Free Span Pada Pipa Bawah Laut

Apa yang dimaksud dengan 'free span' ? Sedikit gambarannya kira-kira seperti ini

Free span adalah bentang pipa di antara 2 tumppuan (support). Panjang bentangan ini ada kriterianya yang harus diikutin. Misalnya untuk kondisi installation, maksimal panjang free span adalah 30m, jadi ketika ada free span dengan panjang lebih dari 30m maka free span itu harus direktifikasi. Rektifikasi ini tujuannya untun mengurangi panjang free span, bisa dengan memasang grout bag di tengah free span sehingga panjangnya jadi setengahnya, atau bisa dengan menghancurkan salah satu support nya supaya pipa nya turun. Ada ada banyak cara lainnya. Pokoknya bagaimana itu free span bisa hilang atau berkurang panjangnya.

Di project stage, sebelum pipa mulai dipasang, kita sebagai pipeline engineer diminta untuk memprediksi berapa jumlah free span yang melebihi kriteria. Dan bagaimana saja bentuk free span yang mungkin terjadi (panjang, tinggi dan tipe free span). Semua ini adalah prediksi sehingga kita bisa membuat rencana mau diapain itu free span dan berapa biayanya.  Memprediksi free span agak kompleks karena banyak sekali variable yang tidak jelas, seperti jenis tanah, kontur seabed, residual tension yang ada di pipa setelah laying, dan lainnya. 

Jenis tanah mungkin bisa kita dapat, tapi apakah semua akurat? Belum tentu. Dengan jarak cuma 1 meter aja jenis tanah bisa berbeda . Pas kita bor untuk ambil sample tanahnya lembek (dan ini yang kita pakai di analysis), tapi pas masa installasi ternyata 1 meter di sebelah lokasi bor itu ada batu-batuan (ini gak kita pertimbangkan dalam analysis karena gak ada data sebelumnya). Terus kontur seabed, ketika seabednya berupa sand, maka sudah dpastikan kontur seabed akan mudah berubah. Kontur seabed dari hasil survey yang nantinya kita gunakan dalam analisa belum tentu sama dengan kontur seabed sebenarnya nanti saat instalasi. Dan juga masalah residual tension yang ada di pipe dari hasil proses laying, belum ada rumus yang akurat yang bisa menghitung berapa nilainya. Jadi intinya, semuanya serba asumsi untuk melakukan analisa prediksi terjadinya free span ini. 

Software yang bisa digunakan untuk menganalisis free span ini salah satunya OFFPIPE, ini salah satu yang umum dipakai oleh para pipeline engineer dalam mendesain pipa. Dalam OFFPIPE, kita akan memasukkan variabel-variabel untuk dianalisis, nah untuk free span ini ada 4 input utama :

• Pipe properties (*PIPE)
• Kontur seabed, x dan y koordinat (*SPAN),
• Soil stiffness dan soil friction (*SOIL),
• Residual tension (*TENS),
• Untuk flooded case bisa dimasukan fluid density (*FLUI)

Dengan input sederhana tersebut berapa panjang free span yang akan terjadi bisa diprediksi. Namun sekali lagi kontur dan fitur seabed itu random banget, jadi tetap berbagai asumsi harus dilakukan dalam analisis software.. semakin banyak variabel yang diperhitungkan akan semakin baik tentunya

Pipeline Laying Methods

Laying pipe on the seafloor can pose a number of challenges, especially if the water is deep. There are three main ways that subsea pipe is laid -- S-lay, J-lay and tow-in -- and the pipelay vessel is integral to the success of the installation.

Buoyancy affects the pipelay process, both in positive and negative ways. In the water, the pipe weighs less if it is filled with air, which puts less stress on the pipelay barge. But once in place on the sea bed, the pipe requires a downward force to remain in place. This can be provided by the weight of the oil passing through the pipeline, but gas does not weigh enough to keep the pipe from drifting across the seafloor. In shallow-water scenarios, concrete is poured over the pipe to keep it in place, while in deepwater situations, the amount of insulation and the thickness required to ward of hydrostatic pressureis usually enough to keep the line in place.

Tow-In Pipeline Installation

While jumpers are typically short enough to be installed in sections by ROVs, flowlines and pipelines are usually long enough to require a different type of installation, whether that is tow-in, S-lay or J-lay.

Tow-in installation is just what it sounds like; here, the pipe is suspended in the water via buoyancy modules, and one or two tug boats tow the pipe into place. Once on location, the buoyancy modules are removed or flooded with water, and the pipe floats to the seafloor.

Surface Tow Pipeline InstallationSource: www.pipelife.no

There are four main forms of tow-in pipeline installation. The first, the surface tow involves towing the pipeline on top of the water. In this method, a tug tows the pipe on top of the water, and buoyancy modules help to keep it on the water's surface.

Using less buoyancy modules than the surface tow, the mid-depth tow uses the forward speed of the tug boat to keep the pipeline at a submerged level. Once the forward motion has stopped, the pipeline settles to the seafloor.

Off-bottom tow uses buoyancy modules and chains for added weight, working against each other to keep the pipe just above the sea bed. When on location, the buoyancy modules are removed, and the pipe settles to the seafloor.

Lastly, the bottom tow drags the pipe along the sea bed, using no buoyancy modules. Only performed in shallow-water installations, the sea floor must be soft and flat for this type of installation.

S-Lay Pipeline Installation

When performing S-lay pipeline installation, pipe is eased off the stern of the vessel as the boat moves forward. The pipe curves downward from the stern through the water until it reaches the "touchdown point," or its final destination on the seafloor. As more pipe is welded in the line and eased off the boat, the pipe forms the shape of an "S" in the water.

S-Lay Pipeline InstallationSource: www.pbjv.com.my

Stingers, measuring up to 300 feet (91 meters) long, extend from the stern to support the pipe as it is moved into the water, as well as control the curvature of the installation. Some pipelay barges have adjustable stingers, which can be shortened or lengthened according to the water depth.

Pipe being lowered into the water via a stinger for S-lay installationSource: www.nord-stream.com

Proper tension is integral during the S-lay process, which is maintained via tensioning rollers and a controlled forward thrust, keeping the pipe from buckling. S-lay can be performed in waters up to 6,500 feet (1,981 meters) deep, and as many as 4 miles (6 kilometers) a day of pipe can be installed in this manner.

J-Lay Pipeline Installation

Overcoming some of the obstacles of S-lay installation, J-lay pipeline installation puts less stress on the pipeline by inserting the pipeline in an almost vertical position. Here, pipe is lifted via a tall tower on the boat, and inserted into the sea. Unlike the double curvature obtained in S-lay, the pipe only curves once in J-lay installation, taking on the shape of a "J" under the water.

J-Lay Pipeline InstallationSource: www.technip.com

The reduced stress on the pipe allows J-lay to work in deeper water depths. Additionally, the J-lay pipeline can withstand more motion and underwater currents than pipe being installed in the S-lay fashion.

J-Lay Pipelay Vessel S7000Source: www.hydro.com

Types Of Pipelay Vessels

There are three main types of pipelay vessels. There are J-lay and S-lay barges that include a welding station and lifting crane on board. The 40- or 80-foot (12- or 24-meter) pipe sections are welded away from wind and water, in an enclosed environment. On these types of vessels, the pipe is laid one section at a time, in an assembly-line method.

On the other hand, reel barges contain a vertical or horizontal reel that the pipe is wrapped around. Reel barges are able to install both smaller diameter pipe and flexible pipe. Horizontal reel barges perform S-lay installation, while vertical reel barges can perform both S-lay and J-lay pipeline installation.

Vertical Reel BargeSource: www.jee.co.uk

When using reel barges, the welding together of pipe sections is done onshore, reducing installation costs. Reeled pipe is lifted from the dock to the vessel, and the pipe is simply rolled out as installation is performed. Once all of the pipe on the reel has been installed, the vessel either returns to shore for another, or some reel barges are outfitted with cranes that can lift a new reel from a transport vessel and return the spent reel, which saves time and money.

Sumber : www.rigzone.com