INSTALLING COATED PIPE - CONCLUSION FOAM INSULATION GETS FIRST REELED INSTALLATION OFF AUSTRALIA

Michael I. Mollison
Esso Australia Ltd.
Melbourne

Development in 1989 of Seahorse and Tarwhine fields in the Bass Strait, offshore Australia, promoted the first reeled installation of the thermally insulated pipeline system known as pipe-in-pipe (PIP).

This system consists of an inner steel pipe coated with high-density polyurethane (HDPU) foam inside an outer steel carrier pipe. The offshore lines for which PIP was finally chosen are 7 and 10.8 miles, respectively, connecting the Barracouta offshore platform and subsea wellheads at each field location.

Several alternative coating systems were evaluated, as reported in Part 1 (OGJ May 11, p. 52) of this series. This conclusion chronicles coating application and shore fabrication of PIP for this project.

Before development of PIP, the high cost of reel laying available insulation-coating systems capable of withstanding the high contact loads reduced the cost benefits of reeling for insulated pipelines

In early 1988, the Esso Australia-BHP Petroleum joint venture was considering the feasibility of installing a number of pipelines, including two insulated lines, by the reeling method. The insulated lines were required for the development of the Seahorse and Tarwhine offshore oil fields.

In addition to the insulated lines, several uninsulated lines were required for the concurrent development of three other fields.

To allow reeling to be used for all these lines and to ensure that the overall cost saving was not offset by the cost of coating the two insulated lines, a less expensive reelable insulation coating option was sought.

The abilities of the various candidate systems to withstand the high contact loads and strains imposed during reeling and straightening were tested on a purpose-built frame (Part 1, Fig. 1) which simulated the reeling process on full-scale specimens (Part 1, Table 1).

Successful development and installation of PIP by reeling have demonstrated that a much cheaper alternative to the expensive coating system is available and has made reeling of insulated lines much more attractive.

COATINGS, CONFIGURATIONS

The pipeline configurations adopted for the Seahorse and Tarwhine pipelines are detailed in Table 1.

In each case, forged and machined bulkheads are installed at the ends of the pipelines to seal the annuluses between the inner and outer pipes. The bulkheads are particularly strong because they must carry the large shear load developed in-service by the thermal expansion of the inner pipe against the outer pipe.

In order to minimize stress concentration at the bulkheads, their cross-section incorporates radiuses and long transitions at locations where wall thickness changes occur (Fig. 1).

The polyurethane foam coating system consists of foam applied to a grit-blasted pipe. The foam in turn is coated with a thin film of water-based paint to protect it from rain and ultraviolet degradation during transport and external storage.

Foam was spray applied to the pipe following grit blasting and heating of the pipe. The spray chemicals were supplied by the manufacturer in two components, an isocyanate and a polyol. The two components were mixed at the spray-head.

The pipes were held stationary but rotated on their axes for foam application. The spray head was attached to a moving cart which traversed past the pipe at a fixed distance from the pipe. Adjusting pipe rotation and cart traverse speeds fine-tuned the foam thickness and spray pattern (Fig. 2).

Temperature control of both the chemicals and the pipe surface was critical in obtaining the correct foam density which determined material strength and insulation capability. In addition, variations in temperature and humidity within the factory affected the success of coating application.

Inexperience with application of foam of relatively high density resulted in delays at the beginning of the job due to late modifications to plant and chemicals.

The nature of the chemicals combined with spray application required that all personnel within the factory wear masks and full protective clothing.

The quality of the end product was high, with good control of foam density and thickness eventually achieved. Because the coating had to be applied within fairly tight thickness and density tolerances, the task of inspection was even more important than usual.

FABRICATION

Fabrication of the pipeline ashore in preparation for reeling by the Apache required formulation of some unique construction procedures. Fabrication had to take into account a number of features of PIP construction:

The relative ease with which the foam could be damaged
The method by which foam field joints were made
The maximum length each PIP sub-stalk could be made being the maximum distance the inner pipe could be dragged through the outer pipe
The configuration of tie-in joints between PIP substalks and method of construction.

The inner polyurethane foam-coated pipe and outer carrier pipe were both fabricated on parallel "firing" lines in 200-m sections (Fig. 3). This length was adopted following pull-through trials and observations of little damage to the foam after being dragged over 200 m and in particular the weld beads on the inside face of the carrier pipe.

In order to prevent contact damage to the foam coating, the pipe roller supports in the "firing line" for the foam consisted of pneumatic tires set in a V-configuration. The field joints in the foam were fabricated with split sleeves manufactured by the pipe coating contractor.

The sleeves were sealed by injection of small amounts of foam mixed on site. The joint was then wrapped with self-adhesive tape to hold the sleeve assembly in place,

Prior to insertion of the 200-m sections of inner pipe into the outer pipe, the sections of outer pipe were drifted and cleaned with a pig assembly consisting of a drift, a brush pig, and a foam pig.

This was done to ensure there were no obstructions and that scale and moisture were, as far as possible, removed prior to pipe insertion.

The inner pipe was pulled through the outer pipe with a cable and winch. A centralizing head was welded to the leading end of each inner section to ensure that the leading edge of the foam was not damaged during pull-through.

The outer pipe was on most occasions dried with a hot-air dryer to minimize the amount of moisture inside the carrier.

When several 200-m sections of PIP had been made up, they were assembled into 1,400-m stalks ready for reeling. The tie-in between sections was achieved by wrapping a ceramic fiber tape over the uncoated field joint area following welding of the inner pipe. The outer pipes were then pulled together for welding with sidebooms.

The ceramic fiber tape was used because it was unaffected by the heat of welding of the outer pipe and provided the required thermal insulation at the field joint.

Tie-ins between the 1,400-m strings during reeling of the pipe were performed in similar fashion, The only difference was that the two outer pipes were brought together with a jacking assembly fitted to the free ends of the two pipes 1,400 m from the joint.

The jack pulled against the inner pipe and pushed the outer pipe forward towards the joint. This procedure minimized tie-in time by allowing a relatively simple joint configuration to be adopted (Fig. 4).

During reeling, little movement was observed between the inner and outer pipes at the free end. Inward movement of the inner pipe of typically 30 mm occurred. Most of this was attributed to partial relaxation of tension in the inner pipe following extension of the pipe during jacking for the tie-in connection.

STRAIGHTENING TRIALS

Straightening trials of a 130-m PIP string were performed with the reelship. This was to check the straightness of the PIP assembly following reeling and straightening and to assess the reelability of intermediate bulkheads between the inner and outer pipes.

The trial involved loading and reeling the trial string onto the reelship followed by straightening and measurement of the string.

The PIP reeled and straightened satisfactorily and in the same fashion as a single line. Acceptable straightness of the PIP assembly was achieved after minor adjustments to the straightener setting.

Although the Seahorse and Tarwhine pipelines were laid without any intermediate bulkheads, a fullscale trial of their reelability was performed for future reference. Introduction of an intermediate bulkhead into the PIP string did not affect how the line reeled or straightened.

The length over which a bulkhead occurs is so short that the increased plastic bending resistance at that point does not affect the shape of the plastically deformed pipe on the reel or its behavior through the straightener.

A series of nondestructive examinations of the bulkheads following reeling and straightening established that no defects were introduced into the bulkhead or weld connections by the process. The trial established that bulkheads can be reeled without difficulty.

PIPELINE INSTALLATION

The pipelines could not be laid in one length due to the length and weight limitations of the reelship. This called for the development of tie-in procedures and a connection configuration which would minimize tie-in time offshore without compromising the integrity of the connection.

The tie-ins at the midline connections were made by a butt-weld connection of the inner pipe which was radiographically inspected. The uncoated length at the weld area was then wrapped with ceramic fiber tape.

A steel sleeve which had been slipped over the outer pipe prior to welding of the inner pipe was then slid over the joint and lapped over both outer pipes. The connections between the carrier and sleeve were then made by two circumferential fillet welds. The ceramic tape was used because it could withstand the heat of welding and provided adequate thermal insulation at the joint (Fig. 5).

The two pipelines are now successfully in operation. The thermal insulation provided by the PIP system has been greater than expected, a fact attributed primarily to the added benefit of the air gap between the coated inner pipe and the outer pipe.