GB2220462A

GB2220462A - Apparatus and method for landing and retrieval of subsea modules

Info

Publication number: GB2220462A
Application number: GB8914989A
Authority: GB
Inventors: Carlos Villanueva; Trevor Donaldson Caldwell
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1988-07-02
Filing date: 1989-06-29
Publication date: 1990-01-10
Also published as: AU3704989A; NO892724L; BR8903258A; NO892724D0; GB8914989D0; GB8815785D0

Abstract

A hydraulically operated soft-landing cylinder comprises a first piston (1), and a second piston (2) within a housing (3). The pistons move to absorb both the initial shock on landing (piston (1) retracts) followed by controlled landing (piston (2) retracts). The cylinders may be installed in modules to allow both their landing and retrieval to and from subsea templates, retrieval following initial introduction of liquid to extend piston (2). <IMAGE>

Description

APPARATUS AND METHOD FOR LANDING AND RETRIEVAL OF SUSSEA This invention relates to an apparatus and method for the and retrieval of subsea modules.

A considerable proportion of the world's remaining oil and gas reserves is believed to be offshore lender water depths in excess of 200 metres in hostle environements. In these ciroumstances the cost of conventional offshore recovery systems rises rapidly and soon becomes uneconomie.

For this reason attention has been given to subsea production systems in which the well control and production equipment is placed on the sea bed. In order to do this a structure known as a template is employed. This is a large frame with guide tubes for drilling which is deposited and secured on the sea bed in a desired location.

After drilling, well control and production equipment are mounted on the frame and these facilities remain on the sea led.

The well control and production equipment is generally located using guidalines, however it is desirable to dispense with guidelines due to the necessity of using powerful winches when operating in deep waters.

Our UK Patent Application No 2195686-A describes a subsea production system in which the components are located swithin modules which are self locating thaws removing the need for guidelines.

The modules are deployed from a dynamically positioned support vessel by suspension from a remotely guided vehicle (RGV). The manoeuvres the nodule into its correct position above the template before lowering into place.

Due to sea motion on the support vessel vertical movement is encountered during the deployment and retrieval of the modules on the subsea template. This is particuliarly pronounoed in beavy seas and oritaical at the moment of landing if damage to both modules and template is to be avoided.

It is therefore desirable to minimise impact during landing of the modules and to be able to exeroise control during both the final landing and retrieval procedures.

We lave now devised an apparatus and method for the soft zing and controlled jacking of modules which may be incorporated into the subsea production system described in UK Patent Application No 2195686-A.

Thus according to one aspect of the present invention there is provided a hydraulically operated soft-landing cylinder, the cylinder comprising (a) an outer housing provided with an internal stop, the housing comprising (b) a first piston in which the piston bead is provided with a flange, (c) a second piston concentric with the first in which the piston bead is provided with a flange and the lower extremity with a stop, and (d) means for controlling the pressure acting on said pistons to cushion the landing of said cyliner, the first piston being located inside and retractable within the secnd, its extension being limited bp the stop on the second piston, the second piston being looated inside and retractable within said housing, its estnsion being limited by the internal stop on the The pressure acting on the cylinder is controlled by suitable fluid which may be pumped into the cylinders or allowed to vent into an accumulator or into the sea.

The landing of the cylinder takes place in two phases, shock absorption and controlled landing.

The first smaller piston moves for shock absorption dissipating the kinetio energy on initial landing and deoelerating the module.

The stroke of the first piston is such that after shock absorption the first piston has retracted to be almost level with the end of the second. The secOtxl larger piston then moves for controlled landing.

By strove we mean the distance each piston is able to retract within the cyijider determined by the sizing of each piston. The sizing of each piston may vary depending upon the type of module to be landed.

In order to land a suea module a number of landing cylinders nay be employed mounted in the base of the module.

The preferred landing system uses four cyli^ers located at the corners of the module.

The soft landing system may be incorporated into any module suitable for use in a subsea system and nay also be employed in the The cylinders are connected by pipework to the accumlator and valves situated in the RGV for controlling the flow of fluid in the cylinders.

Suitable fluid may be standard hydrallic fluid such as a water based fluid such as Marston Bentley Ococic HW54O.

During the shock absorption phase damping may be provided by use of a fixed orifice restrictor and pipework restriction as the fluid is displaced from the cylinder into the accumulator.

At the end of the shock absorption phase the final equilibrium pressure in the system has risen to a value 'tich acting on the first piston supports the module weight.

In this position the interfaces to be connected will be in a stand off position.

For controlled landing fluid is wetted at a controlled rate the restrictor and allowed to discharge into the sea.

The second piston retracts until the two interfaces are in contact.

To complete the installation landing stools to nate with the cylinders are fitted to the structure on which the module is lanes.

These provide the necessary bard points to react with the nodule landing loads snd also, by the use of a rubber pad below the reaction plate, allow a limited misalignment capability in the event of the module landing at an angle.

The rate at which the module lands is deteemeeed by the size of the restrictor.

The cylinder housing and first piston may be manufactured in Ferralium 225, a duplex stainless steel.

The send piston nay be manufactured in Hidurch 191 a high strength cupro-nickel material.

These materials elinate the requirement for special bearings and corrosion resistant coatings.

The accumulator may be a standard propriety unit in low alloy steel quiped with a low permsability bladder. The low alloy steel nay be protected intern-lly and externally with subsea standard epoxy Paint.

The accumulator may be coupled to the landing cyliners by rigid stainless steel pipework via a manifold.

Because duplex stainless steel may suffer from hydrogen embrittlement if used with a structure Which incorporates a Cathodic protection system, the cylinders may be electrically isolated from the module by the use of non-metallic collars and washers. Isolation may also be provided between the cylinders anS the stateless steel pipework.

According to another aspect of the present invention there is provided a method of landing a module on a suba template which method comprises the steps of (a) mounting at least one of the landing cylinders as hereinbefore described in the base of the module, (b) glowering the nodule until contact is nade between the cylinders.

and the template itself or another module an the template, (c) venting fluid from the cylinders to allow for shock absorption.

and (d) venting further fluid from the cylinders at a controlled rate to achieve final landing.

The nodule nay be lowered to the template by use of a RGV containing the required hydra:ulic system for the operation of the landing cylinders Suitable nodules include Xmas tree, flow control, and production choke modules, production control pods and blow out preventers.

As the same RGv is used with a family of modules the system has been designed to use the same accumulator varying only precharge pressure to suit the nasa of the particular module being run susea.

The cylinders of the present invention may also be used for the controlled jacking up of the modules from the template before retrieval to the surface bp means of the Thea according to another aspect of the present invention there is provided a method for the jacking up of a module containing one or more cylinders as hereinbefore described from a subsea template which method involves increasing the pressure acting on the cylinders to extend the second piston and raise the module from the template.

The distance the module is raised is dependnet upon the stroke of the second piston. This allows the module to be raised to a defined height bp controlling the supply pressure only.

After the module has lifted the pressure in the system allows the the first piston to extend.

Fluid is then vented to the sea to return the system pressure to ambient.

The pressure acting on each piston during both the landing and jacking procedures is dependant upon the type of module being mapoeuvred.

The invention is further illustrated with reference to the following drawings wherein Figure 1 is a section of a landing cylinder in the pre-landing position.

Figure 2 represents in section the sequence for the controlled landing and jacking up of the cylinders, and Figure 3 is a diagrammatic view of the hydraulic circuit for the control of the landing system.

The landing cylier comprises a first concentric piston 1 operating within a second concentric piston 2. The second piston moves within the outer housing 3. The upper ends of each piston are fitted with flanges 4 which locate on the stops at the lower extremities 5 of the housing and second piston respectively to ensure the correct stroke is achieved during both shock absorption and controlled ling.

With reference to Figure 2 the sequence for landing and jacking up of the cylinders is illustrated by the following stages: Stage 1 - In the prelanding position both pistons 1 and 2 are extended from the housing 3 ready for the shook absorption stroke indicated by the distance a-b.

Stage 2 - After shock absorption piston 1 has retracted almost within piston 2.

Stage 3 - Controlled landing is achieved over the distance x-y as piston 2 is retracted within housing 3.

Stage 4 - Pressure in the tem extends piston 2 to jack the module.

In Figure 3 the hydrauuio circuit is shown for the landing and jackiog up of either a flow control module (FCM) or the RGV.

The cylinders 7 mounted on the FCM are linked by hydraulic line 8 to both the accumulator 9 and valve 10 situated on the EGv. Operation of valve 10 controls the pressure in the oylinders and allows fluid to be vented into the sea via restrictor 11 during linding In a similar manner oylinders 12 mounted on the RGV are linked to an accumulator 13 and valve 14.

The bydra:ulic system is based on the assumption that the hydraulio supply in the RGV cannot be isolated form the landing

Claims

Claims 1. A hydraulically operated soft-landing cylinder comprising (a) an outer housing provided with an internal stop, the housing comprising (b) a first piston in which the piston bead is provided with a flange, Cc) a second piston concentric with the first in which the piston bead is provided with a flange and the lower extremity with a stop, and (d) means for controlling the pressure acting on said pistons to cushion the larding of said cylinder, the first piston being located inside and retractable within the second, its extension being limited bp the stop on the second piston1 the second piston being located inside and retractable within said housing, its extension being limited by the internal stop on the housing
2. A soft-laning cylinder as claimed in claim 1 wherein the first piston moves to absorb the initial shock on landing and the second piston moves for the controlled landing of the said cylinders.
3. A soft-landing cylinder as claimed in either of the preoeeding claims wherein the means for controlling the pressure acting on the psiton comprises an accumulator connected to sais cylinder.
4. A soft-landing cylinder as claimed in claim 3 comprising in addition a restrictor valve connected to said cylinder for releasing the pressure acting on the piston.
5. A subsea module containing one or more softlanding cylinders according to any of the preceding cliaims.
6. A susea module according to claim 5 whereon the mmber of cylinders is four.
7. A method of landing a modle on a subsea template which method involves the steps of (a) mounting at least one of the soft-alnding oylinders according to any of claims 1 to 4 in the base of the module, (b) lowering the module until contact is made between the cylinders and the template itself or another module on the teate, (c) venting fluid from the cylinders to allow for shock absorption and, (d) venting further fluid from the oyclinders at a controlled rate to achieve final laning. I
8. A method for the tacking up of a module containing one or more cylinders according to any of claims 1 to 4 from a subsea template which method involves increasing the pressure acting on the cylinder to extend the second piston and raise the module from the template.
9. A hydraulically operated soft-landing cyleeeer as bereibefore described with reference to the accompanying drawings.
10. A method for the landing and jacking up of a subsea module as hereinbefore described with reference to the accrpanying drawings.