Semi-Submersible Transshipper, Indonesia

Seabulk Systems Inc.

Category: Natural Resources and Industry

Seabulk Systems has developed the Semi-Submersible Transshipper (SST) to transfer bulk cargo from barges into ships on the open sea. A prototype unit has been built and is currently in operation, offloading coal into ships off the coast of Indonesia.

Unlike conventional grab crane operations, the semi-submersible transshipper transfers cargo at 2,000 tonnes per hour with zero tolerance in terms of loss or damage to the material. Seabulk, which has won two previous Canadian Consulting Engineering awards for material handling systems, was involved in the design of the semi-submersible platform and the entire unloading system. The invention is protected by U.S. patent.

The transshipper is designed to operate in most sea conditions that would normally permit coastal barging operations. Numerical models to assess relative motions between the transshipper, bulk carrier and barge, and considering wave height, interval and direction, have shown that in all but extreme cases, the vessels can operate together.

The unique features of the transshipper are as follows.

Hull form

In typical transshippers, twin hull catamaran barges are tied together at the deck level. The barges are not submersible.

This transshipper uses two semi-submersible pontoons. They are tied together at an elevated platform through eight vertical columns that also have a ballast capability.

The design of such a platform required original analysis. The challenge is having to deal with wave forces being transferred from the pontoon to the platform, combined with lateral berthing forces from the barge located between the pontoon. To deal with these problems, special features are included in the semi-submersible transshipper’s design, such as the crown arches connecting the platform and columns, and internal diaphragms in the pontoons. The additional feature of a ballastic pontoon and columns has no precedent. While submersible barges are in existence and submersible platforms with columns also exist, the use of the barge for buoyancy during transit and the ability to ballast the pontoon and columns in order to minimize the water plane area are unique developments.

A second unique feature is the use of asymmetrical hulls in the catamaran. They provide additional buoyancy to offset the weight of the shiploading boom, which is carried entirely by the inboard pontoon. The effect of an asymmetrical hull on towing requirements while the transshipper is being transported presented a new problem that also required original research.

Materials handling

The use of chain bucket reclaimers and shiploading booms is very common in land-based bulk material transshippers. In this case, however, a system was developed to operate at high speed in an open sea environment. The unique features of the unloading system include:

A computer-controlled electro-hydraulic system capable of automatically adjusting the marine leg bucket elevator to deal with material surges. If the bucket digs into a stockpile of heavy material like iron ore, the bucket speed is automatically adjusted to maintain a constant unloading rate. Additionally, the marine leg hoist senses the surge load, automatically lifts the leg out of the pile and generates an overpressure to the hydraulic system to empty out the buckets.

The self-tensioning hydraulic winch automatically compensates when waves cause a barge to shift between the transshipper’s pontoons. The winch also controls the digging force into the material pile, thus minimizing the effect of this operation on the stability of the barge.

The high speed of the reclaim system means that the barge and the transshipper’s platform have to be moved continually in order to unload the barge. This movement must be in a controlled manner if the barge is not to hit and damage the transshipper’s columns or the offloading ship. To achieve this control, there are four hydraulic traction winches and four self-tensioning hydraulic mooring winches that are operated through a central computer station. The winches not only hold the platform and barge against the ship side, but also traverse the barge at an adjustable speed. The traction winches are also used to steer the barge safely between the transshipper’s pontoons.

The unloading system includes two luffing/slewing scrapers attached to the marine leg reclaimer. The scrapers are adjustable and deal with variations in the width, height and configurations of cargo holds in different barges. The scrapers have electro-hydraulic controls that compensate for movements of the barge in relation to the marine leg reclaimer, and also have a surge load protection in the event of the scrapers getting flooded in the cargo pile. These features are unique to marine applications. Land-based systems currently in use do not have these requirements.

A transfer conveyor that pivots around a loading chute luffs up 7 degrees and down 15 degrees based on the position of the marine leg reclaimer, thus minimizing degradation of the material product. The support tower is used as a sampling station.

The telescopic boom is equipped with an articulated chute for loading material into a ship’s cargo hold.

Cost and schedule

Due to the prototype nature of the project, the original budget of US $8 million was exceeded by 12%. The construction schedule of nine months was also delayed when the shippers in Indonesia included new operational requirements.

Seabulk has, however, gained considerable experience from this prototype development and is presently working on a second semi-submersible transshipper that will meet cost and schedule targets. Seabulk’s engineering, procurement and construction management fees totalled US $500,000.CCE

Project name: Semi-Submersible Transshipper

Award winner: Seabulk Systems Inc., Richmond, B.C. (engineering prime consultant)

Project team leaders: Sidney Sridhar, P.Eng., Patrick Kennedy, P.Eng., David Popoff, P.Eng., Dale Chen, P.Eng., Anna Klimek, P.Eng., Peter Phillips, Eric Hurst, Rob Snider, Lu Chang Li, Rafael Santos, Bong Chung

Client/Owner: Semisub Transshippers Inc.

Other key players: Marine Research Institute of China (MARIC) (naval architect); Fleet Technology (marine engineers); Westinghouse (PLC control system)

Semi Submersible Transshipper, Indonesia