cessation
  • 2001-06-22 AU AU2001282692A patent/AU2001282692A1/en not_active Abandoned
  • 2001-06-22 US US10/311,470 patent/US20030095839A1/en not_active Abandoned
  • 2001-06-22 BR BRPI0111822-6A patent/BR0111822B1/pt not_active IP Right Cessation
  • 2001-06-22 WO PCT/NO2001/000266 patent/WO2002000496A1/en active Search and Examination
  • 2001-06-22 EP EP01961431A patent/EP1292491B1/en not_active Expired - Lifetime
  • 2001-06-22 DE DE60103396T patent/DE60103396D1/de not_active Expired - Fee Related

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