ALTERNATIVE ASSESSMENT OF WEATHER CRITERION FOR SHIPS WITH LARGE BREADTH AND DRAUGHT RATIOS BY A MODEL EXPERIMENT: A CASE STUDY ON AN INDONESIAN RO-RO FERRY

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient correponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

ON THE ROLL DAMPING OF AN FPSO WITH RISER BALCONY AND BILGE KEELS

Although the topic of roll damping of vessels at sea is already brought to the attention of naval architects by Froude more than 100 years ago, the physics of it remain intriguing, even today. An accurate prediction of the motions of offshore structures in harsh environments, designed for 25 years continuous operation, is the topic of this paper. Model test experiments for two FPSO’s developed by SBM for Petrobras are discussed. It is shown that the FPSO submerged riser balcony on one side of the vessel contributes to the roll damping through similar physics as the bilge keel does. Flow memory effects are discussed in detail since these are shown to have a noticeable effect on the roll damping coefficients. The paper further employs 3D CFD simulations to enhance the understanding of the fluid behaviour around the FPSO appendages, necessary to construct a rational and accurate roll damping model in the future.

Analisis Pengaruh Jumlah Bilge Keel terhadap Gerakan Rolling pada Kapal Patroli 14 m

AbstrakKapal Patroli merupakan kapal yang memiliki ukuran yang relatif kecil, namun memiliki kecepatan yang cukup tinggi. Bentuk kapal yang kecil akan berpengaruh terhadap gerakan rolling kapal, sehingga menarik untuk dibahas secara mendalam pengaruh pemasangan bilge keel terhadap gerakan kapal. Penelitian ini akan menganalisis pengaruh bilge keel yang ditambah jumlahnya menjadi 2 pasang dan 3 pasang terhadap efek gerakan rolling kapal agar dapat memperbaiki performa kapal menjadi lebih baik. Model kapal patroli yang telah didesain akan dianalisis menggunakan metode BEM (Boundary Element Method) yang terintegrasi untuk mendapatkan nilai rolling pada setiap model yang dianalisis. Pada penelitian ini, didapatkan kesimpulan bahwa penambahan jumlah bilge keel akan menambah RMS rolling sebesar 11-50% pada kecepatan rendah, namun dapat mengurangi RMS rolling sebesar 7-28% pada kecepatan tinggi.Kata kunci: Bilge Keel, Kapal Patroli, Olah Gerak Kapal, Rolling.AbstractA Study on the Effects of Bilge Keel Number on 14 m Patrol Boat’s Rolling Motion: Patrol boats can move at such high speed despite their relatively small size. This relatively small size will affect its rolling motions, hence the effect of bilge keel installation on ship motions is an interesting topic to discuss. In this study, the effect of adding bilge keel pair to 2 and 3 pairs of bilge keel on ship’s rolling motions was studied so that the ship’s performance becomes more optimal. The patrol boat model that had been designed was studied using integrated BEM (Boundary Element Method) to obtain each model’s rolling value. This study concluded that the addition of installed bilge keel pairs increased RMS rolling by 11-50% on low speed, but it reduced 7-28% RMS rolling at high speed.Keywords: Bilge Keel, Patrol boats, Ship motions, Rolling.

Effect of Bilge Radius and Bilge Keel Height on Roll Damping Performance of Floating Structures

A round bilge with a bilge keel structure is a key element which can alleviate roll motions of ships and floating structures by transferring the roll momentum of a floating body into the kinetic energy of water. This study presents a practical guide to properly designing a bilge radius and bilge keel height of a barge-shaped and tanker-shaped FPSOs. A parametric study to figure out the effect of bilge radius and bilge keel height on the roll damping performance is conducted through a series of numerical roll free decay simulations based on Computational Fluid Dynamics (CFD). The bilge radius is normalized by the half breadth of ship, and the bilge keel height is normalized by the maximum bilge keel height which is limited by the molded lines of a side shell and bottom shell. In addition, it is investigated to identify how the roll damping performance of a rectangular section differs from the result of a typical round bilge section with maximum available bilge keel height.

Low-aspect ratio appendages for wind-assisted ships

Wind propulsion for commercial ships has been identified as a key component in the energy transition for the maritime industry. The sailing hybrid ship will operate with leeway (drift) angles to produce a lateral force known as sideforce, for steady operation under sail. In this paper, experimental results for the sailing performance of ships fitted with bilge keel appendages are presented. Systematic variations in appendage height, length, and position were tested, including several special cases (multiple bilge keels). The appendage typology is shown to mitigate the strong ‘destabilizing’ yaw moment that is characteristic of wind-assisted commercial vessels and to promote the non-linear sideforce component. The working principal for bilge keels—promotion of flow separation—can be employed to specify the separation location for components of the vessel vortex wake to improve the sailing performance of the ship.

Alternative Assessment of Weather Criterion for Ships with Large Breadth and Draught Ratios By a Model Experiment: A Case Study on an Indonesian Ro-Ro Ferry

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient corresponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

Rancangan Konstruksi Kapal Penangkap Ikan 60 GT Untuk Nelayan Pelabuhan Perikanan Nusantara Karangantu Kota Serang

Rancangan perhitungan konstruksi kapal penangkap ikan mengacu kepada regulasi klas yaitu Biro Klasifikasi Indonesia (BKI) khusus untuk kapal perikanan yang terbuat dari bahan baja. Ukuran pokok kapal adalah Loa = 25, 45 m, B = 5,40 m, H = 2,20 m, dan T = 1,60 m, alat tangkap pancing yang ramah lingkungan, kecepatan 12,5 knots. Hasil rancangan konstruksi dimaksud dapat dijamin baik dan aman disebabkan perhitungannya mengacu kepada standar klas (BKI). Apalagi tetap mempertahan bagian konstruksi seperti bilge keel, fender dan penebalan ukuran tebal pelat kubu bagian paling atas dan top plate kubu. Bilge keel berfungsi untuk memperlambat periode oleng. Perlambatan periode oleng sangat bermanfaat agar ikan hasil tangkapan yang berada pada palka ikan (cold storage) tidak rusak akibat goncangan atau keolengan kapal. Fender berfungsi untuk melindungi benturan badan kapal dengan dermaga saat berlabuh, serta penebalan bagian paling atas kubu dan top plate kubu berfungsi untuk menambah ketegaran konstruksi kubu sebagai penahan atau pelindung awak kapal saat mancing. Terdapat fungsi lainnya dari ketiga bagian konstruksi tersebut yaitu untuk menambah modulus penampang konstruksi melintang kapal. Penelitian ini dimaksudkan sebagai upaya untuk membantu para nelayan PPN Karangantu agar dapat mengetahui dan memahami kosntruksi kapal perikanan yang terbuat dari bahan baja. Selain itu dapat memotivasi kelompok nelayan Karangantu agar mempunyai armada kapal ikan yang besar dan layak beroperasi pada perairan Samudera (ZEE) dan perairan internasional yang memiliki rumpon berbagai jenis ikan. Jika kelompok nelayan PPN Karangantu memiliki armada kapal penangkap ikan berukuran besar dengan jumlah yang banyak, berdampak kepada kinerja peningkatan produksi perikanan tangkap PPN Karangantu.