Floating platform for offshore drilling or production of hydrocarbons

Abstract

A floating platform for offshore drilling or production of hydrocarbons comprises a topsides 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). During its operation the platform is exposed to wave forces (9) which cause heave motion (v) and roll and pitch motion (p) of the platform. The substructure has a draught (11) which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres, and the ratio between the distance (16) between the columns' centre axes (25), measured along a side of the substructure, and the lower 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.

Description

[0001] 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.

[0002] 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.

[0003] For a floating platform it is normally three of the six motion components which are of the greatest importance, viz. upwardly and downwardly-directed vertical motion, usually called heave, reciprocating rotating motion about a horizontal longitudinal axis, usually called roll, and recprocating rotating motion about a horizontal transversal axis, usually called pitch.

[0004] When engineering a floating platform it is desirable to design the platform in such a manner that the platform has the least possible oscillating motions. This applies in particular to floating platforms which have dry well completion on the main deck. In addition, the design of the platform must also take into consideration space requirements, load capacity requirements, possibly requirements regarding storage capacity for hydrocarbons, and also other characteristics which are desirable for the floating platform.

[0005] It is usually possible to design a floating platform in such a fashion that some of its motion components in the water are moderate. To design a platform where all the motion components are avoided, however, is no easy task, since designing the platform with a view to avoiding one motion component usually results in an increase in the tendency of the platform to acquire another motion component.

[0006] 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.

[0007] 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. One type is known under the name SPAR platform, and 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.

[0008] U.S. Pat. No. 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.

[0009] Other floating platform structures are described in U.S. Pat. No. 3,986,471, U.S. Pat. No. 5,931,602, U.S. Pat. No. 4,913,238, U.S. Pat. No. 5,439,321, U.S. Pat. No. 4,215,950, U.S. Pat. No. 4,793,738, U.S. Pat. No. 4,753,553, U.S. Pat. No. 4,702,321, U.S. Pat. No. 4,194,568 and EP 0 256 177.

[0010] In U.S. Pat. No. 4,829,928 it is described a platform which has a negatively buoyant pontoon suspended from the balance of the platform to increase heave resonant period to at least 25 seconds. Tendons suspend the pontoon to a depth where dynamic wave forces do not materially act directly on it in seas of normally occurring periods of about 15 seconds but do in seas of periods above 15 seconds. Columns and an upper pontoon provide buoyancy for the platform. The platform motion is reduced by increasing its mass and the damping component for the platform. One drawback with this solution, which one would like to avoid, is the complications related to the presence of a large suspended pontoon that cannot be utilised to anything useful and the suspension mechanism. Adding extra structural elements to a platform construction like a suspended pontoon and suspension devises, will add complication in both construction, building, running, maintenance and repairs of the platform, that one wants to avoid.

[0011] 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.

[0012] 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.

[0013] 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. Surprisingly, it has been found that a platform with dimensions and dimensional ratios according to the invention encounters little heave, roll and pitch motion. According to the invention the substructure has a draught which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres. Moreover, the ratio of the substructure's draught to the distance between the columns' centre axes, measured along a side of the substructure is between 1.0 and 1.5, preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35. Calculations show that by means of the invention it is possible 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.

[0014] The invention will now be explained in greater detail in connection with a specific embodiment, and with reference to the accompanying drawings, in which:

[0015] FIG. 1 illustrates a side section through a floating platform according to the invention,

[0016] FIG. 2 illustrates a cross section viewed from above through columns which form part of a floating platform according to the invention, and

[0017] FIG. 3 is a curve showing oscillating motion as a function of oscillating period.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] When the floating platform is located in the water it will acquire a motion which is divided into vertical upwardly and downwardly-directed heave motion, which is indicated in FIG. 1 by the double arrow v, and roll and pitch motion, which means a reciprocating rotation of the platform about horizontal axes. In FIG. 1 the roll and pitch motion is indicated by the double arrow p, and the rotation centre for the roll and pitch motion is indicated by reference numeral 5. It can be seen that the rotation centre 5 is located slightly above the platform's centre of gravity 6.

[0022] 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.

[0023] The driving forces for the pitch and roll motion have many contributors. As described in the literature, for example O. M. Faltinsen: Sea Loads on Ships and Offshore Structures, the wave forces can be divided into mass forces and pressure forces.

[0024] 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.

[0025] On account of the waves' upwardly and downwardly-directed motion, there will also be varying liquid pressure in a wave. A body which is located in a wave will be influenced by forces from this varying liquid pressure, and these forces are called pressure forces.

[0026] 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:

[0027] Moments due to mass forces on the columns

[0028] Moments due to pressure forces on the columns

[0029] Moments due to mass forces on the pontoons

[0030] Moments due to pressure forces on the pontoons

[0031] These components act in different directions and with relative magnitude depending on the oscillating period of the waves, the draught and principal dimensions of the substructure.

[0032] By means of a spectral analysis of a typical wave motion, which can be performed by a computer program of a known type, it is possible to find the extent of excitation conveyed to the platform by the various components in the wave motion. In FIG. 3 this is illustrated in a curve 30, called the wave excitation spectrum. The wave motion's excitation energy, which has the unit m2s, is shown here as a function of the waves' oscillating period t. It is seen that the waves have an oscillating period of between approximately 6 and 25 seconds. The curve 30 has a top 31 for an oscillating period tt of approximately 17 seconds for the sea state represented by the curve. It can be seen that there is little excitation from waves with a oscillating period of over 20 seconds. An analysis of typically occurring wave motion in the sea has shown that waves with an oscillating period of over 20 seconds generally convey little excitation of oscillations.

[0033] 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 tz of approximately 22 seconds. This oscillating period is called the platform's natural period tz for heave motion.

[0034] It has been found that, in order to avoid excitation of heave motion of the platform, 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 tz 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 tz for heave motion should be at least 22, preferably at least 24 and most preferred at least 26 seconds.

[0035] With the natural periods tz for the platform's heave motion which can realistically be expected to be achieved, there will be a certain amount of wave motion. It has been found that the effect of the pressure forces' excitation of heave motion decreases with increasing draught. With the platform according to the invention, therefore, 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.

[0036] In order to avoid excitation of roll and pitch motion of the platform, according to prior art 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.

[0037] It has been found that the mass forces on the pontoon 3 and the columns 4 for the most important of the waves' periods act in the opposite direction. It has been found that the mass forces on the pontoon 3 have a greater influence on the platform's motion than the mass forces on-the columns 4 when the substructure's draught 11 is small relative to the distance 16 between the columns' 4 centre axes 25, measured along a side of the substructure. When the substrucutre's draught 1 is substantially greater than the distance 16 between the columns' centre axes 25, the mass forces on the columns 4 will make a greater contribution to the platform's motion than the mass forces on the pontoon 3. With the platform according to the invention, therefore, the ratio of the substructure's draught 11 to the distance 16 between the columns' centre axes 25 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.

[0038] It has been found that 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.

[0039] It has further been found that an additional reduction of the platform's motions in the water is achieved when the ratio of the total cross sectional area of the columns 4 in the waterline 10 to the platform's total mass is between 0.005 and 0.018 m2/metric ton, and preferably between 0.08 and 0.015 m2/metric ton, where total cross sectional area of the columns 4 in the waterline 10 means total cross sectional area of the columns 4 as they are illustrated in FIG. 2.

[0040] Calculations show that a floating platform which has constructional features according to the invention has little motion in the water, while being capable of being designed so as to have a substantial amount of space and considerable load capacity.

Claims

1. A floating platform for offshore drilling or production of hydrocarbons, comprising a topsides 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), where during its operation the platform is exposed to wave forces (9) which cause heave motion (v) and roll and pitch motion (p) of the platform,

characterized in that

a) the substructure has a draught (11) which is at least 40 metres, preferably at least 50 metres and most preferred at least 60 metres, and

b) the ratio of the substructure's draught (11) to the distance (16) between the columns' centre axes (25), measured along a side of the substructure is between 1.0 and 1.5, preferably between 1.2 and 1.4 and most preferred between 1.3 and 1.35.

2. A floating platform according to claim 1,

characterized in that 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) are adapted so as to give the platform a natural period (tz) for heave motion which is at least 22, preferably at least 24 and most preferred at least 26 seconds.

3. A floating platform according to claim 1 or 2,

characterized in that the ratio of total cross sectional area of the columns (4) in the waterline (10) to the platform's total mass is between 0.005 and 0.018 m2/metric ton, and preferably between 0.08 and 0.015 m2/metric ton.