Installation aspects of flexible riser systems

Installation aspects of flexible riser systems D. R. A. Johnston

Comex Houlder Ltd, Aberdeen, UK

This paper presents a review of the flexible riser systems currently available for use on subsea development projects. It outlines the installation methods associated with each riser system and details certain aspects of these methods.

Keywords: flexible riser systems, components, installation The continued pace of subsea developments together with associated floating production facilities (FPF) has inevitably led to the increased use of flexible riser systems. Flexible riser systems allow for the dynamic response of the vessel and of the riser itself, and offer considerable advantages over rigid riser assemblies. The basic configuration of a flexible riser system may be defined as either single or double catenary. It is to be noted that the number of individual risers required in order to fulfil the field production requirements will normally necessitate that riser groups be formed which will contain two or more individual risers. The riser groups will then be installed as a single entity. For convenience of presentation the riser systems are described below as if for an individual riser; the systems are, however, equally applicable for riser groups.

Single catenary The single eatenary configuration comprises a simple catenary hanging from the FPF structure and laying in a catenary under its own weight until it reaches touchdown on the seabed (see Figure 1).

Double catenary The double catenary configuration includes a form of mid-water buoyant support which allows the riser system to form an upper catenary between the FPF and the mid-water support, and a further lower catenary between the mid-water support and the seabed. There are several variations on the double catenary, each developed for a specific purpose. These have been termed the 'lazy-S', the 'steep-S', the 'lazy wave' and the 'steep wave', the major differences being in the method of providing mid-water support.

Lazy-S. The lazy-S riser system configuration is shown in Figure 2. It is a double catenary configuration consisting of the upper catenary from the FPF to a buoyant moored mid-water arch. The riser is secured over the arch and then descends in a lower catenary from the mid-water arch to its seabed touchdown point. The riser then extends This paper was originallypr~ented at a meeting on 'Flexible risers' held 9 January 1989at UniversityCollegeLondon,UK.

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Eng. Struct. 1989, Vol. 11, October

along the seabed to its tie-in point. A riser stabilization device (RSD) is attached to the riser at a point along its seabed length.

Steep-S. The steep-S riser system configuration is shown in Figure 3. It is a double catenary configuration consisting of an upper catenary from the FPF to a buoyant free mid-water arch. The riser is secured over the arch and then descends in a lower steep catenary from the mid-water arch to a seabed securement point or riser base connector (RBC). The lower catenary does not achieve a natural touchdown on the seabed under its own weight. Lazy wave. The lazy wave riser system configuration is shown in Figure 4. It is a modification of the lazy-S double catenary configuration, consisting of an upper catenary between the FPF and a series of buoyancy modules attached to the riser at the mid-water location. The riser then descends from the mid-water location in a second lower catenary to its seabed touchdown point. Steep wave. The steep wave riser system configuration is shown in Figure 5. It is a modification of the steep-S double catenary configuration, the buoyant mid-water arch being replaced by a series of buoyancy modules. Flexible riser assemblies and components

Individual riser systems are of individual design and feature specifically designed components. There are, however, common components and specific components designed for common purpose.

Common components The following major components may be considered as common to all riser systems.

Flexible riser. The flexible riser itself is the primary common component. A flexible riser is here considered to transport hydrocarbons or ancilliary fluids between the subsea and topside facilities. Floating production facilities are subject to the dynamics of the seaway in which they are situated. Due to their materials and construction, flexible riser systems inherently allow for 0141-0296/89/040254-12/$03.00 © 1989 Butterworth& Co (Publishers) Ltd

Installation aspects of flexible riser systems: D.R.A. Johnston

Sea level FPF JRiser release connector

Flexible riser

Seabed touchdown point

Riser stability d.evice

Riser lower end flonge

Seabed

Figure I

Singlecatenary flexible riser system

L Sea level FPF I .,

release connector

Flexible rise Mid-water arch

Seabed touchdown point

Mooring line

Riser lower end flange

Seabed Gravity bose

Riser stability device

Figure 2 Lazy-S flexible riser system the associated riser dynamics. It is not the intention in this paper to further define the flexible riser. Riser release connector. All flexible riser systems for attachment to an F P F feature a riser release connector (RRC) in order to allow quick disconnection of the riser from the FPF. The RRC consists of an upper half fixed to the F P F structure and a lower half attached to the riser top flange. When mated, the two halves are locked together with a clamping mechanism which may be

hydraulically released to allow the riser to disconnect from the FPF. The fixed location of the RRC upper half is dependent on both the riser system design and on the F P F layout and design. In the past with semi-submersible F P F s the RRC has been positioned either above the waterline at deck level or below the waterline on a brace or pontoon. An RRC may be designed to mate a single riser to the F P F pipework or two or more risers may be connected to a single RRC.

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Installation aspects of flexible riser systems. D.R.A. Johnston

Sea level FPF Riser release connector

Flexible riser Mid-water arch

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release connector

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Seabed Seabed touchdown point

Figure 4 Lazy wave flexible riser system

Flanges. All flexible risers incorporate flanged connections. These are normally of the ring joint type, the specific end fitting and flange design being dependent on the service application for the riser.

riser movement on the seabed. The RSD is placed on the seabed, a clamp is installed onto the riser on the surface, the riser is laid over the RSD and attached to it via securing lines from the pre-installed clamp.

Riser stability device

Mid-water arch assemblies

Riser stability bases would be incorporated in single catenary and double catenary lazy-S and lazy wave riser systems. The RSD is a simple stabilizing weight to prevent

Mid-water arch assemblies are common to the lazy and steepS riser systems. In the lazy-S system the arch is moored to a gravity base on the seabed; in the s t e e p s

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Installation aspects of flexible riser systems: D.R.A. Johnston

Sea level FPF

release connector

Flexible riser Mid-water buoyancy

\

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Figure 5

Steep wave flexible riser system

system the arch is not moored and hence there is no gravity base or mooring line. Gravity base. A gravity base is a simple weighted block which is designed to act as an anchor for the mid-water arch. The construction and general arrangement is to suit the soil characteristics and the riser system requirements. Previous designs have been constructed in the form of sandwiched steel plate, dimensions being in the order of 3000 x 3000 x 500 mm, and the weight being in the order of 50 tonnes. In addition to the attachment point for the mooring line, suitable padeyes need to be incorporated on the upper surface to allow for deployment and deck handling. Consideration also needs to be given to any requirement for ensuring correct positioning and alignment during final installation, particularly if there are to be further adjacent gravity bases installed. Mooring line. The mooring line holds the mid-water arch to the gravity base. Synthetic materials are suitable for their strength characteristics and for their ability to remain stable and not deteriorate during prolonged immersion in seawater. Multi-plait (or braided) construction is likely to be most suitable. The mooring line end fitting or splice and the attachment method to both the gravity base and the arch must be considered, and should allow for easy termination and connection in the field. Mid-water arch. A mid-water arch is designed to sufficiently support the riser catenary weight such that it floats mid-water. The arch supports the flexible riser such that an adequate bending radius is maintained at all times. The arch therefore consists of a shaped guide chute, or arch, over which the riser is secured and sufficient buoyancy to enable the arch to float at its design depth in operational conditions. The buoyancy may be provided by steel buoyancy tanks, by syntactic foam modules or a combination of both. Syntactic foam offers advantages in terms of service life and durability.

Buoyancy modules These components are common to the lazy and steep wave configurations, in which they replace the mid-water arch assemblies described above. They consist of syntactic foam modules or collars which are clamped to the riser over a section of its mid-length such that they support the riser catenary in its mid-water location under operational conditions. Riser base connectors These components are common to the steep-S and steep wave configurations. The lower catenary in both these systems does not allow the riser to touch down on the seabed under its own weight. The riser base connector is the seabed securing point for the lower riser flange and is pre-installed. It is necessary to physically pull the riser lower flange down into position onto the RBC, where it is then secured. Once the riser is connected to the RBC, the riser itself acts as a tether to restrain the mid-water buoyancy.

The installation scope of work The scope of work for flexible riser installation is here considered to consist of all works required to successfully install a riser system into its intended final configuration. This will include all pre- and post-installation work, in addition to riser system installation itself. Pre-installation work will include survey and debris clearance and the installation of seabed items such as the RSD or RBC. Post-installation work will include gauge pigging, pressure testing, subsea tie-in and leak testing.

Installation sequence The installation sequence may vary for specific riser systems; however, the following principles generally apply.

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Installation aspects of flexible riser systems. D.R.A. Johnston The riser system is installed with the FPF already on site. • The riser system is installed by first making the top connection to the FPF and then laying the catenary system to the seabed. • The riser system is transported to site in component or sub-assembly form. The flexible risers are transported on lay reels and the RRCs and any further components such as the mid-water supports, mooring lines and RBCs are transported as individual items. • The riser and component parts are assembled into a system during installation. •

It is to be noted that the sequences given here have been written as though for the installation of a single riser. Where a riser group is to be installed, the installation sequence need not alter but the individual risers within the group require to be deployed simultaneously.

Single catenary The following sequence of operations generally applies for installation of a single catenary riser system (refer also to Figure 6). Pre-installation survey Install RSD Attach RRC to riser Establish pull-in wire Deploy riser

Pull-in winch IFPF

P.ull-in

w~re

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Pull-in RRC to FPF Connect RRC Lay out catenary Overboard riser lower flange Lay riser over RSD Attach riser to RSD Lay out riser on seabed Lay down riser lower flange Gauge pig riser Pressure test riser Tie-in to seabed structure Leak test connections Post-installation survey

Lazy-S The following sequence of operations generally applies for installation of a double catenary lazy-S riser system (refer also to Figure 7). Pre-installation survey Install RSD Attach RRC to riser Establish pull-in wire Deploy riser Pull-in RRC to FPF Connect RRC Lay out upper catenary Deploy gravity base Attach arch

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Eng. Struct. 1989, Vol. 11, October

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Installation aspects of flexible riser systems: D.R.A. Johnston

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Overboard arch Deploy arch to seabed Lay out lower catenary Overboard lower flange Lay riser over RSD Attach riser to RSD Lay riser along seabed Lay down riser lower flange Gauge pig riser Pressure test riser Tie-in to seabed structure Leak test connections Post-installation survey

Steep-S The following sequence of operations generally applies for installation of a double catenary steep-S riser system (refer also to Figure 8). Pre-installation survey Install RBC Attach RRC to riser Establish pull-in wire Deploy riser Pull-in RRC to F P F Connect RRC Lay out upper catenary Attach arch Overboard arch Lay out lower catenary Gauge pig riser

Overboard riser lower flange Establish pull-down wire Pull-down lower flange to RBC Connect to RBC Pressure test riser Post-installation survey

Lazy wave The following sequence of operations generally applies for installation of a double catenary lazy wave riser system (refer also to Figure 9). Pre-installation survey Install RSD Attach RRC to riser Establish pull-in wire Deploy riser Pull-in RRC to F P F Connect RRC Lay out upper catenary Attach buoyancy collars Overboard buoyancy collars Lay out lower catenary Overboard riser lower flange Lay riser over RSD Attach riser to RSD Lay riser along seabed Lay down riser lower flange Gauge pig riser Pressure test riser Tie-in to seabed structure Leak test connections Post-installation survey

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Installation aspects of flexible riser systems." D.R.A. Johnston

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Eng. Struct. 1989, Vol. 11, October

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Installation aspects of flexible riser systems. D.R.A. Johnston

Steep wave The following sequence of operations generally applies for installation of a double catenary steep wave riser system (refer also to Figure 10). Pre-installation survey Install RBC Attach RRC to riser Establish pull-in wire Deploy riser Pull-in RRC to F P F Connect RRC Lay out upper catenary Attach buoyancy collars Overboard buoyancy collars Lay out lower catenary Gauge pig riser Overboard riser lower flange Establish pull-down wire Pull-down lower flange to RBC Pressure test riser Post-installation survey

Pull-in of RRC

Installation methods It is not possible to consider here all possible installation methods for all riser systems. Instead, some of the major operations are described and their applicability to specific riser systems are noted as appropriate. It is also to be noted that the installation methods described would be

MSL

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adapted, altered or alternatives developed to suit the specific needs of an individual riser system and the available installation spread.

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Installotion

wire

vessel

This task is a common requirement for all riser systems installation. The lower half of the RRC is first attached to the riser upper flange on the installation vessel. The pull-in wire is established between the FPF-based pull-in winch and the lower half of the RRC. The riser upper flange with the RRC attached is overboarded from the installation vessel using the vessel crane. By paying out on the riser installation reel and taking up on the FPF-based pull-in winch, the lower RRC is transferred until the riser is hanging vertically from the F P F (see Figure 11). The RRC lower half and the riser are then pulled up and mated with the RRC upper half, which is fixed to the F P F (refer to Figure 12). Once the RRC halves are mated, the hydraulic clamp within the RRC is operated to lock them together. A guide system is normally incorporated in the RRC design to ensure that the two RRC halves are correctly aligned as they are mated together. This may take the form of guide posts and guides. Unless the pull-in wire has been reeved to run through the RRC upper half, it may well be necessary to temporarily stop off the riser at the F P F and re-rig for final pull-in. Final pull-in operations are more easily monitored and controlled if

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Installation sequence for steep wave flexible riser system

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Installation aspects of flexible riser systems. D.R.A. Johnston Pull-in winch

FPF

l J J~\

Pull-inwire

Lay chute

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Fixed riser hard piping See level RRC upper half RRC lower half

Flexible riser

RRC upper half

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post Seabed ~/`\`~/~/~`\\y~X\`~`\X\`///~A\V/~X\\y~/h(\\XV~/~X\~/~\\\y~x\\\y~/~X*Xy~/~\\V///~X~//~6`\w~/~\~\

Figure 11 Initial pull-in o f the RRC (catenary geometry)

lower half the riser system design is such that the RRC upper half is fixed at deck level to the FPF.

Deploy mid-water buoyancy arrangements As previously described, the buoyancy arrangements for the double catenary riser systems differ. The buoyancy module or collar arrangement of the lazy and steep wave configurations require only to be attached at the appropriate point on the riser and overboarded during the lay operation. The mid-water arch arrangement for the steep-S configuration similarly requires to be attached to the riser at the appropriate point and then overboarded during the lay operation. For the lazy-S system the mid-water arch is to be moored to the gravity base. This requires that the gravity base and mooring line be deployed over the side using the vessel crane. The weight of the gravity base will then be transferred to a winch or stopper line whilst the riser is installed and attached to the mid-water arch. The arch is then attached to the mooring line and is overboarded using the crane (see Figure 13). The gravity base may then be lowered until the mooring line is taut; at this point the arch overboarding rigging is detached and the complete assembly is deployed to the seabed by paying out on the riser and lowering on the gravity base. Once in position on the seabed the gravity base rigging may be detached.

~xible riser

Figure 12 Final pull-in of the RRC

overboarded from the installation vessel with a dead man anchor (DMA) attached by a strop. The DMA will have been pre-attached to the riser lower flange. In order to pull the riser lower flange down to the RBC, it is necessary to transfer the pull-down wire to the RBC and to attach the end to an hydraulic puller or winch which is capable ofexerting the required pull-down load (refer to Figure 14). An alternative is to establish a wire from the surface and to attach this to the pull-down wire, which is rigged through the RBC. The pull-down can then be effected from the installation vessel whilst being monitored at the RBC by the diver.

Pull-down of riser onto RBC

Installation engineering

In both the steep-S and steep wave configurations the riser lower flange is connected at the seabed to a pre-installed RBC. The riser lower flange will have been

The engineering requirements for riser systems installation are partially pre-determined by the riser system design and the choice of the installation vessel. The

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Eng. Struct. 1989, Vol. 11, October

Installation aspects of flexible riser systems." D.R.A. Johnston F

~

Mid-waterarcharrangement

Lifting slings for overboarding Buoyancy

Lay chute

riser

Installation vessel Sea level

i

Riser lower end

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Pull-down

Win(:

RBC

Gravity base

Figure 13 Deployment of the mid-water arch (lazy-S system)

Figure 14 Pull-down of the RBC (steep-S and steep wave systems)

engineering process is both interactive and iterative, and requires an integrated approach to all major aspects. Major areas which require to be fully assessed include the following.

Develop installation methods and procedures The initial demand on the engineering function is to define the outline installation methods. This is a prime requirement which has implications for all engineering and operational aspects. Prior to developing full installation procedures it is necessary to confirm the acceptability of these outline methods. In order to do this, it is necessary to investigate the catenary geometry throughout the proposed installation sequence and to determine the associated pull-in loads required. On confirmation of the proposed methods the installation procedures, which detail all stages of the installation operation, may be prepared. In conjunction with procedure preparation, all necessary equipment and rigging will be identified, specified and/or designed. A mobilization plan is developed for the installation spread, equipment layouts are prepared and seafastening requirements are defined.

Catenary geometry analysis The catenary geometry and the forces imposed on the catenary are interdependent, the geometry being defined by the combination of the flexible riser characteristics and the magnitude and direction of the imposed forces. There are an infinite number of potential catenary geometries. In practice it is preferred to control the

catenary geometry and hence the forces, rather than vice versa. This is because the geometry may be more exactly monitored. It is therefore necessary to define an acceptable catenary geometry for stages throughout the installation operation. A static analysis of the catenary is made for sufficient stages to adequately model the installation. The catenary is mathematically modelled for each stage and analysed using conventional catenary theory and equations. The catenary is considered as 'heavy', the drag force is insignificant compared with the weight and hence is ignored. Because of the number of stages to be analysed and their iterative nature, the analysis is usually carried out using a suitable computer program.

Determine pull-in loads Pull-in loads at the FPF may be determined from the completed catenary analysis. Pull-down loads at the RBC for the steep riser configurations will require a separate analysis of this particular operation.

Specification, design and procurement In conjunction with the development of the installation procedures, the equipment, rigging and ancilliary items will be identified and seafastening requirements determined. All rigging will be required to be designed for its appropriate SWL. Seafastening design will require to take into account the installation vessel characteristics. Deck loading limits may affect equipment layouts. Equipment and externally supplied items will require to be sourced and procured at the appropriate time.

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Installation aspects of flexible riser systems. D.R.A Johnston

Installation spread The installation spread constitutes all equipment required to execute the installation scope of work. Selection of the marine spread (or installation vessel) has an impact on all aspects of the installation engineering. The installation spread incorporates all FPF-based equipment such as the pull-in winch and all installation-vessel-based equipment such as the lay reels and chute. An installation vessel is seldom selected specifically for its suitability for purpose. Selection must depend on availability and financial aspects, in addition to the other consideration of the installation contractor. The following factors may be considered as basic requirements with regard to assessing vessel suitability. It must provide a stable platform with adequate deck space and craneage capable of supporting the necessary operations. The vessel must be equipped with an efficient DP system which will enable it to maintain station in close proximity to the FPF. In general a saturation diving system will be required.

Associated operations These incorporate ancilliary operations which are required to execute fully the installation scope of work. These include survey, navigation and positioning, diving, gauging and hydrotesting.

normally require saturation diving techniques and the associated equipment and personnel. In addition, if the riser system design has incorporated the RRC on a lower brace of the FPF, then it is likely that air diving intervention from the FPF will be required to assist during final pull-in of the RRC and possibly during leak testing of the completed connection. This necessitates the provision of an air dive station on the F P F at an allocation convenient to the RRC location.

Gauging and pressure testing On completion of the lay it will be necessary to gauge pig the riser. This operation may be initiated from either end of the riser, depending on the system design and installation methods. Normally the pig will have been pre-installed in the riser and it is only necessary to connect the flooding hose to run the pig. Where possible it is more convenient to pig from the seabed end to the FPF, as this can eliminate the need for subsea recovery of the pig. With the steep-S and steep wave riser configurations pigging may be carried out prior to deploying the riser lower flange to the seabed. It is also necessary to pressure test the installed riser. If this operation is carried out prior to tie-in work, then it will also be necessary to leak test the connections after tie-in.

Operational aspects Survey, navigation and positioning Accurate positioning is essential to the riser installation operations. Control and monitoring of the laying operations requires knowledge of both real vessel position and relative catenary position, the catenary geometry being monitored by the length of line payed out, the vessel position and the position of the RRC during pull-in to the FPF. In order to allow safe diving operations, the vessel DP system should be equipped with three independent reference systems. Typically these would be two taut wire systems and one acoustic system. The latter may be a long baseline system in order to achieve high positional accuracy during lay operations. To determine relative vessel position, it may be possible to utilize an Artemis beacon on an existing platform (depending on the field geographical location) or a suitable Syledis or similar chain may be available. An alternative would be to install an Artemis beacon onto the F P F although, as this is itself a floating and moving structure, the position fixing is not as reliable as it would be from a fixed structure.

The successful completion of riser installation works depends on efficient project organization with defined responsibilities. Two operational aspects which are of considerable importance are safety and communications. The safety aspects of all operations must be carefully considered prior to commencing the work. All procedures should be reviewed with this in mind and any areas of doubt must be clarified. Much of the work to be undertaken during riser installation is of a potentially hazardous nature. It is necessary as a minimum to ensure that all legal requirements are complied with; however, not all eventualities are covered by the legalities and much is subject to interpretation of the requirements. Diving operations in particular must be carefully planned, especially as the work is in close proximity to the F P F and its associated mooring pattern. Reliable and efficient lines of communication do much to ensure the safety of the work being undertaken and it is worth taking care that these may be achieved. Defined responsibilities for specific tasks ensures that all concerned are aware of their roles as the work progresses.

New developments Diving Diving operations are an integral part of the riser installation and are required at various stages throughout the work. The extent of the diving operations depends on the riser system design and the installation methods. All riser installation works will require seabed intervention work, e.g., attachment of the riser to the RSD or pull-down of the riser lower flange to the RBC. These works will generally be carried out from the installation vessel, which therefore requires to be capable of supporting diving operations at the seabed water depth. This will

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This section addresses some areas of new development which have an impact on the installation aspects offlexible riser systems.

Multi-bore risers An alternative to the riser group arrangement described previously is the manufacture of multi-bore risers which contain two or more bores within a single riser. In terms of installation a multi-bore riser would be installed as a single riser, thus eliminating the complexity arising from

Installation aspects of flexible riser systems: D.R.A. Johnston installation of a riser group consisting of two or more individual risers, each requiring their own installation reels and simultaneous deployment.

It is most likely that significant differences may arise in the design, construction and hence installation of the RRCs.

Floating production ships

Concluding observations

The proposed use of floating production vessels of conventional monohull ship shape does not in itself require major deviation from present flexible riser system design or installation philosophy. However, as for all riser systems, they will incorporate components which are for a specific purpose and which may necessitate alteration or adaptation of the installation methods in current use.

Riser system designs and hence their associated installation procedures are not 'off the shelf' commodities. The outline methods for installation are dictated by the design, and the practical aspects of any proposed design must be considered in detail from the outset of the design process if an optimum is to be achieved between the design and the installation requirements.

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