Coursework assignments

100% out of 100% total mark

Offshore Jacket Structures

Course Title:            Offshore Structural Design

Course Code:                 EA50JG

Assignment 1 (25%):                Preliminary design of Jacket structures

Assignment 2 (25%):                Loading and computational analysis

Assignment 3 (25%):                Design of tubular joints and foundation

Assignment 4 (25%):                Computational design of Jacket structures

Assignment 1 (25%): Preliminary design of Jacket structures The coursework project includes:

•   Coursework P1: Framing and bracing configurations and preliminary sizes.

Assignment 2 (25%): Loading and computational analysis

The coursework projects include:

•   Coursework P2: Hydrodynamic and wind loading and preliminary analysis.

•   Coursework P3: Modelling the initially sized jacket in P1 and applying the assumed vertical permanent and variable loading as well as the lateral loading derived in P2.

Assignment 3 (25%): Design of tubular joints and foundation The coursework projects include:

•   Coursework P4: Design of tubular joints.

•   Coursework P5: Foundation design.

Assignment 4 (25%): Computational design of Jacket structures The coursework project includes:

•   Coursework   P6:       Designing    the   jacket    members    using   the    more   accurate computational forces incorporating piles and check for fatigue.

The project is aligned with IStructE Chartered membership exam:

https://youtu.be/lDdOdF2aSFs?si=IrK_Cx3tfg-rzhvA

Submissions: Through submission links in MyAberdeen.

You are required to keep your own electronic and/or ‘hard’ copy of any work submitted.

Assessment criteria:

The assessment of the course works will be based on various items include:

•   A  complete  package  of  calculations  from  concept  design  to  detailed design.

•   Effective    presentation,     conveyance    and     communication    of    the information (2CGS marks for each of the assignments) . As a structural Engineer in design offices, you need to present your calculations to other members of the design team and your calculation results will be used by other Engineers to continue the work until the stage of construction and completion of the project. The way you present your calculations should flow and must be very clear to everybody not just yourself. You need to use sketches, drawings, and diagrams throughout the calculations.

•   Accuracy of the calculations is also very important for the same reason as above.

•   A sample calculation (2CGS marks for each of the assignments) covering the whole process of the design is required for all the hand-calculations. The repetitions can be tabulated.

Missing each of the above criteria may result in reduction of marks by one grade bande.g. if the calculations are complete and accurate but presentation is not satisfactory the grade maybe dropped to a B band.

All calculations can be handwritten or typed in the provided calculation sheets or a structured format of your own choice.

The general project description:

The client requires the conceptual design of an offshore substructure to support a well head platform in a water depth of approximately 34.5m (to LAT). Other alternative  concepts  have  already  been  ruled  out  for  this  site,  leaving  steel jacket structures as the only viable alternative. This assignment covers a range of integrated design courseworks that are required in the conceptual design of a jacket.  Design  data  for the jacket  structure  is  included  accordingly  in  this assignment.

The substructure is required to provide lateral support toten 0.762 m diameter conductors spaced at a minimum of 1.3 m centres. All conductors should be located within the conductor bay outline shown in figure below.

Functional loading:

The  topside  loads  on  the  wellhead   platform.  topside  consist  of  dry   loads, operating loads and live loads.

The  wellhead  deck  should  be  assumed  to  have  a  dry  load  of  2200  te  and operating load of 1500 te. The centre of gravity for the dry and operating loads should be assumed to beat the centre of the topside at 26.5 m above LAT.

The laydown area is shown below and should be assumed to have a load limit of 15 kN/m2.

Tidal levels for the platform. are given below.

MSL = LAT + 1.87m

HAT = LAT + 3.60m

The storm stillwater levels for different return periods are:

SWL (1 year) = MSL + 1.23 m                          SWL (100 year) = MSL + 2.01 m

Assignment 1- Coursework P1 (100%), Framing/bracing configurations and preliminary sizes:

Prepare adesign appraisal with appropriate sketches indicating between three to four distinct and viable structural solutions for the proposed substructure (i.e., from seabed to the +21m level) complying with the requirements of the platform framing  configurations  (20%  marks).  The  different  schemes  should consider aspects such as: the batter angle; the number of bracing bays, and the pattern of the frame. bracing. Preliminary framing and bracing sizes (70% marks) of at least three comparable choices (recommended choices are: 3 and 2-bay, X- bracing compared with 3 and 2-bay, diagonal bracing systems using the same batter angle for all the choices) should lead to and justify (10% marks) selection of one preferred scheme based on weight comparison of 4-sided jacket (i.e., the whole structure), structural performance and practical issues.

Assignment    2-    Coursework    P2    (50%),    In-place    loading    and preliminary analysis:

For the platform. layout given in the general description and the preferred jacket outlined in P1: a) Draw the wave and current velocity profiles. b) Calculate wave, current, wind and vertical forces applied to the jacket. c) Calculate the maximum base shear and overturning moment that must be resisted by the jacket and transferred to the pile foundation.

The design parameters are given in Tables as below.

Assume marine growth of 100 mm below SWL100. 

Platform location: 052°47'11"N, 003°09'36"E

Air Density = 1.24 kg/m3

Sea Water Density = 1025 kg/m3

Design wave parameters:

Design current profile:

Wind design parameters:

Assignment 2- Coursework P3 (50%), Computational modelling/analysis:

Simulate the initially sized jacket in P1 with beam elements employing a finite element software: apply the given permanent and variable loads to the topside platform. and input the environmental wave, current and wind loading tabulated in  P2.  Compare  the  base  shear  and  overturning  moment  derived  from  the computational  analysis  to  those   in  Coursework   P2  approximated   by  hand calculations  for  wave  +  current  +  wind  loads  and  discuss  (20%  marks)  the possible reasons for the difference in results (check with and without the effect of pile sleeves in hand calculations).

Report the computational work using graphics from the final model views, applied loads (detailed input parameters and tables for wave, current, wind and vertical loads) (20% marks) and analysis results including deformed shapes under vertical and lateral loads (20% marks), axial forces (filled diagrams for vertical and lateral load cases at 0, 45, 90-degree directions) (20% marks) and support reactions (show values) (20% marks).

Assignment 3-Coursework P4 (50%), Design of tubular joints:

For the preferred platform. jacket outlined in P1 and analysed in P3: Select and classify the most critical joint at the bottom plan level, based on the most critical load combination, comprising all the in-plane braces at that joint (10% marks). Check the joint detailing and validity range criteria are satisfied (10% marks) and calculate the utilisation factor for at least one set using an appropriate LRFD code  of  practice  (80%  marks).  For  braces  classified  as  K%  and  Y%  joints,  a combined  K/Y  effect  should  be  accounted  for  through  the   corresponding strength and chord force factors (i.e., Qu=K% × Quk  + Y% × QuY and qA=K% × qAk  + Y% × qAY).

The angles between the braces should betaken from your computational model. The joint detailing should be checked accounting for eccentricity/overlap of the braces based on a chosen/measured gap.

If the joint found to be over-utilised, or the joint detailing and validity check criteria  are  not  satisfied,  provide  a  solution  supported  by  engineering  joint detailing sketches of an update to the joint geometry that will enable it to resist the applied loads and satisfy the specified detailing criteria.

Assignment 3-Coursework P5 (50%), Design of pile foundations:

For the preferred platform. jacket outlined in P1 and analysed in P3 and based on the maximum axial and shear forces taken from the computational analysis results considering all the ASD load combinations:

Select a pile diameter and thickness to resist the maximum compressive pile head axial and lateral forces calculated in P3 with an interaction ratio between 1.0 and 1.33. Assume moderate to good pile driving condition (5% marks).

Design the most critical pile using the soil profile shown below, assuming both plugged and unplugged failures and discuss the governing failure. Use a single pile foundation  per  leg  (generally  a smaller  number  of  larger diameter  piles reduces the installation time) (80% marks).

For  both  the  plug  and  unplugged  designs  plot  the  variation  in  the  soil-pile tension and compression capacity to a depth of 70 mbelow seabed (15% marks).

Design soil profile:

Assignment 4-Coursework P6 (100%), Computational design:

For  the  simulated  and  analysed  jacket  in   P3  perform.  the  design  iteration inputting appropriate load combinations for full 360-degree wave approaches:

-    Incorporate the piles (designed in P4) into the computational simulation employing springs for axial and lateral stiffness based on appropriate t-z, Q-zand p-y soil-pile interaction characteristics. Design the most efficient jacket sections with piles (50% marks) with expected demand to capacity ratios (DCRs) of the legs and face braces being greater than 0.7 while lower DCRs may be acceptable for plan braces. The legs and face braces maybe grouped at each bay for practicality purposes.

-    Check  and  redesign  the  jacket  members  and  piles  based  on  updated dimensions/number of the piles to achieve a topside maximum lateral deformation   of  the   minimum   of   100   mm   and   80%   of   the   initial deformation under the operating environmental load with 1-year return period (40% marks).

-    Compare the initial and updated jacket and pile designs and discuss the most efficient solution based on minimum weight and practicality (10% marks).

Report the above computational work using 2D views of the designed sections and their design to capacity ratios of ALL the elevations and plans as well as tables and sketches for comparing the designs.

Note:  Local  buckling should  be avoided for all the designs following the  D/t limitations instructed in the preliminary sizing of the sections.