NO344410B1

NO344410B1 - Method for replacing flexible risers whilst installation vessel is positioned outside the platform safety zone

Info

Publication number: NO344410B1
Application number: NO20151637A
Authority: NO
Inventors: Håvard Strand
Original assignee: Ocean Installer As
Priority date: 2015-12-01
Filing date: 2015-12-01
Publication date: 2019-12-02
Also published as: NO20151637A1; WO2017095229A9; WO2017095229A1

Description

DESCRIPTION

Method for replacing flexible risers whilst installation vessel is positioned outside the platform safety zone

A method for replacing risers, umbilicals and other flexible products (summarily termed risers) was developed in order to prevent the installation vessel performing the operations from needing to be in close proximity of the platform. This provides marine safety benefits in terms of keeping the vessel further away from the platform (hence reducing risk of collision as well as exposure to potential hazardous discharge/leaks from platform), it provides flexibility with heading for the construction/installation vessel which provides improved operability depending on environmental (wind/waves) direction, and as some platforms have exposed riser balconies (hang off areas where risers are hung off on the topside) and prohibited zones where the vessel cannot enter whilst production is ongoing it allows for simultaneous marine operations to replace risers, umbilicals and flexible products whilst production is ongoing at the platform.

The method can be performed either with recovery of risers or installation of risers. This patent covers for the differences in method compared to the normal replacement method which requires the construction vessel to be typically within 20m of the platform. Typical arrangement for operations when vessel is required to be close to platform for topside handshake is shown on Figures 1-6, which show a typical up-close arrangement for topside transfer of a riser topside end to the platform/topside pull in winch wire.

With reference to Statoil Patent (International Publication Number WO 2015/074687 A1), this patent describes installing or replacing flexible products by an improved and simplified transfer of both topside and subsea ends to and from vessel/platform. This is without the use of additional vessel(s), additional winches on the topside/platform, subsea winches or requiring to take topside end near the seabed. This method describes a novel use of a lightweight rope or wire to safely transfer and connect heavy rigging up between installation vessel and platform, with use of wire deflecting sheaves/wheels on both topside/platform and installation vessel.

This patent also describes a way to use buoyancy on the topside end if required in order to reduce winching tension for transfer of topside end between topside/platform and installation vessel.

Several devices/equipment are required for this method. A "vessel wire deflector" device on the installation vessel (described below), a winch with lightweight rope/wire on it on it, rigging for performing the "handshakes" and a topside winch with a "topside wire deflector" on the topside, ref to Figure 7 and 8.

The "vessel wire deflector" is a device which allows a lifting wire to be deflected away from its main lifting axis (this is usually vertical for cranes but may be different for other lifting devices -such as horizontal for winches). This typically involves a sheave but may have a chute or rollers or other similar design to allow the wire to be bent away from the main vertical axis. The sheave/chute/rollers can typically rotate such that there is minimal wear on the lifting wire. The sheave/chute/rollers may also swivel (typically in the vertical axis - but may be different depending upon lifting device) such that it allows for varying angles of the wire relative to the vessel (which provides vessel heading flexibility). This is particularly useful on cranes as the crane wire typically has a restriction in offlead and sidelead angles which prevents large deflections relative to the vertical axis from being allowed. A typical concept sketch for a “vessel wire deflector” is shown in Figure 7.

The "topside wire deflector" is a similar device which allows a lifting wire to be deflected away from the main lifting axis (typically vertical for winches but may be different depending on lifting device and wire routing). This typically involves a sheave but may have a chute or rollers or other similar design to allow the wire to be bent away from the main lifting axis. The sheave/chute/rollers can typically rotate such that there is minimal wear on the lifting wire. The "topside wire deflector" however does not typically need to swivel around an axis as the platform riser angle towards the installation vessel is typically maintained within a small sector and the platform does not typically need heading flexibility. A typical “topside wire deflector” arrangement is shown on the left side of the Figure 8 and if this is not used the normal angle of wire out of the hangoff area is restricted due to clashing of wire and this is shown on the righthand side of Figure 8.

Due to the distances of transfer, the wire tensions and deflection angles can be large. If the wire angle & tension combination during the transfers are larger than what the equipment can safely operate, an alternative method is to incorporate some buoyancy elements on the topside end of the riser in order to reduce the top tension. This can be kept in place during the transfer or removed separately.

The method for recovery of existing risers consists of the following steps:

● Lowering the topside end of the riser through the hang off slot so the riser goes deeper into the water (shown on Figure 9.).

● Installing a “topside wire deflector” on the topside hang off slot such that it allows the winch wire on the platform to be deflected below level of the hang off slot towards the vessel.

● The installation vessel then deploys a lifting wire over the side of the vessel facing the platform (typically crane block but could easily be a winch or other lifting device with wire attached) (shown on Figure 9.).

● This lifting wire from the installation vessel is then inserted into the crane wire deflector and secured in the sheave/chute/rollers.

● A pulling device (typically winch) with lightweight rope/wire (typically synthetic/fibre rope made close to neutrally buoyant) or lightweight wire is then deployed over the same side of the vessel facing the platform (shown on Figure 9.).

● The lightweight rope/wire is equipped with rigging that is attached to the bottom of the installation vessel lifting wire and with a snatch block (or other running device which allows the rope to run through the system) to the topside end of the riser (or rigging above it) (shown on Figure 10).

● The lightweight rope/wire is then paid in to bring the end of the lifting wire from the installation vessel towards the topside end of the riser and this is then connected up to the topside end of the riser through rigging (shown on Figure 11 and 12).

● The lightweight rope/wire is then disconnected and recovered back to the vessel (or in an alternative arrangement can remain connected during the transfer), whilst the winch wire from the platform is paid out and the lifting wire from the vessel is paid in either simultaneously or in steps or in a combination to bring the topside end of the riser towards the installation vessel (shown on Figure 12 and 13).

● The lightweight rope/wire is then deployed again and connected up to the topside end of the riser (or rigging above it) and the snatch block (or other running device which allows the rope to run through the system) is connected up to the winch wire from the platform/topside. This can be also done the other way with the snatch block connected to the topside end of the riser and the end connected to the winch wire from the platform/topside. This is shown on Figure 14.

● The winch wire is then disconnected from the riser and the lightweight rope/wire is paid out to allow the winch wire from the platform/topside to be recovered to the platform/topside area. This is shown on Figure 14.

● The vessel remains at distance from the platform during this whole process.

● The riser is then recovered in the normal method whilst keeping at distance away from the platform/topside.

The method for installation can then involve additional steps in order initiate the subsea end of the riser if the subsea end of the riser is in pliant wave configuration. The method for this initiation of subsea end consists of the following steps:

● The subsea end of the riser is deployed from the installation vessel (shown on Figure 15). ● The pulling device (typically winch) with lightweight rope/wire (typically synthetic/fibre rope made close to neutrally buoyant) or lightweight wire is then deployed over the same side of the vessel facing the platform (shown on Figure 15).

● The lightweight rope/wire with snatch block or other running device attaches to the subsea end of the riser below the vessel and also is connected to the winch wire from the platform/topside (shown on Figure 15).

● The lightweight rope/wire is then paid in to bring the winch wire from the platform/topside across to the subsea end of the riser (shown on Figure 16).

● A subsea connection is then made between the platform wire rigging and the subsea end of riser (shown on Figure 16).

● The lightweight rope/wire is then disconnected from the subsea end and platform/topside wire (shown on Figure 16).

● The winch wire from the platform and riser on the vessel are paid in/out either simultaneously or in steps or in a combination to allow the subsea end of the riser to be brought closer to the platform and then the "sag bend" catenary that is created in the riser is landed on the seabed in correct position with the subsea end suspended on the platform winch wire and the riser is then laid away from the platform (shown on Figure 17 and 18).

● Once the installation vessel is in position, the platform is then moved in a coordinated pattern (typically using the mooring/winch system) and the platform/topside winch wire is lowered (either simultaneously, in steps, or a combination) to enable the subsea end to be lowered towards the seabed and disconnected (shown on Figure 19).

Once the platform winch wire is disconnected from the subsea end and the installation vessel is ready for transfer of the topside end of the riser to the platform/topside, the topside end of the product will typically be hanging from below the vessel on the A&R (Abandonment and Recovery) rigging or the crane rigging (shown on Figure 20 and 21 below). The transfer of the topside end of the riser will then be performed from either the crane/winch rigging or from the A&R rigging and it consists of the following steps:

● The lightweight rope/wire with snatch block or other running device will be deployed and connected up to both topside end of riser and platform winch wire (shown on Figure 22).

● The lightweight rope/wire is then paid in to bring the end of the lifting wire from platform/topside towards the topside end of the riser and is then connected up (shown on Figure 23).

● The lightweight rope/wire is then disconnected and recovered back to the vessel (or in an alternative arrangement can remain connected during the transfer), whilst the winch wire from the platform is paid in and the crane rigging or A&R wire from the vessel is paid out either simultaneously or in steps or in a combination to bring the topside end of the riser underneath the platform/topside (shown on Figure 24).

● Once the load is taken by the platform winch wire, the lightweight rope/wire is connected to the end of the crane rigging or A&R wire and the topside end to disconnect the crane/A&R rigging and then the lightweight rope/wire is paid out to allow the crane/A&R rigging to return to the installation vessel (shown on Figure 24 -26).

● The lightweight rope/wire is then disconnected and the topside end of riser is fully transferred over to the platform/topside (shown on Figure 26 - 27).

Claims

1. A method for removing a riser using an installation vessel positioned and remaining outside the safety zone of an offshore platform characterized in that the method comprises the steps of:

a. The topside end of the riser is lowered below the platform, in the case of the removal of a riser by a first lifting wire connected to a first lifting device, b. A second lifting wire connected to a second lifting device is lowered below the installation vessel in the case of the removal of a riser,

c. A rope or line, light in weight with respect to the first and second lifting wire, is lowered from a winch on the installation vessel and connected to the end of the second lifting wire,

d. A pulley, arranged along the rope or line, is connected to the topside end of the riser, such that the rope or line extends from the winch on the installation vessel, through the pulley attached to the topside end of the riser, and over to the second lifting wire,

e. The rope or line is pulled in by the winch on the installation vessel in coordination with paying out the second lifting wire, whereby the second lifting wire is pulled over to the immediate vicinity of the topside end of the riser,

f. The second lifting wire is connected to the topside end of the riser,

g. The second lifting wire is pulled in by the second lifting device, in coordination with the first lifting wire being paid out by the first lifting device, whereby the topside end of the riser is moved to a position below the second lifting device,

Whereby the riser may then be moved vertically to a desired position by the second lifting device.

2. The method for removing a riser according to claim 1, characterized in that the topside end of the riser comprises connection rigging for connecting the pulley and/or the second lifting wire to the topside end of the riser.

3. The method for removal of a riser according to claim 1 or 2, characterized in that the desired position of the topside end of the riser is at a moonpool or at the side of the installation vessel.

4. The method for removing a riser according to one of the preceding claims further comprises the step of pulling in the first lifting wire by the first lifting device, in coordination with paying out the rope or line, in order to safely return the first lifting wire to a position below the first lifting device.

5. The method for removing a riser according to one of the preceding claims, whereby either or both of the first or the second lifting wire pass through a deflecting sheave arranged to permit the lifting wire to achieve an angle of deflection greater than the angle of deflection otherwise tolerated by its respective lifting device.

6. The method for removing a riser according to one of the preceding claims, characterized in that one or more of the connections of the first or second lifting wires, and/or the rope or the line, is performed by a ROV.

7. The method for installation of a riser using an installation vessel positioned and remaining outside the safety zone of an offshore platform characterized in that the method comprises the steps of:

a. A first lifting wire connected to a first lifting device is lowered from the platform b. The topside end of the riser is lowered below the installation vessel, using a second lifting wire

c. A rope or line, light in weight with respect to the first and second lifting wires is lowered from a winch on the installation vessel and connected to the end of the first lifting wire,

d. A pulley, arranged along the rope or line, is connected to the topside end of the riser, such that the rope or line extends from the winch on the installation vessel, through the pulley attached to the topside end of the riser, and over to the first lifting wire, e. The rope or line is pulled in by the winch on the installation vessel in coordination with paying out the first lifting wire, whereby the first lifting wire is pulled over to the immediate vicinity of the topside end of the riser,

f. The first lifting wire is connected to the topside end of the riser,

g. The first lifting wire is pulled in by the first lifting device, at the same time as the second lifting wire is being paid out by the second lifting device, whereby the topside end of the riser is moved to a position below the first lifting device,

Whereby the riser may then be moved vertically to a desired position by the first lifting device.

8. The method for installing a riser according to claim 7, characterized in that the desired position of the topside end of the riser is at a hang off point on the platform.

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