Buoyant substructure for offshore platform

A buoyant substructure to float upright in water (e.g. for an offshore platform) and comprising at least three spaced apart columns [103] joined together in generally aligned relationship, in which at least one of the columns is adapted to be ballasted at its end [107] which is to float low down in the water, in which the columns are joined together by discrete cross members [111,113] which are widely spaced apart along the length of the columns, and in which the discrete cross members [111,113] have relatively small dimensions in the direction of alignment of the columns as compared with the total length of the columns.

TECHNICAL FIELD OF THE INVENTION
 The invention relates to a buoyant substructure for an offshore platform,
 and to a method of forming that substructure.
 More particularly the invention relates to a buoyant substructure which can
 be arranged to float upright over an offshore oil and/or gas field, in
 order to support drilling and/or production facilities.
 BACKGROUND OF THE INVENTION
 Oil and/or gas production in deep water may be carried out from facilities
 set on buoyant substructures. These substructures may comprise ship shaped
 Floating Production Storage and Offloading vessels (FPSO's),
 semisubmersibles, Tension Leg Platforms (TLP's) or spars. When using
 FPSO's or semisubmersibles for production, wellheads are usually located
 on the seabed. For both TLP's and spars, wellheads can be disposed above
 sea level.
 The use of spars for drilling and production is a relatively recent
 development. (The Brent spar was merely for storage and offloading.) The
 Oryx Neptune spar (for production) was installed in August 1996. The
 design, fabrication and installation of that spar were described in OTC
 Papers 8384 and 8385. These papers were presented at the Offshore
 Technology Conference in Houston Tex. during May 1997. Subsequently, spars
 were designed for the Chevron Genesis (drilling and production) and Exxon
 Diana developments.
 The spars referred to above were constructed primarily of steel. It has
 also been suggested that spars should be made of concrete. Spar
 configurations for construction in concrete were described on pages 29--33
 of Offshore Engineer for April 1996.
 In addition to the "spar" substructures described above, proposals have
 been made for other types of floating substructures designed generally on
 the spar principle.
 One such proposal is set out in PCT Patent Specification No WO96/14473.
 This shows a single cylindrical hull, and a downward extension formed of
 four vertical legs of reduced diameter. The cylindrical hull is buoyant,
 and supports a deck. The vertical legs are connected together by diagonal
 truss members. The substructure is held in place by an array of semi taut
 mooring lines.
 Our UK Patent Specification No 2,147,546A describes a multi column floating
 substructure. This has four corner columns which support a deck. The
 columns are buoyant, and are of substantial diameter. At the lower ends of
 the columns there are downwardly extending legs of reduced diameter. The
 downwardly extending legs are connected together by diagonal bracing
 members. The substructure is held in place by a conventional spread of
 (catenary) anchor chains.
 In the substructures illustrated in these two patent specifications, the
 downwardly extending legs have diagonal truss or bracing members between
 them. Moreover, in both cases there are sudden transitions of cross
 section (from the hull or corner columns respectively, to the legs of
 reduced diameter).
 The substructures in the patent specifications referred to above are
 designed to be built in a horizontal attitude. The truss or bracing
 members require significant fabrication activity. Fit up and welding of
 these members adds cost and takes up time in the fabrication schedule.
 When complete, the substructures can be floated out (still in a horizontal
 attitude) to their required locations. At those locations they have to be
 upended. The sudden transitions of cross section may create instability
 during floatout and upending.
 Multi leg substructures have smaller diameter columns, and so need less
 reinforcement to resist hydrostatic crushing loads, than do the hulls of
 spars. This results in lighter substructures to support similar topside
 weights. Additionally, the spacing apart of the columns gives better
 stability characteristics.
 The present invention is intended to take advantage of these
 characteristics of a multi leg configuration, while avoiding the need for
 complex truss or bracing members, extending between the columns. It is
 also intended to reduce the disadvantages of discontinuities in cross
 section in the columns.
 DISCLOSURE OF THE INVENTION
 The invention provides a buoyant substructure to float upright in water
 (e.g. for an offshore platform) and comprising at least three spaced apart
 columns joined together in generally aligned relationship, in which at
 least one of the columns is adapted to be ballasted at its end which is to
 float low down in the water, in which the columns are joined together by
 discrete cross members which are widely spaced apart along the length of
 the columns, and in which the discrete cross members have relatively small
 dimensions in the direction of alignment of the columns as compared with
 the total length of the columns.
 It is preferred that at least the column adapted to be ballasted at its end
 which is to float low down in the water has at least two cylindrical
 portions, one of which portions is above and the other of which portions
 is below at least one of the discrete cross members.
 It is also preferred that at least the column adapted to be ballasted at
 its end which is to float low down in the water has a surface piercing
 portion, a buoyancy portion, and a ballastable portion, and in which (when
 floating upright) the horizontal cross section of the buoyancy portion is
 greater than the horizontal cross section of the ballastable portion.
 It is further preferred that all of the at least three columns have
 generally aligned surface piercing portions, buoyancy portions and
 ballastable portions.
 It is further preferred that all of the columns are of identical external
 configuration.
 It is yet still further preferred that two of the columns have identical
 respective horizontal cross sections along their aligned lengths.
 Preferably the columns are aligned in parallel relationship.
 Preferably the at least three columns are joined together in spaced apart
 relationship at three positions along their aligned lengths.
 In a form in which there is a surface piercing portion, a buoyancy portion
 and a ballastable portion, it is preferred that the horizontal cross
 section of the buoyancy portion is greater than the horizontal cross
 section of the surface piercing portion.
 In this last mentioned form it is further preferred that the at least three
 columns are joined together in spaced apart relationship between the
 surface piercing portions, and at or near the respective opposed ends of
 the ballastable portions.
 The columns may be joined by cross members comprising discrete lattice
 truss frameworks in planes defined by adjacent elongate members.
 Alternatively, the columns may be joined by cross members comprising box
 girder elements in planes defined by adjacent elongate members.
 It is preferred that elements joining the columns together in spaced apart
 relationship constitute riser and/or conductor guides.
 It is also preferred that elements joining the columns together in spaced
 apart relationship constitute heave suppression baffles.
 According to a feature of the invention, a mooring line to keep the
 substructure on station is connected to the substructure by a bridle
 having upper and lower elements which can be controlled to adjust the line
 of action of the force in the mooring line.
 The invention also provides a method of forming a buoyant substructure
 (e.g. for an offshore platform) which includes the steps of constructing
 at least three columns horizontally for joining in a spaced apart
 relationship, floating the substructure so formed horizontally on two of
 those columns, uprighting the substructure by selective ballasting with
 water, adding solid ballast to the lower end of at least one of the
 columns, and securing the substructure in position with mooring lines.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS
 A first embodiment of the invention will now be described with reference to
 FIGS. 1 and 2.
 A buoyant substructure has three columns extending down its whole length.
 In common with conventional spar designs, its stability in the installed
 condition depends on a combination of buoyancy in an upper part of the
 substructure and solid ballast at the base. The columns have relatively
 large diameters in the upper part to provide the required buoyancy, but
 (as shown) may be slightly waisted near the water line, to adjust the
 waterplane area, and hence the natural heave period. For heavier topside
 loads, four columns might be required.
 The geometry of the substructure is shown in FIG. 1. An oblique perspective
 view of the substructure is shown in FIG. 2. Each column is divided into
 five portions, which are either cylindrical or conical. Level K is at the
 base (or Keel) of the substructure. Levels A to D are at peripheral joints
 between the portions. The water line intersects portion DE. A deck (not
 shown) is set on Level E. Portion KA is free flooding, and therefore does
 not need to resist full hydrostatic pressure. The remaining portions have
 hard tanks, able to sustain the full hydrostatic pressure at the depths at
 which they operate.
 The connections between the columns consist of a plan frame at the base and
 two sets of plate girders on portion AK. The plan frame at the base is a
 shallow triangular box, which has guides for the risers, and which
 contains iron ore ballast. (Alternatively, the iron ore ballast could be
 enclosed in the columns, but then more ballast would be needed, as it
 would not be so low down. The triangular box could then be replaced by a
 stiffened plate which has guides for the risers.) This plan frame also
 augments the heave added mass and damping. Any additional added mass at
 the keel increases the heave natural period without incurring any extra
 wave excitation.
 Although riser tension is applied at the top of the well risers, the riser
 guides at the base K mean that the effective point of application of this
 lateral load is at the base. The risers are not shielded, so it is not
 possible to tension them with air cans near the water line. However, deck
 mounted tensioners give the necessary restraint. Optionally, floatation
 sleeves (secured to the risers beneath but close to the substructure) can
 reduce tensioner requirements depending on water depth and presence of
 submerged currents. This well riser system eliminates the flat vertical
 walls around the moon pool inside a conventional spar, thereby achieving
 significant steel weight savings.
 Sea water ballast in portions AB and BC is trimming ballast, and might be
 varied during the running or retrieving of risers, or during the
 installation of a deck (not shown). The amount of ballast in these
 portions is at the discretion of the operator. The lower parts of the
 columns (portion KA) and the plate girders are free flooding.
 The permanent ballast compartment is also free flooding and contains iron
 ore and sea water. Within the plan frame at the base (forming the
 permanent ballast compartment), part of the volume is occupied by iron ore
 saturated with sea water, and the remaining space is taken up with sea
 water alone. If the necessary quantity of iron ore can not fit within this
 plan frame, increasing the depth or the plan area of that frame will
 provide more space.
 The sizing of the substructure is the first stage of the design cycle,
 which includes motions response analysis, mooring design, riser design,
 structural design and detailed weight take off, and engineering for
 fabrication and installation. The steel weights, vertical mooring loads
 and riser tensions will all be subject to iterative change as the design
 progresses.
 A spread sheet may be used to perform all kinds of parametric variations. A
 basic parameter is depth of the draft. A key requirement of the design
 process is to minimise the heave response by keeping the heave natural
 period longer than the maximum wave period.
 The columns may be waisted around the water line to adjust the waterplane
 area and hence the heave period. The conical transitions may attract heave
 excitation, and the efficacy of the design can only be checked following
 the motions response analysis. Another way in which the heave natural
 period may be manipulated is by using plated in plan frames between the
 columns, which significantly augment the heave added mass.
 Design iteration may be used to vary the draft against constraints on
 stability and heave period. Satisfying these two constraints
 simultaneously requires an iterative approach. Significant parameters are
 the structural steel weight and the quantity of solid ballast. Given the
 costs per tonne of these quantities, a draft for minimum cost may be
 found. However, a substructure design which was optimum on this basis
 could turn out to have unacceptable motion characteristics.
 To increase stability (i.e. GM or overturning stiffness), more ballast is
 needed, and more buoyancy higher up in the substructure is needed to
 support the extra ballast. In addition, the draft can be increased. The
 required level of stability is set by the need for manageable inclinations
 under the action of wind, wave and current during storms. Since these
 inclinations are evaluated later in the design cycle, iterations are
 likely to be needed.
 The mooring sheaves (fairleaders) may be set at or near the KG or KB so
 that mooring line loads and pitch motion are minimised. They may be set at
 45% of the draft up from the keel. The exact positioning of the
 fairleaders is a subject for development. There will be a trade-off
 between minimising inclinations of the substructure and high line
 tensions. The fairleader position needs to be compatible with the bracing
 between the columns.
 Spoilers for vortex induced vibration are not necessary as the three spaced
 columns and exposed conductors are expected to disrupt flow conditions.
 The substructure is stable in trim and heel. The generous stability
 minimises the inclinations under the combination of environmental, mooring
 and riser loads. The pitch and heave modes are outside the range of wave
 excitation, so the wave excitation on these modes is low. Thus the motion
 response is good, and the dynamic loads in the mooring lines are low,
 despite the relatively taut mooring. The steady state wind forces place
 the highest load upon the mooring system. Steady state current forces are
 of less concern than first order wave forces. By tuning the mooring system
 to shift surge natural frequency away from the significant wave frequency,
 an optimum solution can be obtained. This optimum is achieved by adjusting
 line size, line pre-tension and line arrangement to find an effective
 mooring system stiffness.
 A second embodiment of the invention will now be described with reference
 to FIGS. 3 and 4.
 A buoyant substructure has three columns, and is an assembly of tubular
 members. The substructure a surface piercing portion (above D'); a
 submerged buoyancy portion (D' A'); and float out and permanent ballast
 portion (A' K'). For heavier topside loads four columns might be required.
 The surface piercing portion (above D') diameter is selected to achieve a
 sufficiently long natural period. The submerged buoyancy portion (D'A')
 diameter and length is selected to support the lightship, deck load,
 ballast, mooring load vertical reaction and conductor tensioner loads. The
 lower portion A'K' gives in-service pitch stability.
 More specifically, the diameter of the surface piercing portion (above D')
 is selected to achieve a sufficiently long heave natural period. The
 submerged buoyancy portion D'A' is selected to minimise vertical force
 excitation. In addition, a minimum diameter for portion above D' is
 required to minimise hydroelasticity for drilling operations. The minimum
 hydroelasticity for well completion drilling is governed by grounding of
 well riser casings, such that heave response is less than survival stroke
 of top tensioners. For full drilling capability, the stuck pipe capacity
 of the drill rig derrick when released will not produce a heave response
 greater than the survival stroke of the top tensioners.
 The three columns consist of cylindrical and conical tubular portions which
 are interconnected using trusses of tubular members at the base and mid
 depth, and a plan frame (shown in FIG. 4) at well deck level. A deck (not
 shown) is rigidly connected to the tops of the columns. The trusses occupy
 only very limited vertical distances with respect to the total height of
 the columns.
 Adding box girders (as shown in FIGS. 1 and 2) in place of trusses could
 reduce heave (in part by added mass). Box girders are useful in reducing
 surge in spite of their deep sections. This can also be explained by the
 added inertia effect.
 Adding a keel plate increases natural heave period. Added mass and drag of
 the keel plate supply damping. Keel plates at the other truss girder
 levels are less effective.
 The largest heave excitation component in the critical 4-18 s range is the
 upper conical portion C'D'. This is closest to the sea surface and hence
 is subject to the highest water particle velocities. For longer periods,
 the lower conical portion A'B' imparts a larger force, as it has a greater
 diameter difference, and the waves take effect deeper in the water.
 Mooring lines damp both heave and surge.
 The rigid well risers extend up from the seabed tie-back to the keel K' of
 the substructure where they are laterally guided before continuing up to
 the well deck. The risers are also laterally guided at mid depth within
 the substructure and at well deck level. Optionally, buoyancy cells are
 attached to the risers below the keel plan guide until there is sufficient
 support to develop tension stabilisation. Well tensioners are provided at
 the well deck level to stabilise the risers from seabed to deck. The BOP
 is mounted on top of a dry tree on the well deck.
 Development of the field begins with construction of wells below the seabed
 using a Mobile Offshore Drilling Unit (or MODU). The MODU drills a mudline
 suspended well that will be tied back to the deck facility of the platform
 using a well completion rig. The well construction programme will be
 performed in two stages: drilling before platform installation; and
 drilling after production (from facilities on the platform).
 The second stage of the well construction programme may be several years
 after first oil, depending on the field development plan. Following well
 construction below seabed, the same MODU (or a DSV) can install suction
 piles and deploy mooring lines on the seabed prior to installation of the
 substructure.
 Once the substructure arrives on location it is upended and soft moored,
 and permanent ballast is added to the keel. The substructure is then
 ballasted to mating draft. The deck (not shown) is floated over the
 substructure, and the substructure is deballasted to mate with and lift
 the deck to operating draft. Chain jacks tighten the mooring lines over a
 specific well location.
 A third embodiment of the invention will now be described in more detail
 with reference to FIGS. 5 to 27.
 FIGS. 5 to 7 show the general arrangement of a substructure for an offshore
 platform. The substructure has three floatation columns 101, 102 and 103.
 For some topside configurations, a forth column may be necessary. The
 columns are arranged in spaced apart side by side relationships, with
 their axes parallel to each other. The columns are externally identical to
 each other.
 Each column has a surface piercing portion 104, a submerged buoyancy
 portion 105, and a ballastable portion 106. These portions 104, 105 and
 106 are cylindrical. At the foot of the ballastable portion 106 there is a
 permanent ballast compartment 107. The cylindrical portions 104, 105 and
 106 are joined by conical transition portions 108 and 109.
 The three columns 101, 102 and 103 are joined at four levels by cross
 members comprising tubular members and box girders.
 Above sea level the columns are joined by horizontal tubular members 111.
 These tubulars support between them a guide plate 112 (FIG. 8), through
 which risers can pass up from the seabed to processing facilities on the
 deck.
 At the tops of the ballastable portions 106 there are box girders 113,
 supporting guide framing 114 (FIG. 9). The box girders 113 are free
 flooding, so that the effects of hydrostatic and hydrodynamic pressures
 can be ignored.
 Half way down the ballastable portions 106 there are further free flooding
 box girders 115, supporting another guide plate 116 (FIG. 10).
 Connecting the permanent ballast compartments 107 there is a third set of
 box girders 117. Further guide framing 118 is supported by these box
 girders 117 to form a keel plate (FIG. 11).
 The permanent ballast compartments 107 are designed to contain iron ore,
 and have appropriate flooding and venting valves. Ballast is pumped in
 from the surface, with the vent valves open. The ballast can be removed by
 air assisted lift.
 It is a feature of the substructure that the flotation columns are spaced
 apart with members that occupy minimal vertical distance, as compared with
 the full depth of the columns. Moreover, where the columns are at their
 widest (at the buoyancy portion 105) the columns are not joined at all.
 Details of the interior of an upper part of column 101 are shown in FIG.
 12. The column encloses a lift 121, a ladder space 122, ventilation
 ducting 123, chain locker pipes 124 and mechanical systems ducting 125.
 The upper part of the column is not designed to withstand high hydrostatic
 pressure.
 The arrangements for stiffening the interior of the columns are a
 significant factor in saving steel weight in the completed substructure.
 Details of internal stiffening for one of the columns are shown in FIGS.
 13A and 13B. The surface piercing portion 104 has ring stiffeners 131
 fixed to the inner cylindrical surface of the shell. The conical
 transition is a shell of thicker material. The buoyancy portion 105 has
 outer shell stiffeners 132 and inner shell stiffeners 133. Water tight
 bulk heads 134 divide the space between inner and outer shells into
 watertight compartments. The inner shell defines a central access tube
 135. Design of the shells and stiffeners is closely controlled to suit the
 hydrostatic pressure expected to be experienced by the parts of the
 columns at that particular depth. Illustrations of the design head
 .DELTA.p are shown alongside the sections of the columns. Two types of
 bulkheads are envisaged for the columns, soft and hard. The hard bulkheads
 are located at the keel elevation and at the bottom of the buoyancy
 column.
 The main compartmentation used for upending and deck mating operations is
 contained in the wide bodied portion 105 and the lower transition cone 109
 of each column.
 Individual compartments are provided with internal and/or external flood
 and venting valves. External valves, on the lowest compartments, are
 actuated by a ROV and enable each compartment to be completely flooded
 individually.
 The wide bodied compartments are manifolded within the central access tube
 135. The manifold is connected to a high pressure ballast pump which is
 used to deballast each compartment individually. A separate internal vent
 line allows the ingress of air into a specific compartment whilst
 deballasting is in progress. All of these compartments have access manways
 to allow periodic inspection and/or maintenance as required. The ballast
 system has twin ballast pumps for redundancy purposes.
 The use of three small diameter columns (as compared with a single large
 diameter hull for a conventional spar) makes the horizontal flats smaller,
 makes the columns easier to stiffen, and so results in a lighter
 substructure for the same buoyancy as a conventional spar.
 A semi taut mooring system, consisting of six lines in three pairs,
 restrains the platform on location. This mooring system can best be seen
 in FIGS. 5 and 29. One particular mooring system is described below. The
 configuration of the mooring system is highly dependent on water depth and
 other factors.
 Each mooring line consists of a chain-wire-chain combination. A chain
 segment 141 starts at deck level and passes through fairleaders 142 and
 143 on the column. The chain 141 then joins a wire 144. Each mooring line
 is anchored to the seabed at 146 with a chain segment 145 that connects to
 the remote end of the wire 144. The wire is a spiral steel strand
 consisting of class A galvanised wires ranging from 4 to 7 mm diameter
 resulting in satisfactory breaking strength, tension fatigue, and axial
 stiffness. The "ordinary rig quality" chain has the same or greater
 minimum breaking load as the wire. (Synthetic mooring ropes would be
 lighter in many circumstances.)
 The mooring anchorages 146 may consist of the following, depending on
 bottom soil conditions; drag enbedment anchors, clump weights, or drilled
 and grouted or suction piles.
 Due to the fairly taut catenary mooring arrangement selected, a tight watch
 circle can be achieved thus reducing loads upon the risers. The mooring
 lines are attached to the centre of gravity of the hull to minimise the
 pitch response.
 Having described the features of the buoyant substructure, the method of
 fabrication, loadout, transportation and installation will now be
 outlined.
 Fabrication
 Individual column cans are rolled and welded in a workshop. The cans are
 laid on roller beds and the long seams welded by submerged arc welding.
 Ring stiffeners are installed with the can in the horizontal or vertical
 position depending on the fabricator's preference. The ring stiffeners'
 sizes and locations will be optimised to take advantage of the
 fabricator/suppliers preferred plate widths. Individual stiffened can
 sub-assemblies are aligned with each other on the roller beds to assure
 `out of roundness` effects are minimised. The cans are then submerged arc
 welded into larger sub-assemblies and transported to the assembly site.
 There are three basic assembly techniques:
 Bent Assembly
 The substructure is assembled on it's side and parallel to a load out quay.
 The two `ground level` columns are positioned adjacent to each other, and
 interconnecting truss members are installed and welded out to form the
 first bent. The `mid-air` column also is assembled adjacent to this bent
 and the inter-connecting members installed, but only the welds at the
 `mid-air` column are completed. This partially completed bent is then
 lifted and rotated into position using a bank of cranes. Temporary
 supports are installed to allow the removal of the cranes. The remaining
 inter-connecting members are then installed and welded out.
 Toast Rack Assembly
 The substructure is divided into discrete sub-assemblies through the
 transverse direction, thus the more complex areas (i.e. the
 inter-connecting members) can be easily fabricated. Depending on the
 fabricator's facility, each sub-assembly may be completed in it's entirety
 in the fabrication workshop and moved to the assembly area. This method
 reduces the volume of `height` work required at the assembly area and
 requires fewer crane resources than the bent assembly method.
 Jackup Assembly
 The columns are assembled and arranged parallel to each other on he ground.
 The "mid-air" column is erected using jacking towers. Alignment frames
 would be erected to support the interconnecting members prior to welding
 out. This method reduces crane requirements and provides dimensional
 control for assembly of the frame.
 Loadout
 There are three possible loadout methods:
 Lifted
 The substructure is assembled adjacent and parallel to the loadout quay. A
 combination of land-based and two floating shear leg cranes lifts the
 substructure off it's supports. The cranes `walk` the substructure to the
 water's edge and lower it into the water. Depending on the unballasted
 floating condition, some ballast may be pumped into the substructure to
 ensure an acceptable floating condition before all the cranes are
 disconnected.
 Sideways Launch
 The substructure is assembled adjacent and parallel to the water's edge and
 is built on slipways. On completion, it is pulled and/or jacked into the
 water. Suitable ballast is added at the appropriate stage during loadout
 to assure an acceptable floating and tow condition.
 Slipway Launch
 The substructure is assembled on the slipway using the Bent or Toast rack
 method. On completion, it is launched down the slipway.
 Transportation
 The self floating substructure is transported horizontally on two columns
 that have a shallow draft. In this position, the longitudinal and
 transverse stability are excellent. Manoeuvring inshore requires two tugs,
 and offshore tow uses a single tug. Towing the substructure to site on two
 legs enables the transit to be made in a stable configuration without
 large amounts of ballast. Towing in a fully assembled condition avoids the
 need to connect up discrete lengths of hull in open water (as has been
 done with conventional spars).
 Alternatively the substructure can be transported on a submersible
 transportation vessel 151, as shown in FIGS. 14-16. The completed
 substructure is loaded directly onto vessel 151; or can be floated out
 from its construction site, and then loaded by ballasting and then
 deballasting the vessel 151 underneath it. Staging 152 to 156 carries the
 substructure on the vessel. Conveniently staging 152 supports the columns
 close to box girder 115; staging 153 supports the columns close to box
 girder 113, and staging 154 and 155 supports the conical ends of column
 portions 105.
 Installation
 There are four major installation phases:
 Mooring Line Installation
 This phase in itself consists of two steps i.e. installation of the seabed
 anchor and installation of the mooring line respectively.
 The seabed anchor may consist of any one of several types, e.g. Bruce or
 Stevpris anchors, suction and/or drilled and grouted or driven piles, or
 gravity boxes. Selection of the preferred anchorage is a function of
 seabed soil conditions, line loads and installation vessel requirements.
 To shorten the overall installation programme, it is advantageous to
 pre-lay the mooring lines. These operations can be performed using two
 large anchor handling vessels suitably equipped with heavy handling gear
 and winches. The mooring lines comprise a large diameter wire and chain
 combination. (Note: It is assumed that the deployment of synthetic mooring
 ropes will require less robust equipment and possibly fewer vessels,
 although greater care would have to be exercised throughout the whole
 handling operation). Depending on the water depth, the mooring line may
 consist of several components which may have to be joined on the vessel
 deck. The `anchor` is lowered from the stern of the vessel and the mooring
 line is progressively paid out during this process. In some circumstances,
 the mooring line may be used to lower the anchor thus eliminating the need
 for a separate lowering system.
 As the line is paid out, the remaining components are shackled up. At a
 pre-determined point, a second vessel may be required to assist in
 supporting the paid out portion of mooring line if the lowering loads
 become excessive. This lowering operation continues until the anchor
 reaches the seabed. The vessels can then move in the laydown direction
 whilst spooling the mooring line onto the seabed.
 Substructure Installation
 The substructure is wet towed to the field and is upended. (Alternatively
 dry transportation may be employed, as shown in FIGS. 14-16.)
 The substructure is upended by differential ballasting using controlled
 flooding of the lower parts of the columns. It will be necessary to pump
 flood the column 103 which is up in the air. In common with conventional
 spars, it is expected that initial changes in trim would be very gradual,
 but that there would then be a rapid rotation to the near vertical. To
 avoid hydrostatic collapse, all soft tanks must be fully flooded prior to
 this rotation. The loads on the substructure during upending are quite
 likely to be the largest in its life. Upending is shown in FIGS. 17-25.
 The design head diagram (on FIGS. 13A and 13B) shows that the columns below
 box girder 113 are governed by the Upending condition. During upending
 columns 101 and 102 (which are floating on the sea surface prior to
 upending) are subjected to a relatively low hydrostatic head, while column
 103 is subject to a substantially greater hydrostatic head. The
 controlling design head for the columns above box girder 113 is the float
 over condition. The internal bulkheads and inner column design heads are
 governed by the 100 year return damage condition above conical portion
 108.
 The substructure is temporarily moored, and permanent iron ore ballast is
 added while it is at low draft. When the substructure is upright and
 ballasted down to its installation draft, it is then towed to the final
 location. The taut wire moorings are then retrieved and connected via
 fairleaders to the mooring points. The mooring lines are tensioned
 sufficiently to constrain excessive platform movement.
 Deck Installation
 The substructure is further ballasted down to the required deck
 installation (mating) draft by additional ballasting (FIG. 26) and the
 mooring lines re-tensioned to compensate for the increase in draft.
 Floatover or direct lift may be used to place the deck. (Heavier decks
 will require float over mating.) The deck is brought over the substructure
 and the mating operation is completed by deballasting the hull thereby
 decreasing the draft. The deck is integrated with the substructure, so
 reducing hull steel requirements. When the deck transport is removed, the
 mooring lines are tensioned to their operational tensions (FIG. 27).
 (Alternatively, the substructure may be upended and deck installation may
 be carried out in sheltered deepwater (e.g. in a fjord), so that the
 completed platform (substructure and topsides) can be towed to its
 intended location in a fully assembled condition.)
 Riser Installation
 The installation of individual well risers 147 is achieved by lowering a
 riser string down through the interior of the substructure. To ensure the
 end of the riser passes through the lower inter-connecting truss frames, a
 messenger line is pre-installed through all riser guides. This messenger
 line is attached to the riser end. As the riser is lowered, the messenger
 line is tensioned thus controlling the lateral movement of the end. When
 the riser is pulled through the lowest guide, the messenger line is
 disconnected by an ROV. The well riser casing is lowered to the seabed and
 latched to the seabed wellhead by the ROV. An export riser 148 leads away
 from the base of the hull.
 The substructure is adapted to being used in different water depths, with
 the same draft but changed displacement. In deeper water (with longer and
 so heavier risers) less iron ore ballast would be specified.
 The substructure is suitable for medium water depths i.e. 120 m to 300 m
 with flexible well risers. The minimum water depth for rigid well risers
 is approximately 300 m.
 The arrangements shown in FIGS. 28 to 32 control the vertical reaction
 point of individual mooring lines, thereby adjusting the line of action of
 the mooring forces.
 FIG. 28 shows the consequence of moving the effective mooring attachment
 point in a vertical direction. An elongate buoyant substructure 200 has a
 deck 201, and floats in a stable configuration with respect to a sea
 surface 202. The substructure has a mooring line 204, attached to the
 substructure by a bridle 203 having upper and lower elements 205 and 206.
 Vertically movable fairleaders 207 and 208 connect the bridle 203 to the
 substructure. (Similar reference numerals will be used in FIGS. 29 to 32.)
 FIG. 29 shows details of one particular arrangement to control the
 effective vertical attachment point of the mooring line. The main mooring
 line is attached to a bridle. Each element of the bridle is connected to a
 fairleader and routed up to a winch (211, 212) located above the sea
 surface. Each element of the bridle can therefore be controlled
 individually, and the attachment point for the main mooring line can be
 shifted in a vertical direction, thereby changing the vertical location of
 the restoring force produced by the mooring.
 The effect of moving the vertical attachment point on opposite mooring
 lines is illustrated in FIG. 30. The mooring lines can be adjusted to
 create a couple which is equal and opposite to the wind overturning
 moment, although both lines are attached well below the level at which the
 wind load is applied. Moments introduced due to weight changes/shifts in
 deck loads can also be corrected for by adjusting the effective vertical
 attachment point of opposite lines until an equal and opposite moment is
 produced.
 An alternative to routing both bridle lines up to the deck is to use a
 continuous loop 214 between two fairleaders as illustrated in FIG. 31. One
 of the fairleaders is powered (for instance with electrical or hydraulic
 power 215) such that part of the closed loop is moved up or down thereby
 shifting the attachment point for the main mooring line in the vertical
 direction.
 One area of interest for the bridle and continuous loop arrangement (in
 FIG. 31) is the hook-up of the main mooring line to the bridle/continuous
 loop. Part of the system is the chain stopper 216 arrangement illustrated
 in FIG. 32. The main mooring line consists of a chain section at the top
 end. This chain is pulled through a ratchet type chain stopper attached to
 the bridle or continuous loop to for instance a work boat on the surface.
 Once the correct pretension is set, the excess chain can be cut by diver
 or remote operated vehicle.
 Advantages of the Invention
 In summary the buoyant substructure described above shows the following
 advantageous features:
 Dry wellheads, thus minimising subsea activities
 Compatible with flexible or rigid risers, dependent on water depth
 Rigid export risers possible in deepwater for particularly aggressive
 fluids
 Well workover from platform deck
 Significant steel weight savings
 Minimum extreme storm condition motion response
 Easy fabrication, loadout, transport, installation and maintenance