perlindungan pipa bawah laut

Banyak hal yang harus di lakukan oleh para engineer pipa bawah laut untuk memproteksi pipa bawah laut tersebut (pipeline protection) yang di akibatkan beberapa faktor antara lain: kapal- kapal yang tenggelam / karam, lego jangkar dan tarikan jangkar serta kejatuhan jangkar, kegiatan pengerukan, kegiatan perikanan oleh para nelayan, juga di sebabkan oleh scouring (perpindahan material di bawah air karena gelombang dan arus) juga bisa di sebabkan oleh tekanan langsung dari objek-objek padat.

Untuk menghindari kemungkinan terjadinya hal-hal tersebut di atas, ada beberapa solusi yang harus di lakukan untuk melindungi pipa bawah laut atau memproteksi pipa bawah laut tersebut, antara lain:

1. Increase wall / concrete thickness. Yaitu dengan cara melapisi pipeline tersebut dengan campuran beton, seperti terlihan pada gambar.1 dibawah ini, fungsi utama dari system ini adalah sebagai pemberat untuk stabilitas pipa di dasar laut. Disamping itu untuk membuat pipa tahan terhadap potential impact damage dari pukat kapal ikan, kejatuhan barang-barang dan sejenisnya.

Gambar.1 pipeline dengan pelapis dari campuran beton

2. Concrete armor cover. Yaitu dengan cara melapisi pipeline atau melindungi pipeline tersebut dengan campuran beton, dengan cara seperti inni diharapkan pipeline didasar laut dapat terlindungi dari kejatuhan benda-benda seperti jangkar dan lain-lain. lihat gambar dibawah ini:

Gambar.2 concrete armor layer

3. Engineering backfill. Dilakukan dengan penutupan beton, bahan-bahan alam, bahan urugan yang direkayasa untuk keperluan ini (engineered backfill material-graded rock). Pipeline di proteksi dengan cara ini agar pipeline terlindungi dari pukulan berulang karena aksi gelombang, dan pukulan jangkar yang dijatuhkan. Gambar.2 dibawah ini menunjukkan gambar sebuah penutup pipa yang terbuat dari beton yang nantinya akan di pasang untuk menutupi pipeline yang di pasang didasar laut. Gambar.3 menunjukkan penutupan pipa bawah laut menggunakan tumpukan material baatu yang di jatuhkan langsung dari kapal barge.

Gambar. 3 penutup pipa yang di buat dari susunan beton

Gambr.4 penutupan pipeline di dasar laut dengan batuan alam

4. Trenching. Yaitu dengan cara membuat parit pipa di dasar laut, pada dasarnya ada dua cara yang di lakukan yaitu open cut method yaitu parit pipa di buat sebelum atau sesudah pemasangan pipa, dan no dig method (tanpa penggalian) dimana pipa melewati rintangan (obstacles) tanpa ada pekerjaan penggalian ataupun pengerukan. Cara lain adalah dengan membangun terowongan (tunnel)atau dibor lubang mendatar di bawah rintangan tersebut, yang bisa jauh di lepas pantai, dan kemudian pipa ditarik terowongan atau lubang tadi. Untuk melakukan pekerjaan ini bisa dilakukan pada semua jenis tanah kecuali gravel dan boulders (maksimal 20%,gravel), atau semua batuan (apabila tidak terlalu banyak pecahan batu). Untuk panjang 1500 meter dapat dibor lubang dengan diameter 20-24”, dan untuk panjang 1000 meter sampai dengan 40”. Ukuran lubang yang dibor biasanya sekitar 1,5x diameter pipa. Tidak diperlukan atau sedikit sekali lapisan coating pada pipa. Peralatan yang di gunakan antara lain: bajak (plough) atau semburan air (water jet), mechanical trenches, trailing suction hopper dredger, cutter suction dredger, grab dredger atau backhoe dredge, dan lain sebagainya.

Gambar.5 parit bawah laut

Gambar. 6 tranching untuk pipa di darat

5. Anchoring for stability. Yaitu dengan cara mengaitkan pipeline dengan anchor (jangkar) di sekitar pipeline tersebut, hal ini dilakukan supaya pipeline lebih kuat dan tidak mudah goyang dan tertekuk. Juga dapat menjadikan stabilitas pipeline tersebut menjadi lebih besar. Lihat gambar di bawah ini:

Gambar.7 anchor pipeline bawah laut

Beban-beban anjungan lepas pantai menurut LRFD

Macam-Macam Beban yang Bekerja Pada Anjungan Lepas Pantai

Pembahasan untuk beban yang bekerja pada anjungan lepas pantai meliputi antara lain nilai nominal pada beban, prosedur untuk mendefinisikan beban eksternal, faktor beban dan metode untuk menghitung gaya internal.

a. Beban Gravitasi.

Setiap member, joint dan pondasi harus dicek kekuatannya untuk gaya internal ( Q ) yang dinyatakan sebagai berikut:

Q = 1,2 D + 1,6 L + 0,5 (Lr or S)

Dimana :

D = Dead Loads

L = Live Loads

Lr = Roof Live Loads

S = Snow Load

Variasi pada supply berat dan lokasi peralatan yang mudah berpindah harus diperhitungkan untuk mencari nilai Q maksimal. Area beban yang spesifik boleh digunakan untuk menyatakan beban gravitasi normal pada deck platform. (API-LRFD hal. 26)

b. Beban Angin, Gelombang dan Arus.

Untuk analisa statis gelombang dimulai dengan spesifikasi tinggi dan periode gelombang, storm water depth dan profil arus. Untuk desain kriteria angin harus didefinisikan berdasarkan analisa data angin. Sedangkan untuk beban arus dengan perhitungan gaya drag melalui persamaan Morison.

Untuk beban angin, gelombang dan arus pada kondisi ekstrim terjadi dalam rentang waktu ± 100 tahun. Beban angin, gelombang dan arus harus diantipasi sedikitnya 8 arah untuk struktur simetris dan 12 arah untuk struktur asimetris. (API-LRFD hal.26-27)

c. Beban Gempa Bumi.

Analisa kekuatan dibutuhkan platform untuk memastikan tidak ada kerusakan struktur secara signifikan karena goncangan gempa.

1. Strength requirement

Setiap member, joint dan pondasi harus dicek kekuatannya untuk gaya internal ( Q ) yang dinyatakan sebagai berikut:

Q = 1,1 D₁ + 1,1 D₂ + 1,1 L₁ + 0,9 E

Dimana:

D₁ ( Dead Load 1 ) merupakan berat struktur itu sendiri yang meliputi berat struktur di udara, berat peralatan dan seluruh obyek permanen yang tidak berubah saat operasi, gaya hidrostatik dan berat air ballast permanen maupun sementara.

D₂ ( Dead Load 2 ) merupakan beban karena berat peralatan dan obyek yang lain meliputi berat peralatan produksi dan pengeboran, berat living quarter, heliport, peralatan life-support, peralatan selam dan peralatan lain yang dapat dipindahkan dari platform.

L₁ ( Live Load 1 ) termasuk beban fluida pada pipa dan tanki.

L₂ ( Live Load 2 ) merupakan gaya berdurasi pendek pada struktur karena operasi lifting pada drill string, lifting oleh crane, operasi mesin, vessel mooring dan beban helikopter.

E adalah tingkat kekuatan gempa. Ketika gaya inersia karena beban gravitasi menentang gaya internal karena beban gravitasi maka Q dinyatakan dengan:

Q = 0,9 D₁ + 0,9 D₂ + 0,8 L₁ + 0,9 E

2. Ductility requirement

Hasil global sistem pondasi struktur harus dinyatakan untuk memenuhi respon. Untuk struktur jaket dengan kaki lebih dari 8 tidak mebutuhkan analisa saluran jika berada di daerah dengan rasio intensitas tingkat kekuatan gempa kurang dari 2 dan sebaliknya.

3. Additional guidelines

Saat desain tingkat kekuatan pergerakan tanah horisontal lebih dari 0,05 gram, joint member struktur utama haru diukur untuk mengatasi beban buckling. Kapasitas joint harus didefinisikan pada dasar beban nominal pada brace.

(API-LRFD hal.44-46)

d. Beban Fabrikasi dan Instalasi.

Yaitu antara lain meliputi:

a. Gaya Lift

b. Gaya Loadout

c. Gaya Transportasi

d. Gaya Launching dan Gaya Uprighting

e. Gaya Pondasi Instalasi

f. Gaya Removal (API-LRFD hal.46-48)

e. Beban Accidental.

Anjungan lepas pantai dapat mengalami berbagai beban accidental seperti: tubrukan kapal dengan barge, pengaruh benda yang terjatuh, ledakan atau kebakaran. Untuk itu perlu ada pertimbangan pada perancangan, layout maupun perubahan pada fasilitas dan peralatan struktur untuk meminimalkan efek dari beban accidental ini. (API-LRFD hal.48)

Kondisi Pembebanan

Kondisi desain beban lingkungan adalah gaya yang bekerja pada platform sesuai dengan pilihan kondisi desain, sedangkan kondisi operasi desain beban lingkungan adalah gaya yang bekerja pada struktur dengan kondisi yang lebih minim yang tidak cukup mewakili kondisi operasi normal.

Kondisi desain pembebanan

Platform harus didesain untuk kondisi pembebanan yang tepat, yang akan memberikan efek yang sangat besar pada struktur. Kondisi pembebanan harus termasuk kondisi lingkungan yang dikombinasikan secara tepat dengan beban hidup dan beban mati. Contoh, pengoperasian kondisi lingkungan dikombinasikan dengan beban mati dan beban hidup maksimal cocok dengan operasi normal pada platform.

1. Kondisi pembebanan sementara

Kondisi ini terjadi terjadi selama fabrikasi, transportasi, instalasi atau perpindahan dan instalasi ulang dari struktur. Untuk kondisi ini kombinasi dari kesesuaian beban mati, beban temporer maksimal dan beban lingkungan harus dipertimbangkan.

2. Pembebanan pada member

Tiap member platform harus didesain untuk kondisi pembebanan yang memberikan tegangan maksimal pada member sebagai pertimbangan tegangan ijin pada kondisi pembebanan yang menghasilkan tegangan ini. (API-LRFD hal.24-25)

Report On Ocean Wave Energy Conversion Projects

Summary

This report presents a brief overview of the recent wave-energy developments in Japan, and summarizes recent work on the "Mighty Whale" floating prototype at the Japan Marine Science and Technology Center (JAMSTEC). It also outlines a part of the author's project on active control methods to optimize energy conversion by floating devices in irregular waves.

Introduction

Serious research on wave-energy extraction methods began in several countries during the 1970's. Wave-energy projects in Japan began drawing attention in the late '70's, after JAMSTEC tested the world's first large-scale offshore floating prototype "Kaimei" in the Sea of Japan. The "vessel" powered 9 generators on board. These were mounted above lengthwise chambers open to the sea at the bottom. Wave action caused the internal water levels to rise and fall, forcing an alternating airflow that was used to drive air turbines. The main idea was first used by Masuda (e.g. Masuda, 1985) in the 1960's, to power light buoys serving as navigational aids to shipping.

The "oscillating water column" (OWC) method has become very popular since the Kaimei tests, and forms the core of a number of prototypes of current interest in Japan and elsewhere. These include the "Mighty Whale" offshore floating prototype, which has been under development at JAMSTEC since 1987. Sea-trials on this device are scheduled to begin in July 1998.

Kaimei and Mighty Whale differ from other large-scale prototypes in Japan and elsewhere in being floating, as opposed to shore/seafloor supported, devices. Other recent large-scale developments in Japan include the Caisson-type Oscillating Water Column prototype, the Pendulor prototypes, the Constant-Pressure Manifold device, and the Water-Valve Rectifier device.

Recent results of ongoing projects worldwide can be found in EU (1996).

Developments in Japan

1. Caisson-type Oscillating Water Column

This was developed by the Port and Harbor Research Institute of the Ministry of Transport. The principal dimensions of each caisson were 20.9 m X 24.3 m X 27.0 m. The operating water depth was approximately 18 m. Each caisson had 4 openings on its front surface facing the predominant wave direction. Corresponding with the frontal openings, partition walls divided the internal space into 4 chambers. Incident waves caused the water level inside each to rise and fall. The piston action of the oscillating water column (OWC) forced reciprocating airflow over power absorbing air turbines. Self-rectifying (i.e. spinning in the same direction in a reciprocating airflow) "Wells" turbines (1.34 m diameter, 16 blades) in tandem configuration were used. The generators were each rated at 60 kW. The prototype tests were conducted in the Sea of Japan, near Sakata Port in Yamagata Prefecture.

2. "Pendulor" Device

This device is being developed by Muroran Institute of Technology and Cold-Region Port and Harbor Research Center. A hinged plate with frontal dimensions 2.0 m X 6.5 m is mounted inside a caisson (with the hinge at the top). Wave action causes oscillation of the plate ("pendulor"), and the pendulor compresses fluid in a hydraulic power take off. The second-generation prototype uses active control for efficient energy conversion. Tests are underway off Muroran Port in Hokkaido.

3. Constant-Pressure Manifold Device

This device was operational from 1988 to 1997 near Kujukuri beach, Chiba Prefecture. It was developed and operated by Takenaka Komuten Co. The device had 10 cylindrical OWC chambers all connected to a constant-pressure air manifold through one-way valves. Additional one-way valves on the OWC chambers allowed only upward water column motion to generate sufficient airflow. The constant-pressure manifold enabled use of a constant-RPM radial air turbine. This was coupled to a 30 kW generator to produce a smooth AC output at 200 Volts, 50 Hz.

4. Water-Valve Rectifier Device

The device has been developed by Tohoku Power Co., and Mitsui Engineering and Shipbuilding Co. This is another OWC-based device producing smooth, constant voltage AC output. The device spans 20 m, and is mounted in a breakwater serving the Haramachi coal-fired power station in Fukushima Prefecture. Rectification and smoothing in this case are both provided by a water-valve placed above the OWC chamber. The water-valve directs airflow to an impulse turbine with tandem rotors. The rated generator output is 130 kW. The device has been in operation since 1996. Current experiments are expected to conclude in spring 1998.

5. Offshore Floating Device "Mighty Whale"

This device has been developed by JAMSTEC, under funding from the Science and Technology Agency (STA). Envisioned applications include fish farming and coastal-water aeration. Work on this device has been in progress since 1987, and the final prototype design is based on results from a number of laboratory tests at progressively increasing scales. The prototype tends to resemble a whale in appearance, and is 50 m X 30 m X 12 m in overall dimensions. It is designed to float at a draft of 8 m at even keel, and is to be moored in a water depth of 40 m. The device has three oscillating water column chambers distributed breadth-wise.

Prototype construction is in steel and structural design is in accordance with NK (Japan Classification Society) Part P regulations for special-purpose floating platforms. The device has been in construction at the Ishikawajima Harima Heavy Industries (IHI) Aioi Shipyard in Hyogo Prefecture. A 1.7 m diameter, Aluminum-alloy, biplane (i.e. with tandem rotors), self-reciprocating Wells turbine is provided for each chamber. Each rotor has 8 camberless blades with the NACA 0021 profile. Four induction generators are used on board. Two of the four generators are rated at 30 kW, one at 50 kW, and one at 10 kW. A 7.5 kW air compressor is also provided. The total rated capacity is 110 kW, and automatically controlled switching circuitry selects the appropriate combination of generators at any given time.

Mooring is by means of 6 lines (each with intermediate weights) terminating in box-type anchors. The test site is just outside the mouth of Gokasho Bay off Mie Prefecture. Experiments are scheduled to begin in July 1998 and continue through July 2000. More than 48 parameters are to be monitored and analyzed on board. Many of the results are expected to be transmitted to shore. In addition to generating technologically important data, the device is also expected to serve as an offshore platform for measurements of interest to Oceanographic and Environmental sciences.

NSF/STA Project: Optimal control of floating wave energy devices

Offshore floating devices such as the Mighty Whale have at least three advantages over shore/seafloor fixed devices: First, significant economy results from the fact that floating hulls experience considerably lower impact loads in extreme wave conditions. Second, available wave energy generally increases with increasing water depth (exceptions are shallow-water regions where refractions due to favorable bottom topography can lead to localized focusing of energy). Third, rigid-body motion of the hull can increase energy absorption in certain wave conditions, due to increased relative motion between OWCs and hull.

Part of this project concerns the third effect. For axisymmetric, primarily heaving buoys supporting OWCs [e.g. McCormick (1976), Masuda (1985)], the buoy heave motion is generally found to increase energy absorption. However, theory and laboratory experiments on a Kaimei-type floating OWC device (Maeda, et al. 1985) have shown the rigid-body motion of the hull to reduce energy absorption at most wave frequencies of interest. Similar observations have also been made with Mighty Whale laboratory models.

For this reason, a system is being developed as a part of this work, whereby the rigid-body motion of the Mighty Whale hull can be utilized to supplement the energy already being generated by the OWCs. The design constraints are: to achieve this without the use of actuated moorings or tethers, to retain the original advantages of a floating hull, and to minimize any energy required to operate such a system.

The system exploits favorable interaction of two or more coupled oscillators (spring-mass systems) whereby one of the masses is locked into zero displacement at a certain frequency. This phenomenon is often used in "dynamic vibration absorbers", and can be utilized in heave compensation systems for drill ships. Real-time control of at least one of the oscillators is necessary in the case of a floating wave-energy device. This is because ocean waves are irregular (when not very large, an irregular wave can be considered to be a superposition of a large number of sinusoidal waves of different frequencies), and the phenomenon above occurs at a single frequency. Control using correctly applied reactive forces allows the phenomenon to be extended to a range of frequencies (Korde, 1998). Thus, when waves and hull motions are not very large, such control produces good hull-motion compensation in irregular waves.

Passive versions of the system were tested on a 1/62.5 scale model of the Mighty Whale device; as well as on a "point-absorber" type axisymmetric buoy consisting of an OWC in a long tube inside a flotation collar. In both cases the system was arranged to provide compensation against hull-heave for a platform placed near the roof of the model, across the vertical airflow passage. With the hull heaving and the platform stationary the piston action of the platform was expected to increase the magnitudes of overall airflow and gage pressure in the chamber. Improvement in energy absorption was noticed for both devices near the frequencies at which effective motion compensation was achieved. The tests also highlighted some essential design aspects to be considered prior to building an active system.

Experiments on the active system are expected to begin shortly once the control hardware is fully fabricated and tested. The active tests are to be carried out with the compensator arranged in the surge mode, because this is the dominant mode for the Mighty Whale at low frequencies, and because considerably more energy is carried by incident waves at low frequencies. For precise quantification of the additional energy-absorption enabled by the system, the model is to be constrained (from above) to pure surge motion. A Scott-Russell mechanism to achieve this is currently being tested. Calculations based on numerically computed hydrodynamic parameters are being carried out to validate/interpret the experimental results.