GB2346842A - Floating substructure with ballasting system - Google Patents

Abstract

A system for controlling the draft of a DDF substructure, the substructure being adapted to support the upper end of at least one riser (120) extending from close to the seabed to above the sea surface, and having provision for moveable ballast to adjust its draft; the system comprising means to introduce ballast in order to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded with no risers installed, and means to expel ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of ballast counteracts this tendency, thus to ensure that the allowable operational pitch angle is not exceeded.

Description

FLOATING SUBSTRUCTURE WITH BALLASTING SYSTEM The invention relates to a system for controlling the draft and attitude of a deep draft floating substructure for an offshore platform.

Deep draft floating substructures have been designed to carry topsides for offshore platforms. These substructures may be of the form of a"spar", such as that described in OTC Papers 8384 and 8385. The substructures may also be of the type shown in our UK Patent Specification No. 2, 147, 546A, which describes and illustrates a multi column floating substructure. In particular, the substructures may be of the kind described and illustrated in our PCT Application W099/10230. For convenience, these Deep Draft Floating substructures will be referred to herein as a"DDF substructure", or more simply as a"substructure".

The topsides may comprise drilling and/or production facilities. Oil and/or gas is brought up from the seabed to these facilities through pipelines extending from the seabed to above the sea surface. These pipelines are known as"risers". The risers are conventionally made of steel. They are able to expand and contract by only very small amounts in response to tension loads.

A DDF substructure has to support the topsides and the risers. In deep water, the weight of the risers can form a substantial proportion of the total weight.

The draft and attitude (e. g. pitch) of a DDF substructure are thus dependent on the topside load, substructure weight, ballast weight, mooring loads and riser tension; and on the buoyancy of the part of the substructure below its watenine.

The invention is concerned with controlling the draft of a DDF substructure in order to keep it at a stable draft with a suitable pitch rotational stiffness. This is necessary so that the allowable operational pitch angle is not exceeded.

The invention provides a system for controlling the draft of a DDF substructure, the substructure being adapted to support the upper end of at least one riser extending from close to the seabed to above the sea surface, and having provision for moveable ballast to adjust its draft; the system comprising means to introduce ballast in order to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded with no risers installed, and means to expel ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of ballast counteracts this tendency, thus to ensure that the allowable operational pitch angle is not exceeded.

It is preferred that at least some of the ballast is of solid material, and there is means whereby the solid ballast can be introduced or removed to control the draft of the substructure.

It is further preferred that the means to remove the solid ballast comprises equipment to process the solid ballast into a slurry, and a pump to pump the slurry out of the substructure.

It is still further preferred that there is provision to pump the slurry into a workboat adjacent to the substructure.

The pump to pump the slurry out of the substructure may be used to deballast the substructure for re-upending and subsequent removal.

The invention includes a DDF substructure for an offshore platform, incorporating a system as described above.

The invention also includes a method of controlling the draft of a DDF substructure, the substructure being adapted to support the upper end of at least one riser extending from close to the seabed to above the sea surface, and having provision for moveable ballast to adjust its draft; comprising the steps of introducing ballast to the substructure as the substructure is first installed, and then removing ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of ballast counteracts this tendency, thus to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan of a floating offshore platform, (with its deck and topsides removed); Figure 2 is a side elevation on line 11-II in Figure 1; Figure 3 is a side elevation on line III-III in Figure 1 ; Figure 3A is a corresponding diagram illustrating metacentric height (GM); Figure 4 is a more detailed side elevation of a DDF substructure for the platform shown in Figures 1 to 3, (and corresponding generally to Figure 2); Figure 5 is a plan on level V in Figure 4; Figure 6 is a plan on level VI in Figure 4; Figure 7 is a plan on level Vil in Figure 4; and Figure 8 is a plan on level VIII in Figure 4.

This description will deal in tum with the substructure configuration ; the ballasting provisions; the mooring arrangements ; and the risers.

Figures 1 to 3 show the configuration of a DDF substructure for an offshore platform.

The substructure has three floatation columns 101,102 and 103. (For some topside layouts, a forth column may be necessary.) The columns are disposed in a spaced apart side by side relationship, with their axes parallel to each other. The columns are externally identical to each other.

The use of three small diameter columns (as compared with a single large diameter hull for a conventional spar) makes the horizontal flats smaller, and makes the columns easier to stiffen. This results in the DDF substructure being lighter than a conventional spar for the same buoyancy 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 compartment 107 for solid ballast. The cylindrical portions 104,105 and 106 are joined by conical transition portions 108 and 109.

The three floatation columns 101, 102 and 103 are joined at four levels by cross members comprising tubular members and box girders.

It is a feature of the DDF substructure that the columns are spaced apart with cross members that occupy minimal vertical distance, as compared with the full depth of the columns. (In order to reduce the wall thicknesses of the lower conical transition portions 109, the first box girders below the waterline are set at the lower ends of buoyancy portions 105.) Above sea level the columns are joined by horizontal tubular members 111. These tubular members support between them a (optional) guide plate 112 (Figure 5), through which risers can pass up from the seabed to processing facilities on the deck.

At the lower ends of the buoyancy portions 105 there are box girders 113, which support guide framing 114 (Figure 6). The box girders 113 are free flooding, so that the effects of hydrostatic pressure can be ignored.

Half way down the ballastable portions 106 there are further free flooding box girders 115, which support another (optional) guide plate 116 (Figure 7).

Connecting the solid 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 8).

The DDF substructure has individual columns extending over its whole length. The stability of the substructure in the installed condition depends primarily on a combination of buoyancy in the upper portions; solid ballast at the base; and riser tension. The columns have relatively large diameters in their upper portions to provide the required buoyancy, but are waisted at the water line, to define the water plane area and hence the natural period. The well and export risers are not shielded, so providing wave zone transparency to wave force excitation.

Wave zone transparency is a characteristic of the DDF substructure configuration. The reduced wave loading excitation reduces the oscillatory motion response of the substructure.

The reduced motion in waves reduces the mooring line loads and offset.

Further characteristics of the configuration lie in details of the columns. The variable diameter column is constructed to reduce substructure steel weight and minimise hydrodynamic loading. The columns do not have longitudinal stiffening. Instead they have thicker shells and ring stiffening to resist the hydrostatic pressure and intemal forces. The tank like construction is utilise in the outer column sections and solid ballast compartments. The inner column structure, ballaste portions, tubular bracing and riser guides conform to jacket fabrication practice. The fabricated box girders conform to standard Highway Bridge practice.

The diameter of the surface piercing portion 104 is sized to ensure that the natural heave period exceeds the longest anticipated wave periodicity. In addition, the ratio of the diameter of the surface piercing portion 104 to the diameter of the buoyancy portion 105 is greater than 70% to reduce heave excitation in the wave zone. The surface piercing portions 104 constitute the only hydroelastic element for the DDF substructure, as the vertical mooring stiffness is negligible for the semi taut chain/wire strand/chain lines.

The ballasting provisions are described below.

The solid ballast compartments 107 are designed to contain heavy aggregates such as 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.

Following the invention, ballast can be removed as the risers are added to the platform.

The effect of removing ballast during the riser installation programme is to maintain a pitch offset stiffness. This improves pitch response.

The main water ballast compartmentation is used for upending and deck mating operations. This compartmentation is contained in the wide bodied buoyancy portion 105 and the lower transition cone 109 of each column.

Individual compartments are provided with intemal and/or extemal flood and venting valves. Extemal valves, on the lowest compartments, are actuated by a ROV and enable each compartment to be flooded individually.

The wide bodied compartments are manifolded within a central access tube. The manifold is connected to a high pressure ballast pump which is used to deballast each compartment individually. A separate intemal 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 as a precaution against pump failure.

Features of the invention relate to offshore installation and removal of iron ore. The iron ore can be transferred from a material barge to the solid ballast compartments 107 using conventional slurry pump technology from the mining and dredge industry. Similarly, the iron ore can be transferred back from the compartments 107 up to a material barge. The recovery of the iron ore ballast and subsequent use of pumps to remove water ballast can upend the DDF substructure on the sea surface in a horizontal attitude. This would allow reuse of the DDF substructure at another location.

The mooring arrangements are described below.

The semi taut mooring system has three grouped lines secured to each column. These lines restrain the platform on location. The particular mooring system in this example can best be seen in Figures 1 to 3. This mooring pattern has been chosen for two reasons; it reduces the mooring footprint on the seafloor, and simplifies offshore installation as construction vessels need only relocate twice. The arrangement of the mooring system is highly dependent on water depth and other factors. The particular mooring system in the present example is described in detail below.

Each mooring line consists of a chain/wire strand/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 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 7mm 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.) Mooring anchorages may consist of drag embedment anchors, clump weights, or drilled and grouted or suction piles, depending on bottom soil conditions.

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 substructure at positions to minimise the pitch response.

The methods of fabrication, loadout, transportation and installation of the DDF substructure have been outlined in our PCT Application W099/10230.

The DDF substructure supports multiple well production risers.

These risers connect subsea marine wellheads to surface trees on the deck. The configuration of the DDF substructure is such that it allows an array of well risers 120 to run from the seabed through the center of the substructure to connect to the topside facility (as shown on Figure 3A). The wells could be drilled from the platform, or, optionally, could be predrilled and then tied back to the platform. The tie back option requires only a small platform rig (in terms of weight and wind area) which is removable.

The well design pressure in each riser is resisted by a Christmas tree, casings and flow line. The casings and flow line use threaded connectors for construction from the well completion rig on the deck. The Blow Out Preventer is located on deck below the drilling rig during the tie back and well completion work, or during wireline maintenance of the well.

A bending restrictor consists of an extemal tubular sleeve bonded to the outer casing of the riser. Tapered ends of the bending restrictor permit deployment or removal and limit stress concentration at the end of the restrictor. Low friction surface coating is bonded to the restrictor at all guide levels to eliminate wear.

The stress reducer or flex joint is an integral part of the pressure resisting outer casing and tie-back connector at the seabed wellhead and at the keel plate. Alternative technologies for this joint are available. Typically a titanium element is designed for this service due to reduced elastic modulus that provides the flexibility necessary to reduce stress through rotational deflection of the outer casing of the riser. Titanium also possesses high tensile strength sufficient to resist the well design pressure rating.

The top tensioner load ring is a clamp that transfers the top tension restoring force from hydraulic cylinders to the outer casing of the riser.

The present invention is concerned with Draft and GM management The operating draft of the DDF substructure is controlled by pumping water ballast contained in the outer ballast compartments. The variable deck load consists of consumables, oil production, and equipment for well rig operations and riser installation. Ballast controls are manually operated from the deck.

Ballast in the DDF substructure, particularly the iron ore ballast in compartments 107, is used to ensure the vessel has a high metacentric height (GM). GM is illustrated in figure 3A.

The GM is directly proportional to the pitch/roll inclination due to steady state wind and current.

The total pitch/roll inclination is the summation of steady state inclination and the oscillatory pitch due to wave and wind. By reducing the steady state inclination it is possible to reduce the total pitch/roll inclination of the DDF substructure during the extreme 100 year retum hurricane.

The riser installation programme, water ballast and iron ore ballast are co-ordinated to optimise stability. The restoring force of the riser pre-tension acts at the keel due to the riser guide 118 at that elevation. This means that the larger the pre-tension on the riser, the better the GM and the less pitch inclination.

However, if the risers are not installed, then the GM is reduced. Hence, the GM must be large enough to accommodate the initial riser arrangement. Ore and/or water ballast is placed in the ballast compartments 107 and lower columns 106, with the ore being as low as possible.

For an optimum design, the bottle sections will have very little or no ballast water contained within them when the design topside payload and all risers are connected. The riser installation programme could include the early installation of tie backs on half of the wells.

The association of ballast removal techniques with the riser installation programme could reduce the DDF substructure steel weight by as much as 10%.

Advantageously, use of the invention would maintain the GM (defined on Figure 3A) at greater than 30 feet.

The variable ballast quantity is equal to the total riser top tension and would be added during initial installation. The iron ore would be pumped back into a workboat from the compartments 107 as a slurry during hookup and commissioning of the wells.

For Decommissioning and Removal, the installation process would reversed. This procedure would begin with the plugging of the wells and removal of the tie back risers. The deck facility cranes would remove the well completion rig to transport barges. The deck facility would be lifted or floated off. The iron ore would be pumped from the compartments 107 to the surface and deposited in a material barge.

The reduced draft could expose the chain fairleaders 142 for disconnecting the mooring wire strands and reeling them on to a barge. The DDF substructure would be upended to float on the surface using the iron ore ballast removal pumps and the vent lines in the water ballast compartments.

The Deep Draft Floater concept for deep water fields is similar in cost and schedule efficiency to a typical GOM jacket developed for shallow water locations.

The DDF substructure described above by way of example satisfies the following design objectives :- it is capable of being transported wet or dry using existing submersible ships; it uses easily fabricated shell and ring stiffeners; and the low towout draft means that it can be built in most regional jacket or ship yards. There is a simple mooring system; easy access and inspection provides verifiable reliability ; and the possibility of reuse extends the financial value.

In summary the DDF substructure shows the following advantageous features. It has dry wellheads, thus minimising subsea activities; it is compatible with flexible or rigid risers, dependent on water depth; rigid export risers are possible in deepwater for particularly aggressive fluids ; there are significant steel weight savings; and extreme storm condition motion response is reduced.

Claims (1)

1. Claims 1/A system for controlling the draft of a DDF substructure, the substructure being adapted to support the upper end of at least one riser extending from close to the seabed to above the sea surface, and having provision for moveable ballast to adjust its draft; the system comprising means to introduce ballast in order to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded with no risers installed, and means to expel ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of ballast counteracts this tendency, thus to ensure that the allowable operational pitch angle is not exceeded.

   21 A system as claimed in claim 1 in which at least some of the ballast is of solid material, and there is means whereby the solid ballast can be introduced or removed to control the draft of the substructure.

   3/A system as claimed in claim 2 in which the means to remove the solid ballast comprises equipment to process the solid ballast into a slurry, and a pump to pump the slurry out of the substructure.

   41 A system as claimed in claim 3 in which there is provision to pump the slurry into a workboat adjacent to the substructure.

   5/A system as claimed in claim 3 or claim 4 in which the pump to pump the slurry out of the substructure can be used to deballast the substructure for re-upending and subsequent removal.

   6/A DDF substructure for an offshore platform, incorporating a system as claimed in any one of the preceding claims.

   7/A DDF substructure substantially as hereinbefore described by way of example with reference to and as shown in the accompanying drawing.

   8/A method of controlling the draft of a DDF substructure, the substructure being adapted to support the upper end of at least one riser extending from close to the seabed to above the sea surface, and having provision for moveable ballast to adjust its draft; comprising the steps of introducing ballast to the substructure as the substructure is first installed, and then removing ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of ballast counteracts this tendency, thus to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded.

   Amendments to the claims have been filed as follows 1/A system for controlling the draft of a DDF substructure, the substructure including means to connect with and apply tension to support the upper end of at least one riser extending from close to the seabed to above the sea surface, and the substructure having provision for moveable ballast to adjust its draft ; the system comprising means to introduce ballast during initial installation of the substructure in order to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded with no risers installed, and means to remove ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of at least some of the ballast introduced during initial installation counteracts this tendency, thus to ensure that the allowable operational pitch angle is not exceeded as the riser (or risers) are connected up and tensioned.

   2/A system as claimed in claim 1 in which at least some of the ballast is of solid particulate material, and there is means whereby the solid particulate ballast can be introduced or removed to control the draft of the substructure.

   3/A system as claimed in claim 2 in which the means to remove the solid particulate ballast comprises equipment to process the solid particulate ballast into a slurry, and a pump to pump the slurry out of the substructure.

   4/A system as claimed in claim 3 in which there is provision to pump the slurry into a workboat adjacent to the substructure.

   5/A system as claimed in claim 3 or claim 4 in which the pump to pump the slurry out of the substructure can be used to deballast the substructure for re-upending and subsequent removal.

   6/A DDF substructure for an offshore platform, incorporating a system as claimed in any one of the preceding claims.

   7/A DDF substructure substantially as hereinbefore described by way of example with reference to and as shown in the accompanying drawings.

   8/A method of controlling the draft of a DDF substructure, the substructure including means to connect with and apply tension to support the upper end of at least one riser extending from close to the seabed to above the sea surface, and the substructure having provision for moveable ballast to adjust its draft; comprising the steps of introducing ballast to the substructure as the substructure is first installed, and then removing ballast when a riser is connected up and tensioned, whereby, as the tension of the riser (or risers) tends to increase the draft of the substructure, removal of ballast counteracts this tendency, thus to keep the substructure at a stable draft with a suitable pitch rotational stiffness so that the allowable operational pitch angle is not exceeded.