GB2306138A - Method of assembling a floating platform - Google Patents

Abstract

A method of assembling a floating platform comprising in combination a semisubmersible vessel 54 having two or more pontoons, buoyant columns upstanding from those pontoons and a deck supported on the columns (the semi-submersible vessel being of a kind known per se), and a raft 53 having a hull portion with a planform greater than the planform of the vessel; which method comprises the steps of ballasting a raft (of the kind described) so that the raft 53 rests on a prepared subsea surface 51 with the upper surface of its hull submerged to a depth greater than the deballasted draft of the vessel, floating the vessel 54 over the hull of the raft, deballasting the raft 53 so that the pontoons of the vessel are raised above water level, and then securing the vessel 54 to the upper surface of the raft 53. The raft may be ballasted to sink to the subsea surface on a falling tide, with the vessel being floated over the raft near high tide, and the raft being subsequently deballasted on a rising tide. Furrows or trenches may be provided to avoid suction forming between the raft and subsea bed during deballasting. Alternatively water jetting may be used.

Description

METHOD OF ASSEMBLY FOR BUOYANT PLATFORM The invention relates to a method of assembling a buoyant plafform for the development of an offshore oil/gas field.

In particular the invention relates to a buoyant plafform formed from an existing semisubmersible vessel secured to an upper surface of a purpose built raft held down against the action of its own buoyancy in the manner of a tension leg plafform (TLP). Plafforms of this type have been described and claimed in our U.K. Patent Specification 2285773A.

Semisubmersible vessels have been used for some years in the offshore industry for the purpose of drilling, production and marine operations. These semisubmersible vessels typically have two parallel spaced apart pontoons with buoyant columns upstanding from those pontoons to support a deck. In transit the vessel is deballasted so that it can float on the pontoons with the columns clear of the water. This allows the vessel to operate as a catamaran. For activities which require a stable offshore plafform, the vessel is ballasted down so that the pontoons are submerged. and only the buoyant columns pierce the water surface thus giving the semisubmersible vessel a substantial buoyancy with a small waterplane area.

This configuration - which is illustrated in U.K. Patent Specification 2068439A - provides a more stable plafform for offshore operations than would be provided by a conventional ship shaped vessel. However, for severe seastates, even a semisubmersible vessel becomes subject to movements which are unacceptable for many offshore operations, including inter alia drilling and production. These unacceptable movements lead to 'downtime' during which all the costs of operating the semisubmersible vessel are incurred, but no useful work is done.

Development of offshore oil and gas fields in hostile areas (in terms of severe sea states) has led to requirements for drilling, production and marine operations to be carried out in progressively more severe sea states, with minimum downtime.

Thus there is a requirement for a buoyant plafform which is capable of continued operation in more severe sea states than can be tolerated by semisubmersible vessels currently in use. This may be combined with a further requirement for oil storage at an open sea site.

In some cases it may be advantageous to employ mobile drilling facilities on a semisubmersible vessel (which is already equipped with power supplies and drilling equipment) to enhance the capabilities of another floating vessel. An example of this enhancement is shown in U.K. Patent Specification 2250767A, in which two floating vessels are located beside each other.

In the present case the semisubmersible vessel is associated with another floating component, herein referred to as a taut" Deck load and area limitations, - which would normally prevent heavy and bulky equipment from being carried, can be overcome by supporting the semisubmersible vessel on the raft.

It may a!so be desirable to drill oillgas wells at a certain location, and then leave those wells tied back to surface wellheads' at that location, while the drilling equipment is removed and reused at another location.

The present invention is concemed with a method of assembly for a buoyant plafform formed from a semisubmersible vessel and a raft.

The invention provides a method of assembly of a floating platform comprising in combination a semi-submersible vessel having two or more pontoons, buoyant columns upstanding from those pontoons and a deck supported on the columns (the semi-submersible vessel being of a kind known per se), and a raft having a hull portion with a planform greater than the planform of the vessel; which method comprises the steps of ballasting a raft (of the kind described) so that the raft rests on a prepared subsea surface with the upper surface of its hull submerged to a depth greater than the deballasted draft of the vessel, floating the vessel over the hull of the raft, deballasting the raft so that the pontoons of the vessel are raised above water level, and then securing the vessel to the upper surface of the raft.

It is preferred that the raft is ballasted down to rest on the prepared subsea surface on a falling tide.

It is also preferred that the semisubmersible vessel is floated over the raft at or near high tide, and the vessel is ballasted down to rest on the upper surface of the raft on a falling tide.

In one form the raft is deballasted at or near low water so that on the subsequent rising tide it will raise the pontoons of the semisubmersible vessel above water level.

In another form the raft is deballasted on a rising tide to raise the pontoons of the semisubmersible vessel above water level.

It is preferred that the semi-submersible vessel is deballasted simultaneously with the raft to raise the pontoons of the vessel above water level.

In a form where there are jet nozzels set into the lower surface of the raft, it is preferred that when the raft is to be deballasted, water is jetted through those nozzels to break any suction holding the raft down on the prepared subsea surface.

Prior to ballasting the raft down to rest on the prepared subsea surface, furrows or trenches may be formed in that subsea surface to avoid the formation of suction beneath the lower surface of the raft.

The invention includes a floating platform when assembled in accordance with the method described above.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of a first buoyant plafform; Figure 2 is a plan view of that plafform; Figure 3 is a side view of a second buoyant plafform; and Figure 4 shows six stages in the assembly of a buoyant plafform.

As shown in Figure 1, a floating plafform includes a generally conventional semisubmersible drilling vessel, which is frequently referred to in the offshore industry as a Mobile Offshore Drilling Unit or MODU. This MODU has two elongated pontoons 10, four buoyant columns 11 (upstanding from the ends of each of those pontoons), and a deck 12 supported on the columns. The pontoons 10 are subdivided intemally into selectively ballastable watertight compartments. The MQDU has anchor chain lockers within its comer columns, and there are winches (not shown in Figure 1) on the tops of those columns and fairleaders 15 near the bases of those columns for deploying mooring arrays.

The MODU illustrated by way of example in Figures 1 and 2 is in fact a SEDCO 600 series semisubmersible drilling vessel, of which three were located worldwide in various operating roles at the time of making this application. This particular MODU is configured for end bay drilling.

The MODU is releasably secured to a purpose built raft 21. The raft has a generally flat hull 22 with a planform larger than the planform of the MODU. The hull 22 has three caissons 23 (located at two adjacent corners and midway along its opposite side), and these caissons stand. slightly higher above the upper surface of the hull 22 than the vertical draft of the pontoons 10 of the MODU in their deballasted condition. A raft having a generally flat hull and at least two upstanding caissons at or near its lateral extremities will be referred to as a raft of the kind described. (In some embodiments the raft may have no caissons.) In this instance, the MODU is secured to the upper surface of the hull 22 between the caissons 23. In this way the raft 21 gives additional buoyancy to the MODU, and also improves the stability characteristics of the MODU. Fixing the MODU to the raft 21 reduces the wave loads applied to the MODU, because these loads are reacted largely by the raft. Thus fatigue lives in structural elements of the MODU will be significantly improved.

The raft 21 has intemal compartments for ballast water and the storage of oil. These compartments are not shown in detail in the Figures, but if the raft were to be made of steel, these would be constructed like conventional compartments of an ocean going VLCC (Very Large Crude Carrier). If the raft were to be made of concrete, the compartments would be formed like cells in a conventional concrete structure.

The raft is fitted with pipework and one or two pump rooms as required for ballast and oil transfer operations. The raft pump rooms are typically located directly below the pump rooms in the MODU pontoons with watertight access from these pontoons. Electrical feeders and pipework to the raft pump rooms can therefore easily be run via the normal routes up through the columns of the MODU.

To compensate for changes in buoyancy in a steel raft, counter flooding arrangements would be necessary, so that sea water could replace or be replaced by oil as the compartments were emptied or filled. Advantageously a flexible compartment liner could physically separate the oil and the sea water to be used as ballast. This concept is the subject of our co-pending Application 96,21302. Because of the large mass of a concrete raft, it is unlikely that counterflooding would be necessary. However, if dry oil storage was adopted, it might be necessary to fill the tops of the storage compartments with an inert gas.

The raft 21 has a moonpool through which drill strings 24 and subsequently rigid or flexible riser systems 25 can be run to the seabed, as shown particularly in Figure 1. The drill strings 24 are deployed from a drilling derrick 32 on the deck 12 of the MODU. The drilling equipment (power supplies, mud tank etc.) is mounted on the MODU, and only minimal facilities are mounted on the caissons 23 of the raft 21. In this case the raft will be required to remain on location over a drilling template or production manifold 26, which is secured to the seabed. When the wells and wellheads are completed, they can be transferred (under tension) from the deck of the MODU to a caisson 23 of the raft 21.

As an altemative to the use of a moonpool, flexible risers may be terminated at raft level.

Produced fluids may then be hard piped via the raft and thence within the columns of the MODU to process equipment above sea level.

In a preferred method of keeping the plafform on location, the raft 21 is held down against the action of its own buoyancy in the manner of a Tension Leg Platform or TLP. In this case tethers 28 extend down from tensioning devices 29 beneath the caissons 23 to foundation templates 31 on the seabed. Conventional mooring lines 20 are deployed from the winches of the MODU and through fairleaders 15 to anchors on the seabed. The mooring lines 20 are set out to a conventional eight anchor mooring array (seen best in Figure 2). The mooring lines 20 can be used to position the combined floating plafform (i.e. MODU plus raft) over the required drilling location prior to commencement of drilling operations.

An important feature of the invention relates to an altemative mooring arrangement. The fairleaders shown in Figure 1 can be repositioned on the caissons 23, and can be arranged to damp the mooring lines 20, so that the mooring loads are carried in the raft 21 rather than in the MODU itself. In use the winches (not shown in Figure 1) are used to adjust the mooring lines 20, which are then damped in the fairleaders 15 on the caissons. This concept is the subject of our co-pending Application 96,21303.

In a variation of the basic configuration of the raft, the hull 22 may be slightly dome shaped so that its draft at its centre is less than its draft around its periphery. With a dome shaped raft, the plafform will ride the waves better, and so in theory a smaller air gap would be required between the nominal sea level and the lowest elements of the deck.

Turning now to the construction and assembly of the plafform, many MODU's already exist which could be used as the upper part of the plafform. The purpose built raft 21 is of simple construction, and could be built in sections in conventional shipyards and then assembled in sheltered water. Altematively, the raft could be formed of concrete. Using conventional shipbuilding or concrete pouring techniques, the raft (with internal compartments for the storage of oil) could be constructed and assembled very economically.

To assemble the complete plafform, the raft 21 could be ballasted down so that only the tops of the caissons 23 were above sea level. The MODU could be de-ballasted so that it floated only on its pontoons 10, and could then be positioned over the raft 21. The raft 21 could be deballasted so that the upper surface of its hull 22 was above sea level and the MODU was clear of the water. In this condition the pontoons of the MODU could be secured to the upper surface of the hull.

As an altemative, the raft could be ballasted down to rest on a specially prepared subsea surface. The MODU would then be floated over the raft as before.

As a further variation the MODU could be deballasted at high tide and floated into a position over the grounded raft. As the tide fell, the MODU would be lowered by the falling tide to rest on the top of the raft. The raft would then be deballasted at low tide, so that as the tide rose, both MODU and raft could be floated off the prepared subsea surface. This concept is the subject of the present Application.

When drilling operations are completed, and the drilling denick 32 and associated drilling equipment are no longer required at that particular location, the MODU would be released from the raft 21, and floated off to another location.

When the MODU has been released, the raft could be drawn down towards the seabed by selective ballasting and/or adjustment of its moorings. The raft could be totally submerged to a depth no deeper than that at which diver intervention on the wellheads is possible. The raft could either be secured at a mid water depth position, or could rest on the seabed if the depth of the sea at that point was shallow. This feature would be of value at locations which are threatened in winter months by drifting ice. A raft for use at such a location would advantageously have its caissons suppressed, so that there would be minimal obstruction to passing icebergs or iceflows. The MODU could later retum for further drilling or work over operations (e.g. during the following summer).

In one form of the invention, the raft - once in position on site - could be permanently located below the water surface.

The arrangement has the advantage that a single MODU can be used at several locations where surface or near surface wellheads are required. The sophisticated mechanical equipment on the MODU could be used sequentially in combination with several low cost rafts.

The raft(s) provide extra buoyancy and greater stability to the MODU while drilling is in progress, so that drilling can continue in more severe sea states than would otherwise be possible.

A second buoyant platform is shown in Figure 3, which illustrates an important feature of the invention. This feature is applicable to the embodiments already recorded in our U.K.

Specification 2285773A and our unpublished Application 96,11740, and relates to the mooring arrangement.

The MODU illustrated by way of example in Figure 3 is in fact a SEDCO 700 series semisubmersible drilling vessel, of which 14 were located worldwide in various operating roles at the time of making this application. This particular MODU is configured for end bay drilling.

An example of an application of the invention is given at the end of the specific description.

As shown in Figure 3, fairleaders 47 on caissons 43 can be arranged to clamp the mooring lines 40, so that the mooring loads are carried in the raft 41 rather than in the MODU itself. In use the winches 44 are used to adjust the mooring lines 40, which are then clamped in the fairleaders.

A method of assembly in accordance with the invention will now be described with reference to Figure 4. This figure comprises sketches a) to f) showing six stages in the assembly of a platform.

In sketch a), a level area 51 is formed on the seabed in shallow water, Furrows 52 extend across this level area to avoid suction forces resisting the float-off of the completed plafform.

Sketch b) shows a raft 53 deballasted, and floating over the level area at high tide.

Sketch c) shows the raft 53 fully ballasted, and sunk onto the level area at low tide.

On a subsequent high tide a MODU 54, at an inspection draft of 5m, is floated over the submerged hull of the raft 53, as shown in sketch d).

The MODU 54 is then ballasted, and on a falling tide descends to rest on the upper surface of the hull of the raft 53. This arrangement, with both MODU 54 and raft 53 ballasted down, is shown in sketch e).

Sketch f) shows the situation in which both MODU and raft have been deballasted and floated off the level area on a high tide. (Jetting arrangements in the under surface of the raft may be used to destroy suction forces between the under surface of the raft and the level area on the seabed.) In this configuration the MODU 54 can be secured to the upper surface of the hull of the raft 53.

EXAMPLE BASED ON A SEDCO 700 MODU A specific example based on a Sedco 700 series MODU (as illustrated in our U.K. Patent Specification 2285773 and in Figure 3) is provided to illustrate the scale of the concept.

The conceptual design of the example was carried out for a storage capacity of 70.000m3.

This storage requirement results in å raft 116 meter long by 96 meter wide and 7 meter deep.

The raft is designed with corner radius of 2 meter to reduce drag loading and hence minimise the impact on the mooring system. To allow drilling, a moonpool of 15 x 15 meter is located in the centre of the raft. The raft has four comer caissons to assist the mating operation and to increase stability during ballasting between inspection draft and operation draft.

The raft is assumed to be fabricated in steel and is designed with a ballast system and oil storage system allowing the compartments to be used both for oil and ballast water. To account for the density variance between salt water and oil, a total of 8000 tonnes of solid ballast is located in the bottom of the raft. The solid ballast also further increases the stability of the MODU plus raft combination.

The raft compartmentation is dictated mainly by damage stability requirements. Though.

the raft is assumed almost full, the compartment size is controlled by the density variation between oil and salt water and the condition where only water is stored.

The plafform is designed for an inspection draft of 5 meter, and the original operational draft of the Sedco 700 of 24.4 meter is maintained. A survival draft is not required since the motion behaviour is excellent with respect to minimum air gap. Further, the existing mooring spread on the MODU is assumed to be used with minor modifications for mooring of the complete plafform.

The motion performance predicted for the complete plafform can be summarised by: excellent motion for drilling activities; acceptable motion in 100 years condition; and no air gap problem.

Claims (10)

1. A method of assembly of a floating platform comprising in combination a semi submersible vessel having two or more pontoons, buoyant columns upstanding from those pontoons and a deck supported on the columns (the semi-submersible vessel being of a kind known per se), and a raft having a hull portion with a planform greater than the planform of the vessel; which method comprises the steps of ballasting a raft (of the kind described) so that the raft rests on a prepared subsea surface with the upper surface of its hull submerged to a depth greater than the deballasted draft of the vessel, floating the vessel over the hull of the raft, deballasting the raft so that the pontoons of the vessel are raised above water level, and then securing the vessel to the upper surface of the raft.

2. A method as claimed in Claim 1 in which the raft is ballasted down to rest on the prepared subsea surface on a falling tide.

3. A method as claimed in Claim 1 or Claim 2 in which the semisubmersible vessel is floated over the raft at or near high tide, and the vessel is ballasted down to rest on the upper surface of the raft on a falling tide.

4. A method as claimed in any one of the preceding claims in which the raft is deballasted at or near low water so that on the subsequent rising tide it will raise the pontoons of the semisubmersible vessel above water level.

5. A method as claimed in any one of claims 1 to 3 in which the raft is deballasted on a rising tide to raise the pontoons of the semi-submersible vessel above water level.

6. A method as claimed in any one of the preceding claims in which the semi-submersible vessel is deballasted simultaneously with the raft to raise the pontoons of the vessel above water level.

7. A method as claimed in any one of the preceding claims in which there are jet nozzels set into the lower surface of the raft, and when the raft is to be deballasted, water is jetted through those nozzels to break any suction holding the raft down on the prepared subsea surface.

8. A method as claimed in any one of the preceding claims in which prior to ballasting the raft down to rest on the prepared subsea surface, furrows or trenches are formed in that subsea surface to avoid the formation of suction beneath the lower surface of the raft.

9. A method substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

10. A floating plafform when assembled in accordance with any one of the preceding claims.