BUNDLED PIPE SPEEDS OFFSHORE LAYING

John Brockbank
Avon Industrial Polymers Ltd.
Melksham, Wiltshire
U.K.

Technology which allows pipelines to be installed in bundles is expediting pipelay operations, especially in the North Sea.

The "piggyback" system was recently used on 60 km of North Sea gas pipelines for three major projects: Britoil's Amethyst project, ARCO's Welland, and Hamilton Bros.' Ravenspurn (Fig. 1).

For the past 7 years the practice of installing two or more pipelines in one operation has become an established practice for North Sea offshore oil and gas projects. The technique, commonly referred to as a "piggyback" operation, reduces installation costs, improves operation reliability, and cuts maintenance time.

Avon Industrial Polymers Ltd., a wholly owned subsidiary of Avon Rubber plc., has been involved in the detailed design, testing, manufacture, and installation supervision for a number of these major piggyback installations.

INSTALLATIONS

To date, more than 350 km of piggyback pipeline have been installed.

The first North Sea project to use the technique was the Conoco Victor development. This is an unmanned gas platform in the southern sector of the North Sea producing gas and condensate.

This mixture is transported back to the nearby Viking complex through a 13-km, 16-in. nominal ID concrete-coated line. To ensure hydrate control and reduce internal corrosion of the pipeline, methanol is injected at the platform.

This methanol is reprocessed and returned through a 3-in. nominal ID fusion-bonded epoxy (FBE) line. This 3-in. methanol line is piggybacked on top of the 16-in. line. The installation was carried out in the spring of 1984 by Saipem's Castoro Sei (C6) lay barge.

The piggyback system in this case involved use of special cast nylon blocks and a high-strength, nonmetallic Kevlar strapping system.

Since this first installation, more than 12 major piggyback lines (i.e., lines of 20 km or longer) have been installed incorporating a number of different materials.

Hamilton Bros. and Maersk Oil & Gas A.S. used the system in 1985, and Shell, Phillips, British Petroleum, Norske Hydro, and Mobil have followed suit.

As operators have recognized the overall cost savings in using totally unmanned platforms and bringing unprocessed oil and gas either to mainland production and processing facilities or to existing platforms, the number of small service lines coupled together with production lines has increased.

In 1987 the concept was developed further for the dual lay of 6-in. lines for the Mobil Ness project (Fig. 2).

The concept has now been used for dual lay of small-diameter lines (i.e., 6 in. nominal bore) and consideration is being given for laying umbilicals and cables linked to rigid pipes.

The technique leads to savings in maintenance costs because ROV inspection of a dual-pipe system becomes a single operation rather than two separate ones. Such regular inspection of operational pipelines in the U.K. North Sea is mandated by the U.K. Department of Energy (DOE).

DESIGN CONSIDERATIONS

Designing a cost-effective and technically reliable piggyback attachment system requires that all aspects relating to the requirements of the operator, the pipelay contractor, and the system manufacturer be considered.

The operator. The conditions for pipeline operation in the North Sea are well documented. Additionally, the U.K. Department of Energy (DOE) has issued operational guidelines that must be followed.
The piggyback system is not directly affected by these guidelines, but no system should run counter to the overall statutory requirements.
The vast majority of pipelines are buried in a trench to provide an element of safety from dropped objects and fishing trawl boards.
Trenching (which is normally a post-lay operation) is of course expensive. One advantage of piggyback systems and dual-lay pipes is that only one trenching operation is required, with the associated cost saving.
Therefore, any piggyback system has to be capable of resisting any of the forces resulting from the trenching operation.
The operator is responsible for the pipeline throughout its operational lifetime which can be expected in some cases to exceed 30 years. For maintenance reasons and structural integrity, he will require that the piggyback system have a lifetime equal to or greater than the pipelines themselves.
Therefore, close attention must be paid to the materials used and their lifetime expectancy in a subsea environment. In addition, none of the materials used should damage the pipe coating.
Further considerations involving the operator are maintenance requirements, the spoolpiece tie-ins, and the overall efficiency and integrity of the system during the pipelay operation. This aspect is important as it relates to the overall installation cost.
An important consideration for all pipeline engineers is "upheaval buckling" resulting from thermal expansion of the pipeline. in the case of piggyback systems in which there could be a temperature difference between the two lines, it is important that the lines be free to move axially relative to each other.
The pipelay contractor. Currently, there are four major pipelay contractors operating in the North Sea: European Marine Contractors (a Saipem/Brown & Root joint venture) and the ETPM/McDermott joint venture. Both operate conventional laybarges.
Allseas bv operates its specialist DP laybarge Lorelay, and Stena (formerly Santa Fe) operates its reel barge the Apache.
In general terms, the conventional lay-barge operators handle pipeline installations of 24 in. and larger. The smaller diameter lines have been laid by the Lorelay and the Apache.
All these contractors have been involved with piggyback systems.
In each case the requirement from the contractor for the system may be significantly different from a design viewpoint. At the same time, it must achieve the operators' specification requirements for the final installed pipeline.
The major concern of all the installation contractors is assembly and installation time for the piggyback assembly. The time taken for assembly should not hinder the overall laying rate. The system should use simple tools suitable for use by offshore personnel.
The system manufacturer. The system manufacturer must provide a system which meets the requirement of the operator and the installation contractor. At the same time, the system must be capable of being manufactured in a relatively short time-scale using known and proven manufacturing techniques and good quality-control techniques.
Often design changes in pipe and anode sizes occur within a relatively short period before installation is to begin.
In the case of the Hamilton Bros.' Esmond field project, more than 12,000 piggyback assemblies were designed, tested, manufactured, and delivered in a 6-week period.

DESIGN CRITERIA

As an example, the design criteria for the recent ARCO Welland project specified as follows:

The MEG line is to be secured to the export pipeline on the lay barge following infilling of the export line field joint. The design of the supports and straps and the method by which they are installed shall avoid hindering the pipelay operation.

Fitting time shall be less than 2 min/support.

Following installation, the pipeline will be trenched. The design of the supports and straps shall be such that they are unobtrusive during trenching.

Because sections of the pipeline will be subject to burial by rock dumping, the design of the support straps shall preclude their being damaged by the rock dump.

The design life of the support and straps is 25 years. Neither supports nor straps shall require maintenance over this period and should suffer no deterioration from corrosion or from chemical or biological degradation.

Each support and strap arrangement shall resist vertical and horizontal (not simultaneously) forces of 3 tons applied through the axis of the MEG line.

The support and straps shall be able to withstand forces applied during pipelay. The support and strap arrangements shall permit longitudinal movement of the MEG line relative to the supports.

Two supports, each 3 m from the field joint, shall be used on each length of the main pipeline.

The strap arrangement shall be such that there are no free ends. Straps may be trimmed, following installation, to satisfy this requirement.

PIGGYBACK CLAMP, MATERIALS

The piggyback clamp design arrangement consists of a nonmetallic pipe-support block together with a suitable strapping system. In each case the system is designed to suit the particular application taking into account the design considerations and criteria discussed earlier. The clamps can be classified into three categories:

Piggyback saddles, which attach and secure a service line on top of a main flow line (Fig. 3), as used by Tenneco Oil offshore Gabon (Fig. 4).
Spacers, which are used to attach and secure two horizontally adjacent pipelines (Fig. 5), as used for Mobil's Ness project,
Bundles, which will secure three or more pipelines together in any configuration (Fig. 6), as proposed for Norske Hydro's Togi project.

In most cases the clamps have been manufactured in an elastomeric material, predominantly rubber, although nylon and polyurethane have been used.

The Avon elastomer clamps are manufactured by an extrusion process from an ethylene-propylene-diene terpolymer (EPDM) rubber compound. This is a synthetic rubber extensively used and well proven in marine environments.1

EPDM has a high-impact resistance and therefore absorbs shock loads which could be experienced during installation and the operational life of the pipeline clamping system.

The flexibility of the rubber allows the saddle to conform to the contour of the main flow line without imposing any detrimental point loads.

The high level of friction experienced between the base of the clamp and the main flow line gives firm positioning and thereby resists rolling and twisting forces, which can be experienced during the laying operation.

The rubber block can be used on any form of pipe coating without damage, even if the friction is exceeded.

STRAPPING

Strapping is used in order to ensure that the integrity of the pipeline clamping system and the pipelines are maintained.

There are six parameters which determine the choice of strapping:

The arrangements and number of pipelines
The stresses incurred during the laying operation
The operational life of the pipeline's clamping system
The coating of the main flow line pipe
The method of assembly and installation time
The type of installation vessel.

In the past, two major categories of strapping have been used:

Metallic banding, which includes conventional carbon steel, stainless steel 316, and alloy 625; and nonmetallic strapping which includes monofilament Kevlar-reinforced straps and monofilament polyester-reinforced straps. The particular type of banding chosen depends upon the combination of strength and corrosion resistance required. For the Phillips Audrey Phase I project, three straps were used: two in conventional steel and one in alloy 625.

The metallic banding is secured with a crimp of the same metal chosen for the band. Pneumatic tools with a preset torque will both tension and crimp the band.

Metallic banding has been used on all types of pipelines even with sensitive coatings. For Hamilton Bros.' Ravenspurn project, the FBE coating was protected by lagging the pipe locally with square woven fabric-reinforced rubber sheets prior to application of the straps.

The particular type of nonmetallic strapping used depends on the combination of strength and extension required. Single straps with nominal breakloads exceeding 7 tons have been used.

A tensioner consisting of two bobbins and a capscrew manufactured in stainless, duplex stainless, or nickel-based alloys is used to secure and tension the strapping. The operational lifetime requirement for the pipeline clamping system determines the choice of metal for the tensioner. This system was used in 1987 for the BP Villages project incorporating Kevlar straps of 5 tons breakload and Ferralium 525 tensioners.

SYSTEM VALIDATION

After the design has been completed and calculations made to prove that the system will theoretically achieve the design requirement, a system validation test is usually conducted. At the same time this is a useful opportunity for the contractor to ascertain the actual assembly time for each unit.

A typical validation test will include direct loads on a system assembly incorporating two short lengths of pipe and the piggyback clamp. This is achieved by a simple test rig.

The systems have been designed specifically for fast and simple installation. The one-piece rubber saddle is fitted by deformation of the saddle around the small-diameter pipe, in effect hinging back on itself.

The strapping is wrapped around the whole system and tensioned in place with a combination air-powered strapping tool. Care has to be taken to ensure the tool is preset at the correct level to create the right pre-tension.

Fig. 1 shows the assembly on board the Saipem Castoro Sei lay barge for the Britoil Amethyst project. With two men working on each side of the pipe, each assembly was taking less than 1 min.

To date, all the piggyback lines have been installed successfully and are in full operation. In some instances, the piggyback lines have been subjected to impact by trawl boards, both during installation and in service.

In some of these cases, no damage was experienced; in others, the straps actually failed but were easily replaced by divers.

This happened during the installation of Maersk Oil & Gas' Rolf pipeline by Brown & Root's Semac vessel.

There was no permanent damage experienced by the pipeline.

ACKNOWLEDGMENT

The author wishes to express appreciation to the following for their cooperation in preparing this article: European Marine Contractors Ltd., Allseas bv, and McDermott International.