BACKGROUND OF THE INVENTION The power needed to operate a semi-submersible rig is provided by engines, typically internal combustion engines such as a diesel engines, each of which is known as a prime mover. These prime movers provide the necessary power required for running the main operations of the rig, for example drilling a well, and electrical power for lighting, heating and machinery located on the deck. A number of prime movers are provided on the deck and these contribute significantly to noise and vibration at deck level and take up large areas of the deck.

SUMMARY OF THE INVENTION In accordance with the invention, there is provided a semi- submersible structure including pontoon means and carrying at least one prime mover, wherein the prime mover is located in the pontoon means.

Preferably the pontoon means comprises a plurality of pontoons at least one of which accommodates a prime mover. The location of the prime mover in the pontoon means is of advantage as this reduces the noise and vibration on the rig deck, and also releases space for additional machinery or allows the structure to be designed to have a reduced area of deck.

The invention is of particular relevance to a semi-submersible rig comprising a deck supported on two pairs of columns, each pair of columns being carried by a respective pontoon.

As a number of prime movers are usually required, each pontoon will typically carry a plurality of prime movers and the engine support equipment, such as the related power generation and distribution machinery, is preferably also located in the pontoons. This provides further noise reduction on deck and the release of additional space.

Preferably fuel tanks for the prime movers are also located within the pontoons, and connected to fuel supply means for pumping fuel to the prime movers.

Preferably, the fuel supply means also includes a back-up gravity fuel feed for supplying fuel to the prime movers if the transfer/pumping of fuel is interrupted.

With the removal of the prime mover to the pontoon, there is a need for adequate ventilation and cooling of the prime mover.

Therefore preferably ducting for exhaust gases and air for the prime mover is provided in the columns of the structure. At least two outlet ducts and one inlet duct are preferred: an inlet duct for air intake, a first outlet duct for removal of heated air and a second outlet duct for removal of engine exhaust. Desirably two outlet ducts and one inlet duct are provided for each prime mover.

Due to the length of the columns, the ducts must be large enough to ensure that adequate flow is maintained despite friction losses.

To further assist in overcoming frictional effects, the air intake duct is preferably over-pressured by forcing air into the duct, such as by the use of fans or blowers, and the structure is provided with extraction means for urging gases through the exhaust duct to prevent a build-up of back pressure which would prevent or inhibit the operation of the prime mover.

The ducts are preferably provided with integrity devices to prevent ingress of potentially explosive gases and to ensure that water cannot enter the ducts if the rig should become unbalanced.

Due to the position of the prime mover in the pontoon, the air around the prime mover may be relatively cool compared to the air from the inlet duct, therefore preferably the air inlet duct provides means for chilling the air or reducing its moisture content, so as to reduce the effect of condensation when the inlet air reaches the prime mover.

Instead of cooling the prime movers primarily via a through- flow of air, the cooling required for the prime mover may be provided via closed loop heat exchangers with a combination of salt water/fresh water coolant heat exchangers.

In order to provide maintenance access to the prime mover (s), the columns within the semi-submersible structure may be provided with elevators to reach the pontoons, the elevators being dimensioned to allow removal of sections of a prime mover for maintenance.

These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.

DRAWINGS FIG. 1 shows a perspective view of a semi-submersible rig with two pontoons and four columns ;

FIG. 2 shows a cross section through the columns, with the upper surface of the pontoons cut away to show the positioning of prime movers; and FIG 3 shows a section through a column and part of the pontoon.

MODE (S) FOR CARRYING OUT THE INVENTION In FIG. 1, a semi-submersible rig 10 in accordance with the present invention is shown when in an operational position. The rig 10 comprises a deck area 12 which is attached via two rear columns 14 and two front columns 15 to two pontoons 16. The columns 14,15 and pontoons 16 are interconnected and braced by means of four cross bracing struts 20. The pontoons 16 are typically twice as deep as those pontoons provided in conventional six column rigs of comparable size and the rig 10 has an improved ballasting system as described in the United Kingdom Patent application No. 9724063 filed 15th November 1997.

By providing pontoons which are approximately twice the depth of conventional pontoons, but with approximately the same horizontal cross sectional area, buoyancy of the rig is not compromised.

Six diesel engines constitute the prime movers 22 which are located within the pontoons 16 (three in each pontoon) in dedicated engine compartments 24 as shown in Figure 2. The related power generation and distribution switch gear and machinery 26,30 associated with the prime movers is similarly located within the pontoons 16 in compartments 32. Fuel tanks 34,36 are also located in the pontoons 16 with piping connecting the fuel tanks 34,36 to the prime movers 22.

The location of the prime movers 22 and ancillary equipment within the pontoons 16 releases deck space and removes a major

source of noise and vibration from the deck area 12. It will be appreciated that removing a large source of weight from the deck area to the pontoons effectively lowers the center of gravity of the rig 10, providing the advantage of increased stability, and allowing additional heavy machinery optionally to be added to the deck area 12 without compromising stability. Thus by locating the prime movers and related power generation and distribution switchgear and machinery in the pontoons, the variable deckload capacity is increased and the steel weight of the vessel for a given displacement is reduced.

Three prime movers 22 are shown in each pontoon 16, although as will be appreciated the number of prime movers can be selected depending on the power requirements of the rig. An elevator 40 is located within each front column 15 so that access to the prime movers 22 is readily available from the deck area 12, with the elevators being suitably dimensioned to allow removal of a partially disassembled prime mover 22 from the pontoon 16 for maintenance or replacement. Connections (not shown) are provided between the engine rooms 32 via the struts 20 connecting the columns 14,15. This ensures that in the case of electrical failure in the prime movers of one pontoon, electrical energy is still available from the other pontoon.

Figure 3 is a detailed section through the pontoon 16 showing the prime mover 22 when located within the pontoon 16. The prime mover is a conventional prime mover as is used on rigs, but additional air and exhaust ducts are required to ensure adequate ventilation and cooling of the engine compartment 24.

An exemplary air inlet duct 40, air outlet duct 42 and exhaust duct 44 are shown passing along the internal space of the column 15, although it will be appreciated that typically at least three of each duct is provided for each engine compartment.

Air inlet duct 40 supplies the air required for combustion and is provided with a series of coarse filters and fine filters.

The air taken to the engine compartment 24 by duct 40 is over pressured by means of a fan, or blower, at the deck area 12 to ensure an adequate through-flow of air to both supply the prime mover engine 22 and to cool the engine room 24. The air intake duct is large to prevent friction losses stopping the air from reaching the engine room. As the engine room 24 is now within the pontoon and not on the deck surface, it is cooled relative to the air on deck due to the surrounding sea water. Therefore the air taken in through the duct 40 is dried by a drying unit (not shown) to remove moisture, and to reduce the amount of condensation which occurs within the engine room 24 when the relatively warm intake air meets the relatively cool air in the engine compartment 24.

Additional forced ventilation may also be provided for the engine room 24 as necessary, either via air inlet duct 40 or through an additional duct (not shown). Again the air is provided via a series of coarse filters and fine filters and is supplied in sufficient quantity to adequately remove any excess/waste heat generated by the prime movers 22 and related machinery, as well as all ancillary and propulsion equipment and machinery.

The air inlet and ventilation ductwork is arranged in a configuration which ensures adequate air movement within the engine room 24 to prevent the accumulation of heat, vapors, and moisture. The ventilation exhaust is returned to surface via ductwork and exhausted to a safe area.

A duct intake 46 for the duct 40 is located on the deck area 12, and is situated along the side of the deck box in such a way as to not interfere with the main deck space, and so as to draw air from a safe area.

To maintain a through-flow of air through the engine compartment 24, the warm air outlet duct 42 leads back up the column 15 to

an outlet 50. The outlet duct 42 is provided with an insulating sleeve to try and prevent heat from the duct causing additional heating in the engine room 24.

The prime mover exhaust is conveyed to the main deck level via shielded exhaust manifolds and duct 44. The shielding is designed to minimize the heat radiated in the machinery spaces, void spaces, column etc.

All intakes to the engine room 24, such as the intake 46, outlet 50 and exhaust duct 44, are fitted with the necessary equipment to ensure that wind/wave borne water spray does not invade the ductwork and are also fitted with the equipment required to ensure the watertight integrity of ductwork and the engine compartment 24 containing the prime mover 22 should they become submerged, or when gas ingress occurs. The exhaust duct 44 is also fitted with means to isolate the prime mover exhaust manifolds in the event of downflooding and spark arresting facilities. The exhaust air is not treated or filtered.

Fuel is supplied to the prime movers 22 by fuel tanks 34,36 and associated supply lines. Fuel is initially loaded at main deck level via a fuel loading system, and stored in the main fuel tanks 34 situated in each pontoon 16. From these tanks, it is pumped to, treated/filtered and stored in secondary tanks (day tanks) 36 periodically as required. The day tanks extend to a higher level than the engines and from these secondary tanks 36, the treated fuel is pumped to the engines on demand, via a series of supply lines which are fitted with inline filters.

Fuel delivered to the engines in excess of the demand is returned to the secondary storage tank 36 via a fuel return line. All fuel tanks are vented at the main deck level.

In the event that the fuel pumping system is inoperable, the fuel is transferred to the engines 22 on demand by gravity transfer. This is achieved via a secondary fuel supply line

(not shown) which bypasses any inline filters and occurs when a normally closed electric solenoid valve is de-energized either through loss of power (black out), on command from the engine/power management system (automatic, or operator requested), or via manual intervention.

The control of the prime movers 22 within the pontoons 16 occurs via an engine and power management system, which incorporates facilities for automatic, remote and manual operation of the engines, as well as alarm and supervisory functions. The system is designed to selectively start or shut down prime movers 22 as required, as well as initiating emergency shutdown by isolating fuel, air and exhaust ducting as required to ensure that the rig integrity is maintained.