W22 Offshore Electrical Substation – Part 2 Network Development

PROBLEM DEFINITION

Further to week 21 and Part 1 of electrical substation planning, we will continue in our preparation of Electrical Substation programme development following the GPCC process / work flow commencing from step 3 (Identify / capture all activities). Week 21 blog and Part 1 has satisfied step 3 and the  identification of activities and scope definition using Omniclass table 21 as the basis.This weeks blog and Part 2 will detail methods and approaches to finalisation of Activity Sequencing and ensuring a high-quality network.

Part 3 will focus on the development and assignment of resources as well as the different methods for calculating activity durations

Part 4 will calculate the critical path and float based on target completion dates including validation checks

Part 5 will consider the development of a schedule risk analysis for the works

Part 6 will consider available options to respond to high potential for negative float within the programme

The development of the Logic Networks must consider thethe following and constraints and works sequences;

  1. Works can only commence upon approval of Design drawings
  2. Activity RC structure will be constructed using Precast Elements (Columns, beams, floors, roof) with topping screed for floor and roof areas.
  3. An external Precast manufacture will be appointed. After Appointment, it is expected that the columns and beams will be progressively delivered to site. First delivery is expected within 28 days after appointment however all precast must be complete before the commencement of erection activities for the Roof Structure inclusive of 1st floor columns.
  4. Columns and Structure for roof can only commence after 1st floor slab has been completed to avoid risk of working overhead.
  5. Permitter Scaffold erection can commence when 50% of Ground floor columns and beams are installed
  6. Columns for Transformer bay can only be installed until after 75% of Roof precast has been installed to ensure access for crane is maintained.
  7. Ground floor Block work may commence immediately after erection of 1st floor columns and beams and erection of permitter scaffold
  8. Scaffold cannot be removed until all cement rendering and painting has been completed for all external walls
  9. Assume that internal walls and external wall will commence simultaneously
  10. Internal finishing works can only commence after service rough in has been completed
  11. Internal finishing work sequence shall follow Wall, ceiling then flooring.
  12. External Handrail, access ladders and fixtures to be installed upon removal of scaffold
  13. Target Dates are as follows
    • Building to be “watertight by the 15th April 2018
    • Overall Completion by the 1st June 2018

FEASIBLE ALTERNATIVES

To develop the sequence and network logic we will consider the following methods

  1. PDM (Activity on Node)
  2. ADM (Activity on Arrow)

This blog will not select the preferred option – this will be undertaken in part 4

DEVELOPMENT OF OUTCOMES

To ensure that above constraints are accurately reflected within the network logic, suitable activity relationships must be selected. Table 1 below is a summary of the logic used within the sequence and network development (PDM).

Using the relationships and logic outlined in table 1, the following PDM network has been created – split into 3 sheets.

When considering the same constraints within a ADM network however, we cannot use the same Logical relationships between the activities as ADM method requires that all relationships be based on Finish to Start.

This means that lags and leads indicated in the PDM network to reflect the partial completion of certain activities before another activity can commence cannot be modelled in the same way.

For example, a constraint (No. 6) has been placed on the erection of Transfer columns such that this activity cannot commence until at least 75% of the roof has been installed due to crane position requirements for hoisting roof slab segments. Under a PDM network, we have conveniently used the S-S relationship on the Roof erection activity with a lag of 75% complete to reflect this constraint.

Under the ADM network, this cannot be modelled the same way as all relationships must be F-S in nature. Thus under the ADM, we have split the roof erection activities into > Erect Roof Slabs to GL 5 (75%) > Erect roof slab t GL 8 (50%) > Roof slab topping works. This will now ensure that the column erection cannot commence until 75% of the roof has been installed whilst maintaining the F-S relationships between all activities. The full ADM network is outlined below;

SELECTION CRITERIA

The following checklist (adapted from GAO best practices) will be used in assessing the quality of the proposed sequence and logic network

1. The schedule contains complete network logic between all activities so that it can correctly forecast the start and end dates of activities within the plan.
2. The majority of relationships within the detailed schedule are finish-to- start.
3. Except for the start and finish milestones, every activity within the schedule has at least one predecessor and at least one successor.
4. Any activity that is missing predecessor or successor logic—besides the start and finish milestones—is clearly justified in the schedule documentation.
5. The schedule contains no dangling logic. That is,
 – Each activity (except the start milestone) has an F–S or S–S predecessor that drives its start date.
– Each activity (except the finish milestone and deliverables that leave the project without subsequent effect on the project) has an F–S or F–F successor that it drives.
6. The schedule does not contain start-to-finish logic relationships.
7. Summary activities do not have logic relationships because the logic is specified for activities that are at the lowest level of detail in the schedule.
8. Instead of SNET constraints, conditions of supply by an outside vendor or contractor are represented as actual activities in the schedule.
9. Date constraints are thoroughly justified in the schedule documentation. Unavoidable hard constraints are used judiciously and are fully justified in reference to some controlling event outside the schedule.
10. Lags are used in the schedule only to denote the passage of time between two activities.
11. Instead of lags and leads, every effort is made to break activities into smaller tasks to identify realistic predecessors and successors so that logic interfaces are clearly available for needed dependency assignments.
12. If included in the schedule, lags and leads are used judiciously and are justified by compelling reasons outside the schedule in the schedule documentation.
13. The schedule is assessed for path convergence. That is, activities with many predecessors have been examined to see whether they are needed and whether alternative logic can be used to link some predecessors to other activities.

COMPARISON OF THE ALTERNATIVES

Using the GAO – best practices as a guideline we will compare / assess both the PDM and ADM logic networks.

PERFORMANCE MONITORING

After reviewing the above PDM network against the GAO best practices, the use of summary activities is not advised with SS and FF relationships / lags used to reflect column erection and precast activities could be improved or even eliminated through the use of lowest level activities making for a more reliable network and overall duration estimate.

Consider updating the PDM network for Part 4 blog and calculation of critical path and float.

References

  1. Chapter  7.4 – Create the Logical Realtionship ands & Sequence relationships – Guild of project controls compendium and reference (CaR) | Project Controls – planning, scheduling, cost management and forensic analysis (Planning Planet).  Retrieved from http://www.planningplanet.com
  2. O’brien, J.J, Plotnick, F.L, (2006). CPM in Construction Management-6th Edition Chapter 4 , McGraw-Hill
  3. GAO Schedule Assessment Guide, December 2015, Appendix IV