簡介：對于安裝海洋平臺上部模塊來說，系泊系統(tǒng)能起到兩個作用，第一固定船舶在固定位置，等待合適的海上作業(yè)天氣，第二是能夠為船只進(jìn)入殼體槽提供動力控制。Dockwise 利用半潛平臺和重型海工起重船進(jìn)行安全的海上模塊安裝。

One of the major challenges in a floatover installation is designing appropriate mooring systems. During a floatover, mooring systems serve two purposes. First, the system holds the vessel in position while waiting for a weather window. Second, the system pulls the vessel into the jacket slot while controlling the motions.

Floatover installations are quickly becoming the method of choice for installing platforms in excess of 10,000 metric tonnes, which is the maximum lift capacity of current crane vessels. With this technique, Dockwise uses semisubmersible and heavy-lift vessels to safely install offshore structures onto pre-installed jackets or floating hulls.

Improving performance

On Aug. 4, 2009, the Black Marlin semisubmersible heavy-lift vessel installed the Muda processing platform (MDPP) in the Gulf of Thailand for operator Carigali-PTTEPI Operating Co. Sdn Bhd (CPOC). The Black Marlin was nominated as the ideal vessel to accommodate the weight of the topsides ? 17,200 metric tonnes ? and the width of the 138.8-ft (42.3-m) jacket slot.

The vessel’s 137.8-ft (42-m) wide hull enabled her to fit perfectly between the sway fenders leaving less than a 6-in. gap on each side. To keep the Black Marlin (including the MDPP deck) in the desired position throughout the floatover, the vessel had to be hooked up to a pre-installed mooring system on the Muda field.

For this installation, two combined mooring systems were used. The anchor moorings held the vessel in the stand-off position while preparations were being made and the company awaited favorable weather conditions for the floatover operation. The mating moorings were used to move the vessel into the jacket slot and keep her in position for the mating operation.

To provide a safe working environment, the layout of the deck was designed such that all of the equipment was placed on an elevated working platform creating sufficient clearance from the clearly marked hazardous areas (e.g., the mooring lines under tension and running over the deck). In compliance with safety rules and regulations, all designs were checked by the Marine Warranty Surveyor, Flagstate, and the Classification Society.

The mooring and mating analyses showed the capacity of the mooring system and determined the particulars for the sway fenders, the leg mating units (LMUs) and deck support units (DSUs). Mooring and deck-mating equipment (winches, wires, anchors, tugs) allowed for better maneuvering of the vessel into the jacket slot.

Meeting challenges with innovation

Three challenges were encountered while designing the anchor mooring system. One was the squalls, short gusts of high winds that are common in the Gulf of Thailand. Second, pipelines on the seabed restricted where anchors could be placed. Third, previously installed platforms obstructed the ideal layout of the anchor pattern.

Data from the nearby Dulang site illustrated the need to consider an extreme squall wind speed of 62 ft/sec (19 m/sec hourly mean). This compared to the “typical” wind speeds, which remain below 39 ft/sec (12 m/sec) for more than 99% of the measurements. With the topside on the vessel, the total transverse wind area was nearly 75,000 sq ft (7,000 sq m), making the vessel behave like a large sail. Designing for the squalls would have been a very expensive option, with a minimal increase in operability.

Dockwise found a better solution. Two of the onsite tugs (which were needed for other operations) assisted the anchor moorings during squalls.

To deal with the pipelines, damage-prevention buoys were installed to maintain a minimum clearance. The positioning of these buoys was critical as the vessel position changed, so the buoys were not always directly above the pipelines. Selecting an appropriate position for each buoy is not as straightforward as for a stationary system. In the end, the selected positions provided the required clearance of 33 ft (10 m) vertically at the critical stages of the operation.

Dockwise supervised the two anchor-handling tugs (AHTs) that laid the four, 12.5-metric-ton mooring anchors well in advance of the floatover operation. The anchor moorings encompassed a system of four lines, composed of 52-mm steel wire and 76-mm chain. The maximum tension in the lines was calculated at approximately 115 tonnes.

In the design of the mating moorings, the main issue was to position the vessel and dampen the vessel motions. Due to the relatively short length of the mating mooring lines, stretchers were used to increase the flexibility and decrease the tensions in the lines. A stretcher is a flexible section of the mooring line, made of nylon or some other synthetic fiber. One disadvantage of this method is that it can result in increased motions. Proper selection of the stiffness in the mating mooring lines resulted in reduced vessel motions and simultaneously reduced mooring line tensions. The mating moorings had maximum tensions calculated at a little less than 100 metric tons.

During the floatover operation, the environmental conditions were near the limit for 3-ft (1-m) following seas and a maximum tension approximately 100 metric tons was observed, confirming the calculated values.

The stern entry guide (SEG) with integrated cross-line catchers (CLC) was a significant innovation introduced by Dockwise. This was the first time such a device was implemented in a floatover operation. In a typical floatover, the aft mating moorings must be disconnected and reconnected once they cross, converting them from transverse lines that control stern motions to longitudinal lines that balance the forward mating lines. This step of disconnecting and reconnecting the lines was found to be unnecessary during the floatover process for the MDPP jacket.

The CLC effectively removed the need for this disconnection and reconnection by guiding the lines toward the rollers. Removing this step created the added advantage that each of the mating wires remained connected to its original connection point on the jacket and prevented possible confusion for the operators during positioning. Once the lines were against the rollers, they could be bent into the longitudinal mating position. The CLC had to be designed to handle all of the components passing through it ? the stretcher, steel wire, and joining shackle ? as well as the vessel motions. By integrating the CLC with the SEG, a one-piece structure was created, which saved precious deck space and kept the lines safety away from personnel.

Achieving a milestone

Once the Black Marlin entered the mating position, Dockwise used advanced ballasting systems to ballast the vessel and lower the topside atop the jacket legs. The eight OKI designed LMUs and DSUs transferred the load gradually from the vessel onto the MDPP jacket. By absorbing the impact loads caused by vessel movements onto the topside and jacket and by guiding the stabbing cone into the receptor pieces, the topside was perfectly installed and aligned onto the jacket.

The successful execution of the transport and floatover installation of CPOC’s MDPP processing platform is a milestone. Dockwise is currently preparing for four future floatovers. 

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