GB2250253A - Mooring arrangement for production loading system - Google Patents

Abstract

In an offshore production scheme for exploiting subsea oil reservoirs, a shuttle tanker mooring system comprising a CALM buoy 11 (of a kind known per se) and a hawser 20 connected successively along its length to a float 27, a flexible production hose 21 and an anchor weight 35; in which mooring system, when the tanker is not riding to the CALM buoy, the float is pulled down beneath the wave effected zone by the anchor weight and so holds a hose connector housing between the wave effected zone and the seabed; and when the tanker is riding to the CALM buoy, the float is lifted out of the water by a winch on the tanker and the anchor is lifted off the seabed; the arrangement being such that the flexible production hose connector housing is free to move over the seabed as the tanker drifts around the CALM buoy, and is held in a catenary shape below the water surface and above the seabed when the tanker is away from the CALM buoy. <IMAGE>

Description

MOORING ARRANGAMENT FOR PRODUCTION LOADING SYSTEM The invention relates to a mooring arrangement for a production loading system.

The invention provides in an offshore production scheme for exploiting subsea oil reservoirs, a shuttle tanker mooring system comprising a CALM buoy (of a kind known per se) and a hawser connected successively along it's length to a float, a flexible production hose connector housing and an anchor weight; in which mooring system, when the tanker is not riding to the CALM buoy, the float is pulled down beneath the wave effected zone by the anchor weight and so holds the flexible production hose connector housing between the wave effected zone and the seabed; and when the tanker is riding to the CALM buoy, the float is lifted out of the water by a winch on the tanker and the anchor is lifted off the seabed; the arrangement being such that the flexible production hose connector housing is free to move over the seabed as the tanker drifts around the CALM buoy, and is held in a catenary shape below the water surface and above the seabed when the tanker is away from the CALM buoy.

It is preferred that the connector housing is configured to hold one end of the flexible production hose, so that the hose end can be drawn out of the housing and attached to a fitting on the shuttle tanker.

It is further preferred that the one end of the flexible production hose is attached to a hose pick-up line connected to a marker buoy.

It is also preferred that the float is attached to a hawser pick-up line connected to a marker buoy.

The hose pick-up line and the hawser pick-up line may be connected to the same marker buoy.

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 schematic diagram of a tanker based production system located offshore over a subsea oil reservoir.

Figure 2 is a sectional view (from sea surface to seabed) showing a mooring system in place with a tanker riding to a loading buoy, and production of crude oil taking place through a flexible production hose; Figure 3 is a sectional view corresponding to Figure 2 with the mooring system in a parked configuration awaiting for a tanker to arrive at the loading buoy; Figure 3A is a detail in Figure 3; and Figure 4 is a plan view on the forecastle of the tanker.

Figure 1 shows a shuttle tanker 10 (with production facilities forming no part of this invention) riding to a CALM buoy 11, and loading crude oil from a subsea reservoir through a flexible production riser 12. (The subsea wells and manifold shown on the seabed at 14 also form no part of this invention).

The mooring and loading system for intermittent loading of the tanker will now be described in more detail with reference to Figures ? and 3.

As shown in Figure 2, the field mooring and loading system includes the CALM buoy 11 from which the flexible riser 12 is suspended. In the centre of the CALM buoy 11 (mounted on its deck), is a turntable and fluid swivel, which permits continued production while the moored tanker 10 is rotating about the CALM buoy 11. The tanker 10 is moored to the CALM buoy 11 via a bow hawser 20. The production line is brought from the CALM buoy 11 to the tanker 10 via a flexible hose 21 hanging in a catenary shape from the CALM buoy to the tanker forecastle.

The mooring and loading system on the tanker forecastle consists of a mooring winch 32 and bowstopper 31 to handle the mooring hawser 20. There is also a hose handling station 22 incorporating a winch for taking the production line on-board; a quick release coupling for the production and water injection hoses; and an electric/ hydraulic coupling for the control umbilical. (The arrangement on the forecastle of the tanker is shown in more detail on Figure 4, and will be described later).

The mooring and loading system illustrated is based on modification and revision of a specific CALM buoy. However, other existing CALM buoys or a purpose new-built unit could be equally suited for the proposed production system.

The CALM buoy 11 is moored by six chain legs 23 spaced at 60 intervals around the buoy. Each of these chain legs is approximately 500m long and terminates at a highholding-power anchor 24. Anchor point radius is approximately 460m. All chains are 102mm diameter grade 3 chain with a breaking load of approximately 750 tonne.

The force-restoring characteristic for the CALM buoy 11 with the above mooring configuration can be compared to the force-restoring characteristic for a currently operating CALM facility located in the Central North Sea in 75m of water. The comparison indicates almost identical spring characteristics for restoring forces in the interval from 500-2000 kN. (The spring characteristics in this load range are of importance on the requirement for tanker disconnect. A softer CALM buoy anchor system spring characteristic will reduce the peak tensions in the tankermooring hawser and improve the operability).

Referring now to Figure 3, the mooring hawser 20 consists of one length of polypropylene/polyester line 120m long. One end is permanently secured to a mooring point on the CALM buoy 11 and the other end terminates in a chafing chain 25. A pick-up line 26 is shackled to the chafing chain 25. A support float 27 is secured to the chafing chain 25, and a marker buoy 28 to the pick-up line 26.

The 3'S in. chafing chain has six links hardfaced to prevent wear by movement in fairlead 29 on the forecastle of the tanker. The polypropylene/polyester hawser 20 will have excellent fatigue properties, and based on past North Sea experience will require replacement approximately once a year.

The deck layout of the forecastle of the tanker is shown in Figure 4. The hawser 20 from the CALM buoy 11 is secured on the forecastle in bowstopper 31. The bowstopper is placed right aft of the fairlead 29. The fairlead is covered with non-spark material where the chain is likely to come in contract with it to reduce explosion risk. Aft of the bowstopper 31 is placed a mooring winch 32 to be used to retrieve the pick-up line 26.

The bowstopper 31 is fitted with a continuous tension measuring system 33 for monitoring tension in the hawser 20. The hawser tension Is displayed on indicators both on an Emergency Release Unit in the forecastle and on the Remote Emergency Release Control panel in the control room. Additionally, a paper recorder is installed in the control room for time history tracking of the hawser tension. The tension history assists the officers to estimate when to leave the buoy due to bad weather conditions.

The production hose assembly (21) is attached to the hawser system (20) and can be retrieved with the hawser as indicated in Figures 2 and 3. The hose loading assembly comprises a high pressure hose 21 (Coflexip 5" or similar), a subsea buoy 34 and a dead man anchor 35. The flexible hose 21 is negatively buoyant and will have a catenary shape both when attached at 22 on the tanker bow during loading (Figure 2), and also when parked and connected to the subsea buoy 34 (Figure 3).

The catenary shape will limit the length of hose in the wave zone and prevent damage to the hose.

The control umbilical (and also the possible high pressure water injection hose) will be connected to the production hose. For simplicity, the control umbilical and water injection hose are not shown.

The production hose, umbilical and water injection hose are fitted with an MIB QC/DC end connector with integral valve. The hose connection 36 hooks into the other parts of the MIB connector located at the hose handling station 22 on the port side of the tanker forecastle.

The hose handling station includes a winch 37 (independent of winch 32) for pulling in the hose, using pull-in line 38 and a jacking system for final engagement of the two hose coupling parts.

The mooring and hose connection equipment have a manually controlled hydraulic system, supplied from hydraulic power packs inside the forecastle. One diesel hydraulic power unit supplies the hawser winch 32 and hose winch 37, and one electric-hydraulic power unit supplies the hose connection system 36 and bowstopper 31.

The electric-hydraulic supply to the connection system is backed up by the diesel power pack in case of pressure drop. Non-return valves in the hydraulic lines to the hose and hawser connection parts will maintain the pressure for some tlme.

An alarm system will continuously monitor the loading and mooring equipment functions throughout the production/loading period.

The alarm station is displayed in the 'Remote Emergency Release Control Panel' in the Control Room. The following alarm functions are incorporated in the alarm system: Diesel unit shut-down Electric-motor stopped Low hydraulic pressure Low oil level Oil temperature Electric supply failure Hawser load exceeding threshold value A 'Remote Emergency Release System' is provided which operates via solenoid valves installed in the hydraulic pipes to control the disconnect functions i.e.: Hose loading valve Hose coupling Hose coupling jacks Bowstopper arm.

The 'Remote Emergency Release System' is intended for use only in emergency situations, when normal hand controlled disconnection of the production hose 21 and release of the hawser 20 cannot be carried out from the hydraulic hand control in the mooring control cabin on the forecastle.

In an emergency situation the RERS will, once activated, perform the following activities in sequential order.

Close valves in hose coupling Release coupling Release hose assembly Release hawser As opposed to the normal release operation, where the hose 21 has a controlled descent, in an emergency release operation the hose 21 will be pulled away by the dead man anchor 35. However, the subsea buoy 34 incorporated in the hose system will prevent the hose from free-falling onto the seabed, and hence prevent likely damage.

In operation, the field will be served by a dedicated field support vessel (FSV) which has sufficient anchor handling capacity to handle the hawser 20 and hose assembly (21) including the dead man anchor 35. Prior to the production tanker's approaching, the FSV will inspect the CALM buoy 11 and the hawser pick-up line 26.

The FSV will also relocate the hawser 20 and hose assembly (21) if required, to ensure that the tanker 10 can approach the CALM buoy 11 up-wind.

The FSV will retrieve the pick-up line 26 and attached a messenger line to it. The messenger line will be passed to the tanker 10 using a pneumatic gun. The tanker will then haul the hawser pick-up line 26 on-board. Attached to the hawser pick-up line is the hose pick-up line 38. This will be passed to the hose winch 37.

Once the hawser chafing chain 25 is on-board it will be locked off in the bow stopper 31. The hose assembly (21) is pulled in with the hose pick-up line 38. The MIB hose end-connector will be jacked into the receptacle and engaged with the tanker mounted connector part. Both ball valves fitted in each part of the connector are actuated from a single hydraulic actuator mounted in the upper (fixed) half of the QC/DC connector.

The lower valve is driven open and closed by a gear drive attachment ensuring exact synchronisation of both valves, and thus will guarantee identical starts. When the ball valves are opened and checks have been carried out for the subsea control system, production can commence.

During production, the production and water injection hose assembly (21) will be kept suspended in a catenary shape between the CALM buoy 11 and the bow of tanker 10. In calm weather conditions the tanker might be running slow astern to maintain tension in the hawser 20 and to avoid overrunning the CALM buoy 11; a constant distance of 120m to the CALM buoy will therefore be maintained at all times.

Disconnection from the CALM buoy 11 will take place in the opposite order to the connect operation. The tanker 10 will be running slow astern while the hawser pickup line 26 is being winched out maintaining sufficient tension In the line to keep the hawser 20 straight, such that the dead man anchor 35 lands on the seabed at the maximum distance from the CALM buoy, hence insuring that the hose 21 will stay parked between sea surface and seabed in a satisfactory catenary shape.

The fact that the dead man anchor 35 can be dropped anywhere within an 80-140m radius from the CALM buoy 11 requires that the well flow lines are buried or otherwise protected within this radius range.

Operating limits for the tanker 10 on the CALM buoy 11 are based on tension in the mooring hawser 20, and are not directly related to wind speed and sea state.

Current North Sea experience shows that there is no reliable correlation between wind speed/sea state and hawser tension. Because the mooring hawser 20 is the determining factor, it will be continuously monitored on-board the tanker 10.

The system will be designed to commence production shutdown when hawser load reaches 100 tonnes and tanker disconnect will take place when the hawser load reaches 120 tonnes. Although wind speed and s-ea states that cause these tensions will vary significantly, typical conditions are 20m/sec wind speed with 5m significant wave height sea state.

These disconnect criteria are very similar to an existing CALM buoy facility operating in the Central North Sea, which has a reported uptime of 88-90%. Similar or better uptime can be expected for the proposed field arrangement. Improvement in uptime can be expected due to the smaller size tanker proposed.

Overall field efficiency can be better than suggested above as the reservoir might regenerate pressure during periods of shutdown.

Claims (6)

1. In an offshore production scheme for exploiting subsea oil reservoirs, a shuttle tanker mooring system comprising a CALM buoy (of a kind known per se) and a hawser connected successively along it's length to a float, a flexible production hose connector housing and an anchor weight; in which mooring system, when the tanker is not riding to the CALM buoy, the float is pulled down beneath the wave effected zone by the anchor weight and so holds the flexible production hose connector housing between the wave effected zone and the seabed; and when the tanker is riding to the CALM buoy, the float is lifted out of the water by a winch on the tanker and the anchor is lifted off the seabed; the arrangement being such that the flexible production hose connector housing is free to move over the seabed as the tanker drifts around the CALM buoy, and is held in a catenary shape below the water surface and above the seabed when the tanker is away from the CALM buoy.

2. A system is claimed in Claim 1 in which the connector housing is configured to hold one end of the flexible production hose, so that the hose end can be drawn out of the housing and attached to a fitting on the shuttle tanker.

3. A system as claimed in Claim 2 in which the one end of the flexible production hose is attached to a hose pick-up line connected to a marker buoy.

4. A system as claimed in any one of the preceding claims in which the float is attached to a hawser pick-up line connected to a marker buoy.

5. A system as claimed in Claim 3 and Claim 4 in which the hose pick-up line and the hawser pick-up line are connected to the same marker buoy.

6. A system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.