GB2187778A - Offshore platform with two jacket portions - Google Patents

Abstract

A support structure to form part of an offshore platform, and comprising a template (23) on the seabed, having a portion (24) through which subsea wells may be drilled, and sequentially installed first and second jacket portions (21 and 22 respectively), each fixed to the template (23) at their respective bases, and arranged to be joined by a further portion (25) at or above the height of the highest expected wave. <IMAGE>

Description

SPECIFICATION Offshore structures The invention relates to the arrangement of a support structure for an offshore platform.

An in-house development exercise has established the technical feasibility and economic viability of using two lift-installed fourleg jackets to support a module support frame (MSF) and topsides for an offshore platform (instead of utilising a single large eight-leg barge-launched jacket). The twin jacket concept is shown in Fig. 1.

The twin lift-installed jackets together support the MSF and topsides arrangements and thus represent a direct substitution for the single eight-leg barge-launched jacket. Each liftinstalled jacket could be designed for the controlling lift capacity of a Balder/Hermod class Heavy Lift Vessel (HLV), and would have a weight similar to that envisaged for a single MSF lift.

While this can be considered a radical departure from current jacket concepts, it remains within the context of a single platform offshore development. It utilises the HLV capacities already intended for the MSF and topsides, and it does not introduce any new engineering or operational requirements. Furthermore, it is not without precedent. Multi-jacket supports for single decks have been used in a number of shallow water developments in the Middle East. A similar twin jacket concept was also used for several platforms in the 70m-80m depths of the Bass Straits, offshore Australia. A twin jacket concept for deep water applications is shown in UK Patent Specifications 2133447.

The present invention is concerned with twin jackets which are joined at their bases by a single seabed template which may be utilised for drilling oil or gas wells to be operated from the platform.

The invention provides a support structure to form part of an offshore platform, and comprising a template on the seabed through which subsea wells may be drilled, and first and second jacket portions, each fixed to the template at their respective bases, and arranged to be joined by a further portion at or above the height of the highest expected wave.

In one form it is preferred that adjacent facing panels of the jacket portions are vertical.

In this form it is preferred that the template is used to space apart the bases of the jacket portions.

In another form it is preferred that adjacent facing panels of the jacket portions are battered away from each other as they rise from the seabed.

In this other form it is preferred that the template is used to provide a common foundation point for the bases of the jacket portions.

The invention also provides a method of constructing an offshore platform, comprising the steps of founding a template on the seabed, docking a first jacket portion with that template, docking a second jacket portion with that same template, and joining the tower portions at a level at or above the height of the highest expected wave.

Advantages offered by the twin lift-installed jacket concept include: O Reduced fabrication weight, by 40%; O Reduced complexity of fabrication and erection because of reduced overall dimensions and reduced size of major structural elements such as legs, pile clusters and main nodes; O Reduced fabrication programme, from two years to one year, which would allow 100% design maturity, and deferred capital expenditure; O Reduced offshore spread, from one expensive H1 10/H1 14 class launch barge to two relatively inexpensive cargo barges; O Greater case of operational inspection and maintenance, with clear access between the two jackets via a platform-based diving spread, and direct external access to all major nodes, a majority of caissons and risers, etc.;; O Greater flexibility and economy for built-in topsides weight growth margins because of direct load paths from deck to piles; A more detailed description of the present twin lift-installed jacket concept, its behaviour and its structural, foundations and installation requirements is presented below.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is an isometric view of an offshore platform comprising two iift-installed four-leg jackets joined at their tops by a module support frame (MSF), and at their bases by a subsea template; Figure 2 is a side elevation of the jackets shown in Fig. 1.; Figure 3 is a side elevation corresponding to Fig. 2, but showing another embodiment of the invention; Figure 4 is a side elevation of the MSF shown in Fig. 1 (with the addition of a transportation brace); Figure 5 is a side elevation of an alternative deck having a two piece MSF with an offshore welded field joint; Figure 6 is a side elevation of another alternative deck having a two piece MSF, with a pinned link between the two pieces;; Figure 7 is a diagrammatic view of a jacking system for jacket levelling; and Figures 8 to 19 are sketches of successive stages in the installation of the jackets.

As shown in Fig. 1, an offshore platform has twin jacket portions 21 and 22, which are joined at their respective bases by a template 23 having a well receptacle portion 24. The tops of the jacket portions 21 and 22 are joined by a module support frame (MSF) 25.

(Variants of the MSF are shown in Figs. 4 to 6.) The MSF supports modularised topsides 26 surmounted by a drilling rig 27.

Each of the joint portions 21 and 22 have four main legs 31, which terminate at their bases in pile sleeves 32.

The lift weight for the two jackets 21 and 22, including all appurtenances and slings, is expected to be 10500 tonnes. Compared to an estimate for the launch weight of an eightleg barge-launched jacket at 18300 tonnes, this shows a fabrication saving of 7800 tonnes.

For a general introduction to the subject of lift-installed jackets for the North Sea, reference should be made to an article on the subject in the March 86 edition of the magazine 'The Oilman'.

The installation and alignment of the twin jackets can be achieved in a number of ways.

Common to ail methods is the extension in size to an existing subsea template design, and the provision of sea bed levelling jacks on the jackets themselves. The degree of template extension and the type and number of jacks depend on the design configurations of the twin jackets. However, the use of jacks is already included on template designs, and with a seabed slope of 1 in 75 it might be necessary to use seabed pile climbing jacks for a conventional eight-leg barge-launched jacket concept.

The methods for docking the jackets 21 and 22 to the template 23 will be similar to those planned for the eight-leg jacket, except that the masses and inertia will be considerably less for the lift-installed jackets 21 and 22.

The structural interaction between the two jackets through different MSE's has been studied for a range of MSF stiffness and strength parameters, for two sets of jacket layouts.

The preferred solution is to augment the MSF 25 with a plan brace frame between the two jackets at the first plan level above the sea.

This attracts a maximum plan shear force of around 700 tonnes in an extreme storm. As an indication of the full interaction envelope for all types of behaviour, it shows that the interactive loads are not significant in terms of normal jacket member sizes.

The basic structural options for the jackets are shown on Figs. 2 and 3. At present the vertical faced jacket option (Fig. 2) is favoured, but the final choice should be examined in the first part of a configuration study.

There is a major saving in fabrication cost and time if two four-leg jackets are built instead of one eight-leg jacket. The fabrication period may be reduced from two years to only one year.

A saving of 7800 tonnes of jacket weight yields a primary saving of 20 million for fabricatin and 4m in the steel order (at 1986 prices). There are secondary savings from reduced construction management costs and from an estimated average of six months float on fabrication funding commitment. These secondary savings may be somewhat offset by additional costs for jacket offshore installation. Overall there should be a saving of about 24m to the project.

There are a number of additional advantages offered by the twin jacket arrangement for the layout and disposition of risers. For access and inspection these are most conveniently positioned on an outside face of a jacket frame, but this is not allowed on eight-leg jackets because of the risk of damage from ship impact. However, the twin jackets shown in Figs. 1 and 2 have free access faces for risers in the central gap and these are shielded from ship impact.

The clear route for divers to these inner faces also means that all the major structural nodes on the jackets are readily accessible for inspection.

One preliminary estimate for the lift weight of both jackets was 11000 tonnes. This was based on designs to support 20-25000 tonne topsides. It was then necessary to investigate how the jacket weights would change if they were installed close enough together to substitute for the proposed single eight-leg jacket, and to carry the same MSF and topsides as one package. The main areas of interest for the twin jackets were positioning and alignment during installation, and loading interaction during operation. The results of the work were very promising and indicate an allin lift weight for both jackets of about 10,500 tonnes.

In the following discussion the two jackets 21 and 22 are referred to as 'Drilling' and 'Process' respectively.

This section discusses the loading interaction for two different jacket arrangements with varying MSF linking stiffness and strength.

In the first instance, two arrangements for the jackets were designed and sized, assuming low interaction loadings. One arrangement has a constant gap of 26m between jackets, giving both individual jackets one vertical face (Fig. 2). The other arrangement has basically th same sized jacket framing to match the MSF, but the jackets have equally battered legs with the nearest legs on the two jackets joined into combined foundations at the seabed. (Fig. 3).

The MSF arrangement was sized to cater for the topside loads. The MSF members over the gap between the jackets were varied in size and stiffness in successive analyses.

(Figs. 4 to 6).

A series of analyses were run using plane frames and also in full three dimensions.

Wave load, wind load and mass loads were generated and member forces and deck deflections were derived. In addition, control estimates on behaviour were derived by manual calculation and on a microcomputer stress program. The analysis included elastically modelled foundations and all the risers, caissons and conductors.

The plane frame analyses were used to determine the moment carry over through the MSF. In this work the main horizontal connections were on the MSF at elevation +1 2m.

The 3D analyses were used to investigate the racking effect for waves in other directions, and the main horiziontal connections between the jackets were at +1 0m elevation.

The results show that the interaction between the jackets does not have a significant effect on jacket weight. The total wave force on the jacket varies from 6000 tonnes up to 9000 tonnes, depending on wave direction.

The jacket member sizes initially selected were not generally overloaded in the analyses, and overall some weight reduction might be possible.

From these preliminary stiffness models, the first determination of platform natural period gave 3.5 seconds. This implies no special level of difficulty for dynamic behaviour and fatigue design, although dynamic analyses will be required in detailed design.

The most significant differences in structural behaviour between the various configurations appear in the vertical deflections at the top of the MSF legs with the storm wave. Two separate effects were identified. The first effect was due to the jackets, and the second was due to the strength of the MSF linking the jackets.

Firstly, for the case of the weak MSF interaction, the legs of the downstream vertical face jacket rose 20mm and the legs on the upstream one fell 20mm. The equivalent behaviour on the joined foot jackets was a 5mm rise and 5mm fail.

Increasing the MSF interaction strength by increasing the size and weight of the central members modifies the displacements. In the case of the strong interaction, the verticalfaced jacket vertical leg deflections change from: +20 +20 -20 -20mm to +20 +20 -12 -20mm.

A similar effect is obtained for the joinedfoot jackets. The MSF can therefore be engineered to provide for a range of vertical deflection behaviour.

The storm-induced module support deflections were compared with those for other large North Sea jackets, and were seen to be within the ranges normally found acceptable to module packages by way of relative movement.

A major topic for the design of the twin jacket substructure will be the quantification of, and design for, differential foundation settlement. The amount of differential settlement can be decreased by increases in foundation strength. The effect of differential settlement can be reduced by the choice of jacket and MSF layout and linking stiffness.

(Figs. 4 to 6 illustrate configurations of the MSF having different stiffness characteristics).

The problem is believed to be not serious if fully considered from the outset, and it should prove to be not much different from skirt piled jacket behaviour. Costed contingency procedures to rectify the effects of excessive settlement could be developed for consideration in a financial risk analysis for the twin jacket concept.

In summary, the jacket interactions are satisfactorily low, the member sizes adopted throughout are suitable for jacket weight estimates, and there are a number of options available for detailed configurations work, all within the present weight and cost estimates for the substructure.

Foundation schemes have been investigated for each of the twin-jacket options. These schemes provide an indication of the piling requirements for twin-jackets with total separation of legs and foundations at mudline toned only by the template 23), and for twin jackets with adjacent pairs of legs joined in a common foundation at mudline.

From a structural viewpoint the main advantage of joining the adjacent pairs of legs at mudline is that it results in a more efficient foundation system under certain load conditions, and ensures equal displacements of the adjacent jacket legs. Installation advantages are discussed later.

For the separate jacket option, foundation loads relating to a diagonal wave have been used for pile design. This results in maximum vertical and horizontal loads at each of the eight legs of around 12,500 tonnes and 850 tonnes respectively.

To resist these loads a vertical four pile group is proposed at each leg, with all four piles located external to the jacket leg. Each pile could be of 2134mm 0.3x63 to 80mm w.t. and driven to a penetration of 47 metres.

As illustrated in Figs. 1 to 3, vertical piles are preferred to raked piles, because vertical piles are simpler to install and require no guidance appurtenances on the jacket legs. This significantly reduces jacket weight, fabrication complexity and cost, as well as wave loading and corrosion protection requirements. The effect of using vertical as opposed to raked piles is to reduce the lateral stiffness of the pile group and to increase pile wall stresses due to bending. However, in relatively good soil, neither of these factors is considered significant, and the installation and weight saving advantages of vertical piles determine the con figuration.

To minimise the bending moments on vertical piles induced by lateral loads, the leg eccentricity with the pile group and the lower plan brace eccentricity with the mudline will be adjusted to counteract each other.

Loads relating to an East-West wave have been used to design the piled foundations for two jackets joined at mudline by a common leg foundation. This results in maximum loads on each of the two common leg foundations of around 23000 tonnes vertically and 2000 tonnes horizontally. For the four outer legs the loads are estimated at 10000 tonnes and 1000 tonnes respectively. To resist these loads it is proposed to use a group of six 2134mm ODx63 80mm w.t. piles driven to 52m penetration at each of the two common foundations. For each of the four outer legs two 2134mm OD piles driven to 62m penetration are proposed. Should this penetration prove to be unattainable, larger piles would be driven to a shallower penetration.

Alignment of piles, whether raked or vertical, depends to a large extent on the method of installation of the pile groups. For the two combined centre leg foundations, a preinstalled foundation template is envisaged as the most suitable system, into which the jacket legs are then stabbed and grouted. This permits any alignment of pile to be used and piles could be raked and aligned as near as possible to the jacket legs.

The jackets would be fabricated by conventional assembly and frame roll-up techniques.

They could be built away from the load-out quay and moved up to it when complete. This would avoid congestion at the quay wall, and would permit both jackets to be loaded out over the same prepared ways, in close succession.

The scheme for fabrication and load-out adopts the use of temporary works and seafastening trusses or box beams to support the jackets for load out and sea transport. This is an attractive option for use with flat topped cargo barges, since it becomes unnecessary to use larger and more expensive jacket launch barges.

The structures are designed to be lifted at Els -16m and -72m by double slings attached to trunnions or directly around the jacket legs. The clearances between jacket and crane vessel have been checked for the SSCV's Hermod and DB102, and exceed 3m to all parts.

The expected jacket lift weights are 5365 tonnes for the 'process' jacket 22 and 5110 tonnes for the 'drilling' jacket 21. The liftability for the DB102 presents no difficulties, and a lift using the Hermod is very satisfactory.

For the present jacket configuration the most highly loaded crane on the Hermod is at 76% of its quoted lift limit when lifting the 5365 tonne 'process' jacket. The other crane is at 56%. This tandem lift arrangement has the 5000 tonne crane in revolving mode lifting to the El -72m jacket level at 33m radius.

The 3000/4000 crane is in fixed mode at 38m working radius lifting at the El - 1 6m jacket level.

Foundation arrangement and installation options are largely dependent upon which twin jacket configuration is used. The twin jackets with combined foundations for adjacent legs could be accomplished using pre-installed centre leg supports, onto which each jacket is docked, whilst installation of the separate jackets can follow conventional jacket installation procedures with support piles at all eight legs installed after jacket placement on the sea bed. Pile climbing jacket levelling jacks are included for all installation options. The numbers and type will vary from one option to another. A typical jack is shown in (Fig. 7).

Two Separate Jackets With the independently based jackets 21 and 22 there are two basic options available for foundation configuration and installation.

i) Scheme S1 This involves the installation of a drilling template 23 with a simple extension tongue to reach the 'process' jacket 22. Each jacket is docked into the template using the conventional two pin pile method. (Figs. 14 to 17).

This ensures accurate alignment of the two jacket bases in plan-form. This scheme has the advantage that it uses conventional installation methods and has relatively few installation steps.

ii) Scheme S2 A second option for the separate jacket system is to pre-install the drilling template on the sea bed, then dock a separate jacket guide frame onto the first template using a pin and bucket system. The two jackets are then docked into the guide frame as before.

The docking pile guide frame is fairly light (around 175 tonnes) and need not be installed until the jacket installation season. This allows the type of template required for the eight-leg jacket option to be built in its original form two years earlier without special modification, since the twin jacket guide frame could link into the system originally provided for docking the eight-leg jacket.

Two Jackets Joined at Mudline Using a common foundation system for the adjacent pairs of legs joined at the mudline there are again two main options.

i) Scheme J1 The 'drilling' jacket 21 would be docked onto a pre-installed drilling template 23 having the same design as already adopted for the single large eight-leg barge-launched jacket.

There would be no extension pieces to the template. The 'drilling' jacket 21 would be provided with the large size pile foundation clusters needed for the joined-footings. These pile sleeve clusters would have a central leg sleeve into which the 'process' jacket legs would be docked and grouted. In effect the 'process' jacket 22 has pile sleeve clusters 32 on two legs only, until it has been installed.

The grouted connection between the 'process' jacket legs and the joint sleeve cluster has been checked to the rules used for pile sleeve grouted connections.

The advantages of this scheme are that the jackets are almost fully aligned in one operation and only require one final jacking operation to bring the jacket tops to the correct spacing.

ii) Scheme J2 This scheme employs a large seabed frame which includes the pile sleeve clusters for the joined footings. This frame could include the drilling template, or could be added to a preinstalled drilling template later.

Each cluster would have two central sleeves to accept the central legs of the two jackets for docking and later grouting. The cluster piles would be installed before jacket installation.

The recommended solution is a twin tower concept having a large drilling and jacket foundation template 23 to suit the joined-foot jacket option. This option offers the least installation jacking alignment work, and due to the structural shape it ensures a very minimum of jacket/MSF loading interaction. However, study work shows that the other concept options do not generate very significant jacket/MSF interactions and require perhaps only 200 tonnes of extra deck steel to cater for the full interaction if a less rigid connection were unacceptable. An unjoined jacket concept would require more subsea alignment jacks. Further design concept refinement applied to the separately footed jackets could narrow the balance of advantages in their favour.Indeed, the facility of having two vertical faced jackets with easier support for key appurtenances could outweigh all other considerations in the choice of which concept to adopt.

Overall there is little to choose between the types of jacket arrangement from a platform engineering point of view. However a planned development programme which called for template installation in May 1987 would require template fabrication to start in January 1987.

This four month fabrication time is based on the template design for an eight-legged platform. Any template which is larger than the present design will require a longer fabrication period. Therefore there is insufficient time for constructing anything significantly more extensive than the present design. This should not preclude a simple extension to reach across to the process jacket for docking pin reception, but it does eliminate some of the interesting options for the joined foot jackets.

The programme for the design of the twin jacket concept will be very similar to that given for the single large eight-leg bargelaunched jacket, provided the preliminary study for the twin jackets starts early in the design process.

The major difference from the programme based on an eight-leg jacket support structure is that the fabrication time for each lift-installed jacket is only one year compared to the two years allowed for the eight leg jacket.

Although the two jackets could be built in different yards it may be preferable to have them built simultaneously in the same yard to concentrate all jacket management and site support in one place.

Shortening the fabrication programme by one year within the Development programme offers an excellent opportunity to use the time for a measured progress through jacket material procurement, and fabrication contract award based on 100% complete jacket drawings having obtained Lloyds Design Certification. It would spread out the contract award proceedings for the project over a longer period and optimise the use of key staff. On the financial side it delays commitment of jacket construction funds by what is estimated to be an average of 6 months.

The offshore installation for the twin jackets is planned to take 3 weeks, and 4 weeks has been allowed. This is one week more than is currently being allowed for major 8-leg jacket installations.

Claims (1)

1. A support structure to form part of an offshore platform, and comprising a template on the seabed through which subsea wells may be drilled, and first and second jacket portions, each fixed to the template at their respective bases, and arranged to be joined by a further portion at or above the height of the highest expected wave.

2. A support structure as claimed in claim 1 in which adjacent facing panels of the jacket portions are vertical.

3. A support structure as claimed in claim 2 in which the template is used to space apart the bases of the jacket portions.

4. A support structure as claimed in claim 1 in which adjacent facing panels of the jacket portions are battered away from each other as they rise from the seabed.

5. A support structure as claimed in claim 4 in which the template is used to provide a common foundation point for the bases of the jacket portions.

6. Method of constructing an offshore platform, comprising the steps of founding a template on the seabed, docking a first jacket portion with that template, docking a second jacket portion with that same template, and joining the tower portions at a level at or above the height of the highest expected wave.

7. A support structure substantially as hereinbefore described with reference to and as shown in Figs. 1 and 2 or in Fig. 3, and in Figs. 4 to 19 of the accompanying drawings.

8. A method of constructing an offshore platform, substantially as hereinbefore described with reference to the accompanying drawings.

CLAIM Claim 1 above has been deleted or textually amended.

New or textually amended claims have been filed as follows:

1. A support structure to form part of an offshore platform; and comprising a template on the seabed through which subsea wells may be drilled, and sequentially installed first and second jacket portions, each fixed to the template at their respective bases, and arranged to be joined by a further portion at or above the height of the highest expected wave.