Cho em hỏi về cách tính toán cao độ mặt sàn H Deck platform

Mình bổ sung chút cho hoàn thiện topic, để tiện refer sau này khi cần
Trong tính toán thiết kế các giàn khoan cố định công trình biển có 2 cao độ cần xác đinh cho chuẩn bao gồm 1. Cao độ đáy của sàn thấp nhất trên topside
2. Độ sâu nước tính toán
Cả 2 thông số này đều ảnh hưởng tới các bài toán trong giai đoạn in-service nói chung, Những sai số nếu có sẽ ảnh hưởng tới khả năng khai thác vận hành giàn sau khi xây dựng xong (ví dụ giàn sẽ bị nước biển bắn lên khi hoạt động, hoặc boat khó tiếp cận khi giàn tke cao độ độ sâu nước quá cao....)
Có những tiêu chuẩn và quy phạm hướng dẫn cách tính, dưới đây là một số thông tin tham chiếu.
Hình 1: Sketch

Hình 2: Tính độ sâu nước tính toán Water depth

Hình 3: Tính cao độ sàn thấp nhất trên Topside

 

API RP 2A, 2014
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5.3.4.3 Deck Clearance
Large forces result when waves strike a platform’s deck and equipment. To avoid this, the bottom of the lowest deck should be located at an elevation that will clear the calculated crest of the design wave with adequate allowance for safety. For new platforms in the Gulf of Mexico, the elevation for the underside of the deck shall not be lower than the 1000-year return period maximum crest elevation provided in API 2MET or site-specific data developed in accordance with the requirements of API 2MET. For new L-3 platforms, the deck may be located below the 1000-year return period maximum crest elevation only if the entire topsides are located below the calculated crest elevation of the design wave designated for L-3 structures. In this case, the full wave and current forces on the topsides shall be considered. API 2SIM provides guidance for predicting the wave/current forces on the deck and topsides.
An air gap, the distance between the maximum crest elevation used for deck clearance and the bottom of
steel on the lower deck, shall be provided for any known or predicted long term seafloor subsidence, both
regional and that due to hydrocarbon extraction. An additional air gap should be allowed to account for
structures that experience significant structural rotation or “set down.”
In general, no platform components, piping or equipment should be located below the lower deck.
However, when it is unavoidable to position such items as minor subcellars, sumps, drains, or production
piping below bottom of steel on the lower deck, provisions should be made for the wave forces developed
on these items. These wave forces may be calculated using the crest pressure of the design wave
applied against the projected area. These forces may be considered on a “local” basis in the design of the
item. These provisions do not apply to vertical members such as deck legs, conductors, risers, etc., which
normally penetrate the air gap.

 

Một hình ảnh minh họa khác

Why do we need to make sure our topsides is above the water, and won't be hit by the waves?
1. Safety. To avoid the operator from falling into the sea when waves hit the topsides
2. Operational. Ensure equipment do not trip when submerged under water.
So, how do we calculate the air gap?
1. Maximum wave height, Hmax. It used to be based on the 100 year Hmax. Nowadays it can be 10,000 years wave height depending on the codes you used. Note: Highest point is 0.5m*Hmax, the distance between the peak and midpoint between trough and peak of the wave
2. Highest Astronomical Tide, HAT above the Mean Sea Level, MSL
3. Storm surge, caused by wind
4. Subsidence
5. 0.5 m clearance above max wave height
Thus you end up with
0.5*Hmax + HAT + Storm surge+ subsidence+ 0.5m
Note: 0.5Hmax is assuming Airy Wave Theory. 0.6 if using Stokes.
Assuming Hmax=10m, HAT=2.5m, Surge=1m, subsidence=0
The air gap, or bottom of Cellar Deck is
= 5m+ 2.5m+1m+0.5m = 9m