PRODUCTION OR RELATED USE Field of the invention

The present invention relates to floating installations for petroleum exploration or production, or related use. More specifically, the invention relates to a floating installation having specific properties making it particularly feasible for drilling or field development of large petroleum fields offshore.

Background of the invention and prior art

Initially, drilling for petroleum reserves or production of petroleum reserves offshore made use of steel jacket structures and gravity base concrete structures. As the water depths for new prospects increased, floating

installations were used, first as monohull ships. However, significant heave, pitch and yaw in large waves led the industry to develop semisubmersible vessels.

In principle, a semi submersible obtains its buoyancy from submerged ballasted pontoons below the water line. At larger draught, as semi submerged, the movements are reduced. However, the movements can still be a severe problem and the capacity of the design with respect to load and deck space are limiting factors. For larger capacity floating installations, the two main choices are either a floating ship, such as an FPSO (Floating Production, Storage and Offloading), or a cylindrical design as offered by Sevan Marine, for example according to the patent publication WO 02/090177 A1 . For an FPSO or other ship based design to be feasible for harsh conditions, a turret with fixed single point mooring is incorporated in the hull. The vessel can then weathervane freely around the turret. However, the swivel of the turret is a limitation with respect to well shut in pressure, power transfer capacity and number of risers possible. For shut in pressure above 490 barg and power transfer capacity above 10 MW, no proven solution exists for an FPSO.

Furthermore, for gas developments it is a preference to use large diameter (OD > 12") free hanging steel risers; however, these are vulnerable to structural damages, including fatigue, due to wave-induced motions, particularly heave and roll. Accordingly, FPSOs find limited use in large offshore field

developments.

For hulls having cylindrical shape, the above limitations can be avoided, however the cylindrical hulls have limitations with respect to hull size since heave tend to increase with increasing hull size. A cylindrical hull shape typically means a closed bottom or in substance closed bottom cylindrical structure, with no opening in the hull bottom or only a moonpool of limited diameter in the bottom center of the hull. The largest cylindrical hull size presently available has a diameter of 90 m and a deck space of about 6500 m ² . A diameter of about 100 m can be a practical limitation in many waters for a cylindrical hull design. The diameter limits the deck area, which again limits the topsides capacity. For most planned future production platforms, a deck space of more than 15 000 m ² will be required, in order to include sufficient process facilities, methanol and glycol (MEG) injection system and storage, in addition to hydrocarbon liquid storage capacity and chemical storage capacity. For medium and harsh environment developments (Hs100yr>8m), the main selection criteria are concerned with wave induced motions, liquid storage capacity, payload capacity and topside deck area. In patent publication FR 2 299 209 a floating platform shaped as a hollow ring is described and illustrated. However, the structure is flat, as seen in cross section of the ring, which results in structural limitations, in addition to severe limitations with respect to allowable weather and wave height. Another floating structure is described and illustrated in patent publication US 7086810 B2, with an inclined transition portion at the waterline between a very large submerged part and a much smaller emergent part over the waterline.

Due to the limitations described above, a demand exists for a floating installation design feasible for large capacity petroleum field developments.

Summary of the invention

The invention meets the demand by providing a floating installation for petroleum exploration or production, or related use, comprising a deck feasible for having a deck area larger than 6500 m2. The floating installation is distinctive in that

the hull is ring-shaped ,

inside the ring of the ring-shaped hull the installation is open up to a deck structure elevation,

the hull extends from below the waterline to the deck structure elevation; wherein a ratio H/W is larger than 1 , where H is height and W is width of the ring in cross section, excluding any protrusions or flanges on the structure below the waterline or radial openings above the waterline but below the deck elevation.

Preferably, the ratio h/w is at least 2, most preferably about 3.

A ratio h/w larger than 1 distinguish over the teaching of FR 2 299 209, which results in a stiffer construction with increased structural strength and elevated level for deck for the installation of the present invention. Accordingly, the installation of the invention is feasible for far worse weather, higher wave heights and increased load capacity compared to the structure of FR 1 299 209.

Preferably, the floating installation comprises an inwards protrusion in the ring shaped hull, below the waterline, preferably at the lower end of the hull.

Preferably, the inwards protrusion extends inwards to reduce an inner diameter of the hull, ID, by at least 5 %, more preferably at least 10 %, even more preferably at least 20 %, compared to ID without said inwards protrusion.

Preferably, the inwards protrusion increases the buoyancy at normal level of the waterline on the hull, by at least 5 %, more preferably at least 0 %, even more preferably at least 20 %, compared to the same hull without said inwards protrusion. The cross section area of the inwards protrusion is at least 5 % of the ring cross section, more preferably at least 10 %, even more preferably at least 20 %, compared to the same hull without said inwards protrusion. The inwards protrusion has two major technical effects. Firstly, the buoyancy and load capacity increases. Secondly, and most important for service in harsh conditions, the inlet to the volume inside the ring shaped hull is restricted, limiting the flow of water in and out, thereby limiting the heave of the water column inside the hull. Thirdly, the structural stiffness increases. Fourthly, heave is reduced due to the damping effect of the protrusion.

Preferably, the floating installation comprising a flange or protrusions in the ring shaped hull, below the waterline, preferably at the lower end of the hull, extending outwards.

More preferably, the floating installation comprising a flange or protrusions in the ring shaped hull, below the waterline, preferably at the lower end of the hull, extending both inwards and outwards.

Preferably, the outwards protrusion extends outwards to increase an outer diameter of the hull, OD, by at least 5 %, more preferably at least 0 %, even more preferably at least 20 %, compared to OD without said outwards protrusion. Preferably, the outwards protrusion increases the buoyancy at normal level of the waterline on the hull, by at least 5 %, more preferably at least 10 %, even more preferably at least 20 %, compared to the same hull without said outwards protrusion. The cross section area of the outwards protrusion is at least 5 % of the ring cross section, more preferably at least 10 %, even more preferably at least 20 %, compared to the same hull without said outwards protrusion. The outwards protrusion has two major technical effects. Firstly, the buoyancy and load capacity increases. Secondly, and most important for service in harsh conditions, the resistance to heave, yaw, pitch and roll increases, providing a more stable platform. Thirdly, the structural stiffness increases. Fourthly, heave is reduced due to the damping effect of the protrusion. Fifthly, the righting moment is improved, particularly if the outwards protrusion extends above the waterline on one side of the installation during roll.

The advantages of inwards and outwards protrusions as combined are the sum of the advantages described separately for inwards and outwards protrusion above.

The inwards protrusion and the outwards protrusion, separate or combined, extends in substance radially outwards from the ring shaped hull for outwards protrusions and radially inwards from the ring shaped hull for inwards protrusions, respectively, without inclined transitions.

The floating installation preferably comprises a deck, the deck has restricted opening for airflow in or out of the volume inside the ring over the waterline, restricting the relative movements of the ring shaped hull and the water inside the ring, controlled to minimise the movements of the installation. The effect is to dampen the fluctuations of the water level inside the hull, and thereby also the fluctuations or movements of the hull, thereby minimising the movements of the installation. Secondly the effect of this deck is to mitigate wave impact loads from so called Moonpool piston effect during conditions with excessive wave height. Preferably, the installation comprises a topside deck arranged on a deck frame consisting of a truss structure with channels or space for ventilation below. The topside deck may be lifted or floated over the floater in one or several parts. Preferably, the hull has an outer diameter OD and an inner diameter ID, as measured above inwards and/or outwards protrusions, a ratio OD/ID is less than about 1 ,4 or 1 ,35. A ratio OD/H preferably is about 3 or less. In a preferable embodiment, the diameter of the encompassed area is equal to or larger than the thickness of the ring shaped hull. The freeboard preferably is above 15 m, more preferably above 20 m, such as about 25 m.

A ratio OD/H preferably is from about 1 .5 to about 3 more preferably 1 ,5 to 2,5, most preferably about 2.

OD is the outer diameter of the ring-shaped hull at the waterline and h is the height of the ring-shaped hull.

Outwards means radially outwards from the ring whilst inwards means radially inwards in the ring.

Most preferably, a ratio OD/H preferably is from about 1.5 to about 3 and OD at the waterline is from about 90 m and larger, such as 100 m , 130 m or 140 m.

The thickness of the ring shaped hull means thickness in radial direction.

The draft of the present floating installation is preferably about 20-30 m for a steel installation and down to about 45 m for a concrete installation. The main deck is in principle at the same elevation above water surface in any case. The OD is constant from below the waterline to above the waterline.

The ratio OD at the waterline to draft T is about 30/40 for concrete and about 130/20-30 for steel. Existing concepts have much deeper draft, smaller diameter, inclined OD and typically inclined ID.

The floating installation of the invention has OD/T of 2,5 or larger, such as about 3,25 or about 4, where OD is outer diameter at the waterline and T is draft. The floating installation of the invention comprises vertical sidewall, from optional protrusions below the waterline up to a deck structure. Preferably, no inclined structure exists on the installation of the invention, which facilitates construction by slip forming concrete or joining steel. For floating installations of the invention made of concrete, the immersed protrusions are in principle at same elevation as for a steel installation, with respect to the upper elevation thereof. However, for concrete the thickness of the protrusion is larger, so the protrusions are going deeper down, to

compensate for increased weight. Hence, the draft increases for concrete.

OD/ID below waterline preferably is 1 ,4 or less, such as 1 ,35 or less.

OD/ID at and above waterline preferably is 1 ,4 or less, such as 1 ,35 or less.

Surprisingly, the ring shaped hull design according to the invention will reduce significantly or eliminate the limitations mentioned above, according to calculations and simulations. For example based on simulations and

experience data, heave for the floating installation compared to a pure cylindrical installation of same loading capacity of 1 Mbbl and same wave condition, is 4 m for the installation of the invention compared to 18 m for the cylindrical installation, whilst roll had an insignificant increase if water depths above 100m are assumed.

Without wishing to be bound by theory, it is assumed that the encompassed area or volume inside the ring shaped hull acts to dampen the movements by having a different and probably destructive resonant frequency compared to said frequency of the ring shaped hull. Preferably, a ratio of a harmonic frequency or period of the floating installation to said frequency or period of the water column inside the ring, including inwards and outward protrusion(s) if any, is an odd number, most preferably a prime number such as 2, 3 or 5.

Accordingly, with the floating installation of the invention, there is in principle no limit with respect to size or deck space in rough waters, since heave will be limited and other movements like yaw, pitch and roll still will be at low level. Installations with deck space of 5 000 m ² , 20 000 m ² or 30 000 m ² for example, can be built and operated with success at deep waters, also with steel risers and comprehensive liquid storage capacity onboard.

The floating installation of the invention comprises a ring shaped hull wherein the waterline area encompassed by the hull, the Moonpool, is at least 25 % of the ring of the ring shaped hull waterline area, preferably at least 50 % of said hull waterline area, more preferably at least 75 % of said hull waterline area, even more preferable at least 100 % of said hull waterline area, most preferable the waterline area encompassed by the hull is larger than said waterline area of the hull, such as 125 %, or 150 % of said hull waterline area.

The floating installation of the invention as seen from above has shape as a circular ring, a quadrate, a near quadrate rectangle, a quadrate having truncated corners, a polygonal ring structure or combinations or shapes in between. Said shape can be elongated or non-symmetrical in other ways, for example having shape like a traditional ship. However, most preferably said shape is circular, quadratic or symmetrical in other ways in order to facilitate fixed mooring without introducing momentums caused by weathervaning motions. Most preferably, each of the four quadrants, as seen from above when viewing the floating installation, have the same shape. All of said shapes are in this context considered to be a "ring shape". Fixed moorings are preferably attached at or close to the outer edges of the hull structure. As an alternative, thrusters and dynamic positioning can be used for position keeping, particularly in deep and very deep waters.

The topside deck of the floating installation is preferably arranged on a truss structure with space for restricted or free ventilation below. Alternatively or in addition, conduits, channels or openings in other ways radially out below the deck or through the deck are provided for restricted ventilation or free ventilation. The term free ventilation means that the movement dampening effect on the hull of the wave action in the encompassed area shall not be significantly influenced by pressure build up and air flow restrictions. For example, at least about 5 %, more preferably about 10 % or more of the encompassed area cross section are available as cross sectional area in openings for ventilation through truss structure, channels or other ventilation openings, for free ventilation. In contrast, less than 5 %, more preferably less than 3 or 2% of said cross sections are available for restricted flow of air, which provides a damping effect on the water column fluctuations inside the ring shaped hull.

In a preferable embodiment, the deck of the floating installation is without openings. Preferably, the ring shaped hull comprises radial openings above the waterline but below the deck elevation.

Preferably, the deck of the floating installation is without openings but the ring shaped hull comprises radial openingsabove the waterline but below the deck elevation, which openings provide radial release of energy from waves inside the ring, that is the Moonpool, particularly relevant with any excessive waves inside the Moonpool. One such embodiment comprises a closed topside deck on a highest elevation of an open truss structure extending between the top of the ring of the ring-shaped hull structure up to said deck. The main deck of the ring shaped hull preferably is a closed structure; whilst radial openings are provided through the ring of the ring-shaped hull above the waterline but below the deck structure, and/or radial openings are provided in the lower parts of an open truss deck structure. This combination of features can be vital in severe sea states with high waves, since the deck elevation and all equipment thereon, including living quarters, are protected from the impact of any oversize wave.

The floating installation has typical dimensions as follows:

Topside weight capacity: 30 000 metric tons

Hull weight: 50 000 metric tons or heavier

Liquid storage capacity: 163 000 metric tons (1.0 Mbbl)

Diameter: 130m, at the waterline

Topside deck length = 50m

Topside deck width =150m

Topside area=22 500m ²

A further preferable feature of the floating installation of the invention is that the ring-shaped hull has constant outer diameter OD from below the waterline up to the deck elevation. Preferably, the OD of the ring shaped hull is constant from the lowest elevation to the highest elevation thereof, excluding any optional outwards protrusions or flanges on the structure below the waterline or radial openings above the waterline but below the deck elevation. Preferably, also the inner diameter ID of the ring shaped hull is constant from the lowest elevation to the highest elevation thereof, excluding any optional inwards protrusions or flanges on the structure below the waterline or radial openings above the waterline but below the deck elevation.

In other words, excluding any optional inwards protrusions or flanges on the structure below or above the waterline or radial openings above the waterline but below the deck elevation, the ring-shaped hull is cylindrical.

The production is simplified thereby, whether the hull is made of steel or concrete.

The ring-shaped hull may comprise protrusions or flanges above the waterline but below the deck structure, if such protrusions or flanges are present they are excluded from the H W - ratios that are distinguishing over the prior art installations. Such protrusions or flanges may be part of an optional

intermediate structure between the deck structure and the ring-shaped hull.

In a preferable embodiment the floating installation of the invention comprises dry wellheads, which provides the option for any further drilling to take place from a deck elevation of the installation.

The invention also provides use of the floating installation according to the invention, for exploration or production of petroleum fields offshore, preferably for production of larger petroleum fields offshore at deep waters, meaning petroleum fields requiring a platform with deck area larger than 6500 m ² at water deeper than 100 m, such as deeper than 500 m. A preferred use of the floating installation of the invention is as a drilling platform with dry wellheads and drilling from the installation. Other uses of the floating installation of the invention are as fundament for bridges, structure for offshore aquaculture, such as fish farming, and other uses.

A very favourable embodiment of the invention is for fish-farming offshore or elsewhere. This is because the ring-shaped structure is in substance closed for water, fish lice, spill and pollution spreading out from the ring and inflow of fish lice into the ring, since the draft typically can be down to at least about 40 m for the embodiments modelled so far, meaning that all water exchange may take place at 40 m depth. This will limit or eliminate the fish lice problem, such as salmon lice problem and may also allow control of any pollution problem or poisonous medicine problem since the water inside the ring can be confined until medicines are harmless and pollution can in substance be held inside the ring, such as on an inwards flange from where it can be taken up periodically.

Figures

The invention is illustrated with nine figures, of which:

Fig. 1 is one embodiment of a floating installation of the invention, in cross section and as obliquely viewed from slightly above, without deck.

Fig. 2 is a further embodiment of a floating installation of the invention, in cross section and as obliquely viewed from slightly above and slightly below, without deck.

Fig. 3 is a further embodiment of a floating installation of the invention, in cross section and as obliquely viewed from slightly above and slightly below, without deck.

Fig. 4 is a further embodiment of a floating installation of the invention, in cross section and as obliquely viewed from slightly above and slightly below, without deck.

Fig. 5 is a further embodiment of a floating installation of the invention, with deck.

Fig. 6 is a further embodiment of a floating installation of the invention, with radial openings through the ring-shaped hull.

Fig. 7 is a further embodiment of a floating installation of the invention, with fish cage or net, preferably easily retrievable.

Fig. 8 is a further embodiment of a floating installation of the invention, with derrick.

Fig. 9 is a further embodiment of a floating installation of the invention, as bridge foundation.

Detailed description

Figures 1-4 are identical except for that each Figure illustrates one specific embodiment of the floating installation of the invention.

Figure 1 illustrates a floating installation (1 ) for petroleum exploration or production, or related use, feasible for having deck area larger than 6500 m ² . The hull (2) is shaped as a ring, the cross section of the ring is having a height H and a width W, a ratio H W is larger than 1 , which is clearly seen from the Figure. The ratio H/W is about 3 and the hull, in cross section of the ring, includes no protrusions or flanges on the structure. However, the installation comprises a deck (3). Figure 2 is in principle identical to Figure 1 , except that the illustrated

embodiment comprises an inwards protrusion (4). Only new features on Figures 2-5, compared to features on Fig. 1 are provided with reference numericals.

Figure 3 is in principle identical to Figure 1 , except that the illustrated

embodiment comprises an outwards protrusion (5).

Figure 4 is in principle identical to Figure 1 , except that the illustrated

embodiment comprises a protrusion (6) extending both inwards and outwards, which is an inwards and outwards protrusion.

Figure 5 illustrates a floating installation of the invention with a quadratic deck structure (3). The deck structure can be supported by beams (7) as illustrated on Fig. 5, or a truss structure (8) as illustrated briefly on Figures 1-4. In addition to what can be seen in Figures 1-5, Figure 6 illustrates radial openings through the ring-shaped hull, Figure 7 illustrates a fish cage or net, preferably easily retrievable by including a winch functionality, not illustrated. With the net retrieved to close the opening at the bottom of the installation, all fish can be kept inside the ring, facilitating lice removal and reducing pollution if medicines are used.

Figure 8 illustrates a derrick on the installation and Figure 9 illustrates the installation used as a bridge foundation.

The floating installation of the invention can comprise any feature here described or illustrated, in any operative combination, each such operative combination is an embodiment of the present invention.