RISER, PIPELINES INSTALLED IN GRIFFIN FIELD

A mooring riser and flow lines along with a 67-km, 8 in., gas export pipeline have been installed offshore Australia for BHP Petroleum's Griffin field development.

The 66 km gas line will carry Griffin field gas to an onshore gas processing plant.

Completing the projects ahead of schedule was Clough Stena joint Venture (Asia), Perth. BHP awarded the contracts in early 1993; the project was completed in January this year.

HEADQUARTERS

Two project engineering teams assembled in Clough Stena's Perth headquarters, center for all engineering, procurement, subcontract, and project management operations.

Mobilization and logistic operations were carried out in Europe, Singapore, Malaysia, the U.S., and Dampier because Clough Stena was also responsible for the transportation of such materials as flow lines, control umbilicals, mooring chains, drag embedment anchors, gas export line pipe, mid depth buoys, and clump weights.

Clough Stena has said that the technically difficult mooring and flow line contract required installation of six 860 m, 4 in. diameter chain mooring legs with piggyback drag embedment anchors pretensioned to a set load of up to 500 metric tons.

After completion of mooring leg installations, the 1,200 metric ton mooring riser was towed into the field, upended, and connected to preinstalled mooring chains (Fig. 1). The mooring riser then received final heavy ballasting.

Clough Stena adopted its previously used method of mooring a converted flat top construction barge alongside the dynamically positioned construction support vessel, Essar Stena 1, to install the mooring system (OGJ, Aug. 9, 1993, p. 33).

All temporary works and installation aids for the mooring and flow lines contract were fabricated in Singapore by Clough Petrosea. Mooring installation equipment, supplied by BHP, arrived at Singapore and was loaded onto the construction barge which was mobilized to the field in early September 1993.

PIPELINE INSTALLATION

After completion of the mooring, the construction barge was demobilized, and the Essar Stena 1 began installation of the flow lines (Fig. 2).

Clough Stena says this part of the project required installation of six mid depth buoys (Fig. 3), segmented clump weights weighing up to 400 metric tons, and 82 km of flexible flow line and control umbilical.

Anticipated installation loads of the mid depth buoys and clump weights required mobilization to the field of a derrick barge with a crane capacity greater than 500 metric tons.

Clough Stena subcontracted McDermott's derrick lay-barge DB17 from Dampier on a bare boat basis. The vessel was crewed and mobilized by Clough Stena to transport 2,000 metric tons of permanent materials to the field. The vessel was also employed for the dual purpose of installing the mid-depth buoys and clump weights and for laying the 8-in. gas export pipeline.

That phase of the work marked Clough Stena's first rigid pipelay, says the company. Each mid depth buoy and clump weight system was assembled and connected aboard the DB17. In a two part lift operation, the buoys and clump weights were lowered to the seabed.

Acoustic transponders were used to position the clump weights; remotely operated hydraulic shackles released the mid depth buoy lift rigging before recovery of the rigging to the surface.

The entire deployment was engineered as a diverless operation.

GAS LINE LAY

After completion of the mid depth buoy and clump weight deployment operation, the laybarge was immediately mobilized to the gas-export pipeline shore crossing location.

The 3 km shore crossing section of the pipeline, installed by Clough Stena, was performed by bottom towing of six 500 m long pipe strings which had been assembled on a beach lay-down area.

On arrival at the shore crossing location, the laybarge recovered the preinstalled pipeline and began pipelay operations out to the Griffin field. Pipe haul was carried out with the use of supply vessels loaded out from Dampier harbor.

The entire 62 km pipeline installation was completed in early November 1993 before pigging and gauging of the line. An expansion spool was installed to effect the tie in of the gas pipeline to the subsea production skid before hydrostatic testing.

The 3.8 km nearshore section of pipeline was stabilized with a special purpose air water jet sled supported by a small construction barge (Fig. 4).

An advanced subsea plough system and special purpose support vessel, subcontracted from Northern Ocean Services and mobilized from the North Sea, was used to stabilize the pipeline from kilometer point (KP) 4.5 to KP 39.

Two advanced rockbolting drilling rigs were designed, constructed, and commissioned by Clough Stena to anchor the pipeline mechanically to the seabed in areas considered unploughable because of seabed conditions. The pipeline was fully stabilized by mid January 1994, 2 months after pipeline installation.

The project's teams managed installation operations which involved up to 400 construction personnel, deployed both ashore and at sea, and mobilized 15 construction vessels from the North Sea, Singapore, and Australia to the Griffin field.

Construction vessels mobilized included the dynamically positioned dive support vessel Essar Stena 1, the derrick laybarge DB17, an advanced subsea plough-support vessel Northern Explorer, specially converted construction barges, the heavy lift vessel Project Europa, and numerous anchor handling and accommodations support vessels.